Angewandte Chemie International Edition

Reconstitution of Low-Density Lipoproteins with Fatty Acids for the Targeted Delivery of Drugs into Cancer Cells ()
Low-density lipoproteins (LDLs) represent a novel class of nanocarriers for the targeted delivery of therapeutics into aberrant cells that overexpress the LDL receptor. Here we report a facile procedure for reconstituting the hydrophobic core of LDLs with a binary fatty acid mixture. Facilitated by the tumor targeting capability of the apolipoprotein, the reconstituted, drug-loaded LDLs can effectively target cancer cells that overexpress the LDL receptor while showing minor adverse impact on normal fibroblasts. According to a hypothesized mechanism, the reconstituted LDLs can also enable metabolism-triggered drug release while preventing the payloads from lysosomal degradation. This study demonstrates that LDLs reconstructed with fatty acids hold great promise to serve as effective and versatile nanocarriers for targeted cancer therapy.
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Palladium-Catalyzed Fluoroarylation of gem-Difluoroalkenes ()
Pd-catalyzed fluoroarylation of gem-difluoroalkenes with aryl halides is disclosed. By taking advantage of the in situ generated α-CF3-benzylsilver intermediates derived from the nucleophilic addition of silver fluoride onto gem-difluoroalkenens, the present strategy bypasses the employment of strong base, thus enabling a mild and general synthetic protocol for the ready access of non-symmetric α,α-disubstituted trifluoroethane derivatives.
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Manganese catalyzed C-H functionalizations involving an unexpected heteroaryl-shift ()
A manganese catalyzed regio- and stereoselective hydroarylation of allenes is reported. The C-H functionalization protocol offers access to various alkenylated indoles with excellent yields. Moreover, a hydroarylation/cyclization cascade involving an unexpected C-N bond cleavage and aryl shift has been developed which provides a new synthetic approach to substituted pyrroloindolones.
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A Highly Efficient Synthesis of Z-Macrocycles using Stereoretentive, Ruthenium-Based Metathesis Catalysts ()
A highly efficient, Z-selective ring-closing metathesis system for the formation of macrocycles using a stereoretentive, ruthenium-based catalyst supported by a dithiolate ligand is reported. This catalyst is demonstrated to be remarkably active as observed in initiation experiments showing complete catalyst initiation at -20 °C within 10 min. Macrocyclization reactions generated Z-products from easily accessible diene starting materials bearing a Z-olefin moiety. This stereoretentive approach provides a more efficient and selective route to Z-macrocyles than in previously reported systems. Reactions were completed in appreciably shorter reaction times, and turnover numbers of up to 100 could be achieved. Macrocyclic lactones ranging in size from twelve-membered to seventeen-membered rings are synthesized in moderate to high yields (68 - 79% yield) with excellent Z-selectivity (95% - 99% Z).
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Heterogeneous Microtesla SABRE Enhancement of 15N NMR Signals ()
The hyperpolarization of heteronuclei via Signal Amplification by Reversible Exchange (SABRE) was investigated under conditions of heterogeneous catalysis and microtesla magnetic fields. Immobilization of [IrCl(COD)(IMes)], [IMes = 1,3-bis(2,4,6-trimethylphenyl), imidazole-2-ylidene; COD = cyclooctadiene] catalyst onto silica particles modified with NH2(CH2)3- linkers engenders an effective heterogeneous SABRE (HET-SABRE) catalyst that was used to demonstrate ~102-fold enhancement of 15N NMR signals in pyridine at 9.4 T following parahydrogen bubbling within a magnetic shield. No 15N NMR enhancement was observed from the supernatant liquid following catalyst separation, which along with XRD characterization, supports that the effects result from SABRE under heterogeneous catalytic conditions. The technique can be developed further for producing catalyst-free agents via SABRE with hyperpolarized heteronuclear spins, and thus is promising for biomedical NMR and MRI applications.
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Bacteriophage Tail Tube Assembly Studied by Proton-Detected 4D Solid-State NMR ()
Obtaining unambiguous resonance assignments remains a major bottleneck in solid-state NMR studies of protein structure and dynamics. Particularly for supramolecular assemblies with large subunits (>150 residues), the analysis of crowded spectral data presents a challenge, even if three-dimensional (3D) spectra are used. Here, we present a proton-detected 4D solid-state NMR assignment procedure that is tailored for large assemblies. The key to recording 4D spectra with three indirect carbon or nitrogen dimensions with their inherently large chemical shift dispersion lies in the use of sparse non-uniform sampling (as low as 2%). As a proof-of-principle we acquired 4D (H)COCANH, (H)CACONH and (H)CBCANH spectra of the 20 kDa bacteriophage tail tube protein gp17.1 in a total time of two and a half weeks. These spectra were sufficient to obtain complete resonance assignments in a straightforward manner without use of previous solution NMR data.
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The Cope Rearrangement of 1,5 Dimethylsemibullvalene-2(4)-d1: Experimental Evidence for Heavy Atom Tunneling ()
As an experimental test of the theoretical prediction that heavy-atom tunneling is involved in the degenerate Cope rearrangement of semibullvalenes at cryogenic temperatures, mono-deuterated 1,5-dimethyl¬semibullvalene isotopomers were prepared and investigated by IR spectroscopy, using the matrix isolation technique. As predicted, the less thermodynamically stable isotopomer rearranges at cryogenic temperatures in the dark to the more stable one, while broadband IR irradiation above 2000 cm-1 results in an equilibration of the isotopomeric ratio. Since this reaction proceeds with a rate constant in the order of 10-4 s-1 despite an experimental barrier of Ea = 4.8 kcal mol-1 and with only a shallow temperature-dependence, the results are interpreted in terms of heavy-atom tunneling.
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Ceria-Zirconia Nanoparticles as Enhanced Multi-Antioxidant for Sepsis Treatment ()
The two oxidation states of ceria nanoparticles, Ce3+ and Ce4+, play a pivotal role in scavenging reactive oxygen species (ROS). In particular, Ce3+ is largely responsible for removing O2- and *OH that are associated with inflammatory response and cell death. Here, we report the synthesis of 2 nm ceria-zirconia nanoparticles (CZ NPs) that possess a higher Ce3+/Ce4+ ratio and faster conversion from Ce4+ to Ce3+ than those exhibited by ceria nanoparticles. The obtained Ce0.7Zr0.3O2 (7CZ) NPs greatly improve ROS scavenging performance, thus regulating inflammatory cells in a very low dose. Moreover, 7CZ NPs are demonstrated to be effective in reducing mortality and systemic inflammation in two representative sepsis models. These findings suggest that 7CZ NPs have the potential as a therapeutic nanomedicine for treating ROS-related inflammatory diseases.
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A Critical Assessment of the Direct Catalytic Oxidation of Methane to Methanol ()
Despite the emerging number of disparate approaches for the direct selective partial oxidation of methane, none of them has translated into an industrial process. The oxidation of methane to methanol is a difficult yet intriguing and rewarding task as it has the potential to eliminate the prevalent natural gas flaring by providing novel routes to its valorisation. This review considers the synthesis of methanol and methanol derivatives from methane by homogeneous and heterogeneous pathways. In establishing the severe limitations related to the direct catalytic synthesis of methanol from methane, we highlight the vastly superior performance of systems, which produce methanol derivatives or incorporate specific measures such as the use of multi-component catalysts to stabilise methanol. We thereby identify methanol protection as being indispensable in homogeneous and heterogeneous catalysis with regard to future research on this topic.
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Heterocorrole Conformations: Little Saddling, Much Ruffling ()
10-Heterocorrole complexes with oxygen, sulfur, and selenium at position 10 of the macrocycle, and with the divalent ions of nickel, copper, and palladium were prepared and investigated. The focus was set onto the size adaptation and matching mechanisms of cavity sizes vs. ionic radii in corrole-type macrocycles. A full set of single crystal X-ray analyses was obtained and revealed that in all but one case the N4 binding site of the ring-contracted tetrapyrrole is larger than necessary to bind the metal ion without deformation. In plane size adaptation through M-N bond length elongation of 2.5%-3.2% is effective, in addition to pronounced out-of-plane ruffling of the macrocycle for those compounds with a more severe size mismatch. Such ruffling had been excluded for corroles before but is apparently the most efficient mechanism to adapt to small central ions.
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Selenium-Doped Carbon Quantum Dots (Se-CQDs) for Free Radical Scavenging ()
Heteroatom doping is an effective way to adjust the fluorescent properties of carbon quantum dots. However, selenium-doped carbon dots have rarely been reported even though selenium has unique chemical properties like redox-responsive properties owing to its special electronegativity. Herein, a facile and high-output strategy to fabricate selenium-doped carbon quantum dots (Se-CQDs) with green fluorescence (quantum yield 7.6 %) is developed through the hydrothermal treatment of selenocystine under mild conditions. Selenium heteroatoms endow the Se-CQDs with redox-dependent reversible fluorescence. In addition, free radicals such as *OH can be effectively scavenged by the Se-CQDs. Once Se-CQDs are internalized into cells, harmful high levels of reactive oxygen species (ROS) in the cells are reduced. This property makes the Se-CQDs capable of protecting the biosystems from oxidative stress.
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Phosphaborenes: accessible reagents for C-C/P-B isosteres ()
Formal exchange of C=C units with isoelectronic B=N or B=P units can provide access to molecules with unique electronic or chemical properties. Here, we report the simple solution-phase generation of highly reactive phosphaborenes, RP=BR, and demonstrate their use for the introduction of P=B units into organic systems. A P-B containing cyclobutene isostere can be ring opened to access unique 1,4-bora-phospha-butadiene systems with conjugated main-group multiple bonds.
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Single-enzyme biofuel cells ()
Here, we firstly demonstrate that the intramolecular electron transfer within signle enzyme molecule is an important alternative pathway which can be harnessed to generate electricity. By decoupling the redox reactions within a single type of enzyme (e.g., Trametes versicolor laccase), we harvested electricity efficiently from unconventional fuels including recalcitrant pollutants (e.g. bisphenol-A and hydroquinon) in a single-laccase biofuel cell. The intramolecular electron-harnessing concept was further demonstrated with other enzymes, including power generation during CO2 bioconversion to formate catalyzed by formate dehydrogenase from Candida boidinii. The novel single-enzyme biofuel cell is shown to be potentially feasible utilizing wastewater as fuel as well as generating energy in tandem with driving bioconversion of chemical feedstock from CO2.
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Unusual Internal Electron Transfer in Conjugated Radical Polymers ()
Nitroxide-containing organic radical polymers (ORPs) have captured attention for their high power and fast redox kinetics. Yet a major challenge is the polymer's aliphatic backbone, resulting in a low electronic conductivity. Recent attempts that replace the aliphatic backbone with a conjugated one have not met with success. The reason for this is not understood until now. We examine a family of polythiophenes bearing nitroxide radical groups, showing that while both species are electrochemically active, there exists an internal electron transfer mechanism that interferes with stabilization of the polymer's fully oxidized form. This finding directs the future design of conjugated radical polymers energy storage and electronics, where careful attention to the redox potential of the backbone relative to the organic radical species is needed.
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Associating and dissociating nanodimer analysis for quantifying ultra-small amounts of DNA ()
Amplification- and enzyme-free quantification of DNA at ultra-low concentrations, on the orders of 10-1,000 targets, is highly beneficial, but is extremely challenging. To address this challenge, true detection signals should be reliably discriminated from false or noise signals. Here, we developed the dynamically associating and dissociating nanodimer analysis (ADNA) method that allows for collecting maximum detection signals from true target-binding events while keeping nonspecific signals at minimum level. In the ADNA assay for ultra-low target concentrations, the whole Au nanoprobes on a lipid micropattern were simultaneously monitored and analyzed in situ, and the newly defined dissociating dimers that are eventually decoupled into monomers after dimer formation were incorporated into detection signals. 10s to 1000s of DNA copies can be reliably quantified with excellent single-base-mismatch differentiation capability using this non-enzymatic, amplification-free ADNA method.
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Tuning the Inter-nanofibril Interaction to Regulate the Morphology and Function of Peptide/DNA Co-assembled Viral Mimics ()
The ability to tune the inter-subunit interaction within the virus capsid may be critical to assembly and biological function. This process has been extended here with peptide/DNA co-assembled viral mimics. The resulting co-assemblies, formed and stabilized by both DNA-peptide nanofibril and peptide nanofibril-nanofibril interactions, have been tuned through hydrophobic packing interactions of the peptide sequences. By strengthening peptide side chain complementarity and/or elongating the peptide chain (from 4 to 8 residues), we report strengthening the inter-nanofibril interaction to create stable nanococoons with high gene transfection efficacy.
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A Novel Electrochemical Biosensor with Dual Signal Outputs: Toward Simultaneous Quantification of pH and O2 in Brain upon Ischemia and in Tumor during Cancer Therapy ()
In this work, we develop a novel methodology for designing electrochemical biosensors with both current and potential signal outputs for simultaneous determination of two important species in the live systems. By designing a new molecule, Hemin-aminoferrocene (Hemin-Fc), we create a single electrochemical biosensor for simultaneous quantification of O2 and pH in brain upon ischemia and in tumor during cancer starvation therapy. The reduction peak current of Hemin group increases with rising concentrations of O2 from 1.3 to 200.6 μM. Meanwhile, this peak potential positively shifts with decreasing pH from 8.0 to 5.5, resulting in simultaneous determination of O2 and pH by monitoring both current and potential signal outputs. The developed single biosensor with high temporal and spatial resolutions, as well as remarkable selectivity and accuracy, is successfully applied in determination of O2 and pH in brain upon ischemia, as well as in tumor during cancer therapy.
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Cesium's off-the-map valence orbital ()
The Td-symmetric [CsO4]+ ion, featuring Cs in an oxidation state of 9, is computed to be a minimum. Cs uses outer core 5s and 5p orbitals to bind the oxygens. The valence Cs 6s orbital lies too high to be involved in bonding, and contributes to Rydberg levels only. From a Molecular Orbital perspective, the bonding scheme reminds of XeO4: an octet of electrons to bind electronegative ligands, and no low-lying acceptor orbitals on the central atom. In this sense, Cs+ resembles hypervalent Xe.
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Facile One Pot Synthesis of Functional Giant Polymeric Vesicles Controlled by Oscillatory Chemistry ()
We introduce a novel application of an oscillatory chemical reaction to the synthesis of block copolymers. The Belousov-Zhabotinsky (B-Z) reaction is coupled with the polymerization of an amphiphilic block copolymer. Radicals generated in the B-Z reaction initiate the polymerization between a polyethylene glycol (PEG) macro reversible addition-fragmentation chain transfer agent and butyl acrylate monomers. The growth of the hydrophobic block on PEG undergoes self-assembly to form spherical micelles. The nanoscale micelles transform into submicron vesicles and grow to giant vesicles as a consequence of the oscillatory behavior of the B-Z reaction. The one pot synthesis of an amphiphilic di-block copolymer and retention of oscillatory behavior for the B-Z reaction with the formation of giant vesicles bring a new insight into possible pathways to generate new opportunities for the synthesis of active functional microreactors in the range from 100's of nm to tens of µm.
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Electrochemical TERS Elucidates Potential-Induced Molecular Reorientation of Adenine/Au(111) ()
Electrochemical surface activity arises from the interaction and geometric arrangement of molecules at electrified interfaces. We present a novel electrochemical tip-enhanced Raman spectroscope that can access the vibrational fingerprint of less than 100 small, non-resonant molecules adsorbed at atomically flat Au electrodes to study their adsorption geometry and chemical reactivity as a function of applied potential. Combining experimental and simulation data for showcase adenine/Au(111), we conclude that protonated physisorbed adenine adapts a tilted orientation at low potentials while it is vertically adsorbed around the potential of zero charge. Further potential increase induces adenine deprotonation and reorientation to a planar configuration. The extension of EC-TERS to the study of adsorbate reorientation significantly broadens the applicability of this advanced spectroelectrochemical tool for the nanoscale characterization of a full range of electrochemical interfaces.
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Antimonene Quantum Dots: Synthesis and Application as Near-Infrared Photothermal Agents for Effective Cancer Therapy ()
Photothermal therapy (PTT) has shown significant potential for cancer therapy. However, developing nanomaterials (NMs)-based photothermal agents (PTAs) with satisfactory photothermal conversion efficacy (PTCE) and biocompatibility remains a key challenge. Herein, a new generation of PTAs based on two-dimensional (2D) antimonene quantum dots (AMQDs) was developed by a novel liquid exfoliation method. Surface modification of AMQDs with polyethylene glycol (PEG) significantly enhanced both biocompatibility and stability in physiological medium. The PEG-coated AMQDs showed a PTCE of 45.5%, which is higher than many other NMs-based PTAs such as graphene, Au, MoS2, and black phosphorus (BP).Through both in vitro and in vivo studies, the PEG-coated AMQDs demonstrated notable NIR-induced tumor ablation ability. This work is expected to expand the utility of 2D antimonene (AM) to biomedical applications through the development of an entirely novel PTA platform.
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Mechanistic Insights into Nanoparticle Surface Adsorption by Solution NMR Spectroscopy in an Aqueous Gel ()
Engineering nanoparticle (NP) functions at molecular level requires a detailed understanding of the dynamic processes occurring at the NP surface. Here we show that the combined analysis of Dark state Exchange Saturation Transfer (DEST) and Relaxation Dispersion (RD) NMR experiments acquired on gel-stabilized samples of NP allows for accurate determination of the kinetics and thermodynamics of adsorption. We used the former approach to describe the interaction of cholic acid (CA) and phenol (PhOH) with ~200 nm ceria NP. We show that, while CA forms weak interactions with the NP, PhOH is tightly bound to the NP surface. Interestingly, we found that adsorption of PhOH proceeds through an intermediate, weakly bound state in which the small molecule has residual degrees of rotational diffusion. We believe the use of aqueous gels for stabilizing NP samples will increase the applicability of solution NMR methods to characterization of nanomaterials.
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From Borane to Borylene without Reduction: Ambiphilic Behavior of a Monovalent Silylisonitrile Boron Species ()
Deprotonation of [(cAAC)BH2(CN)] provided clean access to the stable boryl anion, [(cAAC)BH(CN)]-. Whereas the addition of soft electrophiles occurred at the nucleophilic boron center, harder silyl electrophiles added to the harder terminal cyano nitrogen. The resulting [(cAAC)BH(CNSiPh3)] species behaved both like a silylium boryl nucleophile and a neutral silylisonitrile borylene.
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Visible light photocatalysis of 6π heterocyclization ()
Photo-mediated 6π cyclization is a valuable method for the formation of fused heterocyclic systems. Here we demonstrate that irradiation of cyclic 2-aryloxyketones with blue LED light in the presence of an Ir(III) complex leads to efficient and high yielding arylation across a panoply of substrates by energy transfer. 2-arylthioketones and 2-arylaminoketones also cyclize effectively under these conditions. Quantum calculation demonstrates that the reaction proceeds via conrotatory ring closure in the triplet excited state. Subsequent suprafacial 1,4-hydrogen shift and epimerization leads to the observed cis-fused products.
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Organogelator - Cellulose Composite for Practical and Eco-friendly Marine Oil Spill Recovery ()
Marine oil-spills pose serious threats to the ecosystem and economy. There is much interest in developing sorbents that can tackle such spills. We have developed a novel sorbent by impregnating cellulose pulp with a sugar-derived oleogelator, 1,2:5,6-di-O-cyclohexylidene-mannitol. The gelator molecules mask the surface-exposed hydroxyl groups of cellulose fibrils by engaging them in H-bonding and expose their hydrophobic parts making the fibers temporarily hydrophobic (water contact-angle 110°). This sorbent absorbs oil effectively, selectively and instantly from oil-water mixtures due to its hydrophobicity. Then the gelator molecules get released uniformly in the oil and later self-assemble to fibers, as evident from SEM analysis, congealing the oil within the matrix. This hierarchical entrapment of the oil by non-covalent polymeric fibers within a covalent polymer matrix makes the gel very strong (230-fold increase in the yield-stress) and rigid, making it suitable for practical use.
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Size Dependence of Doping via Vacancy Formation in Copper Sulfide Nanocrystals ()
Doping semiconductor nanocrystals (NCs) is a key yet under-explored approach for tuning their electronic properties. An important route for doping in NCs is vacancy formation. Herein, the size and concentration dependence of doping is studied in Cu2S NCs through a redox reaction with iodine leading to vacancy formation accompanied by a localized surface plasmon response. X-ray spectroscopy and diffraction reveal transformation from Cu2S to Cu depleted phases along with CuI formation. Higher efficiency of the reaction is observed for larger NCs. This behavior is attributed to interplay of the vacancy formation energy, which decreases with size, and the growth of CuI on the NC surface that is favored on well-defined facets of the larger NCs. This doping process thus allows wide tuning of the plasmonic properties by varying the NC size and the iodine concentration. Such tuning by controlled vacancy doping of NCs, opens a path for their tailoring for optoelectronic applications.
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Simulated isotope exchange patterns enable protein structure determination ()
Understanding the myriad protein-protein interactions required for cell function requires efficient leveraging of biophysical data to drive computational docking. The detailed insight into protein interfaces provided by isotope exchange endows this experimental technique with a unique importance for docking approaches. However, progress in coupling these methods is hindered by the inability to interpret the complex exchange patterns in relation to protein structure. A method to simulate protein isotope exchange patterns from docking outputs is described and its utility to guide the selection of native assemblies demonstrated. Unique signatures are generated for each docking pose allowing high throughput ranking of whole docking simulations by pairwise comparison to experimental outputs. Native assemblies are obtained using nothing but their simulated profiles and experimental difference-data for individual proteins are sufficient to drive structure determination for the whole complex.
