Angewandte Chemie International Edition

Importance of Inherent Substrate Reactivity in Enzyme Promoted Carbocation Cyclization/Rearrangements ()
The importance of inherent substrate reactivity for terpene synthase enzymes is discussed, with a focus on recent experimental tests of predictions derived from computations on gas phase reactivity of carbocations.
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Cr(0), Mo(0) and W(0) isocyanide complexes as luminophores and photosensitizers with long-lived excited states ()
Group 6 d6 metal complexes with arylisocyanide ligands are earth-abundant alternatives to photoactive complexes made from precious metals such as Ru(II), Re(I), Os(II), or Ir(III). Some of them have long-lived 3MLCT excited states that exhibit luminescence with good quantum yields combined with nano- to microsecond lifetimes, and they are very strongly reducing. Recent studies have demonstrated that Cr(0), Mo(0), and W(0) arylisocyanides have great potential for application in luminescent devices, photoredox catalysis, and dye-sensitized solar cells.
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Fervent Hype behind Magnesium Batteries: An Open Call to Synthetic Chemists - Electrolytes and Cathodes Needed ()
Magnesium metal is an ideal anode which has double the volumetric capacity of lithium metal and has a negative reduction potential of -2.37V vs. the standard hydrogen electrode. The major advantage of magnesium is the apparent lack of dendritic formation during charging which is one of the major concerns of using a lithium metal anode. In this review, we highlight the major research in the development of electrolytes and cathodes and discuss some of the major challenges which must be overcome in realizing a practical magnesium battery.
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Cationic Polymerization: From Photoinitiation to Photocontrol ()
During the last 40 years, researchers investigating photoinitiated cationic polymerizations have delivered tremendous success in both industrial and academic settings. A myriad of photoinitiating systems have been developed, allowing for the polymerization of a broad array of monomers (e.g., epoxides, vinyl ethers, alkenes, cyclic ethers, and lactones) under practical, inexpensive, and environmentally benign conditions. More recently, owing to progress in photoredox catalysis, photocontrolled cationic polymerization has emerged as a means to precisely regulate polymer chain growth. This review provides a concise historical perspective on cationic polymerization induced by light and discusses the latest advances in both photoinitiated and photocontrolled processes. The latter are exciting new directions for the field that will likely impact industries ranging from micropatterning to the synthesis of complex biomaterials and sequence-controlled polymers.
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Transition Metal-Catalyzed Utilization of Methanol as C1 Source in Organic Synthesis ()
Methanol represents one of the privileged and widespread platform chemicals, which is used in many science sectors such as chemistry, biology, medicine and energy. This essential alcohol serves as common solvent, cost-effective reagent and sustainable feedstock for value-added chemicals, pharmaceuticals and materials. Among different applications, the utilization of methanol as C1 source for the formation of carbon-carbon, carbon-nitrogen and carbon-oxygen bonds continues to be important in organic synthesis and drug discovery. Particularly, the synthesis of C-methylated, N-methylated and O-methylated products is of central interest because these motifs are found in a large number of life science molecules as well as fine and bulk chemicals. In this Minireview, we summarize the utilization of methanol as C1 source in C-methylation, N-methylation, C-methoxylation, N-formylation, methoxycarbonylation and oxidative methyl esterification reactions for the synthesis of C-methylated products, N-methylamines, formamides, urea derivatives, ethers, esters and heterocycles.
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Dynamic Macromolecular Material Design - The Versatility of Cyclodextrin Based Host/Guest Chemistry ()
Dynamic and adaptive materials are powerful constructs in macromolecular and polymer chemistry with a wide array of applications in drug-delivery, bioactive systems or self-healing materials. Very frequently dynamic materials featuring non-covalent interactions driven by supramolecular chemistry are based on carefully tailored cyclodextrin (CD) host/guest interactions. The precise incorporation of these host and guest moieties into macromolecular building blocks allows for the formation of complex macromolecular structures that can be utilized to form higher level assemblies executing specific pre-defined functions. Thus, dynamic materials with extraordinary adaptive property profiles - responsive to thermal, chemical and photonic fields - become accessible. In the current critical review the hierarchical formation of dynamic materials and complex macromolecular structures is explored from the molecular to the macromolecular on to the colloidal and macroscopic level, with a specific emphasis on the functionality and outer field responsiveness of the assemblies, specifically in biological contexts.
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Applications of the Wittig-Still rearrangement in organic synthesis ()
This review traces the discovery of the Wittig-Still rearrangement and its applications in organic synthesis. Its relationship to Wittig rearrangements is discussed along with detailed analysis of E/Z- and diastereoselectivity. Modifications of the products arising from the Wittig-Still rearrangement are reviewed in the context of increased complexity in intermediates potentially useful in target oriented synthesis. Early applications of the Wittig-Still rearrangement to modifications of steroids are reviewed as are applications to various terpene and alkaloid natural product targets and miscellaneous compounds. To the best of our knowledge, the literature is covered through December 2016.
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New Modalities for Challenging Targets in Drug Discovery ()
An ever increasing understanding of biological systems is providing a range of exciting novel biological targets whose modulation may enable novel therapeutic options in many diseases. These targets include protein-protein and protein-nucleic acid interactions, which are, however, often refractory to classical small molecule approaches. Other types of molecules, or modalities, are therefore required to address these targets, which has led several academic research groups and pharmaceutical companies to increasingly use the concept of so-called 'New Modalities'. This review defines for the first time the scope of this term, which includes novel peptidic scaffolds, oligonucleotides, hybrids, molecular conjugates as well as new uses of classical small molecules. We provide herein a journey through the most representative examples of these modalities to target large binding surface areas such as those found in protein-protein interactions and for biological processes at the center of cell regulation.
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Advanced Biofuels and Beyond: Chemistry Solutions for Propulsion and Production ()
Sustainably produced biofuels are being discussed intensively as one possible component in the energy scenarios for future ground transportation, especially when they are derived from lignocellulosic biomass. Traditionally, research activities on their production focus on the synthesis process, while leaving their combustion properties to subsequent evaluation by a different community. The present article adopts an integrative view of engine combustion and fuel synthesis, focusing on the chemical aspects as the common denominator. We wish to demonstrate that fundamental understanding of the combustion process can be instrumental to derive design criteria for the molecular structure of fuel candidates that can then be targets for the analysis of synthetic pathways and the development of catalytic production routes. With such an integrative approach to fuel design, it will be possible to improve systematically the entire system, spanning biomass feedstock, conversion process, fuel, engine, and pollutants with a perspective to improve the carbon footprint, increase efficiency, and reduce emissions of the transportation sector along the whole value chain.
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Nanostructured Materials for Heterogeneous Electrocatalytic CO2 Reduction and Related Reaction Mechanisms ()
The gradually increased concentration of carbon dioxide (CO2) in the atmosphere has been recognized as the primary culprit for the raise of the global mean temperature, thus resulting in the aggravated desert formation and extinction of species. In recent years, development of the routes for highly efficient conversion of CO2 has received numerous attentions. Among them, the reduction of CO2 with electric power is an important transformation route with high application prospect, due to its high environmental compatibility and good combination with other renewable energy sources such as solar and wind energy. This review describes recent progress on the design and synthesis of solid state catalysts (i.e., heterogeneous catalysts) and their emerging catalytic performances in the CO2 reduction. The significance for catalytic conversion of CO2 and the advantages of CO2 electroreduction will be presented in the introduction section, followed by the general parameters for CO2 electroreduction and the summary of reaction apparatus. We also discuss various types of solid catalysts according to CO2 conversion mechanisms. Furthermore, we summarize the crucial factors (particle size, surface structure, composition and etc.) determining the performance for electroreduction. These studies in improvement of solid state catalysts for CO2 reduction offer numerous experiences for developing potential industrialized CO2 electroreduction catalysts in the future. Additionally, the abundant experience for controllable synthesis of solid state catalysts could effectively guide the rational design of catalysts for other electrocatalytic reactions.
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Catalytic Dearomatization of N-Heteroarenes with Silicon and Boron Compounds ()
Dearomatized N-heterocycles provide a class of structural motifs important in the fields of synthetic organic chemistry 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, scattered precedents on the catalytic reduction of N-heteroaromatics with silane or borane reducing agents have been reported thus far, and a handful of elegant catalytic procedures for the reduction of N-aromatics have emerged only recently as a viable tool in organic synthesis. This review presents recent advances in the catalytic reduction of unactivated N-heteroarenes using hydrosilanes, hydroboranes, silaboranes, and diboranes. The focus presents the general chemical behavior and selectivity of transition-metal or metal-free organocatalyst systems for the dearomative homogeneous reduction of N-heteroarenes. In addition, the working modes of these catalysis will be described particularly on the basis of the experimental mechanistic insights.
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Modern Inorganic Aerogels ()
Essentially, the term aerogel describes a special geometric structure of matter. It is neither limited to any material nor to any synthesis procedure. Hence, the possible variety of materials and therefore the multitude of their applications are almost unbounded. In fact, the same applies for nanoparticles. These are also just defined by their geometrical properties. In the past decades nano-sized materials were intensively studied and possible applications appeared in nearly all areas of natural sciences. To date a large variety of metal, semiconductor, oxide and other nanoparticles are available from colloidal synthesis. However, for many applications of these materials an assembly into macroscopic structures is needed. Here we present a comprehensive picture of the developments that enabled the fusion of the colloidal nanoparticle and the aerogel world. This became possible by the controlled destabilization of pre-formed nanoparticles, which leads to their assembly into three-dimensional macroscopic networks. This revolutionary approach makes it possible to use precisely controlled nanoparticles as building blocks for macroscopic porous structures with programmable properties.
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Uniform Supersonic Chemical Reactors: 30 Years of Astrochemical History and Future Challenges ()
The interstellar medium attracts our great attention, as the place where stars and planets are born and from where, probably, the molecular precursors of life have come to Earth. To understand the chemical pathways to the formation of stars, planets, and biological molecules, astronomical observations, astrochemical modelling, and laboratory astrochemistry should go hand in hand. In this paper we review the laboratory experiments devoted to investigations of the reaction dynamics of species of astrochemical interest at the temperatures of the interstellar medium and performed by using one of the most popular techniques in the field, CRESU. We discuss new technical developments and scientific ideas for CRESU, which, if realized, will bring us one more step closer to an understanding of the astrochemical history and future of our Universe.
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Luminescence, Plasmonic and Magnetic Properties of Doped Semiconductor Nanocrystals: Current Developments and Future Prospects ()
Introducing few atoms of impurities or dopants in semiconductor nanocrystals can drastically alter the existing or even introduce new properties. For example, mid-gap states created by doping tremendously affect photocatalytic activities and surface controlled redox reactions, generate new emission centres, show thermometric optical switching, make suitable FRET donors by enhancing the excited state lifetime and also create localized surface plasmon resonance induced low energy absorption. In addition, researchers have more recently started focusing their attention on doped nanocrystals as an important and alternative material for solar energy conversion in order to meet the current demand for renewable energy. Moreover, electrical as well as magnetic properties of the host are also strongly altered on doping. These dopant-induced beneficial changes in material properties suggest that doped nanocrystals with proper selections of dopant-host pairs may be helpful for generating designer materials for a wide range of current technological needs. Such exciting properties related to various aspects of doping a variety of semiconductor nanocrystals are summarized and reported in this mini review.
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Protein-templated fragment ligations - from molecular recognition to drug discovery ()
The understanding and manipulation of molecular recognition events is the key to modern approaches in drug discovery. Protein-templated fragment ligation is a novel concept to support drug discovery and can help to improve the efficacy of already existing protein ligands. Protein-templated fragment ligations are chemical reactions between small molecules ("fragments") that utilize a protein´s surface as a template to combine and to form a protein ligand with increased binding affinity. The approach exploits the molecular recognition of reactive small molecule fragments by proteins both for ligand assembly and for the identification of bioactive fragment combinations. Chemical synthesis and bioassay are thus integrated in one single step. In this article we portrait the biophysical basis of reversible and irreversible fragment ligations and the available methods to detect protein-templated ligation products. The scope of known chemical reactions providing templated ligation products is reviewed and the possibilities to extend the reaction portfolio are discussed. Selected recent applications of the method in protein ligand discovery are reported. Finally, the strengths and limitations of the concept are discussed and an outlook on the future impact of templated fragment ligations on the drug discovery process is given.
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Black Phosphorus Rediscovered: From Bulk to Monolayer ()
Phosphorus is a non-metal with several allotropes, from the highly reactive white phosphorus to the thermodynamically stable black phosphorus (BP) with a puckered orthorhombic layered structure. The bulk form of BP was synthesized for the first time more than a century ago, in 1914, not receiving much attention until very recently rediscovered, in 2014, joining the new wave of 2D layered nanomaterials. BP can be exfoliated to a single sheet structure with tunable direct band, semiconducting, high carrier mobility at room temperature and in-plane anisotropic layered structure. Surface chemistry degradation can still be a holdback for the advancement of BP applications, thus compelling efforts to achieve effective BP passivation are ongoing, such as its integration in van der Waals heterostructures. Currently, BP has been tested as a novel nanomaterial in batteries, transistors, sensors and photonics related fields. In this review we take a look back at BP origin story, taking the path from bulk to nowadays few/single layer. Physical and chemical properties are summarized, highlighting the state-of-the-art in BP applications.
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Xenobiology meets enzymology: Exploring the potential of unnatural building blocks in biocatalysis ()
Xenobiology (XB) aims to design biological systems endowed with unusual biochemistries, while enzymology concerns the study of enzymes, the workhorses of biocatalysis. Biocatalysis employs enzymes and organisms to perform useful biotransformations in synthetic chemistry and biotechnology. During the past years, the effects of incorporating noncanonical amino acids (ncAAs) into enzymes with potential applications in biocatalysis have been increasingly investigated. Here we provide an overview of the effects of new chemical functionalities that have been introduced into proteins to improve various facets of enzymatic catalysis. We also discuss future research avenues that will complement unnatural mutagenesis with the standard protein-engineering toolbox for producing novel and versatile biocatalysts with applications in synthetic organic chemistry and biotechnology.
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Carbon-Carbon Bond Formation in a Weak Ligand Field: Leveraging Open Shell First Row Transition Metal Catalysts ()
Unique features of Earth abundant transition metal catalysts are reviewed in the context of catalytic carbon-carbon bond forming reactions. Aryl-substituted bis(imino)pyridine iron and cobalt dihalide compounds, when activated with alkyl aluminum reagents, form highly active catalysts for the polymerization of ethylene. Open shell iron and cobalt alkyl complexes have been synthesized that serve as single component olefin polymerization catalysts. Reduced bis(imino)pyridine iron- and cobalt dinitrogen compounds have also been discovered that promote the unique [2+2] cycloaddition of unactivated terminal alkenes. Electronic structure studies support open shell intermediates, a deviation from traditional strong field organometallic compounds that promote catalytic C-C bond formation.
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Fullerenes in space ()
In 1985 the football structure of $\textrm{C}_{60}$, buckminsterfullerene was proposed and subsequently confirmed following its macroscopic synthesis in 1990. From the very beginning the role of $\textrm{C}_{60}$ and $\textrm{C}_{60}^{+}$ in space was considered, particularly in the context of the enigmatic diffuse interstellar bands. These are absorption features found in the spectra of reddened star light. The first astronomical observations were made around one hundred years ago and despite significant efforts none of the interstellar molecules responsible have been identified. The absorption spectrum of $\textrm{C}_{60}^{+}$ was measured in a $5\,\textrm{K}$ neon matrix in 1993 and two prominent bands near $9583\,\textrm{\AA}$ and $9645\,\textrm{\AA}$ were observed. On the basis of this data the likely wavelength range in which the gas phase $\textrm{C}_{60}^{+}$ absorptions should lie was predicted. In 1994 two diffuse interstellar bands were found in this spectral region and proposed to be due to $\textrm{C}_{60}^{+}$. It took over 20 years to measure the absorption spectrum of $\textrm{C}_{60}^{+}$ under conditions similar to those prevailing in diffuse clouds. In 2015, sophisticated laboratory experiments led to the confirmation that these two interstellar bands are indeed caused by $\textrm{C}_{60}^{+}$, providing the first answer to this century old puzzle. Here, we describe the experiments, concepts and astronomical observations that led to the detection of $\textrm{C}_{60}^{+}$ in interstellar space.
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From composition to cure: A systems engineering approach to anti-cancer drug carriers ()
The molecular complexity and heterogeneity of cancer has led to a persistent, and as yet unsolved, challenge of developing cures for this disease. Although the pharmaceutical industry focuses the bulk of its efforts on the development of new drugs, an alternative —and in many ways complementary— approach is the improvement of the delivery of existing drugs with drug carriers that can manipulate when, where, and how a drug exerts its therapeutic effect. For the treatment of solid tumors, systemically delivered drug carriers face significant challenges that are imposed by the pathophysiological barriers that lie between their site of administration and their site of therapeutic action in the tumor. Furthermore, drug carriers face additional challenges in their translation from pre-clinical validation to clinical approval and adoption. Addressing this diverse network of challenges requires a systems engineering approach to anti-cancer drug delivery. Such a perspective is needed to better enable the rational design of optimized vehicles that navigate the trade-offs of this complex system to create next generation anti-cancer drug carriers that have a realistic prospect for translation from the lab to the patient.
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Methyltransferase directed labeling of biomolecules and its applications ()
Methyltransferases (MTases) compose a large family of enzymes which have the ability to methylate a diverse set of targets, ranging from the three major biopolymers DNA, RNA and protein to small molecules. Most of these MTases use the cofactor S-Adenosyl-L-Methionine (AdoMet) as their methyl source. Given the important biological role of methylation, e.g. in epigenetic regulation of gene activity and the vast potential of targeted functionalization in biology, diagnostics and nanotechnology, it should come as no surprise that recent years have seen significant efforts in the development of AdoMet analogues with the aim of transferring moieties other than simple methyl groups. Two major classes of AdoMet analogues currently exist- the doubly-activated and aziridine based molecules- each of which employs a different approach for transalkylation, as opposed to transmethylation, of the target molecule. In this review, we discuss the various strategies for labelling and functionalizing biomolecules using AdoMet-dependent MTases and AdoMet analogues. We cover the synthetic routes to AdoMet analogues, their stability in biological environments and their application in transalkylation reactions. Finally, some perspectives are presented for the potential use of AdoMet analogues in biology research, (epi)genetics and nanotechnology.
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Synthetic Biology - The Synthesis of Biology ()
Synthetic biology envisages the engineering of man-made living biomachines from standardized components that can perform pre-defined functions in a (self-)controlled manner. Different research strategies and interdisciplinary efforts are pursued to implement engineering principles to biology. The "top-down" strategy exploits nature's incredible diversity of existing, natural parts to construct synthetic compositions of genetic, metabolic or signalling networks with predictable and controllable properties. This mainly application-driven approach results in living factories that produce drugs, biofuels, biomaterials and fine chemicals and results in living pills that are based on engineered cells with the capacity to autonomously detect and treat disease states in vivo. In contrast, the "bottom-up" strategy seeks to be independent of existing living systems by designing biological systems from scratch and synthesizing artificial biological entities not found in nature. This more knowledge-driven approach investigates the reconstruction of minimal biological systems that are capable of performing basic biological phenomena, such as self-organization, self-replication and self-sustainability. Moreover, the syntheses of artificial biological units, such as synthetic nucleotides or amino acids, and their implementation into polymers inside living cells currently set the boundaries between natural and artificial biological systems. In particular, the in vitro design, synthesis and transfer of complete genomes into host cells and the application of efficient genome-wide intervention techniques point to the future of synthetic biology: the creation of living designer cells with tailored desirable properties for biomedicine and biotechnology.
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50 (and more) Years of Propellane Chemistry - From a Chemical Curiosity to "Explosive" Applications in Material Sciences and Natural Product Chemistry ()
Propellanes are a unique class of compounds with well over 10000 members with an inverted tetrahedral configuration featuring a weak single bond between two carbon atoms linked by three additional bridge. Not only structurally interesting, these propella-type compounds exhibit unusual reactivities. In this review, we highlight their synthesis and applications in material sciences, natural product chemistry and medicinal chemistry. We feature the chemistry of [1.1.1]propellane like the synthesis of oligomeric and polymeric structures derived from it such as bicyclo[1.1.1]pentanes and staffanes. A selected number of natural products are discussed in detail. Heteropropellanes and inorganic propellanes are also addressed. The historical background is given in brief showing the pioneering work of David Ginsburg, Günther Snatzke, Günter Szeimies, Kenneth Wiberg and others.
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Label-free molecular imaging of biological cells and tissues by linear and non-linear Raman spectroscopic approaches ()
Raman spectroscopy is an emerging technique in bioanalysis and imaging of biomaterials due to its unique spectroscopic fingerprint capabilities. Imaging of cells and tissues by Raman microspectroscopy represents a non-destructive and label-free approach. All components of cells or tissues contribute to the Raman signals giving complex spectral signatures. Long acquisitions times are often required due to the relatively small Raman scattering cross sections. To overcome these limitations, Raman signal enhancing methods like resonance Raman scattering and surface enhanced Raman scattering can be applied that also reduce the spectral complexity because the enhancement is often restricted to selected bands. Raman-active labels can be introduced to increase specificity and multimodality. In addition, non-linear coherent Raman scattering such as coherent anti-Stokes Raman scattering and stimulated Raman scattering offer higher sensitivities which enable rapid imaging of larger sampling areas. Finally, fiber based imaging techniques open the way towards in vivo applications of Raman spectroscopy. This critical review summarizes theory, instrumentation, data processing and progress of medical Raman imaging since 2012.
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The Stability Challenges of Oxygen Evolving Electrocatalysts: Towards a Common Fundamental Understanding and Mitigation of Catalyst Degradation ()
The electrochemical oxygen evolution reaction (OER) is an important reaction in the industrial production of numerous inorganic chemicals. However, over the last decade it has also received growing attention as scalable proton- and electron-providing process for use in the production of solar fuels, such as hydrogen, hydrocarbons, or alcohols carried out in water- and CO2-electrolyzers. To operate these devices efficiently and economically , active and stable electrocatalysts are required. While advances have been made in understanding and tuning OER efficiency and activity, the stability of OER catalysts and the reduction of their degradation continue to be major challenges. While most stability studies limit themselves to short-term testing in idealized three-electrode set ups, a much stronger focus on advancing our understanding of degradation of OER catalysts in realistic Membrane Electrode Assemblies (MEAs) with industrial current densities, is critically needed. This review addresses the technical challenges, their scientific basis, as well as recent progress and the road ahead regarding stability and degradation of OER catalysts operating at electrolyzer anodes in acidic MEA environments. First, we start clarifying the complexity associated with the term "catalyst stability", cover today's performance targets and outline major catalyst degradation mechanisms and their mitigation strategies. Then we evaluate suitable in-situ experimental methods to get insight into catalyst degradation and describe achievements in tuning OER catalyst stability. Finally, we highlight the importance of identifying universal figures of merit for stability and develop a comprehensive accelerated life test (ALT) that would yield comparable performance data across labs and catalyst types. As a whole, this review will help to disseminate and highlight the important relations between structure, composition and stability of OER catalysis under different operating conditions.
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Power-to-Syngas - an enabling technology for the transition of the energy system? Production of tailored synfuels and chemicals using renewably generated electricity. ()
Power-to-X concepts promise a significant reduction of greenhouse gas emissions and simultaneously guaranteeing a safe energy supply even at high share of renewable power generation, thus becoming a cornerstone of a sustainable energy system. Power-to-Syngas, i.e. the electrochemical conversion of steam and carbon dioxide with the use of renewably generated electricity to syngas for the production of synfuels and high-value chemicals, offers an efficient technology to couple different energy-intense sectors, such as 'traffic and transportation' and 'chemical industry'. Consequently, co-electrolysis can be regarded as a key-enabling step for a transition of the energy system that offers additionally features of CO2-valorization and closed carbon cycles. In this Minireview, we outline and discuss advantages and current technical limitations of low- and high-temperature co-electrolysis. Advances in both, a fundamental understanding of the basic reaction schemes and in stable high-performance materials are essential to further promote co-electrolysis.
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Copper-Catalyzed [2+2+2] Modular Synthesis of Multisubstituted Pyridines: Alkenylation of Nitriles with Vinyliodonium Salts ()
A [2+2+2] modular synthesis of multisubstituted pyridines, with excellent regioselectivity, has been realized by copper catalysisand involves three distinct components: vinyliodonium salts, nitriles, and alkynes. The reactions proceeded with the facile formation of an aza-butadienylium intermediate by alkenylation of the nitrile with a vinyliodonium salt. Moreover, the alkynes in the reaction were extended to alkenes, which are an advantage of expense and relative scarceness of alkynes. Three to one! A [2+2+2] modular synthesis of multisubstituted pyridines with excellent regioselectivity has been realized by copper catalysis and involves three distinct components: vinyliodonium salts, nitriles, and alkynes. The reactions proceeded with the facile formation of an aza-butadienylium intermediate by alkenylation of the nitrile with a vinyliodonium salt.
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A DNAzyme Feedback Amplification Strategy for Biosensing ()
We report a signal amplification strategy termed DNAzyme feedback amplification (DFA) that takes advantage of rolling-circle amplification (RCA) and an RNA-cleaving DNAzyme (RCD). DFA employs two specially programmed DNA complexes, one composed of a primer and a circular template containing the antisense sequence of an RCD, and the other composed of the same circular template and an RNA-containing substrate for the RCD. RCA is initiated at the first complex to produce RCD elements that go on to cleave the substrate in the second complex. This cleavage event triggers the production of more input complexes for RCA. This reaction circuit continues autonomously, resulting in exponential DNA amplification. We demonstrate the versatility of this approach for biosensing through the design of DFA systems capable of detecting a microRNA sequence and a bacterium, with sensitivity improvements of 3–6 orders of magnitude over conventional methods. Molecular ping pong: Special DNA assemblies were designed to enable autonomous multistep cyclic actions by a DNA polymerase (Pol) and a DNAzyme (DZ) that can turn a limited number of molecular recognition events into large amounts of DNA amplicon for biosensing applications. Blue=circular DNA template, red=DNA primer, green=RNA-cleaving DNAzyme.
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Chiral Tertiary Sulfonium Salts as Effective Catalysts for Asymmetric Base-Free Neutral Phase-Transfer Reactions ()
Although chiral quaternary ammonium and phosphonium salts are commonly used for asymmetric organocatalysis, the catalytic ability of chiral tertiary sulfonium salts has yet to be demonstrated in asymmetric synthesis. Herein, we show that chiral bifunctional trialkylsulfonium salts catalyze highly enantioselective conjugate additions of 3-substituted oxindoles to maleimides under base-free neutral phase-transfer conditions. Sulfonium catalyst: Whereas chiral quaternary ammonium and phosphonium salts are commonly used for asymmetric organocatalysis, chiral tertiary sulfonium salts have not been employed for such purposes. It is now shown that chiral bifunctional trialkylsulfonium salts catalyze the enantioselective conjugate addition of 3-substituted oxindoles to maleimides under base-free neutral phase-transfer conditions.
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A Photocatalytically Active Lubricant-Impregnated Surface ()
Lubricant impregnated surfaces (LISs) exhibit sliding angles below 5°. A LIS is presented that possesses photocatalytic activity as well as improved liquid repellency. In a single-step reaction, the surface of photocatalytic mesoporous TiO2 substrate is modified by grafting polydimethylsiloxane (PDMS) brush and the residual non-bound PDMS serves as lubricant. Since the lubricant and the hydrophobic layer are chemically identical, the grafting PDMS layer is stably swollen by the lubricant PDMS, which inhibits direct contact of liquid drops to the solid substrate. Liquid drops such as water, methanol, and even low-surface-tension fluorocarbons, slide on the surface with tilt angles below 1°. The surface exhibits long-term stable photocatalytic activity while retaining its liquid repellency. This photocatalytic activity allows photocatalytic chemistry, for example, decomposition of organics, on LIS to be carried out. Can't touch this: A lubricant-impregnated surface with long-term stable photocatalytic activity and improved liquid repellency is presented. The mesoporous TiO2 substrate surface is modified by grafting polydimethylsiloxane (PDMS) brush, and residual non-bound PDMS serves as lubricant. Liquid drops such as water, methanol, and even low-surface-tension fluorocarbons slide on the surface with tilt angles below 1°.