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Two-Dimensional Metal-Organic Framework Nanosheets for Membrane-Based Gas Separation ()
Metal-organic framework (MOF) nanosheets could serve as ideal building blocks of molecular sieve membranes due to their structural diversity and minimized-mass transfer barrier. Till now, discovery of appropriate MOF nanosheets and facile fabrication of high performance MOF nanosheet-based membranes remain as great challenges. Herein, we present a modified soft-physical exfoliation methodology to disintegrate lamellar amphiprotic MOF into nanosheets with high aspect-ratio. Consequently sub-10 nm-thick ultrathin membranes were successfully prepared and demonstrated a remarkable H2/CO2 separation performance with separation factor up to 166 and H2 permeance up to 8 ×10E-7 mol/m2∙s∙Pa at elevated testing temperature due to a well-defined size exclusion effect. Our results suggest that this nanosheet-based membrane holds great promise as the next generation of ultrapermeable gas separation membrane.
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An Activatable Photosensitizer Targeted to γ-Glutamyltranspeptidase ()
We adopted a spirocyclization-based strategy to design γ-glutamyl-hydroxymethyl selenorhodamine green (gGlu-HMSeR) as a photo-inactive compound that would be specifically cleaved by the tumor-associated enzyme γ-glutamyltranspeptidase (GGT) to generate a potent photosensitizer, HMSeR. gGlu-HMSeR takes a colorless, spirocyclic structure, and does not show marked phototoxicity to low-GGT-expressing cells or normal tissues upon irradiation with visible light. In contrast, HMSeR predominantly takes an open colored structure, and generates reactive oxygen species upon irradiation. Thus, the γ-glutamyl group serves as a tumor-targeting moiety for photodynamic therapy (PDT), switching on tumor-cell-specific phototoxicity. Photoirradiation after gGlu-HMSeR treatment resulted in selective ablation of implanted tumor spheroids, without damage to healthy tissue. gGlu-HMSeR is the first activatable photosensitizer targeting an aminopeptidase for highly tumor-selective PDT.
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High-Field NMR Spectroscopy Reveals Aromaticity-Modulated Hydrogen Bonding (AMHB) in Heterocycles ()
Abstract: From DNA base pairs to drug-receptor binding, hydrogen (H-)bonding and aromaticity are common features of heterocycles. Herein, the interplay of these bonding aspects is explored; H-bond strength modulation due to enhancement or disruption of heterocycles' aromaticity is revealed by comparing homodimer H-bond energies of aromatic heterocycles with analogues that have the same H-bonding moieties but lack cyclic π-conjugation. NMR studies of dimerization in C6D6 find aromaticity-modulated H-bonding (AMHB) energy effects of ca. ±30%, depending on whether they enhance or weaken aromatic delocalization. The attendant ring current perturbations expected from such modulation are confirmed by chemical shift changes in both observed ring C-H and calculated nucleus-independent sites. In silico modeling confirms that AMHB effects outweigh those of hybridization or dipole-dipole interaction.
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Controlling Reaction Selectivity via Surface Termination of Perovskite Catalysts ()
Although perovskites have been widely used in catalysis, tuning their surface terminations to control reaction selectivities has not been well established. In this work, we employ multiple surface sensitive techniques to characterize the surface termination (one aspect of surface reconstruction) of SrTiO3 (STO) after thermal pretreatment (Sr-enrichment) and chemical etching (Ti-enrichment). We show, using the conversion of 2-propanol as a probe reaction, that the surface termination of STO can be controlled to greatly tune catalytic acid/base properties and consequently the reaction selectivities in a wide range, which are inaccessible using single metal oxides, either SrO or TiO2. Density functional theory (DFT) calculations well explain the selectivity tuning and reaction mechanism on different surface terminations of STO. Similar catalytic tunability is also observed on BaZrO3, highlighting the generality of the finding from this work.
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Visible Light-Driven Aza-ortho-Quinone Methide Generation Enables a Multicomponent Reaction ()
A visible light photoredox-catalyzed and radical-mediated strategy for the in situ generation of aza-ortho-quinone methides from readily accessible alkenylanilines and alkyl radical precursors has been accomplished for the first time. This protocol enables an efficient multicomponent cascade reaction of alkenylanilines, halides and sulfur ylides, and shows wide substrate scope and high functional group tolerance with respect to each component. Treatment of the initially formed cycloaddition products with a base leads to a convergent and streamlined synthesis of densely functionalized indoles in a single flask operation.
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Switch of C−H Activation Mechanism for Full Selectivity Control in Cobalt(III)-Catalyzed C−H Alkylations ()
Selectivity control in hydroarylation-based C−H alkylation has been dominated by steric interactions. In contrast, we unravel a conceptually distinct strategy that exploits the programmed switch in the C−H activation mechanism by means of cobalt catalysis, setting the stage for expedient C−H alkylations with unactivated alkenes. Detailed mechanistic studies provided compelling evidence for a programmable switch in the C−H activation mechanism from a linear-selective ligand-to-ligand hydrogen transfer (LLHT) to a branched-selective base-assisted internal electrophilic-type substitution (BIES).
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Decarboxylative Alkynylation ()
The development of a new decarboxylative cross-coupling protocol that affords terminal and substituted alkynes from various carboxylic acids is described using both nickel- and iron-based catalysts. The use of N-hydroxytetrachlorophthalimide (TCNHPI) esters is crucial to the success of the transformation, and the reaction is amenable to in situ carboxylic acid activation. Additionally, an inexpensive, commercially-available alkyne source is employed in this formal homologation process that serves as a surrogate for other well-established alkyne syntheses. The reaction can be conducted on a mole scale, and the conditions are mild, operationally simple, and broad in scope while providing succinct avenues to previously reported synthetic intermediates.
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Phosphonium Salts as Pseudohalides: Regioselective Ni-Catalyzed Cross-Coupling of Complex Pyridines and Diazines. ()
Heterobiaryls are important pharmacophores that are challenging to prepare by traditional cross-coupling methods. An alternative approach is presented where pyridines and diazines are converted to heteroaryl phosphonium salts and coupled with arylboronic acids. Nickel catalysts are unique for selective heteroaryl transfer, and the reaction has a broad substrate scope that includes complex pharmaceuticals. Phosphonium ions also display orthogonal reactivity in cross-couplings compared to halides enabling chemoselective palladium and nickel-catalyzed coupling sequences.
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Segmental, domain-selective perdeuteration and small angle neutron scattering for structural analysis of multi-domain proteins ()
Multi-domain proteins play critical roles in fine-tuning essential processes in cellular signaling and gene regulation. Typically, multiple globular domains that are connected by flexible linkers undergo dynamic re-arrangements upon binding to protein, DNA or RNA ligands. RNA binding proteins (RBPs) represent an important class of multi-domain proteins, which regulate gene expression by recognizing linear or structured RNA sequence motifs. Here, we employ segmental perdeuteration of the three RNA recognition motif (RRM) domains in the RBP TIA-1 using Sortase A-mediated protein ligation. We show that domain-selective perdeuteration combined with contrast-matched small-angle neutron scattering (SANS), SAXS and computational modelling provides valuable information to precisely define relative domain arrangements. The approach is generally applicable to study conformational arrangements of individual domains in multi-domain proteins and changes induced by ligand binding.
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Stretchable Electrochemical Sensor for Inducing and Monitoring Cell Mechanotransduction in Real-Time ()
Existing methods offer little direct and real-time information about stretch-triggered biochemical responses during cell mechanotransduction. Herein, we report a novel stretchable electrochemical sensor that takes advantage of a hierarchical percolation network of carbon nanotubes and gold nanotubes (CNTs- Au NTs). This hybrid nano-structure provides the sensor with excellent time-reproducible mechanical and electrochemical performances while granting very good cellular compatibility, making it perfectly apt to induce and monitor simultaneously transient biochemical signals. This is validated by monitoring stretch-induced transient release of small signaling molecules by both endothelial and epithelial cells cultured on this sensor and submitted to stretching strains of different intensities. This work demonstrates that the hybrid CNTs-Au NTs platform offers a versatile and highly sensitive way to characterize and quantify short-time mechanotransduction responses.
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The First Total Synthesis of Ovafolinin A and B: Unique Polycyclic Benzoxepin Lignans via a Cascade Cyclization ()
Ovafolinins A and B, isolated from Lyonia ovalifolia var. elliptica, are lignans which contain a unique penta- and tetracyclic benzoxepin bridged aryl tetralin structure. We report the first total synthesis of these natural products initially utilizing an acyl-Claisen rearrangement to construct the lignan backbone with correct relative stereochemistry. Judicious use of a bulky protecting group placed reactive moieties in the correct orientation resulting in a cascade reaction forming the benzoxepin bridged aryl tetralin from a linear precursor in a single step. Modification of this route allowed an enantioselective synthesis of (+)-ovafolinin A and B which confirmed the absolute stereochemistry and comparison of optical rotation suggests these compounds are found as scalemic mixtures in nature.
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Selective CF3 Substitution for Affecting the Physical and Chemical Properties of Gold Corroles ()
Gold corroles have been hardly accessible and display no interesting physical or chemical properties. We now introduce a facile methodology for obtaining selectively CF3-substituted gold(III) corroles and show that the introduction of these groups has immense effects on the structures of the complexes, their photophysical and redox properties, and on their ability to participate in catalytic processes.
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Spectroscopic Observation of the Triplet Diradical State of a Cyclobutadiene ()
We report the first direct observation by electron paramagnetic resonance (EPR) spectroscopy of a triplet diradical state of a CBD derivative 2 which is formed by a thermal excitation of the CBD singlet 1. Both experiment and theory support a thermal equilibrium between rectangular singlet 1 and the square triplet diradical 2 with a singlet-triplet energy gap of 13.9 kcal mol ➖1.
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Oxygen Vacancy-Mediated Photocatalysis of BiOCl: Reactivity, Selectivity and Perspective ()
Semiconductor photocatalysis is a trustworthy approach to harvest clean solar light for energy conversions, while state-of-the-art catalytic efficiencies are unsatisfactory because of the finite light response and/or robust charge carriers' recombination. Along with the development of modern material characterization techniques and electronic-structure computations, oxygen vacancies (OVs) on real photocatalysts surface, even in infinitesimal concentration, are found to play a more decisive role in determining the kinetics, energetics and mechanisms of photocatalytic reactions. This review endeavors to clarify the inherent functionality of OVs in photocatalysis at the surface molecular level using 2D BiOCl as the platform. Structure sensitivity of OVs on reactivity and selectivity of photocatalytic reactions is intensely discussed via confining OVs onto prototypical BiOCl surfaces of different structures. The critical understanding of OVs chemistry in this review can help to consolidate and advance the fundamental theories of photocatalysis, and also offer new perspectives and guidelines for the rational design of catalysts with satisfactory performance.
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A novel DNA structure containing AgI-mediated G:G and C:C Base Pairs ()
Metallo-base pairs have been extensively utilized in many research fields, including genetic code extension, novel therapeutics development, and various nanodevice designs. Compared to other cations, AgI is more flexible in pairing with natural base pairs. Herein, we present a DNA structure containing two C-AgI-C pairs and one first time reported G-AgI-G pair in a short 8mer DNA strand. This structure not only discloses the detailed insights into these AgI-base pairing patterns in DNA, but also represents the first nonhelical DNA structure driven by the heavy metal ions, further contributing to the structural diversity of DNA. This unique complex structure is highly sequence dependent, implying its functional potentials as a new DNA aptamer that can bind and recognize silver ions. These results not only advance our understanding on the interactions between AgI and nucleobases, but also provide a unique structural component for the rational design of new DNA nanodevices.
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The Viologen Cation Radical Pimer: A Case of Dispersion-Driven Bonding ()
Pi bonds between organic radicals have generated recent excitement as an orthogonal interaction for designing self-assembling architectures in water. Here, we provide the first a systematic investigation of the effect of the viologen cation radical structure on the strength and nature of the pimer bond. A striking and unexpected feature of the this pi bond is that the bond strength is unchanged by substitution with electron-donating groups or withdrawing groups or with increased conjugation. Furthermore, the interaction is undiminished by sterically bulky N-alkyl groups. Theoretical modelling indicates that strong dispersion forces dominate the interaction between the radicals, rationalizing the insensitivity of the bonding interaction to substituents: The stacking of polarizable pi radicals leads to attractive dispersion forces in excess of typical dispersion interactions of small molecules and helps overcome the Coulombic repulsion of bringing two cationic species into contact.
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A Carbene Catalysis Strategy for the Synthesis of Protoilludane Natural Products ()
The Armillaria and Lactarius genera of fungi produce antimicrobial and cytotoxic mellolide, protoilludane, and marasmane sesquiterpenoids. Herein, we report a unified synthetic strategy to access the protoilludane, mellolide, and marasmane families of natural products. The significance of these syntheses lies in a) the organocatalytic, enantioselective construction of key chiral intermediates from a simple achiral precursor, b) the utility of a key 1,2-butanediol intermediate to serve as a progenitor to each natural product class, and c) a direct chemical conversion of a protoilludane to a marasmane via serendipitous ring contraction, providing experimental support for their proposed biosynthetic relationships.
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Iridium(I)-Catalyzed Intramolecular Hydrocarbonation of Alkenes: Efficient Access to Cyclic Systems Bearing Quaternary Stereocenters ()
A catalytic, versatile and atom-economical C-H functionalization process that provides a wide variety of cyclic systems featuring methyl-substituted quaternary stereocenters is described. The method relies on the use of a cationic Ir(I)-bisphosphine catalyst, which promotes a carboxamide-assisted activation of an olefinic C(sp2)-H bond followed by exo-cyclization to a tethered 1,1-disubstituted alkene. The extension of the method to aromatic and heteroaromatic C-H bonds, as well as developments on an enantioselective variant, are also described.
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Catalyst-Controlled, Enantioselective and Diastereodivergent Conjugate Addition of Aldehydes to Electron-Deficient Olefins ()
A chiral amine-catalyzed, enantioselective and diastereodivergent method for aldehyde addition to electron deficient olefins is presented. Hydrogen bonding was used as a control element to achieve unusual anti-selectivity, which was further elucidated through mechanistic and computational studies.
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QM/MM Study on the Photoreactions of Dark- and Light-Adapted States of a Blue-Light YtvA LOV Photoreceptor ()
The dark- and light-adapted states of YtvA LOV domains exhibit distinct excited-state behavior. We have employed high-level QM(MS-CASPT2)/MM calculations to study the photochemical reactions of the dark- and light-adapted states. The photoreaction from the dark-adapted state starts with an S1T1 intersystem crossing followed by a triplet-state hydrogen transfer from the thiol to the flavin moiety that produces a diradical intermediate, and a subsequent internal conversion that triggers a barrierless C-S bond formation in the S0 state. The energy profiles for these transformations are different for the four conformers of the dark-adapted state considered. The photochemistry of the light-adapted state does not involve the triplet state: photoexcitation to the S1 state triggers C-S bond cleavage followed by recombination in the S0 state; both these processes are essentially barrierless and thus ultrafast. The present work offers new mechanistic insights into blue-light photoreceptors.
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Mn(I)-Catalyzed Regio- and Stereoselective 1,2-Diheteroarylation of Allenes: Combination of C-H activation and Smiles Rearrangement ()
Heteroarenes are important structural motif in functional molecules. Reported herein is a Mn(I)-catalyzed 1,2-diheteroarylation of allenes via a C-H activation/Smiles rearrangement cascade. The reaction occurred under additive-free or even solvent-free conditions which allowed the creation of two C-C and one C-N bonds in a single operation. A series of structurally diverse bicyclic or tricyclic compounds bearing an exocyclic double bond were constructed in good to excellent efficiency. The decarboxylative ring-opening of the products led to the facile synthesis of vicinal biheteroaryls. Synthetic applications were demonstrated and preliminary mechanistic studies were conducted.
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Visible Light Irradiated Graphitic Carbon Nitride (g-C3N4) Photocatalyzed Cation Radical Diels-Alder Reactions with Dioxygen as Sustainable Mediator for Photoinduced Electrons ()
Photocatalytic Diels-Alder (D-A) reaction with electron rich olefins is realized by graphitic carbon nitride (g-C3N4) under visible light irradiation and aerobic condition. This heterogeneous photo-redox reaction system is highly efficient, and apparent quantum yield reaches a remarkable value of 47 % for the model reaction. Dioxygen plays a critical role as electron mediator, which is distinct from the previous reports in the homogeneous Ru(II) complex photo-redox system. Moreover, the reaction intermediate vinylcyclobutane is captured and monitored during the reaction, serving as a direct evidence for the proposed reaction mechanism. The cycloaddition process is thereby determined to be the combination of direct [4 + 2] cycloaddition and [2 + 2] cycloaddition followed by photocatalytic rearrangement of the vinylcyclobutane intermediate.
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Mg2+ dependent high mechanical anisotropy of three-way-junction-pRNA revealed by single-molecule force spectroscopy ()
Mechanical anisotropy is ubiquitous in biological tissues but remains hard to be reproduced in synthetic biomaterials. Developing molecular building blocks with anisotropic mechanical response is the key towards engineering anisotropic biomaterials. Here we report that the three-way-junction- (3WJ-) pRNA, derived from φ29 DNA packaging motor, shows strong mechanical anisotropy upon Mg2+ binding. In the absence of Mg2+, 3WJ-pRNA is mechanically weak without noticeable mechanical anisotropy. While in the presence of Mg2+, the unfolding forces can differ by more than 4 folds along different pulling directions, ranging from ~47 pN to ~219 pN. Mechanical anisotropy of 3WJ-pRNA stems from pulling direction dependent cooperativity for the rupture of two Mg2+ binding sites, which represents a novel mechanism for the mechanical anisotropy of biomacromolecules. We anticipate that 3WJ-pRNA can be used as a key element for the construction of biomaterials with controllable mechanical anisotropy.
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ABi2(IO3)2F5 (A= K, Rb and Cs): Combination of Halide and Oxide Anionic Units to Create Large SHG Response with Wide Bandgap ()
A family of nonlinear optical materials that contain the halide, oxide and oxyhalide polar units simultaneously in a single structure, namely, ABi2(IO3)2F5 (A = K, 1;Rb,2 and Cs,3), have been designed and synthesized. They crystallize in the same polar space group of P21 with a two-dimensional double-layered framework constructed by [BiF5]2- and [BiO2F4]5- units connected to each other by four F atoms, in which two [IO3]- groups are linked to [BiO2F4]5- unit on the same side, and a hanging Bi-F bond of [BiF5]2- unit is located on the other side via ionic interaction with the layer-inserted alkali metal ions to form three-dimensional structure. The well-ordered alignments of these polar units lead to a very strong second harmonic generation response of 12 (1), 9.5 (2) and 7.5 (3) times larger than that of KDP under 1064 nm laser radiation. All of them exhibited wide bandgap over 3.75 eV, indicating that they will possess high laser damage threshold.
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Biosynthesis of Complex Indole Alkaloids: Elucidation of the Concise Pathway of Okaramines ()
The okaramines are a class of complex indole alkaloids isolated from Penicillium and Aspergillus species. Their potent insecticidal activity arises from selectively activating glutamate-gated chloride channels (GluCls) in invertebrates, not affecting human ligand-gated anion channels. Okaramines B (1) and D (2) contain a polycyclic skeleton, including an azocine ring and an unprecedented 2-dimethyl-3-methyl-azetidine ring. Due to their complex scaffold, okaramines have inspired many total synthesis efforts, but the enzymology of the okaramine biosynthetic pathway remains unexplored. Here, we identified and characterized the biosynthetic gene cluster (oka) of 1 and 2, then elucidated the pathway with target gene inactivation, heterologous reconstitution, and biochemical characterization. Notably, we characterized an α-ketoglutarate-dependent non-heme Fe(II) dioxygenase that forged the azetidine ring on the okaramine skeleton.
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A polyacrylamide hydrogel electrolyte enabled intrinsically 1000% stretchable and 50% compressible supercapacitor ()
Stretchability and compressibility of supercapacitors constitute essential elements of modern electronics such as flexible and wearable devices. Widely-used polyvinyl alcohol-based electrolytes are neither very stretchable nor compressible, which fundamentally limit the realization of high stretchability and compressibility of supercapacitors. We present a new electrolyte that is intrinsically super-stretchable and compressible. Vinyl hybrid silica nanoparticle cross-linkers are introduced into polyacrylamide hydrogel backbones to promote dynamic crosslinking of the polymer networks. These cross-linkers serve as stress buffers to dissipate energy when strain is applied, providing an ultimate solution to the intrinsic high stretchability and compressibility problems of a supercapacitor. As a result, our supercapacitor with this electrolyte can be stretched up to the unprecedented 1000% strain with enhanced performance, and compressed to 50% strain with initial performance well retained.
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The covalent functionalization of layered black phosphorus ()
Layered black phosphorus has been attracting great attention due to its interesting material properties which lead into a plethora of proposed applications. Here we show several approaches for covalent chemical modifications of layered black phosphorus in order to form P-C and P-O-C bonds. We demonstrate that the nucleophilic reagents are highly effective for chemical modification of black phosphorus and that it leads to formation of P-O-C as well as P-C bonds on black phosphorus' surface. Further derivatization approaches investigated were based on the radical reactions. These reagents seem to be not effectively for surface covalent modification of black phosphorus, especially in comparison with nucleophilic reagents. The influence of black phosphorus covalent modification on electronic structure was investigated using ab-initio calculations showing a strong effect on the resulting electronic structure including band-gap energy and leading to spin-polarization.
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Molecular Antimony Complexes for Electrocatalysis: Activity of a Main Group Element in Proton Reduction ()
Main group complexes are shown to be viable electrocatalysts for the H2-evolution reaction (HER) from acid. A series of antimony porphyrins with varying axial ligands were synthesized for electrocatalysis applications. The proton-reduction catalytic properties of the compound TPSb(OH)2 (TP = 5,10,15,20-tetra(p-tolyl)porphyrin) with two hydroxyl axial ligands was studied in detail, demonstrating catalytic H2 production. Experiments, in conjunction with quantum chemistry calculations, show that the catalytic cycle is driven via the redox activity of both the porphyrin ligand and the Sb center. This study brings insight into main group catalysis and the role of redox-active ligands during catalysis.