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The Lightest Element Phosphoranylidene: NHC-Supported Cyclic Borylidene–Phosphorane with Significant B=P Character ()
A borylidene–phosphorane, the lightest phosphoranylidene, which is stabilized by an N-heterocyclic carbene ligand, was synthesized and fully characterized. Experimental electron density analysis and DFT calculations indicate an enhanced ylene character (rather than an ylide character) with an exceptionally strong BP π-back bonding related to the less electronegative boron compared to phosphorus. Opposites attract: A phosphorus bora-ylide stabilized by an N-heterocyclic carbene ligand was synthesized. Experimental electron density analysis and DFT calculations indicate enhanced phosphorane character (rather than ylide character) with an exceptionally strong BP π-back bonding compared to related compounds; this is due to the electronegativity difference between boron and phosphorus atoms.
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Discovery and Characterization of a New Family of Diterpene Cyclases in Bacteria and Fungi ()
Diterpene cyclases from bacteria and basidiomycete fungi are seldom studied. Here, we presented the identification and verification of EriG, a member of the UbiA superfamily, as the enzyme responsible for the cyclization of the cyathane skeleton in the mushroom Hericium erinaceum. Genome mining using the EriG protein sequence as a probe led to the discovery of a new family of ubiquitous UbiA-related diterpene cyclases in bacteria and fungi. We successfully characterized seven new diterpene cyclases from bacteria or basidiomycete fungi with the help of an engineered Escherichia coli strain and determined the structures of their corresponding products. A new diterpene with an unusual skeleton was generated during this process. The discovery of this new family of diterpene cyclases provides new insight into the UbiA superfamily. Full circle: EriG, a member of UbiA superfamily, was identified as the enzyme responsible for the cyclization of the cyathane skeleton in the mushroom Hericium erinaceum. Genome mining using the EriG protein sequence led to the identification of a new family of widespread UbiA-related diterpene cyclase genes in bacteria and fungi.
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Biosynthesis of Modular Ascarosides in C. elegans ()
The nematode Caenorhabditis elegans uses simple building blocks from primary metabolism and a strategy of modular assembly to build a great diversity of signaling molecules, the ascarosides, which function as a chemical language in this model organism. In the ascarosides, the dideoxysugar ascarylose serves as a scaffold to which diverse moieties from lipid, amino acid, neurotransmitter, and nucleoside metabolism are attached. However, the mechanisms that underlie the highly specific assembly of ascarosides are not understood. We show that the acyl-CoA synthetase ACS-7, which localizes to lysosome-related organelles, is specifically required for the attachment of different building blocks to the 4′-position of ascr#9. We further show that mutants lacking lysosome-related organelles are defective in the production of all 4′-modified ascarosides, thus identifying the waste disposal system of the cell as a hotspot for ascaroside biosynthesis. From the waste bin: C. elegans uses simple building blocks from primary metabolism to construct a modular library of signaling molecules, the ascarosides. It is demonstrated that lysosome-related organelles, which are essentially cellular waste disposal centers, play a major role in the assembly of these compounds, which requires the activation of building blocks by a specific acyl-CoA synthetase.
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A Reversible Fluorescent Probe for Real-time Quantitative Monitoring of Cellular Glutathione ()
The ability to monitor and quantify glutathione (GSH) in live cells is essential in order to gain a detailed understanding of GSH-related pathological events. However, owing to their irreversible response mechanisms, most existing fluorescent GSH probes are not suitable for this purpose. We have developed a ratiometric fluorescent probe (QG-1) for quantitatively monitoring cellular GSH. The probe responds specifically and reversibility to GSH with an ideal dissociation constant (Kd) of 2.59 mm and a fast response time (t1/2=5.82 s). We also demonstrate that QG-1 detection of GSH is feasible in a model protein system. QG-1 was found to have extremely low cytotoxicity and was applied to determine the GSH concentration in live HeLa cells (5.40±0.87 mm). Given the green light: A ratiometric fluorescent probe (QG-1) for monitoring and quantifying variations in cellular glutathione (GSH) was developed. The probe shows a fluorescence shift from red to green upon binding GSH and exhibits specificity and reversibility, with an appropriate dissociation constant for sensing species with high cellular abundance (Kd=2.59 mm) and a fast response time (t1/2=5.82 s).
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Synthesis of Functionalized Cyclopentene Derivatives from Vinyldiazo Compounds and Vinylazides through Sequential Copper-Promoted [3+2] Cycloaddition/Azide Rearrangement ()
The reaction of vinylazides with alkenyldiazo compounds in the presence of [Cu(CH3CN)4][BF4] provided cyclopentene derivatives with retention of the azide functionality. This process likely involves a sequence comprising: 1) decomposition of the diazo component with generation of a copper alkenylcarbene species; 2) stepwise regioselective [3+2] cycloaddition; 3) allylic azide rearrangement. This method is compatible with a broad range of substrates. We also show that the azide-containing cycloadducts can be efficiently converted into the corresponding amine and triazole derivatives. 2×(vinyl)=[3+2]: A copper-promoted reaction of vinylazides and vinyldiazo compounds provides a convenient route to azide-containing cyclopent-1-ene-carboxylic acid derivatives. This transformation likely involves the initial generation of a copper alkenylcarbene species, which then undergoes a [3+2] cycloaddition/allylic azide rearrangement sequence. Further elaboration of the azide functional group allows the synthesis of new nitrogen-containing carbocycles.
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A Membrane-Intercalating Conjugated Oligoelectrolyte with High-Efficiency Photodynamic Antimicrobial Activity ()
A membrane-intercalating conjugated oligoelectrolyte (COE), PTTP, was designed and synthesized with the goal of providing red-shifted absorption spectra relative to previously synthesized COE analogs. Specifically, electron-rich and electron-poor subunits were introduced in the conjugated backbone to modulate the band gap. PTTP exhibits maxima of absorption at 507 nm and of emission at 725 nm. PTTP can also efficiently function to generate singlet oxygen in situ (ΦΔ≈20 %) and has appropriate topology and dimensions to interact with lipid membranes. The resulting rapid membrane insertion and sensitizing ability provide PTTP with a highly efficient antibacterial capability under a low light dose (0.6 J cm−2) toward Gram-negative bacteria E. coli, making it a remarkably efficient optically mediated antimicrobial agent. Light on, bugs out: A membrane-intercalating conjugated oligoelectrolyte (COE), PTTP, was designed to exhibit red-shifted absorption spectra relative to previously synthesized COEs. PTTP accumulates in the membrane of E. coli and is efficient toward singlet-oxygen photogeneration. This combination of properties gives rise to excellent optically mediated antibacterial capability under low light dosage.
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Charges Shift Protonation: Neutron Diffraction Reveals that Aniline and 2-Aminopyridine Become Protonated Upon Binding to Trypsin ()
Hydrogen atoms play a key role in protein–ligand recognition. They determine the quality of established H-bonding networks and define the protonation of bound ligands. Structural visualization of H atoms by X-ray crystallography is rarely possible. We used neutron diffraction to determine the positions of the hydrogen atoms in the ligands aniline and 2-aminopyridine bound to the archetypical serine protease trypsin. The resulting structures show the best resolution so far achieved for proteins larger than 100 residues and allow an accurate description of the protonation states and interactions with nearby water molecules. Despite its low pKa of 4.6 and a large distance of 3.6 Å to the charged Asp189 at the bottom of the S1 pocket, the amino group of aniline becomes protonated, whereas in 2-aminopyridine, the pyridine nitrogen picks up the proton although its amino group is 1.6 Å closer to Asp189. Therefore, apart from charge–charge distances, tautomer stability is decisive for the resulting binding poses, an aspect that is pivotal for predicting correct binding. Where are the protons? Hydrogen atoms are usually difficult to visualize experimentally but are key for a proper understanding of protein–ligand recognition. Using neutron crystallography, it was found that, despite its low pKa, the amino group of aniline picks up a proton upon binding to the archetypical serine protease trypsin. In contrast, 2-aminopyridine becomes protonated at the pyridine nitrogen atom to give the more stable tautomer of this molecule.
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A Biomimetic Phosphatidylcholine-Terminated Monolayer Greatly Improves the In Vivo Performance of Electrochemical Aptamer-Based Sensors ()
The real-time monitoring of specific analytes in situ in the living body would greatly advance our understanding of physiology and the development of personalized medicine. Because they are continuous (wash-free and reagentless) and are able to work in complex media (e.g., undiluted serum), electrochemical aptamer-based (E-AB) sensors are promising candidates to fill this role. E-AB sensors suffer, however, from often-severe baseline drift when deployed in undiluted whole blood either in vitro or in vivo. We demonstrate that cell-membrane-mimicking phosphatidylcholine (PC)-terminated monolayers improve the performance of E-AB sensors, reducing the baseline drift from around 70 % to just a few percent after several hours in flowing whole blood in vitro. With this improvement comes the ability to deploy E-AB sensors directly in situ in the veins of live animals, achieving micromolar precision over many hours without the use of physical barriers or active drift-correction algorithms. Broadcast live: A biomimetic surface employing phosphatidylcholine head groups (red spheres) greatly improves the baseline stability of electrochemical aptamer-based (E-AB) sensors in whole blood and in live rats, reducing the baseline drift from 70 % to less than 10 % under these challenging conditions. MB=methylene blue, T=target, dark blue ribbon=aptamer, yellow strip=electrode, eT=electron transfer.
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Completely Recyclable Monomers and Polycarbonate: Approach to Sustainable Polymers ()
It is of great significance to depolymerize used or waste polymers to recover the starting monomers suitable for repolymerization reactions that reform recycled materials no different from the virgin polymer. Herein, we report a novel recyclable plastic: degradable polycarbonate synthesized by dinuclear chromium-complex-mediated copolymerization of CO2 with 1-benzyloxycarbonyl-3,4-epoxy pyrrolidine, a meso-epoxide. Notably, the novel polycarbonate with more than 99 % carbonate linkages could be recycled back into the epoxide monomer in quantitative yield under mild reaction conditions. Remarkably, the copolymerization/depolymerization processes can be achieved by the ON/OFF reversible temperature switch, and recycled several times without any change in the epoxide monomer and copolymer. These characteristics accord well with the concept of perfectly sustainable polymers. A sustainable plastic was synthesized by dinuclear chromium-complex-mediated copolymerization of CO2 with an epoxide that could be recycled back into the monomer in quantitative yield under mild conditions. Temperature was used as a reversible switch in the recycle process. These characteristics are in accordance with the concept of perfectly sustainable polymers.
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A Cascade of Redox Reactions Generates Complexity in the Biosynthesis of the Protein Phosphatase-2 Inhibitor Rubratoxin A ()
Redox modifications are key complexity-generating steps in the biosynthesis of natural products. The unique structure of rubratoxin A (1), many of which arise through redox modifications, make it a nanomolar inhibitor of protein phosphatase 2A (PP2A). We identified the biosynthetic pathway of 1 and completely mapped the enzymatic sequence of redox reactions starting from the nonadride 5. Six redox enzymes are involved, including four α-ketoglutarate- and iron(II)-dependent dioxygenases that hydroxylate four sp3 carbons; one flavin-dependent dehydrogenase that is involved in formation of the unsaturated lactone; and the ferric-reductase-like enzyme RbtH, which regioselectively reduces one of the maleic anhydride moieties in rubratoxin B to the γ-hydroxybutenolide that is critical for PP2A inhibition. RbtH is proposed to perform sequential single-electron reductions of the maleic anhydride using electrons derived from NADH and transferred through a ferredoxin and ferredoxin reductase pair. Hit for six: Four α-ketoglutarate-dependent dioxygenases (red) that hydroxylate sp3 carbons, a flavin-dependent dehydrogenase (blue) that forms the unsaturated lactone, and the highly unusual ferric-reductase-like enzyme RbtH (green) are involved in the biosynthesis of rubratoxin A. RbtH regioselectively reduces one of the maleic anhydride moieties in the intermediate rubratoxin B to the γ-hydroxybutenolide group that is critical for the biological activity.
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A Two-Stage Dissociation System for Multilayer Imaging of Cancer Biomarker-Synergic Networks in Single Cells ()
The monitoring of cancer biomarkers is crucial to the early detection of cancer. However, a limiting factor in biomarker analysis is the ability to obtain the multilayered information of various biomarker molecules located at different parts of cells from the plasma membrane to the cytoplasm. A two-stage dissociation nanoparticle system based on multifunctionalized polydopamine-coated gold nanoparticles (Au@PDA NPs) is reported, which allows for the two-stage imaging of cancer biomarkers in single cells. We demonstrate the feasibility of this strategy on sialic acids (SAs), p53 protein, and microRNA-21 (miRNA-21) in MCF-7 breast cancer cells by two custom-built probes. Furthermore, the multicolor fluorescence information extracted is used for the monitoring of biomarker expression changes under different drug combinations, which allows us to investigate the complex interactions between various cancer biomarkers and to describe the cancer biomarker-synergic networks in single cells. A two-stage dissociation system based on multifunctionalized polydopamine-coated gold nanoparticles (AuNPs) for the multilayer imaging and monitoring of various biomarkers and their interactions is described.
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Total Syntheses of Sesterterpenoid Ansellones A and B, and Phorbadione ()
Ansellane-type sesterterpenoids including, ansellones A-G and (+)-phorbadione are structurally novel marine secondary metabolites which exhibit anticancer and anti-HIV activity. The first, asymmetric total syntheses of three structurally representative members, (−)-ansellones A and B and (+)-phorbadione, were accomplished in 16–23 steps from (+)-sclareolide. The route features the first regioselective cyclization of vinyl epoxides with internal alcohol nucleophiles in a 1,4-addition manner (SN2′). Additionally, the allylic C−H oxidation was exploited at a late stage of the synthesis of (−)-ansellone A and (+)-phorbadione. This strategy is expected to be applicable to the synthesis of other ansellane sesterterpenoids. Minding manners: Unusual regioselective cyclization of 1,3-cyclohexadiene epoxides with internal alcohol nucleophiles in a 1,4-addition manner enabled the first asymmetric total syntheses of (−)-ansellones A and B and (+)-phorbadione in 16–23 steps from (+)-sclareolide. This strategy is expected to be applicable to the synthesis of other ansellane sesterterpenoids.