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Dendronized semiconducting polymer as photothermal nanocarrier for remote activation of gene expression ()
Regulation of transgene systems is imperative to develop innovative medicines. However, noninvasive remote control of gene expression has been rarely developed and remains elusive. We herein synthesize a near-infrared (NIR) absorbing dendronized semiconducting polymer (DSP) and utilize it as a photothermal nanocarrier not only to efficiently deliver genes but also to spatiotemporally control gene expression in conjunction with heat-inducible promoter. DSP has a high photothermal conversion efficiency (44.2%) at 808 nm, permitting fast transduction of NIR light into heat signals for intracellular activation of transcription. Such a DSP-mediated remote activation can rapidly and safely result in 25- and 4.5-fold increments in the expression levels of proteins in cells and living mice, respectively. This study thus provides a promising approach to optically regulate transgene systems.
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Imprinted photonic hydrogels for the size and shell-selective recognition of nanoparticles ()
Sensors based on responsive photonic hydrogels have recently attracted considerable attention for visual medical diagnostics, pharmaceutical bioassays and environmental monitoring. However, the use of these promising materials for the detection of nanoparticles (NPs) has never been explored so far, although the sensing of nano-objects is a rapidly-evolving new area of research. To address this issue, we have combined the concepts of inverse opal hydrogels and nanoparticle-imprinted polymers. By this way, we could obtain a NP-imprinted photonic hydrogel, consisting in a three-dimensional, highly-ordered poly(methacrylic acid) macroporous array, where nanocavities complementary to the target NPs, here colloidal quantum dots, are distributed. This novel type of NP-imprinted photonic hydrogel sensor was shown to display a high sensitivity and selectivity, opening new prospects for the development of equipment-free and cost efficient sensing devices for NPs.
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Protonation of Nitramines: Where does the Proton go? ()
The reactions of nitramine, N-methyl nitramine and N,N-dimethyl nitramine with anhydrous HF and the superacids HF/MF5 (M = As, Sb) were investigated at temperatures below -40 °C. In solution, exclusive O-protonation was observed by multinuclear NMR spectroscopy. Whereas no solid product could be isolated from the neat HF solutions even at -78 °C, in the HF/MF5 systems, protonated nitramine MF6- salts were isolated for the first time as moisture-sensitive solids that decompose at temperatures above -40 °C. In the solid state, depending on the counter-ion, O-protonated or N-protonated cations can be formed, in accord with theoretical calculations which show that the energy differences between O-protonation and N-protonation are very small. The [H2N-NO2H][AsF6], [H3N-NO2][SbF6], [MeHNNO2H][SbF6], and [Me2NNO2H][SbF6] salts were characterized by their X-ray crystal structures.
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Opto-electrochemical in situ monitoring of cathodic single-cobalt nanoparticle formation ()
Single particle electrochemistry at a nanoelectrode is explored by dark-field optical microscopy. The analysis of the scattered light allows in situ dynamic monitoring of the electrodeposition of single cobalt nanoparticles down to a radius of 65 nm. Larger sub-micrometer particles are directly sized optically by edges super-localization and the scattered light contains complementary chemical information concerning the particle redox chemistry. This opto-electrochemical approach is used to derive mechanistic insights about electrocatalysis which are not accessible from single particle electrochemistry.
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Ureido-based design of tunable, UCST-type self-oscillating polymer ()
Herein, we designed an upper critical solution temperature (UCST)-type self-oscillating polymer that exhibited rhythmic soluble-insoluble changes induced by Belousov-Zhabotinsky (BZ) reaction. The target polymers were prepared by conjugating Ru(bpy)3, a catalyst for the BZ reaction, to ureido-containing poly(allylamine-co-allylurea) (PAU) copolymers. The Ru(bpy)3-conjugated PAUs exhibited UCST-type phase transition behavior, and the solubility of the polymer changed in response to the alternation in the valency of Ru(bpy)3. The ureido content influence the temperature range of self-oscillation, and the oscillation occurred at higher temperatures than conventional LCST-type self-oscillating polymers. Further, the self-oscillating behavior of the Ru-PAU could be regulated by addition of urea, which is a unique tuning strategy. We envision that novel self-oscillating polymers with widely tunable soluble-insoluble behaviors can be rationally designed based these UCST-type polymers.
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Imaging Endogenous Metal Ions in Living Cells Using a DNAzyme–Catalytic Hairpin Assembly Probe ()
DNAzymes are a promising platform for metal ion detection, and a few DNAzyme-based sensors have been reported to detect metal ions inside cells. However, these methods required an influx of metal ions to increase their concentrations for detection. To address this major issue, the design of a catalytic hairpin assembly (CHA) reaction to amplify the signal from photocaged Na+-specific DNAzyme to detect endogenous Na+ inside cells is reported. Upon light activation and in the presence of Na+, the NaA43 DNAzyme cleaves its substrate strand and releases a product strand, which becomes an initiator that trigger the subsequent CHA amplification reaction. This strategy allows detection of endogenous Na+ inside cells, which has been demonstrated by both fluorescent imaging of individual cells and flow cytometry of the whole cell population. This method can be generally applied to detect other endogenous metal ions and thus contribute to deeper understanding of the role of metal ions in biological systems. Cha cha cha: A catalytic hairpin assembly (CHA) reaction was designed to amplify the signal from photocaged Na+-specific DNAzyme cleavage to detect endogenous Na+ inside cells. After light activation and in the presence of Na+, the NaA43 DNAzyme cleaves its substrate strand and releases a product strand, which becomes an initiator that can trigger the subsequent CHA signal amplification reaction.
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Switching Between Giant Positive and Negative Thermal Expansions of a YFe(CN)6-based Prussian Blue Analogue Induced by Guest Species ()
The control of thermal expansion of solid compounds is intriguing but remains challenging. The effect of guests on the thermal expansion of open-framework structures was investigated. Notably, the presence of guest ions (K+) and molecules (H2O) can substantially switch thermal expansion of YFe(CN)6 from negative (αv=−33.67×10−6 K−1) to positive (αv=+42.72×10−6 K−1)—a range that covers the thermal expansion of most inorganic compounds. The mechanism of such substantial thermal expansion switching is revealed by joint studies with synchrotron X-ray diffraction, X-ray absorption fine structure, neutron powder diffraction, and density functional theory calculations. The presence of guest ions or molecules plays a critical damping effect on transverse vibrations, thus inhibiting negative thermal expansion. An effective method is demonstrated to control the thermal expansion in open-framework materials by adjusting the presence of guests. Guest of honor: Thermal expansion of an open YFe(CN)6 framework is switched from strongly negative to positive by introducing guest potassium ions or H2O molecules. Guests have a damping effect on the transverse vibrations of nitrogen and carbon atoms, thus inhibiting negative thermal expansion.
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Electrically Activated Conductivity and White Light Emission of a Hydrocarbon Nanoring–Iodine Assembly ()
Numerous otherwise difficult applications have been realized with materials, the chemical/physical properties of which can be controlled by external stimuli such as heat, pressure, photo-irradiation, and voltage bias. However, the complexity of design and the lack of easy-to-conduct synthetic methods make the creation of on-demand stimuli responsive materials a formidable task. Here we report an electric-stimuli-responsive multifunctional material, [10]CPP-I: crystalline assembly of a hydrocarbon nanoring ([10]cycloparaphenylene: [10]CPP) as an “electro-responsive porous host” and iodine as a “potentially functional molecule”. Through applying electric stimulus, [10]CPP-I turned to exhibit two attractive properties: electronic conductivity and white light emission. We revealed that electric stimuli trigger the cascade formation of polyiodide chains inside the [10]CPP assembly through charge transfer, leading to the emergence of these properties. This “responsive porous host” approach is expected to be applicable for different stimuli, and opens the path for devising a generic strategy to the development of stimuli-responsive materials. Host–guest systems: An electric-stimuli-responsive multifunctional material can be readily made by mixing a hydrocarbon nanoring ([10]cycloparaphenylene: [10]CPP) and iodine ([10]CPP-I). Through applying an electric stimulus, the assembled material [10]CPP-I is endowed with two attractive properties: turn-on electronic conductivity and white light emission.
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One Model, Two Enzymes: Activation of Hydrogen and Carbon Monoxide ()
The ability to catalyze the oxidation of both H2 and CO in one reaction pot would be a major boon to hydrogen technology since CO is a consistent contaminant of H2 supplies. Here, we report just such a catalyst, with the ability to catalyze the oxidation of either or both H2 and CO, based on the pH value. This catalyst is based on a NiIr core that mimics the chemical function of [NiFe]hydrogenase in acidic media (pH 4–7) and carbon monoxide dehydrogenase in basic media (pH 7–10). We have applied this catalyst in a demonstration fuel cell using H2, CO, and H2/CO (1/1) feeds as fuels for oxidation at the anode. The power density of the fuel cell depends on the pH value in the media of the fuel cell and shows a similar pH dependence in a flask. We have isolated and characterized all intermediates in our proposed catalytic cycles. Enzymatic catalysis: If the oxidation of both H2 and CO could be catalyzed in one reaction pot, this would be a major boon to hydrogen technology since CO is a consistent contaminant of H2 supplies. A catalyst is reported with the ability to catalyze the oxidation of either or both H2 and CO (see picture), based on the chosen pH value.
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Cofactor-Free, Direct Photoactivation of Enoate Reductases for the Asymmetric Reduction of C=C Bonds ()
Enoate reductases from the family of old yellow enzymes (OYEs) can catalyze stereoselective trans-hydrogenation of activated C=C bonds. Their application is limited by the necessity for a continuous supply of redox equivalents such as nicotinamide cofactors [NAD(P)H]. Visible light-driven activation of OYEs through NAD(P)H-free, direct transfer of photoexcited electrons from xanthene dyes to the prosthetic flavin moiety is reported. Spectroscopic and electrochemical analyses verified spontaneous association of rose bengal and its derivatives with OYEs. Illumination of a white light-emitting-diode triggered photoreduction of OYEs by xanthene dyes, which facilitated the enantioselective reduction of C=C bonds in the absence of NADH. The photoenzymatic conversion of 2-methylcyclohexenone resulted in enantiopure (ee>99 %) (R)-2-methylcyclohexanone with conversion yields as high as 80–90 %. The turnover frequency was significantly affected by the substitution of halogen atoms in xanthene dyes. Flavin-containing old yellow enzymes (OYE) are activated by molecular photosensitizers through direct transfer of photoinduced electrons to the prosthetic flavin moiety without any NAD(P)H cofactor. Rose bengal (4,5,6,7-tetrachloro-2′,4′,5′,7′-tetraiodofluorescein, RB) and its xanthene derivatives are explored as photosensitizers to drive OYE-catalyzed reduction of conjugated C=C bonds under visible light illumination.
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A New Wrinkle in Oligoarylene Architecture ()
Bowl me over! A breathtaking bowl-shaped oligoarylene has been created. The structure is analogous to that of corranulene, but instead of twenty sp2-hybridized carbon atoms, the new nanometer-sized molecule has been constructed using twenty 1,3,5-trisubstituted benzene rings.
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α-Radical Phosphines: Synthesis, Structure, and Reactivity ()
A series of phosphines featuring a persistent radical were synthesized in two steps by condensation of dialkyl-/diarylchlorophosphines with stable cyclic (alkyl)(amino)carbenes (cAACs) followed by one-electron reduction of the corresponding cationic intermediates. Structural, spectroscopic, and computational data indicate that the spin density in these phosphines is mainly localized on the original carbene carbon from the cAAC fragment; thus, it remains in the α-position with respect to the central phosphorus atom. The potential of these α-radical phosphines to serve as spin-labeled ligands is demonstrated through the preparation of several AuI derivatives, which were also structurally characterized by single-crystal X-ray diffraction. That's radical! A series of α-radical phosphines stabilized by a cyclic (alkyl)(amino)carbene moiety were synthesized, their structures elucidated, and coordination complexes with gold(I) centers prepared. DFT calculations reveal that spin density is delocalized in the carbene π-system of both the phosphine ligands and their gold complexes, and is largest at the formerly carbene carbon atom.
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Aromatic-Imide-Based Thermally Activated Delayed Fluorescence Materials for Highly Efficient Organic Light-Emitting Diodes ()
Aromatic-imide-based thermally activated delayed fluorescence (TADF) materials with a twisted donor–acceptor–donor skeleton were efficiently synthesized and exhibited excellent thermal stability and high photoluminescence quantum yields. The small ΔEST value (<0.1 eV) along with the clear temperature-dependent delayed component of their transient photoluminescence (PL) spectra demonstrated their excellent TADF properties. Moreover, the performance of organic light-emitting diodes in which TADF materials AI-Cz and AI-TBCz were used as dopants were outstanding, with external quantum efficiencies up to 23.2 and 21.1 %, respectively. No apologies for this delay: Thermally activated delayed fluorescence (TADF) materials with a twisted donor–acceptor–donor skeleton have been developed with an N-phenylphthalimide acceptor moiety (see structure). They exhibit small ΔEST values (<0.1 eV) and excellent TADF properties. Organic light-emitting diodes fabricated with these materials had a high external quantum efficiency of up to 21.1 and 23.2 %.
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Transition-Metal Nitride Core@Noble-Metal Shell Nanoparticles as Highly CO Tolerant Catalysts ()
Core–shell architectures offer an effective way to tune and enhance the properties of noble-metal catalysts. Herein, we demonstrate the synthesis of Pt shell on titanium tungsten nitride core nanoparticles (Pt/TiWN) by high temperature ammonia nitridation of a parent core–shell carbide material (Pt/TiWC). X-ray photoelectron spectroscopy revealed significant core-level shifts for Pt shells supported on TiWN cores, corresponding to increased stabilization of the Pt valence d-states. The modulation of the electronic structure of the Pt shell by the nitride core translated into enhanced CO tolerance during hydrogen electrooxidation in the presence of CO. The ability to control shell coverage and vary the heterometallic composition of the shell and nitride core opens up attractive opportunities to synthesize a broad range of new materials with tunable catalytic properties. Cores and effect: Nanoparticles comprising a Pt shell on a transition-metal nitride core (TiWN) are prepared from nanoparticles containing a metal carbide core (TiWC). The nitride core modulates the Pt electronic structure leading to dramatically improved CO tolerance during hydrogen electrooxidation.
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Vancomycin-Dependent Response in Live Drug-Resistant Bacteria by Metabolic Labeling ()
The surge in drug-resistant bacterial infections threatens to overburden healthcare systems worldwide. Bacterial cell walls are essential to bacteria, thus making them unique targets for the development of antibiotics. We describe a cellular reporter to directly monitor the phenotypic switch in drug-resistant bacteria with temporal resolution. Vancomycin-resistant enterococci (VRE) escape the bactericidal action of vancomycin by chemically modifying their cell-wall precursors. A synthetic cell-wall analogue was developed to hijack the biosynthetic rewiring of drug-resistant cells in response to antibiotics. Our study provides the first in vivo VanX reporter agent that responds to cell-wall alteration in drug-resistant bacteria. Cellular reporters that reveal mechanisms related to antibiotic resistance can potentially have a significant impact on the fundamental understanding of cellular adaption to antibiotics. Can they resist? Drug-resistant bacteria pose a major threat. Vancomycin-resistant enterococci express VanX dipeptidases to remove drug-sensitive building blocks. A series of compounds were used to metabolically label bacterial cell walls of live VRE cells (see picture) to report on structural alterations linked with antibiotic resistance.
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Unraveling the Nature of Sites Active toward Hydrogen Peroxide Reduction in Fe-N-C Catalysts ()
Fe-N-C catalysts with high O2 reduction performance are crucial for displacing Pt in low-temperature fuel cells. However, insufficient understanding of which reaction steps are catalyzed by what sites limits their progress. The nature of sites were investigated that are active toward H2O2 reduction, a key intermediate during indirect O2 reduction and a source of deactivation in fuel cells. Catalysts comprising different relative contents of FeNxCy moieties and Fe particles encapsulated in N-doped carbon layers (0–100 %) show that both types of sites are active, although moderately, toward H2O2 reduction. In contrast, N-doped carbons free of Fe and Fe particles exposed to the electrolyte are inactive. When catalyzing the ORR, FeNxCy moieties are more selective than Fe particles encapsulated in N-doped carbon. These novel insights offer rational approaches for more selective and therefore more durable Fe-N-C catalysts. The chemical nature of secondary sites active for reduction of the H2O2 intermediate generated during indirect oxygen reduction was elucidated for the class of Fe-N-C catalysts. Both N-doped carbon layers encapsulating Fe particles and FeNxCy moieties moderately catalyze H2O2 reduction, while Fe-free N-doped carbons and exposed Fe particles are inactive. Direct and indirect pathways for O2 reduction on Fe-N-C catalysts could thus be established.
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Nitrogen Photofixation over III-Nitride Nanowires Assisted by Ruthenium Clusters of Low Atomicity ()
In many heterogeneous catalysts, the interaction of supported metal species with a matrix can alter the electronic and morphological properties of the metal and manipulate its catalytic properties. III-nitride semiconductors have a unique ability to stabilize ultra-small ruthenium (Ru) clusters (ca. 0.8 nm) at a high loading density up to 5 wt %. n-Type III-nitride nanowires decorated with Ru sub-nanoclusters offer controlled surface charge properties and exhibit superior UV- and visible-light photocatalytic activity for ammonia synthesis at ambient temperature. A metal/semiconductor interfacial Schottky junction with a 0.94 eV barrier height can greatly facilitate photogenerated electron transfer from III-nitrides to Ru, rendering Ru an electron sink that promotes N≡N bond cleavage, and thereby achieving low-temperature ammonia synthesis. III-Nitride semiconductors have a unique ability to stabilize ultra-small ruthenium clusters with a high loading density up to 5 wt %. The metal/semiconductor interfacial Schottky junction facilitates photogenerated electron transfer from III-nitrides to ruthenium, rendering ruthenium an electron sink that promotes N≡N bond cleavage during low-temperature ammonia synthesis.
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Rhodium(I)-Catalyzed Bridged [5+2] Cycloaddition of cis-Allene-vinylcyclopropanes to Synthesize the Bicyclo[4.3.1]decane Skeleton ()
Previously reported was that cis-ene-vinylcyclopropanes (cis-ene-VCPs) underwent Rh-catalyzed [5+2] reaction to give 5,7-fused bicyclic products, where vinylcyclopropane (VCP) acts as five-carbon synthon. Unfortunately, this reaction had very limited scope. Replacing the 2π component of cis-ene-VCPs to allene moiety, the corresponding cis-allene-VCPs did not undergo the expected normal [5+2] cycloaddition to give 5,7-fused bicyclic products. Instead, the challenging bicyclo[4.3.1]decane skeleton was obtained via an unprecedented bridged [5+2] cycloaddition. DFT calculations were applied to understand why this bridged [5+2] reaction is favored over the anticipated but not realized normal [5+2] reaction. Building bridges: Rhodium-catalyzed reaction of cis-allene-vinylcyclopropanes did not give the anticipated fused bicyclic products through normal [5+2] reaction. Instead, these substrates underwent an unexpected bridged [5+2] cycloaddition, by inverse allene insertion, compared to normal allene insertion, thus generating the bicyclo[4.3.1]decane skeleton.
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“Super-Reducing” Photocatalysis: Consecutive Energy and Electron Transfers with Polycyclic Aromatic Hydrocarbons ()
Donation welcome: Recent developments in visible-light photocatalysis allow the utilization of increasingly negative reduction potentials. Successive energy and electron transfer with polycyclic aromatic hydrocarbons enables the catalytic formation of strongly reducing arene radical anions, classical stoichiometric reagents for one-electron reduction in organic synthesis.
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Activation-Dependent Breathing in a Flexible Metal–Organic Framework and the Effects of Repeated Sorption/Desorption Cycling ()
A non-interpenetrated metal–organic framework with a paddle-wheel secondary building unit has been activated by direct thermal evacuation, guest exchange with a volatile solvent, and supercritical CO2 drying. Conventional thermal activation yields a mixture of crystalline phases and some amorphous content. Exchange with a volatile solvent prior to vacuum activation produces a pure breathing phase with high sorption capacity, selectivity for CO2 over N2 and CH4, and substantial hysteresis. Supercritical drying can be used to access a guest-free open phase. Pressure-resolved differential scanning calorimetry was used to confirm and investigate a systematic loss of sorption capacity by the breathing phase as a function of successive cycles of sorption and desorption. A corresponding loss of sample integrity was not detectable by powder X-ray diffraction analysis. This may be an important factor to consider in cases where flexible MOFs are earmarked for industrial applications. Taking a deep breath: A metal–organic framework with a paddle-wheel secondary building unit is activated by direct thermal evacuation, guest exchange with a volatile solvent, and supercritical CO2 drying. The solvent exchange step produces a pure breathing phase with high sorption capacity, selectivity for CO2 over N2 and CH4, and substantial hysteresis.
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Tricyanomethane and Its Ketenimine Tautomer: Generation from Different Precursors and Analysis in Solution, Argon Matrix, and as a Single Crystal ()
Solutions of azidomethylidenemalononitrile were photolyzed at low temperatures to produce the corresponding 2H-azirine and tricyanomethane, which were analyzed by low-temperature NMR spectroscopy. The latter product was also observed after short thermolysis of the azide precursor in solution whereas irradiation of the azide isolated in an argon matrix did not lead to tricyanomethane, but to unequivocal detection of the tautomeric ketenimine by IR spectroscopy for the first time. When the long-known “aquoethereal” greenish phase generated from potassium tricyanomethanide, dilute sulfuric acid, and diethyl ether was rapidly evaporated and sublimed, a mixture of hydronium tricyanomethanide and tricyanomethane was formed instead of the previously claimed ketenimine tautomer. Under special conditions of sublimation, single crystals of tricyanomethane could be isolated, which enabled the analysis of the molecular structure by X-ray diffraction. Held incommunicado in an argon matrix, a long-sought ketenimine was detected after irradiation of an appropriate vinyl azide whereas photolysis or thermolysis of the same precursor in solution led to cyanoform, which is also available as single crystals from potassium tricyanomethanide.
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Chemically Tailoring the Dopant Emission in Manganese-Doped CsPbCl3 Perovskite Nanocrystals ()
Doping in perovskite nanocrystals adopts different mechanistic approach in comparison to widely established doping in chalcogenide quantum dots. The fast formation of perovskites makes the dopant insertions more competitive and challenging. Introducing alkylamine hydrochloride (RNH3Cl) as a promoting reagent, precise controlled doping of MnII in CsPbCl3 perovskite nanocrystals is reported. Simply, by changing the amount of RNH3Cl, the Mn incorporation and subsequent tuning in the excitonic as well as Mn d–d emission intensities are tailored. Investigations suggested that RNH3Cl acted as the chlorinating source, controlled the size, and also helps in increasing the number of particles. This provided more opportunity for Mn ions to take part in reaction and occupied the appropriate lattice positions. Carrying out several reactions with varying reaction parameters, the doping conditions are optimized and the role of the promoting reagent for both doped and undoped systems are compared. Programming perovskite doping: Controlled doping of MnII in CsPbCl3 nanocrystals was performed by using alkylamine hydrochloride (RNH3Cl) as doping promoting reagent. The dopant emission intensity is tailored as a function of Mn doping governed by the amount of introduced RNH3Cl. This helped in stabilizing and also controlling the size of CsPbCl3 nanocrystals, which in turn facilitated efficient doping.
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Mechanism-Based Enantiodivergence in Manganese Reduction Catalysis: A Chiral Pincer Complex for the Highly Enantioselective Hydroboration of Ketones ()
A manganese alkyl complex containing a chiral bis(oxazolinyl-methylidene)isoindoline pincer ligand is a precatalyst for a catalytic system of unprecedented activity and selectivity in the enantioselective hydroboration of ketones, thus producing preparatively useful chiral alcohols in excellent yields with up to greater than 99 % ee. It is applicable for both aryl alkyl and dialkyl ketone reduction under mild reaction conditions (TOF >450 h−1 at −40 °C). The earth-abundant base-metal catalyst operates at very low catalyst loadings (as low as 0.1 mol %) and with a high level of functional-group tolerance. There is evidence for the existence of two distinct mechanistic pathways for manganese-catalyzed hydride transfer and their role for enantiocontrol in the selectivity-determining step is presented. Direct or indirect: Two distinct mechanistic pathways are operative for manganese-catalyzed hydride transfer in the selectivity-determining step of the enantioselective hydroboration of ketones. The reaction produces preparatively useful chiral alcohols in excellent yields and enantioselectivities. The catalyst loadings are low and the reaction demonstrates a high level of functional-group tolerance.
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Noncovalent Immobilization of Molecular Electrocatalysts for Chemical Synthesis: Efficient Electrochemical Alcohol Oxidation with a Pyrene–TEMPO Conjugate ()
Electrocatalytic methods for organic synthesis could offer sustainable alternatives to traditional redox reactions, but strategies are needed to enhance the performance of molecular catalysts designed for this purpose. The synthesis of a pyrene-tethered TEMPO derivative (TEMPO=2,2,6,6-tetramethylpiperidinyl-N-oxyl) is described, which undergoes facile in situ noncovalent immobilization onto a carbon cloth electrode. Cyclic voltammetry and controlled potential electrolysis studies demonstrate that the immobilized catalyst exhibits much higher activity relative to 4-acetamido–TEMPO, an electronically similar homogeneous catalyst. In preparative electrolysis experiments with a series of alcohol substrates and the immobilized catalyst, turnover numbers and frequencies approach 2 000 and 4 000 h−1, respectively. The synthetic utility of the method is further demonstrated in the oxidation of a sterically hindered hydroxymethylpyrimidine precursor to the blockbuster drug, rosuvastatin. Linked to performance: A 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) catalyst tethered to pyrene was immobilized on a carbon cloth electrode in situ. The noncovalently immobilized electrocatalyst efficiently oxidizes variously substituted benzyl alcohols.
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Dearomative Intramolecular (4+3) Cycloadditions of Arenes with Epoxy and Aziridinyl Enolsilanes ()
An intramolecular (4+3) cycloaddition of epoxy and aziridinyl enolsilanes with benzene, naphthalene, and anthracene derivatives is reported. Highly functionalized polycyclic alcohols and amines are generated under relatively mild reaction conditions with yields up to 89 %. Optically enriched cycloadducts are obtained from cycloadditions of enantiomerically pure epoxides and aziridines. Breaking the cycle: Arenes react as dienes to undergo intramolecular (4+3) cycloadditions with epoxy/aziridinyl enolsilanes. This stepwise cycloaddition is facile at low temperatures, even for benzenes. Optically active (4+3) cycloadducts are obtained when either enantiomerically enriched epoxides or aziridines are used.
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Structural Transition in Liquid Crystal Bubbles Generated from Fluidic Nanocellulose Colloids ()
The structural transition in micrometer-sized liquid crystal bubbles (LCBs) derived from rod-like cellulose nanocrystals (CNCs) was studied. The CNC-based LCBs were suspended in nematic or chiral nematic liquid-crystalline CNCs, which generated topological defects and distinct birefringent textures around them. The ordering and structure of the LCBs shifted from a nematic to chiral nematic arrangement as water evaporation progressed. These packed LCBs exhibited a specific photonic cross-communication property that is due to a combination of Bragg reflection and bubble curvature and size. Liquid crystal bubbles with active ordering were obtained from fluidic nanocellulose colloids. The structure, texture, topological defects, and temporal evolution of these bubbles were studied both in suspension and solid film, and interesting optical properties were observed.
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Wet and Functional Adhesives from One-Step Aqueous Self-Assembly of Natural Amino Acids and Polyoxometalates ()
Sessile organisms have undergone long-term evolution to develop the unique ability by positioning themselves on wet solid surface through secreting adhesive proteins. The present study reveals that natural amino acid monomers can also exhibit similar adhesion capacity. This kind of biomimetic adhesives were created by the one-step aqueous assembly of basic amino acids with assistance of anionic polyoxometalates. The polyoxometalates not only serve as multivalent scaffold to initiate the supramolecular cross-linking of amino acid molecules, but also function as a redox component, bestowing the wet adhesives with electrochromic features. Perfect partner: Hydrogen bonding and electrostatic interactions between basic amino acids and polyoxometalates together with the zwitterionic self-assembly resulted in the formation of wet adhesives with electrochromic properties. This simple and sustainable strategy opens up new opportunities for creating functional adhesives beyond nature.
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Chemical Recognition of Active Oxygen Species on the Surface of Oxygen Evolution Reaction Electrocatalysts ()
Owing to the transient nature of the intermediates formed during the oxygen evolution reaction (OER) on the surface of transition metal oxides, their nature remains largely elusive by the means of simple techniques. The use of chemical probes is proposed, which, owing to their specific affinities towards different oxygen species, unravel the role played by these species on the OER mechanism. For that, tetraalkylammonium (TAA) cations, previously known for their surfactant properties, are introduced, which interact with the active oxygen sites and modify the hydrogen bond network on the surface of OER catalysts. Combining chemical probes with isotopic and pH-dependent measurements, it is further demonstrated that the introduction of iron into amorphous Ni oxyhydroxide films used as model catalysts deeply modifies the proton exchange properties, and therefore the OER mechanism and activity. Introduction of iron into nickel oxyhydroxide amorphous films deeply modifies the interfacial proton diffusion properties, and tetraalkylammonium cations capture reactive oxygen species. These effects are related to the activity and mechanism of the oxygen evolution reaction. IHP/OHP=inner/outer Helmholtz plane.
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One Model, Two Enzymes: Activation of Hydrogen and Carbon Monoxide ()
A [NiIr]-based catalyst turns the table on CO poisoning by exploiting the gas as a fuel. The system is able to act as a H2 oxidizing catalyst at low pH and as a CO oxidizing catalyst at high pH. At pH 7, there is a crossover where both oxidations are catalyzed simultaneously. In their Communication (DOI: 10.1002/anie.201704864), S. Ogo et al. report a proof-of-concept fuel cell that functioned on a 50:50 supply of H2 and CO. The double function is depicted above as a two-headed bird.
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Inducing Complexity in Intermetallics through Electron–Hole Matching: the Structure of Fe14Pd17Al69 ()
We illustrate how the crystal structure of Fe14Pd17Al69 provides an example of an electron–hole matching approach to inducing frustration in intermetallic systems. Its structure contains a framework based on IrAl2.75, a binary compound that closely adheres to the 18−n rule. Upon substituting the Ir with a mixture of Fe and Pd, a competition arises between maintaining the overall ideal electron concentration and accommodating the different structural preferences of the two elements. A 2×2×2 supercell results, with Pd- and Fe-rich regions emerging. Just as in the original IrAl2.75 phase, the electronic structure of Fe14Pd17Al69 exhibits a pseudogap at the Fermi energy arising from an 18−n bonding scheme. The electron–hole matching approach's ability to combine structural complexity with electronic pseudogaps offers an avenue to new phonon glass–electron crystal materials. The crystal and electronic structure of Fe14Pd17Al69 illustrate an approach to new realizations of the phonon glass–electron crystal concept: the electron–hole matching strategy.
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Electronically Activated Organoboron Catalysts for Enantioselective Propargyl Addition to Trifluoromethyl Ketones ()
A broadly applicable, practical, scalable, efficient and highly α- and enantioselective method for addition of a silyl-protected propargyl moiety to trifluoromethyl ketones has been developed. Reactions, promoted by 2.0 mol % of a catalyst that is derived in situ from a readily accessible aminophenol compound at ambient temperature, were complete after only 15 minutes at room temperature. The desired tertiary alcohols were isolated in up to 97 % yield and 98.5:1.5 enantiomeric ratio. Alkyl-, alkenyl-, alkynyl-, aryl- or heteroaryl-substituted trifluoromethyl ketones can be used. Utility is highlighted by application to a transformation that is relevant to enantioselective synthesis of BI 653048, a compound active against rheumatoid arthritis. Fast and selective: When armed with a trifluoromethyl group, readily accessible chiral organoboron catalysts can promote highly enantioselective addition of a silyl-protected propargylboron compound to a wide range of trifluoromethyl ketones with high efficiency. Utility is demonstrated through a process that may be used for enantioselective synthesis of BI 653048 (active against rheumatoid arthritis).
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From Chemical Topology to Molecular Machines (Nobel Lecture) ()
To a large extent, the field of “molecular machines” started after several groups were able to prepare, reasonably easily, interlocking ring compounds (named catenanes for compounds consisting of interlocking rings and rotaxanes for rings threaded by molecular filaments or axes). Important families of molecular machines not belonging to the interlocking world were also designed, prepared, and studied but, for most of them, their elaboration was more recent than that of catenanes or rotaxanes. Since the creation of interlocking ring molecules is so important in relation to the molecular machinery area, we will start with this aspect of our work. The second part will naturally be devoted to the dynamic properties of such systems and to the compounds for which motions can be directed in a controlled manner from the outside, that is, molecular machines. We will restrict our discussion to a very limited number of examples which we consider as particularly representative of the field. Magic rings: The field of molecular machines has its origins in the synthesis of catenanes and rotaxanes. J.-P. Sauvage describes in his Nobel Lecture the beginnings of this research and the developments that led to the first molecular muscles and machines whose movement can be directed ′′from the outside′′ in a controlled manner.
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A Unified Continuous Flow Assembly-Line Synthesis of Highly Substituted Pyrazoles and Pyrazolines ()
A rapid and modular continuous flow synthesis of highly functionalized fluorinated pyrazoles and pyrazolines has been developed. Flowing fluorinated amines through sequential reactor coils mediates diazoalkane formation and [3+2] cycloaddition to generate more than 30 azoles in a telescoped fashion. Pyrazole cores are then sequentially modified through additional reactor modules performing N-alkylation and arylation, deprotection, and amidation to install broad molecular diversity in short order. Continuous flow synthesis enables the safe handling of diazoalkanes at elevated temperatures, and the use of aryl alkyne dipolarphiles under catalyst-free conditions. This assembly-line synthesis provides a flexible approach for the synthesis of agrochemicals and pharmaceuticals, as demonstrated by a four-step, telescoped synthesis of measles therapeutic, AS-136A, in a total residence time of 31.7 min (1.76 g h−1). Reaction modularity meets flow: Assembly lines have been used to synthesize highly functionalized pyrazoles and pyrazolines on route to agrochemicals and pharmaceuticals through multi-step, continuous flow synthesis.
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Direct Zinc(II)-Catalyzed Enantioconvergent Additions of Terminal Alkynes to α-Keto Esters ()
The addition of terminal alkynes to racemic β-stereogenic α-keto esters was achieved in high levels of stereoselectivity, affording versatile tertiary propargylic alcohols containing two stereocenters. This environmentally benign enantioconvergent reaction proceeds with perfect atom economy, requires no solvent, and is catalyzed by a non-toxic zinc salt. The alkyne moiety can be leveraged in downstream transformations including hydrogenation to the corresponding saturated tertiary alcohol, which represents the product of a formal enantioconvergent aliphatic nucleophile addition. A perfect transformation: The addition of terminal alkynes to racemic β-stereogenic α-keto esters was achieved in high levels of stereoselectivity, affording versatile tertiary propargylic alcohols containing two stereocenters. This environmentally benign enantioconvergent reaction proceeds with high atom economy, requires no solvent, and is catalyzed by a non-toxic zinc salt.
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Switchable Access to Different Spirocyclopentane Oxindoles by N-Heterocyclic Carbene Catalyzed Reactions of Isatin-Derived Enals and N-Sulfonyl Ketimines ()
A novel NHC-catalyzed annulation protocol for the asymmetric synthesis of biologically important β-lactam fused spirocyclopentane oxindoles with four contiguous stereocenters, including two quaternary carbon centers, was developed. Alternatively, spirocyclopentane oxindoles containing an enaminone moiety can be achieved using the same starting materials, isatin-derived enals, and N-sulfonyl ketimines, in the presence of a slightly different NHC catalytic system. This switchable annulation strategy enables the selective assembly of both heterocyclic scaffolds with good yields and excellent enantioselectivities for a broad range of substrates. Flip a switch: The switchable NHC-catalyzed asymmetric reaction between isatin-derived enals and N-sulfonyl ketimines by the homoenolate pathway selectively leads to spirocyclopentane oxindoles bearing either a β-lactam or an enaminone moiety. The reaction proceeds in high yields and with excellent stereoselectivities for a wide range of substrates.
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A Designed Approach to Enantiodivergent Enamine Catalysis ()
The rational design and implementation of enantiodivergent enamine catalysis is reported. A simple secondary amine catalyst, 2-methyl-l-proline, and its tetrabutylammonium salt function as an enantiodivergent catalyst pair delivering the enantiomers of α-functionalized aldehyde products in excellent enantioselectivities. This novel concept of designed enantiodivergence is applied to the enantioselective α-amination, aldol, and α-aminoxylation/α-hydroxyamination reactions of aldehydes. Paired off: The rational design and implementation of enantiodivergent enamine catalysis is reported. A simple secondary amine catalyst, 2-methyl-l-proline, and its tetrabutylammonium salt function as an enantiodivergent catalyst pair delivering the enantiomers of α-functionalized aldehyde products in excellent enantioselectivities. This novel concept of designed enantiodivergence is applied to the enantioselective α-amination, aldol, and α-aminoxylation/α-hydroxyamination reactions of aldehydes.
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A Modular Approach to Inorganic Phosphazane Macrocycles ()
The union of organic and macrocyclic chemistry: The formation of large inorganic macrocycles containing phosphazane units linked by sulfur or selenium becomes straightforward with the methodology described by D. S. Wright et al. in their Communication (10.1002/anie.201702558). This allows the introduction of a range of organic substituents, facilitating the tuning of the steric and electronic host environment, and marks a significant step towards the development of synthetic tools in macromolecular inorganic assembly that mirror those in organic synthesis.
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Controlling the α/γ-Reactivity of Vinylogous Ketone Enolates in Organocatalytic Enantioselective Michael Reactions ()
The first regio-, diastereo-, and enantioselective direct Michael reaction of β,γ-unsaturated ketones with nitroolefins is enabled by Brønsted base/hydrogen-bonding bifunctional catalysis. A squaramide-substituted tertiary amine catalyzes the reaction of a broad range of β,γ-unsaturated ketones to proceed at the α-site exclusively, giving rise to adducts with two consecutive tertiary carbon stereocenters in diastereomeric ratios of up to >20:1 and enantioselectivities generally in the 90–98 % ee range. Direct with a switch: Most catalytic reactions with vinylogous enolate equivalents generated in situ proceed through the γ-carbon atom (conjugation preserved). It is now shown that a bifunctional tertiary amine/hydrogen-bonding catalyst can be used for highly diastereo- and enantioselective α-additions of β,γ-unsaturated ketones to nitroolefins.
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A T-Shaped Nickel(I) Metalloradical Species ()
A T-shaped nickel(I) metalloradical species is prepared using a rigid acridane-based PNP pincer ligand revealing its novel open-shell reactivity. In their Communication (10.1002/anie.201704487), Y. Lee and C. Yoo show that because of its sterically exposed half-filled dx2−y2 orbital, this d9 metalloradical species shows unique reactivity including the homolytic cleavage of various σ-bonds, such as H–H, N—N, and C—C via a binuclear two-electron process.
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Silver(III)⋅⋅⋅Silver(III) Interactions that Stabilize the syn Form in a Porphyrin Dimer Upon Oxidation ()
The interaction between two AgII porphyrins, connected covalently through a highly flexible ethane bridge, in a metalloporphyrin dimer has been investigated upon stepwise oxidation. X-ray structure determination of one and two-electron oxidized complexes has clearly revealed only metal-centered oxidation that results in short Ag−N (porphyrin) distance with large distortion in the porphyrin macrocycle. The 2e-oxidized complex exhibits significant metallophilic interaction in the form of a close AgIII⋅⋅⋅AgIII contact that brings two porphyrin rings more cofacial with syn-conformation, which would otherwise stabilize in an anti-form. The interaction also leads to an intense emission peak at 546 nm at 77 K in the photoluminescence study. Meeting at the bridge: An ethylene-bridged disilver(III) porphyrin dimer exhibits significant metallophilic interaction that brings two porphyrin rings on top of each other. The short AgIII⋅⋅⋅AgIII distance is attributed to the overlap of dz2 metal orbitals. The complex displays an intense emission peak at 546 nm at 77 K in a photoluminescence study.
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Crystalline Hollow Microrods for Site-Selective Enhancement of Nonlinear Photoluminescence ()
In the hands of the monkey god Sun Wukong hollow-structured microcrystals produce spatially resolved optical codes upon illumination with near-infrared light. In their Communication (DOI: 10.1002/anie.201703600), F. Wang et al. reveal that light scattering and reflection by the inner walls of the microrods modulates light intensity across the structure. As a result, the electric field proximal to the inner walls is enhanced and bright optical emissions are observed around the holes.
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Iron-Catalyzed Cyclotrimerization of Terminal Alkynes by Dual Catalyst Activation in the Absence of Reductants ()
Catalyzing C−C bond-forming reactions with earth-abundant metals under mild conditions is at the heart of sustainable synthesis. The cyclotrimerization of alkynes is a valuable atom-efficient reaction in organic synthesis that is enabled by several metal catalysts, including iron. This study reports an effective iron-catalyzed cyclotrimerization for the regioselective synthesis of 1,2,4-substituted arenes (1 mol % catalyst, toluene, 20 °C, 5 min). A dual activation mechanism (substrate deprotonation, reductive elimination) renders the simple FeII precatalyst highly active in the absence of any reductant. Cyclotrimerization reactions of terminal alkynes are enabled by the simple precatalyst Fe(hmds)2 (hmds=1,1,1,3,3,3-hexamethyldisilazide), which does not require the addition of a dedicated reductant but undergoes rapid substrate-induced activation. 1,2,4-Trisubstituted arenes were thus formed in a highly regioselective fashion within a few minutes at 20 °C.
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Synthesis, Solid-State Structure, and Bonding Analysis of a Homoleptic Beryllium Azide ()
[Ph4P]2[Be(N3)4] (1) and [PNP]2[Be(N3)4] (2; PNP=Ph3PNPPh3) were synthesized by reacting Be(N3)2 with [Ph4P]N3 and [PNP]N3. Compound 1 represents the first structurally characterized homoleptic beryllium azide. The electronic structure and bonding situation in the tetraazidoberyllate dianion [Be(N3)4]2− were investigated by quantum-chemical calculations (NPA, ELF, LOL). Nitrogen-rich salts: Be(N3)2 reacts with two equivalents of [Ph4P]N3 and [PNP]N3 (PNP=Ph3PNPPh3) to give [Ph4P]2[Be(N3)4] (1) and [PNP]2[Be(N3)4] (2), which contain the tetraazidoberyllate dianion. Compound 1 was characterized by single-crystal X-ray analysis, and its electronic structure was studied by quantum-chemical calculations. The solution structure of 2 was determined by heteronuclear NMR spectroscopy.