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Erwin Reisner ()

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Increased Synthetic Control—Gaining Access to Predicted Mg2Si5N8 and β-Ca2Si5N8 ()
Nitridosilicates represent an intriguing class of materials and are typically made up of highly condensed tetrahedral network structures. Alkaline-earth nitridosilicates emerged as unique host materials for Eu2+ doped luminophores which found broad application in phosphor-converted (pc)-LEDs. In contrast to common strategies of preparing nitridosilicates by bottom-up syntheses, we have now succeeded to post-synthetically design nitridosilicates by ion exchange in metal halide melts. We describe the syntheses of hitherto unknown but predicted alkaline-earth nitridosilicates, Mg2Si5N8 and β-Ca2Si5N8. Both compounds were obtained by ion exchange starting from pre-synthesized nitridosilicates. In situ investigations of the ion-exchange process show that the Si–N network topology remains preserved. Therefore the reaction offers a significant increase of synthetic control with respect to classical bottom-up syntheses. An exchange is as good as a rest: Nitridosilicates are solid-state materials consisting of cations embedded in highly condensed anionic tetrahedra networks. A new synthetic method enables the exchange of cations while the networks remain preserved. This route gives a significant increase in synthetic control providing the possibility of synthesis planning in nitridosilicate chemistry. The predicted compounds Mg2Si5N8 and β-Ca2Si5N8 were prepared using this route.
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Fluorescent Analogs of Biomolecular Building Blocks. Design and Applications Edited by Marcus Wilhelmsson and Yitzhak Tor. ()
John Wiley and Sons, Hoboken 2016. 448 pp., hardcover, € 182.00.—ISBN 978-1118175866
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Insight into Ion Transfer through the Sub-Nanometer Channels in Zeolitic Imidazolate Frameworks ()
A crack-free sub-nanometer composite structure for the study of ion transfer was constructed by in situ growth of ZIF-90 [Zn(ICA)2, ICA=Imidazole-2-carboxaldehyde] on the tip of a glass nanopipette. The potential-driven ion transfer through the sub-nanometer channels in ZIF-90 is strongly influenced by the pH of the solution. A rectification ratio over 500 is observed in 1 m KCl solution under alkaline conditions (pH 11.58), which is the highest value reported under such a high salt concentration. Fluorescence experiments show the super-high rectification ratio under alkaline conditions results from the strong electrostatic interaction between ions and the sub-nanometer channels of ZIF-90. In addition to providing a general pathway for further study of mass-transfer process through sub-nanometer channels, the approach enable all kinds of metal–organic frameworks (MOFs) to be used as ionic permselectivity materials in nanopore-based analysis. Tipping the balance: By coating the tip of a nanopipette with crystalline zeolitic imidazolate framework-90 (ZIF-90), super high ionic rectification was detected. The rectification results from the interaction between the sub-nanometer channels of the glass-nanopipette-supported ZIF-90 (GNS-ZIF-90) and the ions in solution.
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Oxygen-Incorporated Amorphous Cobalt Sulfide Porous Nanocubes as High-Activity Electrocatalysts for the Oxygen Evolution Reaction in an Alkaline/Neutral Medium ()
A novel OER electrocatalyst, namely oxygen-incorporated amorphous cobalt sulfide porous nanocubes (A-CoS4.6O0.6 PNCs), show advantages over the benchmark RuO2 catalyst in alkaline/neutral medium. Experiments combining with calculation demonstrate that the desirable O* adsorption energy, associated with the distorted CoS4.6O0.6 octahedron structure and the oxygen doping, contribute synergistically to the outstanding electrocatalytic activity. Oxygen incorporated amorphous cobalt sulfide porous nanocubes, benefiting from Co−S dangling bands in the distorted CoS4.6O0.6 octahedral structure, as well as oxygen incorporation into CoSx hosts, show outstanding electrocatalytic activity for water oxidation in alkaline and neutral electrolytes. vac=vacancy; Co blue, S yellow, O red, H white.
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para-Selective C−H Borylation of (Hetero)Arenes by Cooperative Iridium/Aluminum Catalysis ()
para-Selective C−H borylation of benzamides and pyridines has been achieved by cooperative iridium/aluminum catalysis. A combination of iridium catalysts commonly employed for arene C−H borylation and bulky aluminum-based Lewis acid catalysts provides an unprecedented strategy for controlling the regioselectivity of C−H borylation to give variously substituted (hetero)arylboronates, which are versatile synthetic intermediates for complex multi-substituted aromatic compounds. Collect. Select. Reflect: para-Selective C−H borylation of benzamide and pyridine adducts is controlled by a combination of iridium and bulky aluminum-based Lewis acid catalysts. Variously substituted (hetero)arylboronates were prepared, which are versatile synthetic intermediates for complex multi-substituted aromatic compounds.
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Boosting the Performance of the Nickel Anode in the Oxygen Evolution Reaction by Simple Electrochemical Activation ()
The development of cost-effective and active water-splitting electrocatalysts that work at mild pH is an essential step towards the realization of sustainable energy and material circulation in our society. Its success requires a drastic improvement in the kinetics of the anodic half-reaction of the oxygen evolution reaction (OER), which determines the overall system efficiency to a large extent. A simple electrochemical protocol has been developed to activate Ni electrodes, by which a stable NiOOH phase was formed, which could weakly bind to alkali-metal cations. The electrochemically activated (ECA) Ni electrode reached a current of 10 mA at <1.40 V vs. the reversible hydrogen electrode (RHE) at practical operation temperatures (>75 °C) and a mild pH of ca. 10 with excellent stability (>24 h), greatly surpassing that of the state-of-the-art NiFeOx electrodes under analogous conditions. Water electrolysis was demonstrated with ECA-Ni and NiMo, which required an iR-free overall voltage of only 1.44 V to reach 10 mA cmgeo−2. Latest upgrade: After electrochemical activation, a nickel electrocatalyst showed enhanced performance in the oxygen evolution reaction. The new material surpasses the state-of-the-art NiFeOx electrode at practical temperatures and a moderate pH of 10.5.
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Boosting the Energy Density of Carbon-Based Aqueous Supercapacitors by Optimizing the Surface Charge ()
The voltage of carbon-based aqueous supercapacitors is limited by the water splitting reaction occurring in one electrode, generally resulting in the promising but unused potential range of the other electrode. Exploiting this unused potential range provides the possibility for further boosting their energy density. An efficient surface charge control strategy was developed to remarkably enhance the energy density of multiscale porous carbon (MSPC) based aqueous symmetric supercapacitors (SSCs) by controllably tuning the operating potential range of MSPC electrodes. The operating voltage of the SSCs with neutral electrolyte was significantly expanded from 1.4 V to 1.8 V after simple adjustment, enabling the energy density of the optimized SSCs reached twice as much as the original. Such a facile strategy was also demonstrated for the aqueous SSCs with acidic and alkaline electrolytes, and is believed to bring insight in the design of aqueous supercapacitors. AquaMSPC: An efficient surface charge control strategy was developed to remarkably enhance the energy density of multiscale porous carbon-based aqueous symmetric supercapacitors by controllably tuning the operating potential range of MSPC electrodes.
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Highly Selective Synthesis of cis-Enediyne on a Ag(111) Surface ()
Cis-enediyne-type compounds have received much attention as potent antitumor antibiotics. The conventional synthesis of cis-enediynes in solution typically involves multiple steps and various side reactions. For the first time, selective one-step synthesis of cis-enediyne from a single reactant is reported on a Ag(111) surface with a yield up to 90 %. High selectivity for the formation of cis-enediyne originates from the steric effect posed by weak intermolecular interactions, which protect the cis-enediyne from further reaction. A series of comparative experiments and DFT-based transition-state calculations support the findings. The described synthetic approach for directing reaction pathways on-surface may illuminate potential syntheses of other unstable organic compounds. Selective on-surface synthesis of cis-enediyne-type compounds is reported for the first time. Bromide-substituted terminal alkynes transform into cis-enediynes on a Ag(111) surface at 420 K. Coadsorbed bromide atoms help the formed cis-enediynes aggregate into close-packed islands through Br⋅⋅⋅H bonds, which impose a high steric barrier to further reaction of cis-enediyne.
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Enantioselective Palladium-Catalyzed Carbonylative Carbocyclization of Enallenes via Cross-Dehydrogenative Coupling with Terminal Alkynes: Efficient Construction of α-Chirality of Ketones ()
An enantioselective PdII/Brønsted acid-catalyzed carbonylative carbocyclization of enallenes ending with a cross-dehydrogenative coupling (CDC) with a terminal alkyne was developed. VAPOL phosphoric acid was found as the best co-catalyst among the examined 28 chiral acids, for inducing the enantioselectivity of α-chiral ketones. As a result, a number of chiral cyclopentenones were easily synthesized in good to excellent enantiomeric ratio with good yields. From CO to ketones with the introduction of α-chirality: A PdII/Brønsted acid-catalyzed enantioselective carbonylation–carbocyclization of enallenes has been developed, proceeding through an efficient CO and olefin insertion cascade to yield ketones with α-chirality. A number of chiral cyclopentenones could be easily synthesized in good to excellent enantioselectivity and good yields.
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Steering On-Surface Reactions by a Self-Assembly Approach ()
4,4′-Bis(2,6-difluoropyridin-4-yl)-1,1′:4′,1′′-terphenyl (BDFPTP) molecules underwent dehydrocyclization and covalent coupling reactions on Au(111) according to scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations. Self-assembly of the reactants in well-defined molecular domains prior to reaction could greatly enhance the regioselectivity of the dehydrocyclization reaction and suppress defluorinated coupling, demonstrating that self-assembly can efficiently steer on-surface reactions. Such a strategy could be of great importance in surface chemistry and widely applied to control on-surface reactions. Surface-enhanced selectivity: Randomly distributed 4,4′-bis(2,6-difluoropyridin-4-yl)-1,1′:4′,1′′-terphenyl (BDFPTP) undergoes both dehydrocyclization (DHC) and coupling reactions on Au(111). However, the DHC regioselectivity is greatly enhanced and the coupling reaction is completely suppressed when BDFPTP self-assembles on the surface prior to its reactions.
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Palladium-Catalyzed Enantioselective Synthesis of 2-Aryl Cyclohex-2-enone Atropisomers: Platform Molecules for the Divergent Synthesis of Axially Chiral Biaryl Compounds ()
The palladium-catalyzed asymmetric synthesis of enone-based atropisomers from 2-iodo-3-methylcyclohex-2-enones and aryl boronic acid is reported. BoPhoz-type phosphine–aminophosphine ligands showed superior enantioselectivity over other ligands. These cyclohexenone-based atropisomers are useful compounds for further elaboration. The divergent synthesis of biaryl atropisomers with different ortho substituents was demonstrated. A platform to improve the outlook: The palladium-catalyzed asymmetric coupling of 2-iodo-3-methylcyclohex-2-enones and aryl boronic acids provided axially chiral enone–arene systems with several potential sites for further transformation (see scheme). In particular, these cyclohexenone-based atropisomers serve as a valuable platform for the synthesis of biaryl atropisomers with different ortho substituents.
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Chiral Amino Alcohol Accelerated and Stereocontrolled Allylboration of Iminoisatins: Highly Efficient Construction of Adjacent Quaternary Stereogenic Centers ()
We have developed a highly efficient asymmetric allylboration of ketimines with nonchiral γ,γ-disubstituted allylboronic acids by using a chiral amino alcohol as the directing group, which is otherwise challenging. The amino alcohol not only serves as a cheap source of nitrogen and chirality, but also dramatically enhances the reactivity. The versatility of this method was demonstrated by its ability to access all four stereoisomers with adjacent quaternary carbon centers. A reaction model was proposed to explain the diastereoselectivity and the rate-accelerating effect. Collect the full set of four! A chiral amino alcohol was used as the directing group for the asymmetric allylboration of ketimines with nonchiral γ,γ-disubstituted allylboronic acids (see scheme). The amino alcohol not only serves as a cheap source of nitrogen and chirality, but also dramatically enhances reactivity. This versatile method can be used to access all four stereoisomers of a product with respect to the adjacent quaternary carbon centers.
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Engineering High-Energy Interfacial Structures for High-Performance Oxygen-Involving Electrocatalysis ()
Engineering high-energy interfacial structures for high-performance electrocatalysis is achieved by chemical coupling of active CoO nanoclusters and high-index facet Mn3O4 nano-octahedrons (hi-Mn3O4). A thorough characterization, including synchrotron-based near edge X-ray absorption fine structure, reveals that strong interactions between both components promote the formation of high-energy interfacial Mn-O-Co species and high oxidation state CoO, from which electrons are drawn by MnIII-O present in hi-Mn3O4. The CoO/hi-Mn3O4 demonstrates an excellent catalytic performance over the conventional metal oxide-based electrocatalysts, which is reflected by 1.2 times higher oxygen evolution reaction (OER) activity than that of Ru/C and a comparable oxygen reduction reaction (ORR) activity to that of Pt/C as well as a better stability than that of Ru/C (95 % vs. 81 % retained OER activity) and Pt/C (92 % vs. 78 % retained ORR activity after 10 h running) in alkaline electrolyte. Nanocatalysis: Engineering high-energy interfacial structures is achieved by chemical coupling of active CoO nanoclusters with high-index facet Mn3O4 nano-octahedrons. The structures display synergistic effects enhancing their electrocatalytic performance and stability as compared to conventional electrocatalysts for bifunctional oxygen-involving reactions.
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Elemental Boron for Efficient Carbon Dioxide Reduction under Light Irradiation ()
The photoreduction of CO2 is attractive for the production of renewable fuels and the mitigation of global warming. Herein, we report an efficient method for CO2 reduction over elemental boron catalysts in the presence of only water and light irradiation through a photothermocatalytic process. Owing to its high solar-light absorption and effective photothermal conversion, the illuminated boron catalyst experiences remarkable self-heating. This process favors CO2 activation and also induces localized boron hydrolysis to in situ produce H2 as an active proton source and electron donor for CO2 reduction as well as boron oxides as promoters of CO2 adsorption. These synergistic effects, in combination with the unique catalytic properties of boron, are proposed to account for the efficiency of the CO2 reduction. This study highlights the promise of photothermocatalytic strategies for CO2 conversion and also opens new avenues towards the development of related solar-energy utilization schemes. Four in one: Elemental boron is an efficient catalyst for direct CO2 reduction into CO and CH4 in the presence of water under light irradiation through a one-step photothermocatalytic process. The elemental boron material harvests the incident light, converts it into thermal energy, generates hydrogen, and catalyzes the overall process.
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Synthesis of Rationally Halogenated Buckybowls by Chemoselective Aromatic C−F Bond Activation ()
Halogenated buckybowls or bowl-shaped polycyclic aromatic hydrocarbons (BS-PAHs) are key building blocks for the “bottom-up” synthesis of various carbon-based nanomaterials with outstanding potential in different fields of technology. The current state of the art provides quite a limited number of synthetic pathways to BS-PAHs; moreover, none of these approaches show high selectivity and tolerance of functional groups. Herein we demonstrate an effective route to BS-PAHs that includes directed intramolecular aryl–aryl coupling through C−F bond activation. The coupling conditions were found to be completely tolerant toward aromatic C−Br and C−Cl bonds, thus allowing the facile synthesis of rationally halogenated buckybowls with an unprecedented level of selectivity. This finding opens the way to functionalized BS-PAH systems that cannot be obtained by alternative methods. Efficiency that will bowl you over: The activation of aromatic C−F bonds in the presence of more labile C−Br and C−Cl bonds enabled the fully controlled synthesis of halogenated bowl-shaped polycyclic aromatic hydrocarbons through intramolecular aryl–aryl coupling (see picture). Besides its simplicity and high reproducibility, the technique provides access to halogenated bowl-shaped systems that are not accessible by other methods.
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Synthesis of Poly(arylene vinylene)s with Different End Groups by Combining Acyclic Diene Metathesis Polymerization with Wittig-type Couplings ()
A series of end-functionalized poly(9,9′-di-n-octylfluorene vinylene)s (EF-PFVs) with different end groups were obtained by 1) synthesizing EF-PFV with vinyl end groups by acyclic diene metathesis (ADMET) polymerization with a molybdenum catalyst and termination with an aldehyde and 2) subsequent olefin metathesis of the vinyl group with the molybdenum catalyst followed by Wittig-type coupling with another aldehyde. The exclusive formation of EF-PFVs containing a vinyl end group by the ADMET polymerization was confirmed by grafting PEG, and by the synthesis of amphiphilic triblock copolymers by combining atom transfer radical polymerization from the PFV chain end with PEG grafting through a click reaction. Various EF-PFVs with different end groups, such as C6F5, pyridyl, ferrocenyl, and terthiophene, have thus been prepared. Their fluorescence spectra (e.g., intensities, emission wavelengths) were influenced by the end groups and the length of the conjugation. Which is the end? Conjugated polymers based on poly(9,9′-di-n-octylfluorene vinylene) with different end groups were synthesized by combining molybdenum-catalyzed acyclic diene metathesis polymerization with a subsequent Wittig-type coupling. Their fluorescence spectra were influenced by the end groups and the conjugation length.
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Iron(II)-Catalyzed Hydrophosphination of Isocyanates ()
The first transition metal catalyzed hydrophosphination of isocyanates is presented. The use of low-coordinate iron(II) precatalysts leads to an unprecedented catalytic double insertion of isocyanates into the P−H bond of diphenylphosphine to yield phosphinodicarboxamides [Ph2PC(=O)N(R)C(=O)N(H)R], a new family of derivatized organophosphorus compounds. This remarkable result can be attributed to the low-coordinate nature of the iron(II) centers whose inherent electron deficiency enables a Lewis-acid mechanism in which a combination of the steric pocket of the metal center and substrate size determines the reaction products and regioselectivity. Seeing double: Low-coordinate iron(II) complexes have been used in the hydrophosphination of isocyanates to produce mono- and/or diinsertion products yielding phosphinodicarboxamides, a new family of derivatized organophosphorus compounds. Small changes in reaction conditions drastically alter the product selectivity.
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Photochromism and Dual-Color Fluorescence in a Polyoxometalate–Benzospiropyran Molecular Switch ()
The photophysical properties of a Keggin-type polyoxometalate (POM) covalently bounded to a benzospiropyran (BSPR) unit have been investigated. These studies reveal that both closed and open forms are emissive with distinct spectral features (λem (closed form)=530 nm, λem (open form)=670 nm) and that the fluorescence of the BSPR unit of the hybrid is considerably enhanced compared to BSPR parent compounds. While the fluorescence excitation energy of the BSPR reference compounds (370 nm) is close to the intense absorption responsible of the photochromic character (350 nm), the fluorescence excitation of the hybrid is shifted to lower energy (400 nm), improving the population of the emissive state. Combined NOESY NMR and theoretical calculations of the closed form of the hybrid give an intimate understanding of the conformation adopted by the hybrid and show that the nitroaryl moieties of the BSPR is folded toward the POM, which should affect the electronic properties of the BSPR. Small cause, big effect: A covalent polyoxometalate (POM)–benzospiropyran (BSPR) dyad is an unprecedented example of an organic–inorganic molecular switch that displays specific fluorescence feature in each photochromic form. The fluorescence of the BSPR unit of the POM-based hybrid is considerably enhanced compared to BSPR reference compounds attributed to a more easily accessible emitting state in the hybrid.
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Platinum(II)-Crosslinked Single-Chain Nanoparticles: An Approach towards Recyclable Homogeneous Catalysts ()
Recyclable platinum-containing single-chain nanoparticles (SCNPs) retain their form and function when used in platinum-based homogeneous catalysis, paving the way as a new class of advanced catalytic systems. In their Communication (10.1002/anie.201700718) C. Barner-Kowollik, P. W. Roesky, and co-workers show using the amination of allyl alcohol that the novel platinum(II)-SCNP catalyst combines the advantages of homogeneous activity with heterogeneous recyclability.
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A Sodium-Ion-Conducting Direct Formate Fuel Cell: Generating Electricity and Producing Base ()
A barrier that limits the development of the conventional cation-exchange membrane direct liquid fuel cells (CEM-DLFCs) is that the CEM-DLFCs need additional base to offer both alkaline environment and charge carriers. Herein, we propose a Na+-conducting direct formate fuel cell (Na-DFFC) that is operated in the absence of added base. A proof-of-concept Na-DFFC yields a peak power density of 33 mW cm−2 at 60 °C, mainly because the hydrolysis of sodium formate provides enough OH− and Na+ ions, proving the conceptual feasibility. Moreover, contrary to the conventional chlor-alkali process, this Na-DFFC enables to generate electricity and produce NaOH simultaneously without polluting the environment. The Na-DFFC runs stably during 13 hours of continuous operation at a constant current of 10 mA, along with a theoretical production of 195 mg NaOH. This work presents a new type of electrochemical conversion device that possesses a wide range of potential applications. Electrochemical energy conversion: A Na+-conducting direct formate fuel cell (Na-DFFC) was successfully developed and operated without adding OH− and Na+ ions. Contrary to the conventional chlor-alkali process, the Na-DFFC enables to generate electricity and produce base simultaneously without polluting the environment.
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Accelerated Oxygen Atom Transfer and C−H Bond Oxygenation by Remote Redox Changes in Fe3Mn-Iodosobenzene Adducts ()
We report the synthesis, characterization, and reactivity of [LFe3(PhPz)3OMn(sPhIO)][OTf]x (3: x=2; 4: x=3), where 4 is one of very few examples of iodosobenzene–metal adducts characterized by X-ray crystallography. Access to these rare heterometallic clusters enabled differentiation of the metal centers involved in oxygen atom transfer (Mn) or redox modulation (Fe). Specifically, 57Fe Mössbauer and X-ray absorption spectroscopy provided unique insights into how changes in oxidation state (FeIII2FeIIMnII vs. FeIII3MnII) influence oxygen atom transfer in tetranuclear Fe3Mn clusters. In particular, a one-electron redox change at a distal metal site leads to a change in oxygen atom transfer reactivity by ca. two orders of magnitude. Rare finding: Iodosobenzene adducts LFe3(PhPz)3OMn(sPhIO)][OTf]x (x=2 or 3; PhPz=phenylpyrazolate) on multinuclear scaffolds were synthesized and characterized. The reactivity of these rare adducts was investigated in light of remote redox state changes in a triiron core (FeIII2FeIIMnII vs. FeIII3MnII), highlighting significant reactivity differences (>102) in oxygen atom transfer and C−H bond oxygenation.
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Synthesis of Complex Phenols Enabled by a Rationally Designed Hydroxide Surrogate ()
The conversion of aryl halides to phenols under mild reaction conditions is a longstanding and formidable challenge in organic chemistry. Herein, we report the rational design of a broadly applicable Pd-catalyzed method to prepare phenols with benzaldehyde oxime as a hydroxide surrogate. These reactions occur under mildly basic conditions and enable the late-stage hydroxylation of several functionally-dense drug-like aryl halides. The conversion of aryl halides to phenols under mild reaction conditions is a longstanding and formidable challenge in organic chemistry. The rational design of a broadly applicable Pd-catalyzed method to prepare phenols with benzaldehyde oxime as a hydroxide surrogate is now reported. These reactions occur under mildly basic conditions and enable the late-stage hydroxylation of functionally dense aryl halides.
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Different Structural Origins of the Enantioselectivity of Haloalkane Dehalogenases toward Linear β-Haloalkanes: Open–Solvated versus Occluded–Desolvated Active Sites ()
The enzymatic enantiodiscrimination of linear β-haloalkanes is difficult because the simple structures of the substrates prevent directional interactions. Herein we describe two distinct molecular mechanisms for the enantiodiscrimination of the β-haloalkane 2-bromopentane by haloalkane dehalogenases. Highly enantioselective DbjA has an open, solvent-accessible active site, whereas the engineered enzyme DhaA31 has an occluded and less solvated cavity but shows similar enantioselectivity. The enantioselectivity of DhaA31 arises from steric hindrance imposed by two specific substitutions rather than hydration as in DbjA. Two recipes for success: Two distinct mechanisms are described for the enantiodiscrimination of 2-bromopentane by haloalkane dehalogenases. Highly enantioselective DbjA has a very open, solvent-accessible active site. The engineered enzyme DhaA31 has a more occluded and less solvated cavity (see picture) but shows similar enantioselectivity as a result of steric hindrance imposed by two specific residues, rather than hydration as in DbjA.
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Mutant Exon1 Huntingtin Aggregation is Regulated by T3 Phosphorylation-Induced Structural Changes and Crosstalk between T3 Phosphorylation and Acetylation at K6 ()
Herein, we used protein semisynthesis to investigate, for the first time, the effect of lysine acetylation and phosphorylation, as well as the crosstalk between these modifications on the structure and aggregation of mutant huntingtin exon1 (Httex1). Our results demonstrate that phosphorylation at T3 stabilizes the α-helical conformation of the N-terminal 17 amino acids (Nt17) and significantly inhibits the aggregation of mutant Httex1. Acetylation of single lysine residues, K6, K9 or K15, had no effect on Httex1 aggregation. Interestingly, acetylation at K6, but not at K9 or K15, reversed the inhibitory effect of T3 phosphorylation. Together, our results provide novel insight into the role of Nt17 post-translational modifications in regulating the structure and aggregation of Httex1 and suggest that its aggregation and possibly its function(s) are controlled by regulatory mechanisms involving crosstalk between different PTMs. Protein semisynthesis was used to investigate the effect of lysine acetylation and phosphorylation, as well as the crosstalk between these modifications on the structure and aggregation of mutant huntingtin exon1 (Httex1). While phosphorylation significantly inhibits the aggregation of Httext1, acetylation has no effect. However, acetylation at K6 reverses the inhibitory effect of T3 phosphorylation.
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Zirconacyclopentadiene-Annulated Polycyclic Aromatic Hydrocarbons ()
Syntheses of large polycyclic aromatic hydrocarbons (PAHs) and graphene nanostructures demand methods that are capable of selectively and efficiently fusing large numbers of aromatic rings, yet such methods remain scarce. Herein, we report a new approach that is based on the quantitative intramolecular reductive cyclization of an oligo(diyne) with a low-valent zirconocene reagent, which gives a PAH with one or more annulated zirconacyclopentadienes (ZrPAHs). The efficiency of this process is demonstrated by a high-yielding fivefold intramolecular coupling to form a helical ZrPAH with 16 fused rings (from a precursor with no fused rings). Several other PAH topologies are also reported. Protodemetalation of the ZrPAHs allowed full characterization (including by X-ray crystallography) of PAHs containing one or more appended dienes with the ortho-quinodimethane (o-QDM) structure, which are usually too reactive for isolation and are potentially valuable for the fusion of additional rings by Diels–Alder reactions. Zirconocene zipper: An efficient fivefold reductive cyclization demonstrates the promise of a new strategy for the construction of graphene nanostructures. Protodemetalation of the resulting zirconacyclopentadiene-annulated polycyclic aromatic hydrocarbons provides a conceptually unique means for the generation of valuable ortho-quinodimethane structures.