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Photocatalytic Nanosheet Lithography: Photolithography based on Organically Modified Photoactive 2D Nanosheets ()
Harvesting the properties of nanosheets is not only crucial from a fundamental perspective, but also for the development of novel functional devices based on 2D nanosheets. Herein, we demonstrate the processing of organically modified TBAxH1−xCa2Nb3O10 nanosheets into photonic thin films and study their colorimetric sensing properties in response to various aqueous and organic solvent vapors. Building on the enhanced solvent accessibility of TBA-containing nanosheets and their photocatalytic activity under UV irradiation, we develop a new concept for photocatalytic lithography using TBAxH1−xCa2Nb3O10 nanosheets as a negative photoresist to obtain high-fidelity micron-scale patterns of robust inorganic nanosheets. Photocatalytic nanosheet lithography (PNL) therefore adds a new resist-free, resource efficient direct patterning technique to the toolbox of photolithography. The pattern of little sheets: Combining the solvent-uptake capacity of organically modified TBAxH1−xCa2Nb3O10 nanosheets with their inherent photocatalytic activity imparts both vapor-sensing properties and enables their use as negative photoresists for micron-scale patterning.
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A Rapid Total Synthesis of Ciprofloxacin Hydrochloride in Continuous Flow ()
Within a total residence time of 9 min, the sodium salt of ciprofloxacin was prepared from simple building blocks via a linear sequence of six chemical reactions in five flow reactors. Sequential offline acidifications and filtrations afforded ciprofloxacin and ciprofloxacin hydrochloride. The overall yield of the eight-step sequence was 60 %. No separation of intermediates was required throughout the synthesis when a single acylation reaction was applied to remove the main byproduct, dimethylamine. Rapid assembly of five building blocks to make the antibiotic ciprofloxacin was achieved in nine minutes in flow. A total of six reactions took place in five reactors to afford ciprofloxacin sodium salt. Simple offline pH adjustment, filtration and crystallization afforded ciprofloxacin hydrochloride crystal.
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Cold Snapshot of a Molecular Rotary Motor Captured by High-Resolution Rotational Spectroscopy ()
We present the first high-resolution rotational spectrum of an artificial molecular rotary motor. By combining chirped-pulse Fourier transform microwave spectroscopy and supersonic expansions, we captured the vibronic ground-state conformation of a second-generation motor based on chiral, overcrowded alkenes. The rotational constants were accurately determined by fitting more than 200 rotational transitions in the 2–4 GHz frequency range. Evidence for dissociation products allowed for the unambiguous identification and characterization of the isolated motor components. Experiment and complementary quantum-chemical calculations provide accurate geometrical parameters for the C27H20 molecular motor, the largest molecule investigated by high-resolution microwave spectroscopy to date. Supersonic molecular motors: The ground-state structure of a unidirectional artificial molecular rotary motor was determined by using high-resolution microwave spectroscopy and supersonic expansions. These studies revealed the structure of a one-of-a-kind functional nanomachine in the gas phase.
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Qing-Hua Fan ()

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Tailored Ahp-cyclodepsipeptides as Potent Non-covalent Serine Protease Inhibitors ()
The S1 serine protease family is one of the largest and most biologically important protease families. Despite their biomedical significance, generic approaches to generate potent, class-specific, bioactive non-covalent inhibitors for these enzymes are still limited. In this work, we demonstrate that Ahp-cyclodepsipeptides represent a suitable scaffold for generating target-tailored inhibitors of serine proteases. For efficient synthetic access, we developed a practical mixed solid- and solution-phase synthesis that we validated through performing the first chemical synthesis of the two natural products Tasipeptin A and B. The suitability of the Ahp-cyclodepsipeptide scaffold for tailored inhibitor synthesis is showcased by the generation of the most potent human HTRA protease inhibitors to date. We anticipate that our approach may also be applied to other serine proteases, thus opening new avenues for a systematic discovery of serine protease inhibitors. Made to measure: Ahp-cyclodepsipeptides, a class of natural products with specific non-covalent serine protease inhibitory properties, were tailored to generate potent chemical tools for investigating the biomedically relevant human HTRA proteases.
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Renewable Molecular Flasks with NADH Models: Combination of Light-Driven Proton Reduction and Biomimetic Hydrogenation of Benzoxazinones ()
Using small molecules with defined pockets to catalyze chemical transformations resulted in attractive catalytic syntheses that echo the remarkable properties of enzymes. By modulating the active site of a nicotinamide adenine dinucleotide (NADH) model in a redox-active molecular flask, we combined biomimetic hydrogenation with in situ regeneration of the active site in a one-pot transformation using light as a clean energy source. This molecular flask facilitates the encapsulation of benzoxazinones for biomimetic hydrogenation of the substrates within the inner space of the flask using the active sites of the NADH models. The redox-active metal centers provide an active hydrogen source by light-driven proton reduction outside the pocket, allowing the in situ regeneration of the NADH models under irradiation. This new synthetic platform, which offers control over the location of the redox events, provides a regenerating system that exhibits high selectivity and efficiency and is extendable to benzoxazinone and quinoxalinone systems. In the pocket: Through modulation of the active sites of NADH models in redox-active molecular flasks, a new synthetic platform that controls the location of biomimetic hydrogenation and in situ photocatalytic proton reduction to the inner and outer spaces of the pocket, respectively, was developed that echoes the properties of enzymes for the one-pot transformation of benzoxazinones and quinoxalinones using light as a clean energy source.
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Europium Oxybromide Catalysts for Efficient Bromine Looping in Natural Gas Valorization ()
The industrialization of bromine-mediated natural gas upgrading is contingent on the ability to fully recycle hydrogen bromide (HBr), which is the end form of the halogen after the activation and coupling of the alkanes. Europium oxybromide (EuOBr) is introduced as a unique catalytic material to close the bromine loop via HBr oxidation, permitting low-temperature operation and long lifetimes with a stoichiometric feed (O2:HBr=0.25)—conditions at which any catalyst reported to date severely deactivates because of excessive bromination. Besides, EuOBr exhibits unparalleled selectivity to methyl bromide in methane oxybromination, which is an alternative route for bromine looping. This novel active phase is finely dispersed on appropriate carriers and scaled up to technical extrudates, enhancing the utilization of the europium phase while preserving the performance. This catalytic system paves the way for sustainable valorization of stranded natural gas via bromine chemistry. Mind the gap: Europium-oxybromide-based materials are extraordinary catalysts for both HBr oxidation to Br2 and CH4 oxybromination to CH3Br, enabling a closed bromine loop for the sustainable upgrading of natural gas to fuels and chemicals.
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Strain Stiffening Hydrogels through Self-Assembly and Covalent Fixation of Semi-Flexible Fibers ()
Biomimetic, strain-stiffening materials are reported, made through self-assembly and covalent fixation of small building blocks to form fibrous hydrogels that are able to stiffen by an order of magnitude in response to applied stress. The gels consist of semi-flexible rodlike micelles of bisurea bolaamphiphiles with oligo(ethylene oxide) (EO) outer blocks and a polydiacetylene (PDA) backbone. The micelles are fibers, composed of 9–10 ribbons. A gelation method based on Cu-catalyzed azide–alkyne cycloaddition (CuAAC), was developed and shown to lead to strain-stiffening hydrogels with unusual, yet universal, linear and nonlinear stress–strain response. Upon gelation, the X-ray scattering profile is unchanged, suggesting that crosslinks are formed at random positions along the fiber contour without fiber bundling. The work expands current knowledge about the design principles and chemistries needed to achieve fully synthetic, biomimetic soft matter with on-demand, targeted mechanical properties. Biomimetic, strain-stiffening materials were made through self-assembly and covalent fixation of small bisurea bolaamphiphile building blocks to form fibrous hydrogels. These gels are able to stiffen by an order of magnitude in response to applied stress.
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Stable Heterometallic Cluster Ions based on Werner's Hexol ()
Large aqueous ions are interesting because they are useful in materials science (for example to generate thin films) but also because they serve as molecular models for the oxide–aqueous mineral interface where spectroscopy is difficult. Here we show that new clusters of the type M[(μ-OH)2Co(NH3)4]3(NO3)6 (M=Al, Ga) can be synthesized using Werner's century-old cluster as a substitutable framework. We substituted Group 13 metals into the hexol Co[(μ-OH)2Co(NH3)4]36+ ion to make diamagnetic heterometallic ions. The solid-state structure of the hexol-type derivatives were determined by single-crystal XRD and NMR spectroscopy and confirmed that the solid-state structure persists in solution after dissolution into either D2O or [D6]DMSO. Other compositions besides these diamagnetic ions can undoubtedly be made using a similar approach, which considerably expands the number of stable aqueous heteronuclear ions. Heterometallic clusters: New, stable polymetallic ions were prepared by targeted substitutions into the Werner's hexol structure: [Co((μ-OH)2Co(NH3)4)3]6+. The substitutions are made using Group 13 metals and the stability is confirmed by NMR spectroscopy.
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Cytoprotective Encapsulation of Individual Jurkat T Cells within Durable TiO2 Shells for T-Cell Therapy ()
Lymphocytes, such as T cells and natural killer (NK) cells, have therapeutic promise in adoptive cell transfer (ACT) therapy, where the cells are activated and expanded in vitro and then infused into a patient. However, the in vitro preservation of labile lymphocytes during transfer, manipulation, and storage has been one of the bottlenecks in the development and commercialization of therapeutic lymphocytes. Herein, we suggest a cell-in-shell (or artificial spore) strategy to enhance the cell viability in the practical settings, while maintaining biological activities for therapeutic efficacy. A durable titanium oxide (TiO2) shell is formed on individual Jurkat T cells, and the CD3 and other antigens on cell surfaces remain accessible to the antibodies. Interleukin-2 (IL-2) secretion is also not hampered by the shell formation. This work suggests a chemical toolbox for effectively preserving lymphocytes in vitro and developing the lymphocyte-based cancer immunotherapy. Individual Jurkat T cells are encapsulated within TiO2 shells. Tolerance against external stresses is enhanced by the shell formation without compromising the cellular metabolism. The antigen–antibody interaction and IL-2 secretion are preserved, suggesting applications to T-cell cancer therapy.
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A Generalized Strategy for the Synthesis of Large-Size Ultrathin Two-Dimensional Metal Oxide Nanosheets ()
Two-dimensional (2D) nanomaterials show unique electrical, mechanical, and catalytic performance owing to their ultrahigh surface-to-volume ratio and quantum confinement effects. However, ways to simply synthesize 2D metal oxide nanosheets through a general and facile method is still a big challenge. Herein, we report a generalized and facile strategy to synthesize large-size ultrathin 2D metal oxide nanosheets by using graphene oxide (GO) as a template in a wet-chemical system. Notably, the novel strategy mainly relies on accurately controlling the balance between heterogeneous growth and nucleation of metal oxides on the surface of GO, which is independent on the individual character of the metal elements. Therefore, ultrathin nanosheets of various metal oxides, including those from both main-group and transition elements, can be synthesized with large size. The ultrathin 2D metal oxide nanosheets also show controllable thickness and unique surface chemical state. A generalized and facile strategy is used to synthesize large-size ultrathin 2D metal oxide nanosheets by using graphene oxide (GO) as a template in a wet-chemical system. The nanosheets fabricated following the strategy show controllable thickness and a unique surface chemical state.
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1-Substituted 2-Azaspiro[3.3]heptanes: Overlooked Motifs for Drug Discovery ()
The 2-substituted piperidine core is found in drugs (18 FDA-approved drugs), however, their spirocyclic analogues remain unknown. Described here is the synthesis of spirocyclic analogues for 2-substituted piperidines and a demonstration of their validation in drug discovery. Core fitness: The 2-substituted piperidine core is found in many drugs, however, their spirocyclic analogues remain unknown. Described here is the synthesis of spirocyclic analogues for 2-substituted piperidines and their validation in drug discovery.
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Frozen Acrylamide Gels as Dynamic Nuclear Polarization Matrices ()
Aqueous acrylamide gels can be used to provide dynamic nuclear polarization (DNP) NMR signal enhancements of around 200 at 9.4 T and 100 K. The enhancements are shown to increase with crosslinker concentration and low concentrations of the AMUPol biradical. This DNP matrix can be used in situations where conventional incipient wetness methods fail, such as to obtain DNP surface enhanced NMR spectra from inorganic nanoparticles. In particular, we obtain 113Cd spectra from CdTe-COOH NPs in minutes. The spectra clearly indicate a highly disordered cadmium-rich surface. Aqueous acrylamide gels can be used to provide dynamic nuclear polarization (DNP) NMR signal enhancements of around 200 at 9.4 T and 100 K. This new DNP matrix can be used when conventional methods fail, such as to obtain DNP SENS from CdTe-COOH NPs with a highly-disordered cadmium rich surface.
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Studying the Conformation of a Receptor Tyrosine Kinase in Solution by Inhibitor-Based Spin Labeling ()
The synthesis of a spin label based on PD168393, a covalent inhibitor of a major anticancer drug target, the epidermal growth factor receptor (EGFR), is reported. The label facilitates the analysis of the EGFR structure in solution by pulsed electron paramagnetic resonance (EPR) spectroscopy. For various EGFR constructs, including near-full-length EGFR, we determined defined distance distributions between the two spin labels bound to the ATP binding sites of the EGFR dimer. The distances are in excellent agreement with an asymmetric dimer of the EGFR. Based on crystal structures, this dimer had previously been proposed to reflect the active conformation of the receptor but structural data demonstrating its existence in solution have been lacking. More generally, our study provides proof-of-concept that inhibitor-based spin labeling enables the convenient introduction of site-specific spin labels into kinases for which covalent or tight-binding small-molecule modulators are available. Synthesizing and employing a spin-labeled covalent inhibitor led to a versatile and convenient method to introduce paramagnetic residues at a defined site into a protein for EPR studies. The probe allows the determination of the conformation in solution of the near-full-length epidermal growth factor receptor (EGFR) bound to an inhibitor. This study is thus a significant step forward in EPR analysis of complex proteins.
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Catalytic Enantioselective Reaction of Allenylnitriles with Imines Using Chiral Bis(imidazoline)s Palladium(II) Pincer Complexes ()
The first highly enantioselective reaction of allenylnitriles with imines has been developed. Excellent yields and enantioselectivities were observed for the reaction with various imines using chiral Phebim-PdII complexes. This process offers a simple and efficient synthetic route for various functionalized α-vinylidene-β-aminonitriles and their derivatives. Getting a grip on allenyles: The first highly enantioselective reaction of allenylnitriles with imines has been developed (see scheme; SES=silylethanesulfonyl). Excellent yields and enantioselectivities were observed for the reaction with various imines using chiral Phebim–PdII complexes. This process offers a simple and efficient synthetic route for various functionalized α-vinylidene-β-aminonitriles and their derivatives.
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AgHalo: A Facile Fluorogenic Sensor to Detect Drug-Induced Proteome Stress ()
Drug-induced proteome stress that involves protein aggregation may cause adverse effects and undermine the safety profile of a drug. Safety of drugs is regularly evaluated using cytotoxicity assays that measure cell death. However, these assays provide limited insights into the presence of proteome stress in live cells. A fluorogenic protein sensor is reported to detect drug-induced proteome stress prior to cell death. An aggregation prone Halo-tag mutant (AgHalo) was evolved to sense proteome stress through its aggregation. Detection of such conformational changes was enabled by a fluorogenic ligand that fluoresces upon AgHalo forming soluble aggregates. Using 5 common anticancer drugs, we exemplified detection of differential proteome stress before any cell death was observed. Thus, this sensor can be used to evaluate drug safety in a regime that the current cytotoxicity assays cannot cover and be generally applied to detect proteome stress induced by other toxins. A fluorogenic sensor was developed to detect drug-induced proteome stress. In contrast to previous sensors, the fluorogenicity enables direct fluorescence readout for facile detection. Differential proteome stress induced by different anticancer drugs was revealed using this assay, but it is invisible to current cytotoxicity assays.
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Polynitro-Functionalized Dipyrazolo-1,3,5-triazinanes: Energetic Polycyclization toward High Density and Excellent Molecular Stability ()
A new fused N-heterocyclic framework, dipyrazolo-1,3,5-triazinane, was synthesized and the physiochemical properties of its derivatives were investigated to evaluate the integrated energetic performance. In contrast to 1,3,5-trinitro-1,3,5-triazinane (RDX) featuring a distorted chair confirmation, polynitro-functionalized dipyrazolo-1,3,5-triazinanes have nearly planar backbones, thereby enhancing the density and thermal stability. Among these new energetic tricyclic compounds, 5 a and 12 show favorable crystal densities of 1.937 g cm−3 and 1.990 g cm−3 at 150 K, respectively, which rank highest in triazinane-based energetic compounds. Additionally, this synthetic approach was carried out to form seven-membered and eight-membered rings, giving rise to tetranitro dipyrazolo-1,3,5-triazepane (5 b) and tetranitro dipyrazolo-1,3,5-triazocane (5 c), respectively. Energetic backbone: A new fused N-heterocyclic framework, dipyrazolo-1,3,5-triazinane, was synthesized and its physicochemical properties were investigated. Compared to nonaromatic 1,3,5-triazinane, this polycyclic compound exhibits higher density and enhanced molecular stability, which are supported by computational analysis.
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Roald Hoffmann ()

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Doubly Caged Linker for AND-Type Fluorogenic Construction of Protein/Antibody Bioconjugates and In Situ Quantification ()
In situ quantification of the conjugation efficiency of azide-terminated synthetic polymers/imaging probes and thiol-functionalized antibodies/proteins/peptides was enabled by a doubly caged profluorescent and heterodifunctional core molecule C1 as a self-sorting bridging unit. Orthogonal dual “click” coupling of C1 with azide- and thiol-functionalized precursors led to highly fluorescent bioconjugates, whereas single-click products remained essentially nonfluorescent. Integration with FRET processes was also possible. For the construction of antibody–probe conjugates from an anti-carcinoembryonic antigen and a quinone-caged profluorescent naphthalimide derivative, the dual “click” coupling process with C1 was monitored on the basis of the emission turn-on of C1, whereas prominent changes in FRET ratios occurred for antibody–imaging-probe conjugates when specifically triggered by quinone oxidoreductase (NQO1), which is overexpressed in various types of cancer cells. Wait for the second “click”: A doubly caged profluorescent heterodifunctional molecule was identified as a linker for the fabrication of functional protein/antibody bioconjugates through two orthogonal “click” reactions (see picture). Conjugate formation and subsequent activation were quantified in situ on the basis of fluorescence emission intensity and FRET ratio changes. Single-click products remained essentially nonfluorescent.
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Determination of Radical–Radical Distances in Light-Active Proteins and Their Implication for Biological Magnetoreception ()
Light-generated short-lived radial pairs have been suggested to play pivotal roles in cryptochromes and photolyases. Chryptochromes are very probably involved in magnetic compass sensing in migratory birds and the magnetic-field-dependent behavior of insects. We examined photo-generated transient states in the cryptochrome of Drosophila melanogaster and in the structurally related DNA-repair enzyme Escherichia coli DNA photolyase. Using pulsed EPR spectroscopy, the exchange and dipolar contributions to the electron spin–spin interaction were determined in a straightforward and direct way. With these parameters, radical-pair partners may be identified and the magnetoreceptor efficiency of chryptochromes can be evaluated. We present compelling evidence for an extended electron-transfer cascade in the Drosophila cryptochrome, and identified W394 as a key residue for flavin photoreduction and formation of a spin-correlated radical pair with a sufficient lifetime for high-sensitivity magnetic-field sensing. Short-lived spin-correlated radical pairs are essential for the kind of magnetic compass that is believed to be used by migratory birds and insects to perceive direction. Such transient states were studied by pulsed EPR spectroscopy, and the distance-dependent magnetic couplings between the unpaired spins were characterized.
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Continuous Synthesis and Purification by Coupling a Multistep Flow Reaction with Centrifugal Partition Chromatography ()
Continuous-flow multistep synthesis is combined with quasi-continuous final-product purification to produce pure products from crude reaction mixtures. In the nucleophilic aromatic substitution of 2,4-difluoronitrobenzene with morpholine followed by a heterogeneous catalytic hydrogenation, the desired monosubstituted product can be continuously separated from the co- and by-products in a purity of over 99 % by coupling a flow reactor sequence to a multiple dual-mode (MDM) centrifugal partition chromatography (CPC) device. This purification technique has many advantages over HPLC, such as higher resolution and no need for column replacement or silica recycling, and it does not suffer from irreversible adsorption. The desired monosubstituted product of the nucleophilic aromatic substitution of 2,4-difluoronitrobenzene with morpholine followed by a heterogeneous catalytic hydrogenation can be continuously separated from the co- and by-products in a purity of over 99 % by coupling a flow reactor sequence to a multiple dual-mode centrifugal partition chromatography device.
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Highly Luminescent and Ultrastable CsPbBr3 Perovskite Quantum Dots Incorporated into a Silica/Alumina Monolith ()
We successfully prepared QDs incorporated into a silica/alumina monolith (QDs-SAM) by a simple sol–gel reaction of an Al–Si single precursor with CsPbBr3 QDs blended in toluene solution, without adding water and catalyst. The resultant transparent monolith exhibits high photoluminescence quantum yields (PLQY) up to 90 %, and good photostability under strong illumination of blue light for 300 h. We show that the preliminary ligand exchange of didodecyl dimethyl ammonium bromide (DDAB) was very important to protect CsPbBr3 QDs from surface damages during the sol–gel reaction, which not only allowed us to maintain the original optical properties of CsPbBr3 QDs but also prevented the aggregation of QDs and made the monolith transparent. The CsPbBr3 QDs-SAM in powder form was easily mixed into the resins and applied as color-converting layer with curing on blue light-emitting diodes (LED). The material showed a high luminous efficacy of 80 lm W−1 and a narrow emission with a full width at half maximum (FWHM) of 25 nm. CsPbBr3 perovskite quantum dots (QDs) were incorporated into a silica/alumina monolith (QDs-SAM). The material showed high photoluminescence quantum yields (PLQY) up to 90 % and a narrow emission with a full width at half maxima (FWHM) of 25 nm. It has a much superior stability under strong illumination of blue light than the pure QDs.
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Cover Picture: Superelectrophilic Behavior of an Anion Demonstrated by the Spontaneous Binding of Noble Gases to [B12Cl11]− (Angew. Chem. Int. Ed. 27/2017) ()
The molecular ion [B12Cl11]− displays two seemingly incompatible sides, a negative and a positive one, just like the goddess Helja in Norse mythology. Physically, [B12Cl11]− is an anion but it shows chemical reactivity known for highly electrophilic cations. In their Communication on page 7980 ff., S. Grabowsky, J. Warneke et al. report that the electrophilicity of [B12Cl11]− is so high that the noble gases Kr and Xe are bonded spontaneously at room temperature in a reaction that is typical of superelectrophiles.
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Inside Cover: Lithiophilic Sites in Doped Graphene Guide Uniform Lithium Nucleation for Dendrite-Free Lithium Metal Anodes (Angew. Chem. Int. Ed. 27/2017) ()
Lithium-metal anodes are the most promising electrodes for next-generation high-energy-density batteries. In their Communication on page 7764 ff., Q. Zhang and co-workers employed a nitrogen-doped graphene matrix (lotus leaves) with lithiophilic nucleation sites (sitting frogs) to guide Li (tadpoles) nucleation on an anode surface. The result is a uniform distribution of Li on the anode surface. These dendrite-free lithium-metal anodes exhibit an impressive electrochemical performance.
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Inside Back Cover: Significant Enhancement of C2H2/C2H4 Separation by a Photochromic Diarylethene Unit: A Temperature- and Light-Responsive Separation Switch (Angew. Chem. Int. Ed. 27/2017) ()
Adjusting adsorption selectivity and separation in photochromic metal–organic frameworks (MOFs) just by external stimuli is highly important but still rare. In their Communication on page 7900 ff., F. Luo, G.-C. Guo, and co-workers employ a photochromic diarylethene unit as a light-triggered selectivity and separation regulator, leading to ultrahigh adsorption selectivity, for example, for the mixture C2H2/C2H4.
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Back Cover: Formation Mechanism of NF4+ Salts and Extraordinary Enhancement of the Oxidizing Power of Fluorine by Strong Lewis Acids (Angew. Chem. Int. Ed. 27/2017) ()
The strongest neutral one-electron oxidizers are reported by K. O. Christe and co-workers in their Communication on page 7924 ff. The presence of strong Lewis acids enhances the oxidizing power of elemental fluorine by as much as 6.7 eV, and Sb2F11 has an electron affinity of 9.78 eV (PtF6: 7.09 eV). When combined with a typical lattice energy of about 6 eV for a solid product, these Lewis acid/fluorine adducts can “rob” an electron even from highly unreactive compounds and oxidize NF3 to NF3+, for example.
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Frontispiece: Surprising Stability of Larger Criegee Intermediates on Aqueous Interfaces ()
Atmospheric Chemistry J. S. Francisco, X. C. Zeng, and co-workers show in their Communication on page 7740 ff. that Criegee intermediates with hydrophobic substituents (longer than C1) have longer survival times on the surface of water droplets.
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Feeling the Pulse of Chemistry—Already in Practical Laboratory Courses ()
“… The lessons that students can draw from a lab course are unique throughout the entire study of chemistry. Equally important and valuable is the authentic lab experience and the constant repetition of the link between acquiring knowledge, thorough observations, logical thinking, self-critical tenacity, and capacity for success throughout the experiments. The ultimate gain is satisfaction ...” Read more in the Editorial by Roland A. Fischer.
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Graphical Abstract: Angew. Chem. Int. Ed. 27/2017 ()

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Spotlights on our sister journals: Angew. Chem. Int. Ed. 27/2017 ()

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Gareth A. Morris ()
“I advise my students to ignore my advice. They usually do. My favorite way to spend a holiday is walking on the Northumberland coast ...” This and more about Gareth A. Morris can be found on page 7710.
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New Members and Foreign Members of the National Academy of Engineering ()

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Effects of Nanoconfinement on Catalysis. Edited by Rinaldo Poli. ()
Springer International Publishing, 2017. 266 pp., hardcover, € 155.99.—ISBN 978-3319502052
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Photocatalysis Enabling Acceptorless Dehydrogenation of Benzofused Saturated Rings at Room Temperature ()
Sunburned: Visible light facilitates the acceptorless dehydrogenation of saturated hetero- and carbocycles with an annulated benzene ring (see scheme; X=N and CH2). These aromatizations occur at room temperature. The new methods are not only useful for the synthesis of bicyclic heteroarenes and arenes with diverse substitution patterns but also potentially attractive for hydrogen-storage materials.
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Catalytic Dearomatization of N-Heteroarenes with Silicon and Boron Compounds ()
Dearomatized N-heterocycles are an important class of structural motifs for organic synthesis and chemical biology. The catalytic dearomative reduction of unactivated N-heteroarenes using silicon and/or boron-containing compounds as a reductant is one of the most straightforward alternatives to hydrogenation. However, thus far, there are few reported examples on the catalytic reduction of N-heteroaromatic compounds with silane or borane reducing agents. This Review presents recent advances in the catalytic reduction of unactivated N-heteroarenes by hydrosilanes, hydroboranes, silaboranes, and diboranes. The focus is on the chemical reactivity and selectivity of transition-metal or metal-free organocatalyst systems. In addition, the working modes of these catalysis will be described primarily on the basis of experimental mechanistic insight. Alternative to hydrogenation: The catalytic dearomatization of unactivated N-heteroarenes by using silicon and/or boron compounds as a reductant is one of the most straightforward alternatives to hydrogenation. This Review focuses on the chemical reactivity and selectivity of homogeneous catalyst systems in such transformations.
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Surprising Stability of Larger Criegee Intermediates on Aqueous Interfaces ()
Criegee intermediates have implications as key intermediates in atmospheric, organic, and enzymatic reactions. However, their chemistry in aqueous environments is relatively unexplored. Herein, Born–Oppenheimer molecular dynamics (BOMD) simulations examine the dynamic behavior of syn- and anti-CH3CHOO at the air–water interface. They show that unlike the simplest Criegee intermediate (CH2OO), both syn- and anti-CH3CHOO remain inert towards reaction with water. The unexpected high stability of C2 Criegee intermediates is due to the presence of a hydrophobic methyl substituent on the Criegee carbon that lowers the proton transfer ability and inhibits the formation of a pre-reaction complex for the Criegee–water reaction. The simulation of the larger Criegee intermediates, (CH3)2COO, syn- and anti-CH2C(CH3)C(H)OO on the water droplet surface suggests that strongly hydrophobic substituents determine the reactivity of Criegee intermediates at the air–water interface. A drop in reactivity: A hydrophobic methyl substituent on the Criegee carbon atom lowers the proton transfer ability and inhibits the formation of a pre-reaction complex for the Criegee–water reaction. Criegee intermediates with hydrophobic substituents (longer than C1) have longer survival time on the surface of water droplets.
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Unclicking the Click: Metal-Assisted Mechanochemical Cycloreversion of Triazoles Is Possible ()
The mechanochemical cycloreversion of 1,2,3-triazole compounds, which serve as unusually stable building blocks in materials and biomolecular chemistry as a result of mild “click chemistry”, remains puzzling. We show that the hitherto discussed straight-forward retro-click mechanism of the 1,4-disubstituted isomer, even if CuI catalyzed, can be ruled out in view of more favorable activation free energies of destructive pathways. In stark contrast, the 1,5-regioiomer can undergo cycloreversion under rather mild mechanochemical conditions owing to its favorable response to the external force in conjunction with standard RuII catalysis. Mechanochemical unclicking reactions of 1,2,3-triazoles have been a topic of controversy. Based on isotensional quantum-chemical calculations, it is shown that their 1,5-regioisomers easily undergo cycloreversion reactions under standard sonication conditions if they are RuII catalyzed.
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Ultrasensitive Measurement of Ca2+ Influx into Lipid Vesicles Induced by Protein Aggregates ()
To quantify and characterize the potentially toxic protein aggregates associated with neurodegenerative diseases, a high-throughput assay based on measuring the extent of aggregate-induced Ca2+ entry into individual lipid vesicles has been developed. This approach was implemented by tethering vesicles containing a Ca2+ sensitive fluorescent dye to a passivated surface and measuring changes in the fluorescence as a result of membrane disruption using total internal reflection microscopy. Picomolar concentrations of Aβ42 oligomers could be observed to induce Ca2+ influx, which could be inhibited by the addition of a naturally occurring chaperone and a nanobody designed to bind to the Aβ peptide. We show that the assay can be used to study aggregates from other proteins, such as α-synuclein, and to probe the effects of complex biofluids, such as cerebrospinal fluid, and thus has wide applicability. Disruptive influence: To quantify and characterize the membrane-disrupting protein aggregates, associated with neurodegenerative diseases, a high-throughput assay based on measuring the amount of aggregate-induced Ca2+ entry into immobilized lipid vesicles has been developed. Picomolar concentrations of Aβ42 oligomers induce Ca2+ influx, which can be inhibited by the addition of a chaperone and a nanobody.
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The Nature of Ion Conduction in Methylammonium Lead Iodide: A Multimethod Approach ()
By applying a multitude of experimental techniques including 1H, 14N, 207Pb NMR and 127I NMR/NQR, tracer diffusion, reaction cell and doping experiments, as well as stoichiometric variation, conductivity, and polarization experiments, iodine ions are unambiguously shown to be the mobile species in CH3NH3PbI3, with iodine vacancies shown to represent the mechanistic centers under equilibrium conditions. Pb2+ and CH3NH3+ ions do not significantly contribute to the long range transport (upper limits for their contributions are given), whereby the latter exhibit substantial local motion. The decisive electronic contribution to the mixed conductivity in the experimental window stems from electron holes. As holes can be associated with iodine orbitals, local variations of the iodine stoichiometry may be fast and enable light effects on ion transport. Keep calm and carry charge: Using NMR/NQR techniques, tracer diffusion, a reaction cell, and doping experiments, as well as stoichiometric variation, conductivity, and polarization experiments, iodine ions are shown to be the mobile species in CH3NH3PbI3, with iodine vacancies being the mechanistic centers under equilibrium conditions. The decisive electronic contribution to the mixed conductivity stems is shown to be from electron holes.
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High-Flux Carbon Molecular Sieve Membranes for Gas Separation ()
Carbon membranes have great potential for highly selective and cost-efficient gas separation. Carbon is chemically stable and it is relative cheap. The controlled carbonization of a polymer coating on a porous ceramic support provides a 3D carbon material with molecular sieving permeation performance. The carbonization of the polymer blend gives turbostratic carbon domains of randomly stacked together sp2 hybridized carbon sheets as well as sp3 hybridized amorphous carbon. In the evaluation of the carbon molecular sieve membrane, hydrogen could be separated from propane with a selectivity of 10 000 with a hydrogen permeance of 5 m3(STP)/(m2hbar). Furthermore, by a post-synthesis oxidative treatment, the permeation fluxes are increased by widening the pores, and the molecular sieve carbon membrane is transformed from a molecular sieve carbon into a selective surface flow carbon membrane with adsorption controlled performance and becomes selective for carbon dioxide. Oxidize to change selection: The controlled carbonization of a thin polymer blend coating on a ceramic support provides a 125 nm thin 3D high-flux molecular sieve carbon membrane with hydrogen selectivity. By post-synthesis oxidative treatment, the molecular sieve carbon membrane is transformed into a selective surface-flow carbon membrane with carbon dioxide selectivity.
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Lithiophilic Sites in Doped Graphene Guide Uniform Lithium Nucleation for Dendrite-Free Lithium Metal Anodes ()
Lithium (Li) metal is the most promising electrode for next-generation rechargeable batteries. However, the challenges induced by Li dendrites on a working Li metal anode hinder the practical applications of Li metal batteries. Herein, nitrogen (N) doped graphene was adopted as the Li plating matrix to regulate Li metal nucleation and suppress dendrite growth. The N-containing functional groups, such as pyridinic and pyrrolic nitrogen in the N-doped graphene, are lithiophilic, which guide the metallic Li nucleation causing the metal to distribute uniformly on the anode surface. As a result, the N-doped graphene modified Li metal anode exhibits a dendrite-free morphology during repeated Li plating and demonstrates a high Coulombic efficiency of 98 % for near 200 cycles. The matrix: Nitrogen-doped graphene is used as the Li plating matrix to regulate Li metal nucleation and suppress dendrite growth. The N-containing functional groups in the N-doped graphene are lithiophilic, which guide the Li nucleation and give a uniform distribution of Li on the anode surface. The dendrite-free lithium-metal anodes exhibit an impressive electrochemical performance.
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Phase-Selective Syntheses of Cobalt Telluride Nanofleeces for Efficient Oxygen Evolution Catalysts ()
Cobalt-based nanomaterials have been intensively explored as promising noble-metal-free oxygen evolution reaction (OER) electrocatalysts. Herein, we report phase-selective syntheses of novel hierarchical CoTe2 and CoTe nanofleeces for efficient OER catalysts. The CoTe2 nanofleeces exhibited excellent electrocatalytic activity and stablity for OER in alkaline media. The CoTe2 catalyst exhibited superior OER activity compared to the CoTe catalyst, which is comparable to the state-of-the-art RuO2 catalyst. Density functional theory calculations showed that the binding strength and lateral interaction of the reaction intermediates on CoTe2 and CoTe are essential for determining the overpotential required under different conditions. This study provides valuable insights for the rational design of noble-metal-free OER catalysts with high performance and low cost by use of Co-based chalcogenides. Fleeced: Hierarchical CoTe2 nanofleeces were synthesized by using ultrathin Te nanowires as templates. They exhibited excellent electrocatalytic activity and stablity for the oxygen evolution reaction (OER) in alkaline media. The CoTe2 catalyst exhibited superior OER activity to the CoTe catalyst and was comparable to the state-of-the-art RuO2 catalyst.
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Interprotein Electron Transfer between FeS-Protein Nanowires and Oxygen-Tolerant NiFe Hydrogenase ()
Self-assembled redox protein nanowires have been exploited as efficient electron shuttles for an oxygen-tolerant hydrogenase. An intra/inter-protein electron transfer chain has been achieved between the iron-sulfur centers of rubredoxin and the FeS cluster of [NiFe] hydrogenases. [NiFe] Hydrogenases entrapped in the intricated matrix of metalloprotein nanowires achieve a stable, mediated bioelectrocatalytic oxidation of H2 at low-overpotential. Wired up: Self-assembled redox protein nanowires were exploited as efficient electron shuttles for an oxygen-tolerant hydrogenase. [NiFe] Hydrogenases entrapped in the intricate matrix of metalloprotein nanowires achieve a stable, mediated bioelectrocatalytic oxidation of H2 at low overpotential.
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Specific Enhancement of Catalytic Activity by a Dicopper Core: Selective Hydroxylation of Benzene to Phenol with Hydrogen Peroxide ()
A dicopper(II) complex, stabilized by the bis(tpa) ligand 1,2-bis[2-[bis(2-pyridylmethyl)aminomethyl]-6-pyridyl]ethane (6-hpa), [Cu2(μ-OH)(6-hpa)]3+, was synthesized and structurally characterized. This complex catalyzed selective hydroxylation of benzene to phenol using H2O2, thus attaining large turnover numbers (TONs) and high H2O2 efficiency. The TON after 40 hours for the phenol production exceeded 12000 in MeCN at 50 °C under N2, the highest value reported for benzene hydroxylation with H2O2 catalyzed by homogeneous complexes. At 22 % benzene conversion, phenol (95.2 %) and p-benzoquinone (4.8 %) were produced. The mechanism of H2O2 activation and benzene hydroxylation is proposed. Doubled up: A new dicopper complex with a dinucleating ligand, which specifically stabilizes a dinuclear structure, displays enhanced catalytic activity, selectivity, and H2O2 efficiency in the selective hydroxylation of benzene to phenol using H2O2. The dinuclear structure is favorable for the formation of the active species that specifically enhance the catalytic activity.
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Lanthanide Complexes that Respond to Changes in Cyanide Concentration in Water ()
Cyanide ions are shown to interact with lanthanide complexes of phenacylDO3A derivatives in aqueous solution, giving rise to changes in the luminescence and NMR spectra. These changes are the consequence of cyanohydrin formation, which is favored by the coordination of the phenacyl carbonyl group to the lanthanide center. These complexes display minimal affinity for fluoride and can detect cyanide at concentrations less than 1 μm. By contrast, lanthanide complexes with DOTAM derivatives display no affinity for cyanide in water, but respond to changes in fluoride concentration. Lanthanide complexes of phenacyl-DO3A derivatives signal the presence of cyanide in aqueous solution through changes in their luminescence and NMR spectra brought about by cyanohydrin formation. These complexes display minimal affinity for fluoride and can detect cyanide at concentrations below 1 μm.
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From Discrete Molecular Cages to a Network of Cages Exhibiting Enhanced CO2 Adsorption Capacity ()
We have adopted the concept of “cage to frameworks” to successfully produce a Na–N connected coordination networked cage Na-NC1 by using a [3+6] porous imine-linked organic cage NC1 (Nanjing Cage 1) as the precursor. It is found that Na-NC1 exhibits hierarchical porosity (inherent permanent voids and interconnected channel) and gas sorption measurements reveal a significantly enhanced CO2 uptake (1093 cm3 g−1 at 23 bar and 273 K) than that of NC1 (162 cm3 g−1 under the same conditions). In addition, Na-NC1 exhibits very low CO2 adsorption enthalpy making it a good candidate for porous materials with both high CO2 storage and low adsorption enthalpy. Networking: Hierarchically porous structures are prepared by linking discrete shape-persistent porous organic cages with sodium ions. The resulting cage network has an excellent CO2 storage capacity along with low adsorption enthalpy compared to the discrete cage precursor.
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Photocatalytic Self-Doped SnO2−x Nanocrystals Drive Visible-Light-Responsive Color Switching ()
Visible-light-responsive reversible color-switching systems are attractive to many applications because visible light has superior penetration and causes far less damage to organic molecules than UV. Herein, we report that self-doping of SnO2−x nanocrystals with Sn2+ red-shifts their absorption to the visible region and simultaneously produces oxygen vacancies, which can effectively scavenge photogenerated holes and thus enable the color switching of redox dyes using visible light. Wavelength-selective switching can also be achieved by coupling the photocatalytic activity of the SnO2−x NCs with the color-switching kinetics of different redox dyes. The fast light response enables the further fabrication of a solid film that can be repeatedly written on using a visible laser pen or projection printing through a photomask. This discovery represents a big step forward towards practical applications, especially in areas in which safety issues and photodamage by UV light are of concern. Color switching by visible light: Self-doping of SnO2−x nanocrystals with Sn2+ shifts the absorption bands to the visible light region and simultaneously produces oxygen vacancies to act as sacrificial electron donors, thus enabling the visible-light-stimulated color switching of redox dyes. The system can be developed as rewritable solid-state media, which can be written on using a laser pen or projection printed through a photomask.
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One-Dimensionally Disordered Chiral Sorting by Racemic Tiling in a Surface-Confined Supramolecular Assembly of Achiral Tectons ()
The aggregation of (pro)chiral/achiral molecules into crystalline structures at interfaces forms conglomerates, racemates, and solid solutions, comparable to known bulk phases. Scanning tunneling microscopy and Monte Carlo simulations were employed to uncover a distinct racemic phase, expressing 1D disordered chiral sorting through random tiling in surface-confined supramolecularly assembled achiral 4,4′′-diethynyl-1,1′:4′,1′′-terphenyl molecules. The configurational entropy of the 1D disordered racemic tiling phase was verified by analytical modeling, and found to lie between that of a perfectly ordered 2D racemate and a racemic solid solution. Clash of the tectons: Scanning tunneling microscopy and Monte Carlo simulations supported by analytical modeling were used to visualize a distinct racemic phase in surface-confined supramolecularly assembled achiral tectons. The racemic phase expressed 1D disordered chiral sorting by random tiling.
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AJIPHASE®: A Highly Efficient Synthetic Method for One-Pot Peptide Elongation in the Solution Phase by an Fmoc Strategy ()
We previously reported an efficient peptide synthesis method, AJIPHASE®, that comprises repeated reactions and isolations by precipitation. This method utilizes an anchor molecule with long-chain alkyl groups as a protecting group for the C-terminus. To further improve this method, we developed a one-pot synthesis of a peptide sequence wherein the synthetic intermediates were isolated by solvent extraction instead of precipitation. A branched-chain anchor molecule was used in the new process, significantly enhancing the solubility of long peptides and the operational efficiency compared with the previous method, which employed precipitation for isolation and a straight-chain aliphatic group. Another prerequisite for this solvent-extraction-based strategy was the use of thiomalic acid and DBU for Fmoc deprotection, which facilitates the removal of byproducts, such as the fulvene adduct. Extracted, not precipitated: AJIPHASE® is a new method for one-pot peptide synthesis that makes use of solvent extraction during peptide elongation and does not require any isolation steps. This efficient approach leads to peptides of high purity and benefits from significantly reduced solvent consumption.
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Iridium-Catalyzed Formyl-Selective Deuteration of Aldehydes ()
We report the first direct catalytic method for formyl-selective deuterium labeling of aromatic aldehydes under mild conditions, using an iridium-based catalyst designed to favor formyl over aromatic C−H activation. A good range of aromatic aldehydes is selectively labeled, and a one-pot labeling/olefination method is also described. Computational studies support kinetic product control over competing aromatic labeling and decarbonylation pathways. Al(D)hydes: An iridium catalyst system was designed to effect formyl over aryl C−H activation of aromatic aldehydes. Computational studies support kinetic product control over competing aromatic labeling and decarbonylation pathways.