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Multi-Shelled Hollow Metal–Organic Frameworks ()
Hollow metal–organic frameworks (MOFs) are promising materials with sophisticated structures, such as multiple shells, that cannot only enhance the properties of MOFs but also endow them with new functions. Herein, we show a rational strategy to fabricate multi-shelled hollow chromium (III) terephthalate MOFs (MIL-101) with single-crystalline shells through step-by-step crystal growth and subsequent etching processes. This strategy relies on the creation of inhomogeneous MOF crystals in which the outer layer is chemically more robust than the inner layer and can be selectively etched by acetic acid. The regulation of MOF nucleation and crystallization allows the tailoring of the cavity size and shell thickness of each layer. The resultant multi-shelled hollow MIL-101 crystals show significantly enhanced catalytic activity during styrene oxidation. The insight gained from this systematic study will aid in the rational design and synthesis of other multi-shelled hollow structures and the further expansion of their applications. Promising MOF structures: Single-crystalline multi-shelled hollow metal–organic frameworks (MSHMs) were synthesized through step-by-step crystal growth and subsequent etching processes. The cavity size and shell thickness of each layer in the MSHMs was regulated through careful nucleation and crystallization of the metal–organic frameworks. The MSHM crystals show significantly increased catalytic activity.
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Signal-Induced Release of Guests from a Photolatent Metal–Phenolic Supramolecular Cage and Its Hybrid Assemblies ()
The coordination chemistry of plant polyphenols and metal ions can be used for coating various substrates and for creating modular superstructures. We herein explored this chemistry for the controlled release of guests from mesoporous silica nanoparticles (MSNs). The selective adsorption of tannic acids (TAs) on MSN silica walls opens the MSN mesoporous channels without disturbing mass transport. The channel may be closed by the coordination of TA with CuII ions. Upon exposure to light, photolysis of Trojan horse guests (photoacid generators, PAGs) leads to acid generation, which enables the release of payloads by decomposing the outer coordination shell consisting of TA and CuII. We also fabricated a modular assembly of MSNs on glass substrates. The photoresponsive release characteristics of the resulting film are similar to those of the individual MSNs. This method is a fast and facile strategy for producing photoresponsive nanocontainers by non-covalent engineering of MSN surfaces that should be suitable for various applications in materials science. The unique adsorption and coordination characteristics of tannic acid were made use of to fabricate a photolatent supramolecular cage. Mesoporous silica nanoparticles that had been loaded with a photoacid generator and guest molecules were wrapped with a shell of tannic acid and CuII ions. Photoresponsive hybrid assemblies that can release guest molecules on demand were obtained.
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Unravelling a Direct Role for Polysaccharide β-Strands in the Higher Order Structure of Physical Hydrogels ()
The mechanical properties of agarose-derived hydrogels depend on the scaffolding of the polysaccharide network. To identify and quantify such higher order structure, we applied Raman optical activity (ROA)—a spectroscopic technique that is highly sensitive toward carbohydrates—on native agarose and chemically modified agarose in the gel phase for the first time. By spectral global fitting, we isolated features that change as a function of backbone carboxylation (28, 40, 50, 60, 80, and 93 %) from other features that remain unchanged. We assigned these spectral features by comparison to ROA spectra calculated for different oligomer models. We found a 60:40 ratio of double- and single-stranded α-helix in the highly rigid hydrogel of native agarose, while the considerably softer hydrogels made from carboxylated agarose use a scaffold of unpaired β-strands. Agarose hydrogels are stiff but chemical carboxylation of the polysaccharide backbone leads to formation of soft gels. Carboxyl groups prevent interchain organization of the polysaccharide into double helices, thereby promoting a β-strand conformation instead. Raman optical activity was used to identify and quantify the higher order polysaccharide structure directly in the gel phase.
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Germabenzenylpotassium: A Germanium Analogue of a Phenyl Anion ()
A germanium analogue of the phenyl anion was synthesized by the reduction of a stable germabenzene with KC8, under concomitant elimination of the aryl group from the Ge atom. In their Communication (10.1002/anie.201700801), N. Tokitoh and co-workers show its ambident character with contributions from aromatic and germylene resonance structures, which was supported experimentally and theoretically.
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Catalytic Asymmetric N-Alkylation of Indoles and Carbazoles through 1,6-Conjugate Addition of Aza-para-quinone Methides ()
Catalytic asymmetric N-alkylation of indoles and carbazoles represents a family of important but underdeveloped reactions. Herein, we describe a new organocatalytic strategy in which in situ generated aza-para-quinone methides are employed as the alkylating reagent. With the proper choice of a chiral phosphoric acid and an N-protective group, the intermolecular C−N bond formation with various indole and carbazole nucleophiles proceeded efficiently under mild conditions with excellent enantioselectivity and functional-group compatibility. Control experiments and kinetic studies provided important insight into the reaction mechanism. An organocatalytic strategy for the asymmetric N-alkylation of indoles and carbazoles was developed in which in situ generated aza-para-quinone methides are employed as the alkylating reagent. With the proper choice of a chiral phosphoric acid and N-protective group (P), the intermolecular C−N bond formation with various indole and carbazole nucleophiles proceeds under mild conditions with excellent efficiency and enantioselectivity.
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Guest-induced Assembly of Bis(thiosemicarbazonato) Zinc(II) Coordination Nanotubes ()
A ZnII complex of the dianionic tetradentate ligand formed by deprotonation of glyoxal-bis(4-phenyl-3-thiosemicarbazone) (H2gtsp) is a [3+3] trinuclear triangular prism. Recrystallization of this complex in the presence of either CO2, CS2, or CH3CN leads to the formation of [4+4] open-ended charge-neutral tetranuclear coordination nanotubes, approximately 2 nm in length and with internal dimensions large enough to accommodate linear guest molecules, which serve to template their formation. Upon removal of the templating molecules the nanotubes demonstrated reversible sorption of CO2 with an isosteric enthalpy of sorption of 28 kJ mol−1 at low loading. From three to four: Flexible dianionic bridging ligands and zinc(II) centers form either charge-neutral triangular prisms or—in the presence of linear templating molecules, CO2, CS2, and CH3CN—charge-neutral square coordination nanotubes with the templating agent included as a guest. The template can be removed from the inclusion complex while maintaining a porous material capable of adsorbing roughly two molecules of CO2 per nanotube.
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Iridium-Catalyzed Asymmetric Hydrogenation of Benzo[b]thiophene 1,1-Dioxides ()
An efficient iridium-catalyzed asymmetric hydrogenation of substituted benzothiophene 1,1-dioxides is described. The use of iridium complexes with chiral pyridyl phosphinite ligands provides access to highly enantiomerically enriched sulfones with substituents at the 2- and 3-position. Sulfones of this type are of interest as core structures of agrochemicals and pharmaceuticals. Moreover, they can be further reduced to chiral 2,3-dihydrobenzothiophenes. (Sul)fone a friend: The iridium-catalyzed asymmetric hydrogenation of benzothiophene 1,1-dioxides provided access to highly enantiomerically enriched sulfones with substituents at the 2- and 3-position (see scheme). Sulfones of this type are of interest as core structures of agrochemicals and pharmaceuticals. They can also be further reduced to chiral 2,3-dihydrobenzothiophenes, which are not always directly accessible by asymmetric hydrogenation.
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Highly Branched Metal Alloy Networks with Superior Activities for the Methanol Oxidation Reaction ()
Three-dimensional (3D) interconnected metal alloy nanostructures possess superior catalytic performance owing to their advantageous characteristics, including improved catalytic activity, corrosion resistance, and stability. Hierarchically structured Ni-Cu alloys composed of 3D network-like microscopic branches with nanoscopic dendritic feelers on each branch were crafted by a facile and efficient hydrogen evolution-assisted electrodeposition approach. They were subsequently exploited for methanol electrooxidation in alkaline media. Among three hierarchically structured Ni-Cu alloys with different Ni/Cu ratios (Ni0.25Cu0.75, Ni0.50Cu0.50, and Ni0.75Cu0.25), the Ni0.75Cu0.25 electrode exhibited the fastest electrochemical response and highest electrocatalytic activity toward methanol oxidation. The markedly enhanced performance of Ni0.75Cu0.25 eletrocatalyst can be attributed to its alloyed structure with the proper Ni/Cu ratio and a large number of active sites on the surface of hierarchical structures. Hierarchically structured Ni-Cu alloys composed of 3D network-like microscopic branches with nanoscopic dendritic feelers on each branch were crafted by a facile and efficient hydrogen evolution-assisted electrodeposition approach. The resultant structures showed excellent electrochemical response and electrocatalytic activity towards methanol oxidation.
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Synthesis and Properties of Aza-ullazines ()
A range of aza-ullazines, which represent a new heterocyclic core structure, were synthesized through a scalable four-step reaction, including a Sonogashira reaction and metal-free cyclization promoted by p-toluenesulfonic acid. The optical and electrochemical properties of selected derivatives were investigated, they were found to have similar absorption and emission spectra but a higher oxidation potential than the parent ullazine core. Ring, ring: Aza-ullazines, which represent a new heterocyclic core structure, were synthesized through a four-step reaction, including a Sonogashira reaction and metal-free cyclization promoted by p-toluenesulfonic acid. The optical and electrochemical properties of selected derivatives were investigated, they were found to have similar absorption and emission spectra but a higher oxidation potential than the parent ullazine core.
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Fast and Minimal-Solvent Production of Superinsulating Silica Aerogel Granulate ()
With their low thermal conductivity (λ), silica aerogels can reduce carbon emissions from heating and cooling demands, but their widespread adoption is limited by the high production cost. A one-pot synthesis for silica aerogel granulate is presented that drastically reduces solvent use, production time, and global warming potential. The inclusion of the hydrophobization agent prior to gelation with a post-gelation activation step, enables a complete production cycle of less than four hours at the lab scale for a solvent use close to the theoretical minimum, and limits the global warming potential. Importantly, the one-pot aerogel granulate retains the exceptional properties associated with silica aerogel, mostly λ=14.4±1.0 mW m−1⋅K−1 for the pilot scale materials, about half that of standing air (26 mW m−1⋅K−1). The resource-, time-, and cost-effective production will allow silica aerogels to break out of its niche into the mainstream building and industrial insulation markets. Solving the solvent problem: Silica aerogels are ideal thermal insulation materials but are expensive owing to the large solvent consumption during production. A simplified synthesis combines all reagents in a single step and minimizes production time, solvent use, and carbon footprint, but retains the outstanding properties associated with silica aerogels.
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Detection of Carbapenemase-Producing Organisms with a Carbapenem-Based Fluorogenic Probe ()
Antibiotic resistance has become a major challenge to public health worldwide. This crisis is further aggravated by the increasing emergence of bacterial resistance to carbapenems, typically considered as the antibiotics of last resort, which is mainly due to the production of carbapenem-hydrolyzing carbapenemases in bacteria. Herein, the carbapenem-based fluorogenic probe CB-1 with an unprecedented enamine–BODIPY switch is developed for the detection of carbapenemase activity. This reagent is remarkably specific towards carbapenemases over other prevalent β-lactamases. Furthermore, the efficient imaging of live clinically important carbapenemase-producing organisms (CPOs) with CB-1 demonstrates its potential for the rapid detection of antibiotic resistance and timely diagnosis of CPO infections. Lighting up resistance: A fluorogenic probe for the detection of carbapenemase activity has been developed. The utilization of carbapenem as the enzyme recognition motif in this reagent leads to remarkable specificity to carbapenemases over other prevailing β-lactamases, enabling the rapid detection of carbapenemases.
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Bioinspired Asymmetric Synthesis of Hispidanin A ()
The first enantiospecific synthesis of hispidanin A (4), a dimeric diterpenoid from the rhizomes of Isodon hispida, was achieved with a longest linear sequence of 12 steps in 6.5 % overall yield. A key component is the use of the abundant and naturally occurring diterpenoids (+)-sclareolide and (+)-sclareol as starting materials, which enables the gram-scale preparation of the key intermediates totarane (1) and s-trans-12E,14-labdadien-20,8β-olide (2). Subsequently a thermal or an erbium-catalyzed intermolecular Diels–Alder reaction of totarane (1) with labdadienolide (2) provide convergent and rapid access to the natural product hispidanin A (4). The synthetic studies have offered significant impetus for the efficient construction of these architecturally complex natural products. When two become one: An enantiospecific synthesis of hispidanin A, a dimeric diterpenoid from Isodon hispida, was achieved with a longest linear sequence of 12 steps and 6.5 % overall yield. Key component are the use of the diterpenoids (+)-sclareolide and (+)-sclareol as starting materials to prepare the key intermediates totarane (1) and labdadienolide (2), and intermolecular Diels–Alder reaction of 1 with 2 to give hispidanin A.
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Structural Adaptation of a Protein to Increased Metal Stress: NMR Structure of a Marine Snail Metallothionein with an Additional Domain ()
In this study, we present an NMR structure of the metallothionein (MT) from the snail Littorina littorea (LlMT) in complex with Cd2+. LlMT is capable of binding 9 Zn2+ or 9 Cd2+ ions. Sequence alignments with other snail MTs revealed that the protein is likely composed of three domains. The study revealed that the protein is divided into three individual domains, each of which folds into a single well-defined three-metal cluster. The central α2 and C-terminal β domains are positioned with a unique relative orientation. Two variants with longer and shorter linkers were investigated, which revealed that specific interdomain contacts only occurred with the wild-type linker. Moreover, a domain-swap mutant in which the highly similar α1 and α2 domains were exchanged was structurally almost identical. It is suggested that the expression of a three-domain MT confers an evolutionary advantage on Littorina littorea in terms of coping with Cd2+ stress and adverse environmental conditions. A heavy hitter: Metallothioneins are small proteins that are capable of binding heavy metals through Cys residues, thereby forming distinct metal clusters. It is demonstrated that the MT from the snail Littorina littorea displays a novel three-domain structure that increases its metal-loading capacity. This biologically significant feature may help the organism to survive in environments polluted with heavy metals.
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Large Magnetoelectric Coupling Near Room Temperature in Synthetic Melanostibite Mn2FeSbO6 ()
Multiferroic materials exhibit two or more ferroic orders and have potential applications as multifunctional materials in the electronics industry. A coupling of ferroelectricity and ferromagnetism is hereby particularly promising. We show that the synthetic melanostibite mineral Mn2FeSbO6 (R3‾ space group) with ilmenite-type structure exhibits cation off-centering that results in alternating modulated displacements, thus allowing antiferroelectricity to occur. Massive magnetoelectric coupling (MEC) and magnetocapacitance effect of up to 4000 % was detected at a record high temperature of 260 K. The multiferroic behavior is based on the imbalance of cationic displacements caused by a magnetostrictive mechanism, which sets up an unprecedented example to pave the way for the development of highly effective MEC devices operational at or near room temperature. Mn2FeSbO6 ilmenite (R3‾ ) is found to show a massive non-linear magnetoelectric coupling at 260 K, breaking local symmetry through the cationic off-centering under the application of an external magnetic field.