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Strain Coupling of Conversion-type Fe3O4 Thin Films for Lithium Ion Batteries ()
Lithiation/delithiation induces significant stresses and strains into the electrodes for lithium ion batteries, which can severely degrade their cycling performance. Moreover, this electrochemically induced strain can interact with the local strain existing at solid–solid interfaces. It is not clear how this interaction affects the lithiation mechanism. The effect of this coupling on the lithiation kinetics in epitaxial Fe3O4 thin film on a Nb-doped SrTiO3 substrate is investigated. In situ and ex situ transmission electron microscopy (TEM) results show that the lithiation is suppressed by the compressive interfacial strain. At the interface between the film and substrate, the existence of LixFe3O4 rock-salt phase during lithiation consequently restrains the film from delamination. 2D phase-field simulation verifies the effect of strain. This work provides critical insights of understanding the solid–solid interfaces of conversion-type electrodes. The dynamic lithiation process under interfacial strain is investigated using both in situ scanning transmission electron microscopy (STEM) and phase-field simulation. The model system used is a heteroepitaxial Fe3O4 thin film grown on a SrTiO3 (STO) substrate.
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Chemically Controlled Spatiotemporal Oscillations of Colloidal Assemblies ()
We report an autonomous oscillatory micromotor system in which active colloidal particles form clusters, the size of which changes periodically. The system consists of an aqueous suspension of silver orthophosphate microparticles under UV illumination, in the presence of varying concentrations of hydrogen peroxide. The colloid particles first attract each other to form clusters. After a short delay, these clusters abruptly disperse and oscillation begins, alternating between clustering and dispersion of particles. After a cluster oscillation initiates, the oscillatory wave propagates to nearby clusters and eventually all the clusters oscillate in phase-shifted synchrony. The oscillatory behavior is governed by an electrolytic self-diffusiophoretic mechanism which involves alternating electric fields generated by the competing reduction and oxidation of silver. The oscillation frequency is tuned by changing the concentration of hydrogen peroxide. The addition of inert silica particles to the system results in hierarchical sorting and packing of clusters. Densely packed Ag3PO4 particles form a non-oscillating core with an oscillating shell composed largely of silica microparticles. Tunable colloidal oscillations: An autonomous oscillatory system involves Ag3PO4 colloidal particles under UV illumination in the presence of hydrogen peroxide. The particles oscillate between clustered and dispersed states, exhibiting signal propagation and synchronization. The behavior follows an electrolytic self-diffusiophoretic mechanism based on the redox chemistry of silver.
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Enzymatic Engineering of Live Bacterial Cell Surfaces Using Butelase 1 ()
Butelase-mediated ligation (BML) can be used to modify live bacterial cell surfaces with diverse cargo molecules. Surface-displayed butelase recognition motif NHV was first introduced at the C-terminal end of the anchoring protein OmpA on E. coli cells. This then served as a handle of BML for the functionalization of E. coli cell surfaces with fluorescein and biotin tags, a tumor-associated monoglycosylated peptide, and mCherry protein. The cell-surface ligation reaction was achieved at low concentrations of butelase and the labeling substrates. Furthermore, the fluorescein-labeled bacterial cells were used to show the interactions with cultured HeLa cells and with macrophages in live transgenic zebrafish, capturing the latter's powerful phagocytic effect in action. Together these results highlight the usefulness of butelase 1 in live bacterial cell surface engineering for novel applications. Enzymatic cell surface engineering: Butelase-mediated ligation (BML) is used to modify live bacterial cell surfaces with diverse cargo molecules for different applications. The fluorescein-labeled bacterial cells are shown to be useful to visualize pathogen–host interactions with cultured HeLa cells and with macrophages in live transgenic zebrafish, capturing the latter's powerful phagocytic effect in action.
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Visible-Light Photocatalytic Intramolecular Cyclopropane Ring Expansion ()
Described herein is a new visible-light photocatalytic strategy for the synthesis of enantioenriched dihydrofurans and cyclopentenes by an intramolecular nitro cyclopropane ring expansion reaction. Mechanistic studies and DFT calculations are used to elucidate the key factors in this new ring expansion reaction, and the need for the nitro group on the cyclopropane. Expansion project: A new visible-light photocatalytic strategy for the synthesis of enantioenriched dihydrofurans and cyclopentenes through a ring expansion process is presented. Mechanistic and computational studies rationalize the key factors of the reaction.
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Intermolecular Charge-Transfer Interactions Facilitate Two-Photon Absorption in Styrylpyridine–Tetracyanobenzene Cocrystals ()
Cocrystals of 4-styrylpyridine and 1,2,4,5-tetracyanobenzene were successfully prepared by supramolecular self-assembly. Donor–acceptor interactions between the molecular components are the main driving force for self-assembly and contribute to intermolecular charge transfer. The cocrystals possess two-photon absorption properties that are not observed in the individual components; suggesting that two-photon absorption originates from intermolecular charge-transfer interactions in the donor–acceptor system. The origin of two-photon absorption in multichromophore systems remains under-researched; thus, the system offers a rare demonstration of two-photon absorption by cocrystallization. Cocrystal engineering may facilitate further design and development of novel materials for nonlinear optical and optoelectronic applications. Finding light in the darkness: Cocrystals comprising 4-styrylpyridine and 1,2,4,5-tetracyanobenzene (STC) display two-photon absorption. Intermolecular charge-transfer interactions between the molecular components of the supramolecular assembly facilitate two-photon absorption that is absent from the individual molecules.
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Layer-by-Layer Motif Architectures: Programmed Electrochemical Syntheses of Multilayer Mesoporous Metallic Films with Uniformly Sized Pores ()
Although multilayer films have been extensively reported, most compositions have been limited to non-catalytically active materials (e.g. polymers, proteins, lipids, or nucleic acids). Herein, we report the preparation of binder-free multilayer metallic mesoporous films with sufficient accessibility for high electrocatalytic activity by using a programmed electrochemical strategy. By precisely tuning the deposition potential and duration, multilayer mesoporous architectures consisting of alternating mesoporous Pd layers and mesoporous PdPt layers with controlled layer thicknesses can be synthesized within a single electrolyte, containing polymeric micelles as soft templates. This novel architecture, combining the advantages of bimetallic alloys, multilayer architectures, and mesoporous structures, exhibits high electrocatalytic activity for both the methanol oxidation reaction (MOR) and the ethanol oxidation reaction (EOR). Filming in progress: Mesoporous films of Pd, Pt, and PdPt were produced by electrodeposition with good control over their thicknesses. The multilayer, bimetallic PdPt film in particular showed high electroactivity toward the oxidation of both ethanol and methanol.
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Preparation of Ultrathin Two-Dimensional TixTa1−xSyOz Nanosheets as Highly Efficient Photothermal Agents ()
Although two-dimensional (2D) metal oxide/sulfide hybrid nanostructures have been synthesized, the facile preparation of ultrathin 2D nanosheets in high yield still remains a challenge. Herein, we report the first high-yield preparation of solution-processed ultrathin 2D metal oxide/sulfide hybrid nanosheets, that is, TixTa1−xSyOz (x=0.71, 0.49, and 0.30), from TixTa1−xS2 precursors. The nanosheet exhibits strong absorbance in the near-infrared region, giving a large extinction coefficient of 54.1 L g−1 cm−1 at 808 nm, and a high photothermal conversion efficiency of 39.2 %. After modification with lipoic acid-conjugated polyethylene glycol, the nanosheet is a suitable photothermal agent for treatment of cancer cells under 808 nm laser irradiation. This work provides a facile and general method for the preparation of 2D metal oxide/sulfide hybrid nanosheets. Turn up the heat: Ultrathin 2D metal oxide/sulfide hybrid nanosheets in the general form TixTa1−xSyOz were prepared from TixTa1−xS2 layered bulk crystals using an electrochemical Li-intercalation method. The nanosheets exhibit strong absorbance in the near-infrared region, and as such are suitable for photothermal treatment of cancer cells following their modification with lipoic acid-conjugated polyethylene glycol.
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Efficient Electrocatalytic Reduction of CO2 by Nitrogen-Doped Nanoporous Carbon/Carbon Nanotube Membranes: A Step Towards the Electrochemical CO2 Refinery ()
Herein we introduce a straightforward, low cost, scalable, and technologically relevant method to manufacture an all-carbon, electroactive, nitrogen-doped nanoporous-carbon/carbon-nanotube composite membrane, dubbed “HNCM/CNT”. The membrane is demonstrated to function as a binder-free, high-performance gas diffusion electrode for the electrocatalytic reduction of CO2 to formate. The Faradaic efficiency (FE) for the production of formate is 81 %. Furthermore, the robust structural and electrochemical properties of the membrane endow it with excellent long-term stability. Carbon in carbon: A versatile and straightforward method was introduced to fabricate N-doped hierarchical-carbon/carbon-nanotube membrane, which can be directly utilized as a highly active, selective, and stable diffusion electrode for CO2 reduction to formate in aqueous media.
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Lanthanide Metal–Organic Framework Microrods: Colored Optical Waveguides and Chiral Polarized Emission ()
Lanthanide metal–organic frameworks (Ln-MOFs) have received much attention owing to their structural tunability and widely photofunctional applications. However, successful examples of Ln-MOFs with well-defined photonic performances at micro-/nanometer size are still quite limited. Herein, self-assemblies of 1,3,5-benzenetricarboxylic acid (BTC) and lanthanide ions afford isostructural crystalline Ln-MOFs. Tb-BTC, Eu@Tb-BTC, and Eu-BTC have 1D microrod morphologies, high photoluminescence (PL) quantum yields, and different emission colors (green, orange, and red). Spatially PL resolved spectra confirm that Ln-MOF microrods exhibit an optical waveguide effect with low waveguide loss coefficient (0.012≈0.033 dB μm−1) during propagation. Furthermore, these microrods feature both linear and chiral polarized photoemission with high anisotropy. Lightsabers: Three crystalline isostructural lanthanide metal–organic frameworks (Ln-MOFs) with 1D microrod morphologies, high photoluminescence quantum yields, and different emission colors are prepared. The Ln-MOFs feature low waveguide loss as well as high linear- and chiral-polarization anisotropy.
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A Tetracyclic Octaphosphane by Successive Addition, Inversion, and Condensation Reactions ()
An example of an octaphosphane of type R2P8 (R=(DDP)Ga) was isolated by treatment of cage compound (DDP)GaP4 (2, DDP=(2,6-diisopropylphenyl)(4-((2,6-diisopropylphenyl)imino)pent-2-en-2-yl)amide) with (C6F5)2PBr. The initially formed endo-exo butterfly shaped pentaphosphane 7 rapidly rearranges to the more stable exo–exo isomer 8, which undergoes dimerization to decaphosphane 11. Compound 11 unexpectedly eliminates tetraaryldiphosphane 13 to give tetracyclo[3.3.0.02,7.04,6]octaphosphane [(DDP)GaBr]2P8 (12). The reaction steps were confirmed by crystal structure analysis of the key intermediates and supported by kinetic studies using NMR techniques. Building up polyphosphanes: A neutral tetracyclic polyphosphane of the composition R2P8 (R=DDPGa, DDP=chelating N-donor ligand) was isolated by addition of halophosphanes to main-group-element-functionalized white phosphorus. Initially formed pentaphosphane inverts, dimerizes, and eliminates a diphosphane to yield R2P8 selectively.
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Harnessing [1,4], [1,5], and [1,6] Anionic Fries-type Rearrangements by Reaction-Time Control in Flow ()
A series of anionic Fries-type rearrangements of carbamoyl-substituted aryllithium intermediates were controlled by using flow microreactor systems. For the [1,4] and [1,5] rearrangements, the aryllithium intermediate formed before carbamoyl migration and the lithium alkoxide formed after carbamoyl migration can be selectively subjected to subsequent reactions with electrophiles by precisely controlling the residence time and temperature (−25 to −50 °C). In contrast, the [1,6] rearrangement is rather slow even at −25 °C. The absence of crossover products indicates the intramolecular nature of the carbamoyl group migration. Under control: A series of anionic Fries-type rearrangements of carbamoyl-substituted aryllithium species were controlled by using flow microreactor systems. For the [1,4] and [1,5] rearrangements, the aryllithium and lithium alkoxide intermediates formed before or after carbamoyl migration can be selectively subjected to subsequent reactions with electrophiles by precisely controlling the residence time and temperature.
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Hyperconjugation Is the Source of Helicity in Perfluorinated n-Alkanes ()
Hyperconjugative, steric, and electrostatic effects were evaluated as possible sources of the helicity in linear perfluorinated alkanes through analysis of natural bond orbitals and classical electrostatics. Contrary to previous rationalizations, which indicate dominating steric or electrostatic effects, this analysis indicates that hyperconjugative stabilization through σCCσ*CF interactions are the underlying driving force for the origin of the observed helicity in perfluoroalkanes. Hype and twist: It is hyperconjugation (i.e. quantum mechanics), not simple electrostatics, that dictates the helical shape of perfluoroalkanes. This conclusion is contrary to previous rationalizations, which indicate dominating steric or electrostatic effects.
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A Smart Flexible Zinc Battery with Cooling Recovery Ability ()
Flexible batteries are essential for wearable electronic devices. To meet practical applications, they need to be mechanically robust and stable. However, strong or multiple bending may sever the interfacial contact between electrode and electrolyte, causing capacity fading or even battery failure. Herein we present a new cooling-recovery concept for flexible batteries, which involves a temperature-sensitive sol–gel transition behavior of the thermoreversible polymer hydrogel electrolyte. Once a battery has suffered from strong mechanical stresses, a simple cooling process can refresh the electrode–electrolyte interface. The energy-storage capability can be recovered with a healing efficiency higher than 98 %. It is believed that this study not only offers new valuable insights, but also opens up new perspectives to develop functional wearable devices. A cooling recovery function for flexible batteries is proposed based on the thermoreversible gelation behavior of the Pluronic hydrogel electrolyte. A simple cooling process can repair the damaged or cracked electrode–electrolyte interface caused by strong mechanical stresses, with a healing efficiency of up to 98 %.
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Self-Assembled Zinc/Cystine-Based Chloroplast Mimics Capable of Photoenzymatic Reactions for Sustainable Fuel Synthesis ()
Prototypes of biosystems provide good blueprints for the design and creation of biomimetic systems. However, mimicking both the sophisticated natural structures and their complex biological functions still remains a great challenge. Herein, chloroplast mimics have been fabricated by one-step bioinspired amino acid mineralization and simultaneous integration of catalytically active units. Hierarchically structured crystals were obtained by the metal-ion-directed self-assembly of cystine (the oxidized dimer of the amino acid cysteine), with a porous structure and stacks of nanorods, which show similar architectural principles to chloroplasts. Porphyrins and enzymes can both be encapsulated inside the crystal during mineralization, rendering the crystal photocatalytically and enzymatically active for an efficient and sustainable synthesis of hydrogen and acetaldehyde in a coupled photoenzymatic reaction. Chloroplast mimics have been fabricated by the metal-ion-directed self-assembly of cystine and the simultaneous encapsulation of guest molecules, such as porphyrins (e.g. TTPS) and enzymes (ADH). Photocatalytic and enzymatic reactions can be coupled in such biomimetic architectures, which thus mimic the structural and functional features of natural chloroplasts, enabling sustainable fuel synthesis.
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Transformation of Rusty Stainless-Steel Meshes into Stable, Low-Cost, and Binder-Free Cathodes for High-Performance Potassium-Ion Batteries ()
To recycle rusty stainless-steel meshes (RSSM) and meet the urgent requirement of developing high-performance cathodes for potassium-ion batteries (KIB), we demonstrate a new strategy to fabricate flexible binder-free KIB electrodes via transformation of the corrosion layer of RSSM into compact stack-layers of Prussian blue (PB) nanocubes (PB@SSM). When further coated with reduced graphite oxide (RGO) to enhance electric conductivity and structural stability, the low-cost, stable, and binder-free RGO@PB@SSM cathode exhibits excellent electrochemical performances for KIB, including high capacity (96.8 mAh g−1), high discharge voltage (3.3 V), high rate capability (1000 mA g−1; 42 % capacity retention), and outstanding cycle stability (305 cycles; 75.1 % capacity retention). Turning waste into treasure: Rusty stainless steel meshes were utilized as solid-state iron sources with excellent conductivity properties in order to fabricate stable, low-cost, and flexible binder-free potassium-ion battery electrodes. When combined with unique structural design, the reduced graphite oxide-coated electrodes exhibited high capacities, superior rate capabilities, and excellent cycle performance.
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Biomimetic Total Synthesis of (±)-Homodimericin A ()
A biomimetic total synthesis of racemic homodimericin A was achieved in seven steps, including two cascade reactions. Aqueous buffer solutions are found to help both the oxidative dimerization cascade and the intramolecular Diels–Alder cascade. This synthetic sequence validates key steps in the biogenetic proposal of homodimericin A. Magnificent seven: The biomimetic synthesis of homodimericin A was accomplished in seven steps involving two cascade reactions. The biosynthesis proposal was validated with experimental support.
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Bioinspired Total Synthesis of Homodimericin A ()
Homodimericin A is a remarkable fungal metabolite. This highly oxygenated racemic unsaturated polyketide poses a significant synthetic challenge owing to its sterically demanding central cagelike core containing eight contiguous stereogenic centers (including three quaternary stereocenters) and several carbonyl functionalities. On the basis of its proposed biogenetic synthesis, we designed a total synthesis of homodimericin A that proceeds in seven steps and features a double Michael reaction, an intramolecular Diels–Alder reaction, and an ene reaction. On the double: The highly oxygenated and racemic fungal metabolite homodimericin A (1), with a dense cagelike core containing eight contiguous stereogenic centers, was synthesized in just seven steps. The synthetic route was designed on the basis of the proposed biogenetic synthesis of 1 and features a double Michael reaction and an intramolecular Diels–Alder reaction (see scheme).
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Total Synthesis of Homodimericin A ()
We report the concise total synthesis of homodimericin A (1), a recently identified fungal metabolite bearing an unprecedented molecular architecture. The success of the approach hinges on a series of rationally designed and bioinspired transformations, including a Moore rearrangement to assemble the monomeric hydroquinone precursor, homodimerization through double Michael addition to construct the planar A/B/C tricyclic framework, and a tandem Diels–Alder reaction/carbonyl–ene cyclization to forge the congested D/E/F tricyclic cage motif. Unequivocal evidence for the elucidated structure of homodimericin A was also provided by this study. Taking cues from nature: Homodimericin A, a highly complex fungal metabolite, was synthesized from three readily available building blocks in ten steps (see scheme). The success of the approach hinges on a rationally designed Moore rearrangement to construct the monomeric precursor and a sequence of bioinspired transformations, including homodimerization, Diels–Alder, and carbonyl–ene reactions, to assemble the hexacyclic skeleton.
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Significant Enhancement of C2H2/C2H4 Separation by a Photochromic Diarylethene Unit: A Temperature- and Light-Responsive Separation Switch ()
A dual temperature- and light-responsive C2H2/C2H4 separation switch in a diarylethene metal–organic framework (MOF) is presented. At 195 K and 100 kPa this MOF shows ultrahigh C2H2/C2H4 selectivity of 47.1, which is almost 21.4 times larger than the corresponding value of 2.2 at 293 K and 100 kPa, or 15.7 times larger than the value of 3.0 for the material under UV at 195 K and 100 kPa. The origin of this unique control in C2H2/C2H4 selectivity, as unveiled by density functional calculations, is due to a guest discriminatory gate-opening effect from the diarylethene unit. Photochromic diarylethene units were used to identify C2H2, leading to ultrahigh C2H2/C2H4 selectivity and promising application in C2H2/C2H4 separation at low temperature. The photochromic material can be further used as a temperature- and light-responsive switch for C2H2/C2H4 separation.
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Selective and Reversible Fluoride Complexation from Water by a Cyclic Tri(phosphonio)methanide Dication ()
Tri(phosphonio)methanide dication 32+, prepared from a trifluoromethylsulfanylphosphonium dication (12+) via an intramolecular electrophilic aromatic substitution reaction, is an unexpected P-based, water-resistant Lewis acid that is capable to selectively and reversibly bind fluoride ions from organic/aqueous biphasic solution. The formed complex is an unusual fluorophosphorane ([3-F]OTf). The multiple donor–acceptor interactions of 32+ that are crucial for the fluoride fixation have been elucidated by quantum chemical calculation. Compound [3-F]OTf can also be used as a convenient anhydrous fluoride ion source and was probed as a suitable catalyst of the silylotrifluoromethylation of various aldehydes. A highly fluorophilic tri(phosphonio)methanide salt is presented that is capable of selectively and reversibly complexing fluoride ions from an organic/aqueous biphasic solution. The obtained fluorophosphorane salt serves as an excellent catalyst for silylotrifluoromethylation of aldehydes.
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Native Mass Spectrometry from Common Buffers with Salts That Mimic the Extracellular Environment ()
Nonvolatile salts are essential for the structures and functions of many proteins and protein complexes but can severely degrade performance of native mass spectrometry by adducting to protein and protein complex ions, thereby reducing sensitivity and mass measuring accuracy. Small nanoelectrospray emitters are used to form protein and protein complex ions directly from high-ionic-strength (>150 mm) nonvolatile buffers with salts that mimic the extracellular environment. Charge-state distributions are not obtained for proteins and protein complexes from six commonly used nonvolatile buffers and ≥150 mm Na+ with conventionally sized nanoelectrospray emitter tips but are resolved with 0.5 μm tips. This method enables mass measurements of proteins and protein complexes directly from a variety of commonly used buffers with high concentrations of nonvolatile salts and eliminates the need to buffer exchange into volatile ammonium buffers traditionally used in native mass spectrometry. Small nanoelectrospray emitters are used to form protein and protein complex ions directly from high-ionic-strength (>150 mm) nonvolatile buffers with salts that mimic the extracellular environment. Charge-state distributions not obtained with conventional-sized nanoelectrospray emitter tips are resolved with 0.5 μm tips.