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Hierarchical Corannulene-Based Materials: Energy Transfer and Solid-State Photophysics ()
We report the first example of a donor–acceptor corannulene-containing hybrid material with rapid ligand-to-ligand energy transfer (ET). Additionally, we provide the first time-resolved photoluminescence (PL) data for any corannulene-based compounds in the solid state. Comprehensive analysis of PL data in combination with theoretical calculations of donor–acceptor exciton coupling was employed to estimate ET rate and efficiency in the prepared material. The ligand-to-ligand ET rate calculated using two models is comparable with that observed in fullerene-containing materials, which are generally considered for molecular electronics development. Thus, the presented studies not only demonstrate the possibility of merging the intrinsic properties of π-bowls, specifically corannulene derivatives, with the versatility of crystalline hybrid scaffolds, but could also foreshadow the engineering of a novel class of hierarchical corannulene-based hybrid materials for optoelectronic devices. But first, coffee: A rapid ligand-to-ligand energy transfer observed in the first example of a donor–acceptor corannulene-based scaffold is similar to the energy that a cup of coffee could provide. Many components can be added to the corannulene bowl, similar to cream and sugar added to coffee, highlighting the possibility of merging the intrinsic properties of π-bowls with the tunability of hierarchical materials for applications in optoelectronic devices.
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A Symmetric Co(N5)2(H2O)4⋅4 H2O High-Nitrogen Compound Formed by Cobalt(II) Cation Trapping of a Cyclo-N5− Anion ()
The reactions of (N5)6(H3O)3(NH4)4Cl with Co(NO3)2⋅6 H2O at room temperature yielded Co(N5)2(H2O)4⋅4 H2O as an air-stable orange metal complex. The structure, as determined by single-crystal X-ray diffraction, has two planar cyclo-N5− rings and four bound water molecules symmetrically positioned around the central metal ion. Thermal analysis demonstrated the explosive properties of the material. Light my fire: The metal pentazolate anion complex Co(N5)2(H2O)4⋅4 H2O was obtained by reacting (N5)6(H3O)3(NH4)4Cl with Co(NO3)2⋅6 H2O in a methanol/water solution at room temperature. Single-crystal X-ray crystallographic characterization and the physical properties of the comburent material are presented.
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Large-Area Elemental Imaging Reveals Van Eyck's Original Paint Layers on the Ghent Altarpiece (1432), Rescoping Its Conservation Treatment ()
A combination of large-scale and micro-scale elemental imaging, yielding elemental distribution maps obtained by, respectively non-invasive macroscopic X-ray fluorescence (MA-XRF) and by secondary electron microscopy/energy dispersive X-ray analysis (SEM-EDX) and synchrotron radiation-based micro-XRF (SR μ-XRF) imaging was employed to reorient and optimize the conservation strategy of van Eyck's renowned Ghent Altarpiece. By exploiting the penetrative properties of X-rays together with the elemental specificity offered by XRF, it was possible to visualize the original paint layers by van Eyck hidden below the overpainted surface and to simultaneously assess their condition. The distribution of the high-energy Pb-L and Hg-L emission lines revealed the exact location of hidden paint losses, while Fe-K maps demonstrated how and where these lacunae were filled-up using an iron-containing material. The chemical maps nourished the scholarly debate on the overpaint removal with objective, chemical arguments, leading to the decision to remove all skillfully applied overpaints, hitherto interpreted as work by van Eyck. MA-XRF was also employed for monitoring the removal of the overpaint during the treatment phase. To gather complementary information on the in-depth layer build-up, SEM-EDX and SR μ-XRF imaging was used on paint cross sections to record micro-scale elemental maps. Ghent what it used to be: A combination of large- and microscale chemical/elemental imaging supplied compositional information for the entire paint surface of the verso wing panels of van Eyck's Ghent Altarpiece. MA-XRF scanning allowed visualization and assessment of the original paint layers hidden below the overpainted surface. In this way, elemental imaging provided objective arguments supporting the decision for full removal of the overpaint.
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Use of a Traceless Activating and Directing Group for the Construction of Trifluoromethylpyrazoles: One-Pot Transformation of Nitroolefins and Trifluorodiazoethane ()
We disclose an efficient one-pot transformation of trifluorodiazoethane and higher perfluorinated homologues with various nitroolefins. This method takes advantage of the nitro group as a traceless activating and directing group (TADG) that is released in the aromatization step to produce 4-substituted 3-perfluoroalkyl pyrazoles with complete regioselectivity. The potential of this method is further demonstrated by the synthesis of penthiopyrad. Leaves discreetly when the job is done: A one-pot reaction of trifluorodiazoethane and higher perfluorinated homologues with various nitroolefins takes advantage of the nitro group as a traceless activating and directing group that is released in the aromatization step to produce 4-substituted 3-perfluoroalkyl pyrazoles with complete regioselectivity (see scheme). The method was applied to the synthesis of the fungicide penthiopyrad.
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Cover Picture: Formation of Gas-Phase Formate in Thermal Reactions of Carbon Dioxide with Diatomic Iron Hydride Anions (Angew. Chem. Int. Ed. 15/2017) ()
The hydrogenation … … of carbon dioxide involves the activation of the thermodynamically very stable molecule CO2 and the formation of a C−H bond. In their Communication on page 4187 ff., X.-N. Li, S.-G. He, and co-workers describe the mass-spectrometric observation of gas-phase formate as a product of the thermal reaction of CO2 with the metal hydride FeH−. The formation of HCO2− was predicted to proceed by facile hydride transfer.
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Inside Cover: Hydrophobic Water Probed Experimentally at the Gold Electrode/Aqueous Interface (Angew. Chem. Int. Ed. 15/2017) ()
Golden Gaze Persuades Hydrophobic water molecules were detected at the gold electrode/aqueous electrolyte interface using interface-specific vibrational sum frequency spectroscopy. In their Communication on page 4211 ff., R. K. Campen, Y. Tong et al. explain that the population and structure characteristics of this water type are bias-dependent. The mythological Chinese figure, Sun Wukong, is shown wielding a gold-banded rod with the power to orient OH groups in water toward the gold surface.
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Inside Back Cover: Surviving High-Temperature Calcination: ZrO2-Induced Hematite Nanotubes for Photoelectrochemical Water Oxidation (Angew. Chem. Int. Ed. 15/2017) ()
Fe2O3 nanotubes show extraordinary electrical, optical, and magnetic properties. While high-temperature calcination (HTC) is a key technique to produce high-crystallinity materials, it causes structural damage to nanotube arrays. In their Communication on page 4150 ff., T. Wang, J. Gong et al. present a method for fabricating HTC-resistant Fe2O3 nanotube arrays as photoanodes for water splitting. A ZrO2 shell is deposited onto hydrothermal FeOOH nanorods by atomic layer deposition, and a subsequent high-temperature solid-state reaction produces Zr-decorated Fe2O3 nanotubes.
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Back Cover: Divalent Thulium Triflate: A Structural and Spectroscopic Study (Angew. Chem. Int. Ed. 15/2017) ()
Divalent thulium complexes have great potential in reductive organic chemistry and for small-molecule transformations. Nonetheless, they are rare and hard to synthesize, thus their electronic structure is almost unexplored. In their Communication on page 4266 ff., G. Nocton et al. present a first step to the discovery of the submerged part of the iceberg, through unravelling the ground-state nature of divalent thulium triflate by means of luminescence, magnetism, and EPR spectroscopy.
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Frontispiece: Ultrathin Covalently Bound Organic Layers on Mica: Formation of Atomically Flat Biofunctionalizable Surfaces ()
Surface ChemistryIn their Communication on page 4130 ff., H. Zuilhof et al. report the facile and covalent modification of mica by using a catechol-based surface anchor with a flexibly linked amino group.
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Scholarly Integrity ()
“… Scholarly integrity is not only the foundational bedrock of scientific inquiry, it is also the prerequisite for a positive image of scholarship … For individuals, integrity is an aspect of moral character and experience. For institutions, it is about creating an environment that promotes responsible conduct … In the first instance, research institutions must provide guidelines and codes of practice on scholarly integrity …” Read more in the Editorial by J. S. Francisco, U. Hahn, and H. Schwarz.
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Graphical Abstract: Angew. Chem. Int. Ed. 15/2017 ()

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Spotlights on our sister journals: Angew. Chem. Int. Ed. 15/2017 ()

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Ken Tanaka ()
“My favorite drinks are coffee and beer. I admire my previous supervisors ...” This and more about Ken Tanaka can be found on page 4094.
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Grand Prix de la Maison de la Chimie: V. Balzani / Ružička Prize: B. Morandi / Cooperation Agreement between the German and Israel Chemical Societies Honorary Members of the Israel Chemical Society ()

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Photocatalysis. Volumes 1 (Fundamentals and Perspectives) and 2 (Applications). Edited by Jenny Schneider, Detlef Bahnemann, Jinhua Ye, Gianluca Li Puma, and Dionysios D. Dionysiou. ()
RSC Publishing, Cambridge 2016. 936 pp., hardcover, £ 300.00.—ISBN 978-1782627142
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Sulfonimidamides in Medicinal and Agricultural Chemistry ()
The synthesis and evaluation of structural analogues and isosteres are of central importance in medicinal and agricultural chemistry. The sulfonamide functional group represents one of the most important amide isosteres in contemporary drug design, and about 500 such compounds have overcome both the pharmacological and regulatory hurdles that precede studies in humans. The mono aza analogues of sulfonamides, that is, sulfonimidamides, are rapidly gaining popularity as a novel functional group among synthetic chemists involved in the design of biologically active compounds for both pharmaceutical and agrochemical applications. Herein, we review these recent developments to showcase the promise of this functional group. Gaining momentum: Sulfonimidamides are receiving increasing attention as bioisosteres of sulfonamides and as an important functional group in the design of pharmacologically active compounds. In this Minireview, medicinal and agricultural applications of this pharmacophore are summarized and discussed.
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Metal Fluorides as Analogues for Studies on Phosphoryl Transfer Enzymes ()
The 1994 structure of a transition-state analogue with AlF4− and GDP complexed to G1α, a small G protein, heralded a new field of research into the structure and mechanism of enzymes that manipulate the transfer of phosphoryl (PO3−) groups. The number of enzyme structures in the PDB containing metal fluorides (MFx) as ligands that imitate either a phosphoryl or a phosphate group was 357 at the end of 2016. They fall into three distinct geometrical classes: 1) Tetrahedral complexes based on BeF3− that mimic ground-state phosphates; 2) octahedral complexes, primarily based on AlF4−, which mimic “in-line” anionic transition states for phosphoryl transfer; and 3) trigonal bipyramidal complexes, represented by MgF3− and putative AlF30 moieties, which mimic the geometry of the transition state. The interpretation of these structures provides a deeper mechanistic understanding into the behavior and manipulation of phosphate monoesters in molecular biology. This Review provides a comprehensive overview of these structures, their uses, and their computational development. State of play: Metal fluoride complexes are superb analogues of the phosphoryl group, PO3−. They freeze the clock at the transition state for phosphoryl transfer, thus enabling crystallography, NMR spectroscopy, and computation to explore how hydrogen-bond networks provide a core for enzyme catalysis, and enable orbital overlap for GTPase activity.
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Ultrathin Covalently Bound Organic Layers on Mica: Formation of Atomically Flat Biofunctionalizable Surfaces ()
Mica is the substrate of choice for microscopic visualization of a wide variety of intricate nanostructures. Unfortunately, the lack of a facile strategy for its modification has prevented the on-mica assembly of nanostructures. Herein, we disclose a convenient catechol-based linker that enables various surface-bound metal-free click reactions, and an easy modification of mica with DNA nanostructures and a horseradish peroxidase mimicking hemin/G-quadruplex DNAzyme. Catechol-based thin films: The facile and covalent modification of mica is reported using a catechol-based surface anchor with a flexibly linked amino group. Robustly bound, low-roughness, and ultrathin amine-terminated layers are obtained. This approach makes possible highly flexible surface modifications using metal-free click reactions.
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Asymmetric Hydrogenation of In Situ Generated Isochromenylium Intermediates by Copper/Ruthenium Tandem Catalysis ()
The first asymmetric hydrogenation of in situ generated isochromenylium derivatives is enabled by tandem catalysis with a binary system consisting of Cu(OTf)2 and a chiral cationic ruthenium–diamine complex. A range of chiral 1H-isochromenes were obtained in high yields with good to excellent enantioselectivity. These chiral 1H-isochromenes could be easily transformed into isochromanes, which represent an important structural motif in natural products and biologically active compounds. The chiral induction was rationalized by density functional theory calculations. Two metal catalysts: The title reaction is catalyzed by a binary system based on Cu(OTf)2 and a chiral ruthenium–diamine complex. A range of chiral 1H-isochromenes were obtained in high yields with good to excellent enantioselectivities, and were easily transformed into isochromanes.
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Combined Analysis of NMR and MS Spectra (CANMS) ()
Cellular metabolism in mammalian cells represents a challenge for analytical chemistry in the context of current biomedical research. Mass spectrometry and NMR spectroscopy together with computational tools have been used to study metabolism in cells. Compartmentalization of metabolism complicates the interpretation of stable isotope patterns in mammalian cells owing to the superimposition of different pathways contributing to the same pool of analytes. This indicates a need for a model-free approach to interpret such data. Mass spectrometry and NMR spectroscopy provide complementary analytical information on metabolites. Herein an approach that simulates 13C multiplets in NMR spectra and utilizes mass increments to obtain long-range information is presented. The combined information is then utilized to derive isotopomer distributions. This is a first rigorous analytical and computational approach for a model-free analysis of metabolic data applicable to mammalian cells. Complexity is no longer a problem: A model-free isotopomer analysis of 13C-enriched biological cell extracts has been developed. The combination of NMR and MS data in a single analysis enables a highly specific and accurate determination of 13C isotopomers from a complex mixture of metabolites, which can be used for the interpretation of metabolic pathways.