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Iron-Catalyzed Intermolecular 1,2-Difunctionalization of Styrenes and Conjugated Alkenes with Silanes and Nucleophiles ()
The first iron-catalyzed 1,2-difunctionalization of styrenes and conjugated alkenes with silanes and either N or C, using an oxidative radical strategy, is described. Employing FeCl2 and di-tert-butyl peroxide allows divergent alkene 1,2-difunctionalizations, including 1,2-aminosilylation, 1,2-arylsilylation, and 1,2-alkylsilylation, which rely on a wide range of nucleophiles, namely, amines, amides, indoles, pyrroles, and 1,3-dicarbonyls, thus providing a powerful platform for producing diverse silicon-containing alkanes. Iron clad: By employing FeCl2 and di-tert-butyl peroxide (DTBP), divergent alkene 1,2-difunctionalization reactions, including 1,2-aminosilylation, 1,2-arylsilylation, and 1,2-alkylsilylation, are achieved by using different nucleophiles. The method provides straightforward and practical access to 1-amino-2-silylalkanes and other functionalized silicon-containing alkanes with broad substrate scope and high selectivity.
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Phosphoribosyl Pyrophosphate: A Molecular Vestige of the Origin of Life on Minerals ()
In this contribution, we report the formation under prebiotic conditions of phosphoribosyl pyrophosphate (PRPP) as a molecular precursor in the one-pot synthesis of a canonical nucleotide, namely adenosine monophosphate (AMP) from its building blocks (KH2PO4 or Pi, adenine, and d-ribose), on a fumed silica surface. The on-the-rocks approach has been successfully applied to the simultaneous phosphorylation and glycosylation of ribose. The one-pot formation mechanism of AMP involves a two-step pathway via an activated intermediate, namely PRPP, obtained by multiple ribose phosphorylations upon mild thermal activation. AMPing up prebiotic synthesis: The formation of phosphoribosyl pyrophosphate (PRPP) under prebiotic conditions is reported. PRPP was further used as a molecular intermediate in the one-pot synthesis of a canonical nucleotide, namely adenosine monophosphate (AMP), from its building blocks (KH2PO4, adenine, and d-ribose) on a fumed silica surface.
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Formation Mechanism of NF4+ Salts and Extraordinary Enhancement of the Oxidizing Power of Fluorine by Strong Lewis Acids ()
Although the existence of the NF4+ cation has been known for 51 years, and its formation mechanism from NF3 , F2 , and a strong Lewis acid in the presence of an activation energy source had been studied extensively, the mechanism had not been established. Experimental evidence had shown that the first step involves the generation of F atoms from F2 , and also that the NF3+ cation is a key intermediate. However, it was not possible to establish whether the second step involved the reaction of a F atom with either NF3 or the Lewis acid (LA). To distinguish between these two alternatives, a computational study of the NF4 , SbF6 , AsF6 , and BF4 radicals was carried out. Whereas the heats of reaction are small and similar for the NF4 and LAF radicals, at the reaction temperatures, only the LAF radicals possess sufficient thermal stability to be viable species. Most importantly, the ability of the LAF radicals to oxidize NF3 to NF3+ demonstrates that they are extraordinary oxidizers. This extraordinary enhancement of the oxidizing power of fluorine with strong Lewis acids had previously not been fully recognized. The presence of strong Lewis acids (LAs) enhances the oxidizing power of elemental fluorine (3.08 eV) by as much as 6.7 eV, making the resulting LAF radicals the strongest known neutral one-electron oxidizers. When combined with the lattice energy of about 6 eV of the solid product, LAF can easily oxidize compounds, such as NF3 to NF3+.
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Zinc-Catalyzed Synthesis of Allylsilanes by Si−H Bond Insertion of Vinyl Carbenoids Generated from Cyclopropenes ()
Allylsilanes have long been recognized as valuable building blocks for organic synthesis. A zinc-catalyzed reaction of cyclopropenes and hydrosilanes provides a convenient route to these versatile unsaturated organosilanes. In this transformation, ZnBr2 serves as an efficient catalyst, allowing the generation of a zinc vinyl carbenoid intermediate, which is subsequently involved in a Si−H bond insertion. The process shows broad scope, and is amenable to substituted and functionalized cyclopropenes or the functionalization of polysiloxanes. Moreover, zinc-catalyzed carbene insertion into a Ge−H bond is reported for the first time. Unsaturated organosilanes: Allylsilanes were produced by regioselective and atom-economical zinc-catalyzed reactions of cyclopropene and hydrosilane molecules. Insertion of a zinc vinyl carbenoid species into the Si−H bond is posited. The method extends to germanium analogues and an array of cyclopropene and oligomeric siloxanes.
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Carbonyl Activation in Electrophilic Polyene Cyclizations: A Toolbox for the Design of Isoprenoid Libraries ()
An approach to biogenetically overlooked areas of the isoprenoid chemical space is presented. This strategy is based on the generation of a cationic center in functionalized polyolefins by Lewis acid activation of a carbonyl group, rather than by electrophilic attack at a double bond. Starting from the monocyclic humulane trienone zerumbone, polycyclic sesquiterpenoid skeletons which are either not reported as natural products or biogenetically enigmatic in terms of the isoprenoid rule, were obtained by modulating the Lewis acid catalyst. In the course of these studies, the surprising formation of a strained E-cyclooctene motif was observed in a cyclization reaction. Exotic but inhabited: The vast area of the putative isoprenoid chemical space was investigated by cyclizing the humulane dienone zerumbone under carbonyl, rather than olefin, activation. Novel reaction avenues of biogenetic and mechanistic relevance were discovered, thus providing a clue to the enigmatic stenotarsane skeleton and documenting the formation of the strained E-cyclooctene motif in a cyclization reaction.
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Triggering Redox Activity in a Thiophene Compound: Radical Stabilization and Coordination Chemistry ()
The synthesis, metalation, and redox properties of an acyclic bis(iminothienyl)methene L− are presented. This π-conjugated anion displayed pronounced redox activity, undergoing facile one-electron oxidation to the acyclic, metal-free, neutral radical L. on reaction with FeBr2. In contrast, the reaction of L− with CuI formed the unique, neutral Cu2I2(L.) complex of a ligand-centered radical, whereas reaction with the stronger oxidant AgBF4 formed the metal-free radical dication L.2+. Radical relatives: The bis(iminothienyl)methene L− displayed pronounced redox activity in reactions with metal salts (see scheme). Such reactions generated the acyclic neutral radical L., the neutral Cu2I2(L.) complex of a ligand-centered radical, and the metal-free radical dication L.2+, all of which were characterized by crystallographic, spectroscopic, and computational techniques.
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The EPR Spectrum of Triplet Mesitylphosphinidene: Reassignment and New Assignment ()
Low-temperature UV-photolysis of mesitylphosphiranes under highly anaerobic conditions leads to the formation of the triplet mesitylphosphinidene (MesP). The recorded X-band EPR spectrum of triplet MesP and the derived zero-field splitting parameter D=4.116 cm−1 differ significantly from those reported previously for this intermediate. New magnetic parameters of mesitylphosphinidene are discussed along with the results of DFT calculations. Finally caught: The EPR spectrum of triplet mesitylphosphinidene and zero-field splitting parameter D from this spectrum differ significantly from those previously reported in the literature. The presence of the doublet-hyperfine structure in the EPR spectra provides reliable identification of the mesitylphosphinidene molecule in its triplet state.
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Facile Phenylphosphinidene Transfer Reactions from Carbene–Phosphinidene Zinc Complexes ()
Phosphinidenes [R-P] are convenient P1 building blocks for the synthesis of a plethora of organophosphorus compounds. Thus far, transition-metal-complexed phosphinidenes have been used for their singlet ground-state reactivity to promote selective addition and insertion reactions. One disadvantage of this approach is that after transfer of the P1 moiety to the substrate, a challenging demetallation step is required to provide the free phosphine. We report a simple method that enables the Lewis acid promoted transfer of phenylphosphinidene, [PhP], from NHC=PPh adducts (NHC=N-heterocyclic carbene) to various substrates to produce directly uncoordinated phosphorus heterocycles that are difficult to obtain otherwise. Pass it on: ZnCl2-promoted phosphinidene transfer reactions of the sterically unencumbered carbene–phosphinidene adduct MeNHC=PPh to various substrates (S) are demonstrated. These unprecedented reactions provided access to new uncomplexed phosphorus heterocycles, which are difficult to obtain otherwise.
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Divergent Synthesis of Disulfanes and Benzenesulfonothioates Bearing 2-Aminofurans From N-Tosylhydrazone-Bearing Thiocarbamates ()
An efficient and convenient synthesis of valuable disulfanes and benzenesulfonothioates, having a 2-aminofuran framework, has been developed by employing a copper-catalyzed transformation of readily available N-tosylhydrazone-bearing thiocarbamates. This method features an inexpensive metal catalyst, mild reaction conditions, good functional-group tolerance, short reaction times, and delivers valuable and complex products. A copper carbene generated from an N-tosylhydrazone-bearing thiocarbamate is proposed as the key intermediate for the transformation and it triggers the subsequent cascade. Remarkably, the Ts anion released from N-tosylhydrazone further serves as a nucleophile, thus rendering the formation of benzenesulfonothioates under controlled conditions. Waste not: N-tosylhydrazone-bearing thiocarbamates prove to be valuable substrates for access to complex disulfanes and benzenesulfonothioates with a 2-aminofuran framework, when a copper-catalyzed transformation is employed. Remarkably, the 4-toluenesulfonyl (Ts) moiety, which usually is considered a waste product released from N-tosylhydrazone, serves as a nucleophile, thus leading to the formation of benzenesulfonothioates under controlled conditions.
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Silver-Catalyzed Three-Component 1,1-Aminoacylation of Homopropargylamines: α-Additions for Both Terminal Alkynes and Isocyanides ()
The reaction of secondary homopropargylamines, isocyanides, and water in the presence of a catalytic amount of silver acetate and subsequent purification by chromatography on silica gel afforded substituted proline amides in good to excellent yields. Primary homopropargylamines underwent a cyclizative Ugi–Joullié three-component reaction with isocyanides and carboxylic acids to afford functionalized N-acyl proline amides. High diastereoselectivity was observed in the synthesis of 4-alkoxy and 4,5-disubstituted proline derivatives. This work represents the first examples of a three-component cyclizative 1,1-aminoacylation of terminal alkynes. No atom lost: Homopropargylamines react with isocyanides and either carboxylic acids or water in the presence of a catalytic amount of silver acetate to afford substituted proline amides. This three-component reaction proceeds by a cyclizative 1,1-aminoacylation of terminal alkynes and affords good to excellent yields. Boc=tert-butoxycarbonyl.
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Correlation between Chiral Modifier Adsorption and Enantioselectivity in Hydrogenation Catalysis ()
Infrared absorption spectroscopy performed in situ at the solid–liquid interface revealed that the adsorption on platinum supported catalysts of 1-(1-naphthyl)-ethylamine, which is used as a chiral modifier in hydrogenation catalysis, occurs through the amine group, not the aromatic ring as is widely believed. Comparisons were performed against a set of related modifier compounds with targeted substitutions to help identify the key moiety involved in the adsorption. It was determined that neither naphthalene-based modifiers without amine groups nor those with tertiary amine moieties are capable of adsorbing on the metal surface to any significant extent. A direct correlation was also found between the ability of the amines to adsorb on the platinum surface and their performance as chiral modifiers that impart enantioselectivity to the hydrogenation of α-keto esters such as ethyl pyruvate. Chiral modification of platinum hydrogenation catalysts by 1-(1-naphthyl)- ethylamine requires adsorption via the amine nitrogen atom, not bonding through the aromatic ring as is widely believed. Adsorption of the modifier requires NH2 groups (as determined by a study of a family of related compounds by using in situ infrared absorption spectroscopy) and correlates with the enantioselectivity of the catalyst.
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Catalytic Asymmetric Conjugate Addition of Indolizines to α,β-Unsaturated Ketones ()
A catalytic enantioselective conjugate addition of indolizines to enones is described. The chiral phosphoric acid (S)-TRIP activates α,β-unsaturated ketones, thereby promoting an enantioface-differentiating attack by indolizines. Using this reaction, several alkylated indolizines were synthesized in good yields and with enantiomeric ratios of up to 98:2. Alkylated indolizines: Using (S)-TRIP as monofunctional catalyst, several alkylated indolizines were synthesized in good yields and with enantiomeric ratios of up to 98:2.
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Privileged Structures Revisited ()
Privileged structures inspire compound library design in medicinal chemistry. We performed a comprehensive analysis of 1.4 million bioactive compounds, with the aim of assessing the prevalence of certain molecular frameworks. We used the Shannon entropy formalism to quantify the promiscuity of the most frequently observed atom scaffolds across the annotated target families. This analysis revealed an apparent inverse relationship between hydrogen-bond-acceptor count of a scaffold and its potential promiscuity. The results further suggest that chemically easily accessible scaffolds can serve as templates for the generation of bespoke compound libraries with differing degrees of multiple target engagement, and heterocyclic, sp3-rich frameworks are particularly suited for target-focused library design. The outcome of our study enables us to place some of the many narratives surrounding the concept of privileged structures into a critical context. Drug design: Analysis of 1.4 million bioactive compounds revealed an apparent inverse relationship between the sp3 atom and hydrogen-bond-acceptor count of a scaffold and its potential promiscuity in terms of binding a variety of protein targets.
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Synthesis and Trapping of Iminoboranes by M=B/C=N Bond Metathesis ()
Although the metathesis of metal–boron double bonds with elemental chalcogenides is an established process, no similar reactivity has been observed with element–nitrogen bonds. Such a reaction would provide a new route to iminoborane compounds (RB≡NR′), which have recently experienced renewed synthetic interest. Herein, we present the first observation of M=B/C=N metathesis reactions, which led to the isolation of a stable iminoborane in addition to further iminoborane cycloaddition products. Boron and nitrogen: A M=B/C=N metathesis reaction is reported, leading to the isolation of a stable iminoborane in addition to further iminoborane cycloaddition products. This iminoborane is the first BspEsp-hybridized product to be isolated from a metathesis reaction.
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Superelectrophilic Behavior of an Anion Demonstrated by the Spontaneous Binding of Noble Gases to [B12Cl11]− ()
It is common and chemically intuitive to assign cations electrophilic and anions nucleophilic reactivity, respectively. Herein, we demonstrate a striking violation of this concept: The anion [B12Cl11]− spontaneously binds to the noble gases (Ngs) xenon and krypton at room temperature in a reaction that is typical of “superelectrophilic” dications. [B12Cl11Ng]− adducts, with Ng binding energies of 80 to 100 kJ mol−1, contain B−Ng bonds with a substantial degree of covalent interaction. The electrophilic nature of the [B12Cl11]− anion is confirmed spectroscopically by the observation of a blue shift of the CO stretching mode in the IR spectrum of [B12Cl11CO]− and theoretically by investigation of its electronic structure. The orientation of the electric field at the reactive site of [B12Cl11]− results in an energy barrier for the approach of polar molecules and facilitates the formation of Ng adducts that are not detected with reactive cations such as [C6H5]+. This introduces the new chemical concept of “dipole-discriminating electrophilic anions.” Unexpected reactivity: The binding of the noble gases Xe and Kr to the anion [B12Cl11]− is explained with the exceptional properties of the electric field in the vicinity of the undercoordinated boron atom. Whereas polar nucleophiles such as water face a barrier, neutral unpolar species such as N2 or noble gases are preferred, rendering [B12Cl11]− a “dipole-discriminating electrophilic anion”.
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Energy Resonance Crossing Controls the Photoluminescence of Europium Antenna Probes ()
The energy transfer pathways in lanthanide antenna probes cannot be comprehensively rationalized by the currently available models, and their elucidation remains to be a challenging task. On the basis of quantum-chemical ab initio calculations of representative europium antenna complexes, an innovative energy resonance model is proposed, which is controlled by an overall nonet–quintet intersystem crossing on the basis of spin–orbit coupling among the sublevels of the involved states. Going for resonance crossing: An innovative energy transfer model controls the photoluminescence of europium antenna probes. The antenna contributes its adiabatic excitation energy to drive the f electron pairing of Eu3+ through energetically favorable nonet–quintet crossing. The photoluminescence efficiency is governed by spin–orbit coupling among the sublevels of the involved states.
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Selective Si−C(sp3) Bond Cleavage in (Aminomethyl)silanes by Carbanionic Nucleophiles and Its Stereochemical Course ()
Selective cleavage of a silicon–carbon bond in tetraorganosilanes is still a great challenge. A new type of Si−C(sp3) bond cleavage in bench-stable (aminomethyl)silanes with common organolithium reagents as nucleophiles has now been identified. Suitable leaving groups are benzyl, allyl, and phenylthiomethyl groups. A β-donor function and polar solvents are essential for the reaction. Simple switching between α-deprotonation and substitution is possible through slight modifications of the reaction conditions. The stereochemical course of the reaction was elucidated by using a silicon-chiral benzylsilane. The new transformation proceeds stereospecifically with inversion of configuration and can be used for the targeted synthesis of enantiomerically pure tetraorganosilanes, which are otherwise difficult to access. Quantum chemical calculations provided insight into the mechanism of the new substitution. Versatile reactivity: In addition to the well-established α-deprotonation, a new reaction pathway has been identified for (aminomethyl)silanes which also enables synthetic applications of all-C-substituted silanes. Simple variation of the reaction conditions enables carbanionic nucleophiles (Nu) to substitute activated alkyl groups on a Si center. Enantiomerically pure organosilanes are also accessible, which are otherwise difficult to prepare.
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Electronic Structure and Magnetic Anisotropy of an Unsaturated Cyclopentadienyl Iron(I) Complex with 15 Valence Electrons ()
The 15 valence-electron iron(I) complex [CpArFe(IiPr2Me2)] (1, CpAr=C5(C6H4-4-Et)5; IiPr2Me2=1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene) was synthesized in high yield from the FeII precursor [CpArFe(μ-Br)]2. 57Fe Mössbauer and EPR spectroscopic data, magnetic measurements, and ab initio ligand-field calculations indicate an S= 3/2 ground state with a large negative zero-field splitting. As a consequence, 1 features magnetic anisotropy with an effective spin-reversal barrier of Ueff=64 cm−1. Moreover, 1 catalyzes the dehydrogenation of N,N-dimethylamine–borane, affording tetramethyl-1,3-diaza-2,4-diboretane under mild conditions. Iron(I) in action: The 15 valence-electron complex [CpArFe(IiPr2Me2)] is stabilized by the pentaaryl-substituted cyclopentadienyl ligand C5(C6H4-4-Et)5 (CpAr) and the N-heterocyclic carbene 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene (IiPr2Me2). This compound is a new type of iron-based single-molecule magnet owing to an unusual configuration of the 3d orbitals in the ground state.
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Isomer-Selective Generation and Spectroscopic Characterization of Picolyl Radicals ()
Nitrogen-containing resonance-stabilized radicals such as the picolyl radical are important in combustion chemistry and astrochemistry. They have only been scarcely studied because an isomer-selective generation is often difficult. Herein, we present threshold photoelectron spectra of the three picolyl radical isomers, C6H6N, that were obtained with synchrotron radiation. The radicals were selectively generated by flash pyrolysis from aminomethylpyridine precursors through deamination. Ionization energies of 7.70, 7.59, and 8.01 eV were determined for 2-, 3-, and 4-picolyl, respectively. The observed vibrational structure was assigned to an in-plane deformation mode of the aromatic ring. The spectroscopic insight gained in this study can be used to distinguish different picolyl isomers in on-line combustion analysis, for example. The three isomers of the picolyl radical, C6H6N, were generated from aminomethylpyridine precursors by pyrolysis and characterized by threshold photoelectron spectroscopy with synchrotron radiation.
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Cooperative Light-Activated Iodine and Photoredox Catalysis for the Amination of Csp3 −H Bonds ()
An unprecedented method that makes use of the cooperative interplay between molecular iodine and photoredox catalysis has been developed for dual light-activated intramolecular benzylic C−H amination. Iodine serves as the catalyst for the formation of a new C−N bond by activating a remote Csp3 −H bond (1,5-HAT process) under visible-light irradiation while the organic photoredox catalyst TPT effects the reoxidation of the molecular iodine catalyst. To explain the compatibility of the two involved photochemical steps, the key N−I bond activation was elucidated by computational methods. The new cooperative catalysis has important implications for the combination of non-metallic main-group catalysis with photocatalysis. More light, more possibilities: A dual light-activated process with intertwined iodine and photoredox catalysis has been developed for the amination of remote Csp3 −H bonds. While iodine promotes the activation of the Csp3 −H bond, the dye TPT acts as a photoredox catalyst to regenerate the molecular iodine catalyst. The compatibility of the two processes is explained by calculations.
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Engineering a Small HOMO–LUMO Gap and Intramolecular C−H Borylation by Diborene/Anthracene Orbital Intercalation ()
The diborene 1 was synthesized by reduction of a mixture of 1,2-di-9-anthryl-1,2-dibromodiborane(4) (6) and trimethylphosphine with potassium graphite. The X-ray structure of 1 shows the two anthryl rings to be parallel and their π(C14) systems perpendicular to the diborene π(B=B) system. This twisted conformation allows for intercalation of the relatively high-lying π(B=B) orbital and the low-lying π* orbital of the anthryl moiety with no significant conjugation, resulting in a small HOMO–LUMO gap (HLG) and ultimately a C−H borylation of the anthryl unit. The HLG of 1 was estimated to be 1.57 eV from the onset of the long wavelength band in its UV/Vis absorption spectrum (THF, λonset=788 nm). The oxidation of 1 with elemental selenium afforded diboraselenirane 8 in quantitative yield. By oxidative abstraction of one phosphine ligand by another equivalent of elemental selenium, the B−B and C1−H bonds of 8 were cleaved to give the cyclic 1,9-diborylanthracene 9. Gap engineering: An anthryl-substituted diborene was isolated as dark-green crystals with a remarkably small HOMO–LUMO gap (HLG) of 1.57 eV. An unprecedented intramolecular C−H borylation with the B−B bond in the corresponding diboraselenirane was observed after oxidative abstraction of a phosphine ligand.
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