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Programmable Nanoassemblies from Non-Assembling Homopolymers Using Ad Hoc Electrostatic Interactions ()
Robust nanostructures were obtained from polymers that otherwise do not assemble by using a novel approach based on electrostatic self-assembly. The essence of this strategy involves the use of divalent counterions to temporarily perturb the packing features of the ionic groups in a homopolymer, which results in a vesicle-like structure that is captured in situ through a simple crosslinking reaction. The fidelity of the assembly has been tested for molecular transport across the nanomembrane, both for the molecules encapsulated in the lumen and for those trapped in the membrane itself. The membranes are addressable for robust multifunctionalization of their surfaces and for tunable transmembrane molecular transport. Multivalent counterions can be utilized to transform a non-assembling homopolymer into a programmable nanoassembly with tunable interfacial and transport properties. The counterions temporarily perturb the packing features of the ionic groups in the homopolymer, which results in vesicle-like structures.
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Surviving High-Temperature Calcination: ZrO2-Induced Hematite Nanotubes for Photoelectrochemical Water Oxidation ()
Nanotubular Fe2O3 is a promising photoanode material, and producing morphologies that withstand high-temperature calcination (HTC) is urgently needed to enhance the photoelectrochemical (PEC) performance. This work describes the design and fabrication of Fe2O3 nanotube arrays that survive HTC for the first time. By introducing a ZrO2 shell on hydrothermal FeOOH nanorods by atomic layer deposition, subsequent high-temperature solid-state reaction converts FeOOH-ZrO2 nanorods to ZrO2-induced Fe2O3 nanotubes (Zr-Fe2O3 NTs). The structural evolution of the hematite nanotubes is systematically explored. As a result of the nanostructuring and shortened charge collection distance, the nanotube photoanode shows a greatly improved PEC water oxidation activity, exhibiting a photocurrent density of 1.5 mA cm−2 at 1.23 V (vs. reversible hydrogen electrode, RHE), which is the highest among hematite nanotube photoanodes without co-catalysts. Furthermore, a Co-Pi decorated Zr-Fe2O3 NT photoanode reveals an enhanced onset potential of 0.65 V (vs. RHE) and a photocurrent of 1.87 mA cm−2 (at 1.23 V vs. RHE). Survival training: Hematite nanotube arrays (Fe2O3-NTs) surviving high-temperature calcination were prepared by a solid-state reaction between an FeOOH nanorod core and a ZrO2 shell. Their greatly improved photoelectrochemical water oxidation performance originates from their nanostructured morphology and shortened charge collection distance (see picture; W=depletion layer width).
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Enantioselective Palladium-Catalyzed Carbene Insertion into the N−H Bonds of Aromatic Heterocycles ()
C3-substituted indoles and carbazoles react with α-aryl-α-diazoesters under palladium catalysis to form α-(N-indolyl)-α-arylesters and α-(N-carbazolyl)-α-arylesters. The products result from insertion of a palladium-carbene ligand into the N−H bond of the aromatic N-heterocycles. Enantioselection was achieved using a chiral bis(oxazoline) ligand, in many cases with high enantioselectivity (up to 99 % ee). The method was applied to synthesize the core of a bioactive carbazole derivative in a concise manner. Pharmacophores made easy: Palladium catalyzes the insertion of α-arylester carbene ligands into the N−H bonds of heterocycles to afford α-(N-indolyl)arylesters and α-(N-carbazolyl)arylesters in good yields and up to 99 % ee. The method enables a concise route to the core of a bioactive carbazole derivative.
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Composition-Dependent Hot Carrier Relaxation Dynamics in Cesium Lead Halide (CsPbX3, X=Br and I) Perovskite Nanocrystals ()
Cesium-based perovskite nanocrystals (NCs) have outstanding photophysical properties improving the performances of lighting devices. Fundamental studies on excitonic properties and hot-carrier dynamics in perovskite NCs further suggest that these materials show higher efficiencies compared to the bulk form of perovskites. However, the relaxation rates and pathways of hot-carriers are still being elucidated. By using ultrafast transient spectroscopy and calculating electronic band structures, we investigated the dependence of halide in Cs-based perovskite (CsPbX3 with X=Br, I, or their mixtures) NCs on the hot-carrier relaxation processes. All samples exhibit ultrafast (<0.6 ps) hot-carrier relaxation dynamics with following order: CsPbBr3 (310 fs)>CsPbBr1.5I1.5 (380 fs)>CsPbI3 NC (580 fs). These result accounts for a reduced light emission efficiency of CsPbI3 NC compared to CsPbBr3 NC. Significant dependence of the hot-carrier relaxation rate on the halide (Br, I, or their mixture) is mainly attributable to the density of states for holes in the valence band of CsPbX3 nanocrystals. The hot-carrier relaxation rate is observed to decrease from CsPbBr3 to CsPbI3 NC. VB=valence band, CB=conduction band.
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Real-Time In Vivo Hepatotoxicity Monitoring through Chromophore-Conjugated Photon-Upconverting Nanoprobes ()
Drug toxicity is a long-standing concern of modern medicine. A typical anti-pain/fever drug paracetamol often causes hepatotoxicity due to peroxynitrite ONOO−. Conventional blood tests fail to offer real-time unambiguous visualization of such hepatotoxicity in vivo. Here we report a luminescent approach to evaluate acute hepatotoxicity in vivo by chromophore-conjugated upconversion nanoparticles. Upon injection, these nanoprobes mainly accumulate in the liver and the luminescence of nanoparticles remains suppressed owing to energy transfer to the chromophore. ONOO− can readily bleach the chromophore and thus recover the luminescence, the presence of ONOO− in the liver leads to fast restoring of the near-infrared emission. Taking advantages of the high tissue-penetration capability of near-infrared excitation/emission, these nanoprobes achieve real-time monitoring of hepatotoxicity in living animals, thereby providing a convenient screening strategy for assessing hepatotoxicity of synthetic drugs. Drug toxicity: A chromophore-conjugated photon-upconverting nanoprobe was developed for rapid and sensitive detection of the peroxynitrite anion ONOO− in aqueous solution. The nanoprobe was used for real-time monitoring of the hepatotoxicity of synthetic drugs in living animals.
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Rationally Designed Carbon Nanodots towards Pure White-Light Emission ()
We report a rational synthesis of carbon nanodots (CNDs) aimed at tailoring their emission, starting from a reasoned choice of organic precursors. To showcase the potential of this approach in a field such as optoelectronics, we designed experiments aimed at preparing materials that emit across the entire visible spectrum. Specifically, using precursors such as arginine, ethylenediamine, naphthalene dianhydride, and 2,6-dibromonaphtalene dianhydride, in appropriate ratios, it was possible to obtain pure white-light (0.33, 0.33; CIE coordinates) emitting carbon nanodots (WCNDs) through a one-step microwave-assisted synthesis and facile purification. The characterization and properties of this novel nanomaterial is discussed. White light to lighten up my dots: A rational bottom-up synthesis of carbon nanodots with tunable emission is reported. By an appropriate choice and ratio of organic chromophores, carbon dots that emit across the entire visible spectrum and, thus, give white light are obtained. The approach to fabricating this novel material, as well as its characterization, is discussed.
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Dynamics of O2 Chemisorption on a Flat Platinum Surface Probed by an Alignment-Controlled O2 Beam ()
O2 adsorption on Pt surfaces is of great technological importance owing to its relevance to reactions for the purification of car exhaust gas and the oxygen reduction on fuel-cell electrodes. Although the O2/Pt(111) system has been investigated intensively, questions still remain concerning the origin of the low O2 sticking probability and its unusual energy dependence. We herein clarify the alignment dependence of the initial sticking probability (S0) using the single spin-rotational state-selected [(J,M)=(2,2)] O2 beam. The results indicate that, at low translational energy (E0) conditions, direct activated chemisorption occurs only when the O2 axis is nearly parallel to the surface. At high energy conditions (E0>0.5 eV), however, S0 for the parallel O2 decreases with increasing E0 while that of the perpendicular O2 increases, accounting for the nearly energy-independent O2 sticking probability determined previously by a non-state-resolved experiment. Steric effect in O2 chemisorption: The O2 adsorption process on a platinum surface strongly depends on the alignment of an impinging O2 molecule. The origin of low sticking probability at low incident energies and its nearly energy-independent behavior at high energies observed previously by a non-state-resolved experiment, can be accounted for by the different adsorption and scattering processes of parallel and perpendicular O2 molecules.
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Equatorial Ligand Perturbations Influence the Reactivity of Manganese(IV)-Oxo Complexes ()
Manganese(IV)-oxo complexes are often invoked as intermediates in Mn-catalyzed C−H bond activation reactions. While many synthetic MnIV-oxo species are mild oxidants, other members of this class can attack strong C−H bonds. The basis for these reactivity differences is not well understood. Here we describe a series of MnIV-oxo complexes with N5 pentadentate ligands that modulate the equatorial ligand field of the MnIV center, as assessed by electronic absorption, electron paramagnetic resonance, and Mn K-edge X-ray absorption methods. Kinetic experiments show dramatic rate variations in hydrogen-atom and oxygen-atom transfer reactions, with faster rates corresponding to weaker equatorial ligand fields. For these MnIV-oxo complexes, the rate enhancements are correlated with both 1) the energy of a low-lying 4E excited state, which has been postulated to be involved in a two-state reactivity model, and 2) the MnIII/IV reduction potentials. Weaker field, higher rate: Perturbations in the equatorial ligand field of a series of MnIV-oxo complexes shifts the energy of a 4E excited state that has been proposed to mediate hydrogen and oxygen atom transfer reactions. The experimental 4E energy is correlated with reaction rate enhancements.
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Metal-Free Intermolecular Aminoarylation of Alkynes ()
A metal-free aminoarylation of internal alkynes is described, yielding tetrasubstituted enaminoates. The transformation proceeds in good to excellent yields through a tandem conjugate addition/Smiles rearrangement involving aryl and heteroaryl sulfonamides. Substrate scope is very broad under simple, user-friendly conditions, and the reaction can be used to easily access biologically active phenethylamine derivatives. Metal-free and with a traceless linker: Aminoarylation of alkynes can be achieved using a tandem conjugate addition/Smiles rearrangement involving aryl and heteroaryl sulfonamides. These readily available reagents provide amine and aryl components with the SO2 group acting as a traceless linker.
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Formation of Gas-Phase Formate in Thermal Reactions of Carbon Dioxide with Diatomic Iron Hydride Anions ()
The hydrogenation of carbon dioxide involves the activation of the thermodynamically very stable molecule CO2 and formation of a C−H bond. Herein, we report that HCO2− and CO can be formed in the thermal reaction of CO2 with a diatomic metal hydride species, FeH−. The FeH− anions were produced by laser ablation, and the reaction with CO2 was analyzed by mass spectrometry and quantum-chemical calculations. Gas-phase HCO2− was observed directly as a product, and its formation was predicted to proceed by facile hydride transfer. The mechanism of CO2 hydrogenation in this gas-phase study parallels similar behavior of a condensed-phase iron catalyst. The formate anion, HCO2−, was identified in the reaction of CO2 with the metal hydride FeH−. The FeH− anions were produced by laser ablation, and the reaction with CO2 was analyzed by mass spectrometry and quantum-chemical calculations. The formation of gas-phase HCO2− was predicted to proceed by facile hydride transfer.
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Structure of the Complex between a Heparan Sulfate Octasaccharide and Mycobacterial Heparin-Binding Hemagglutinin ()
Heparin-binding hemagglutinin (HBHA) is a 199 amino acid virulence factor at the envelope of Mycobacterium tuberculosis that contributes to latent tuberculosis. The binding of HBHA to respiratory epithelial cells, which leads to extrapulmonary dissemination of the pathogen, is mediated by cell-surface heparan sulfate (HS). We report the structural characterization of the HBHA/HS complex by NMR spectroscopy. To develop a model for the molecular recognition, the first chemically synthesized uniformly 13C- and 15N-labeled HS octasaccharide and a uniformly 13C- and 15N-labeled form of HBHA were prepared. Residues 180–195 at the C-terminal region of HBHA show large chemical shift perturbation upon association with the octasaccharide. Molecular dynamics simulations conforming to the multidimensional NMR data revealed key electrostatic and even hydrophobic interactions between the binding partners that may aid in the development of agents targeting the binding event. Clearly labeled: The structure of the complex between heparan sulfate and the mycobacterial heparin-binding hemagglutinin was characterized by NMR spectroscopy using uniformly 13C- and 15N-labeled forms of the sugar and protein. Notable chemical-shift perturbations occurred at the C-terminal residues of the protein. Molecular dynamics simulations based on the NMR data show electrostatic and hydrophobic interactions between the binding partners.
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Chemoselective Alteration of Fluorophore Scaffolds as a Strategy for the Development of Ratiometric Chemodosimeters ()
Ratiometric sensors generally couple binding events or chemical reactions at a distal site to changes in the fluorescence of a core fluorophore scaffold. However, such approaches are often hindered by spectral overlap of the product and reactant species. We provide a strategy to design ratiometric sensors that display dramatic spectral shifts by leveraging the chemoselective reactivity of novel functional groups inserted within fluorophore scaffolds. As a proof-of-principle, fluorophores containing a borinate (RF620) or silanediol (SiOH2R) functionality at the bridging position of the xanthene ring system are developed as endogenous H2O2 sensors. Both these fluorophores display far-red to near-infrared excitation and emission prior to reaction. Upon oxidation by H2O2 both sensors are chemically converted to tetramethylrhodamine, producing significant (≥66 nm) blue-shifts in excitation and emission maxima. This work provides a new concept for the development of ratiometric probes. Ratiometric sensors: A novel method for designing ratiometric sensors termed chemoselective alteration of fluorophore scaffolds (CAFS) is disclosed. Two proof-of-concept hydrogen peroxide sensors with large excitation and emission shifts (≥66 nm) were obtained by using this approach.
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Construction of Fused Pyrrolidines and β-Lactones by Carbene-Catalyzed C−N, C−C, and C−O Bond Formations ()
A carbene-catalyzed intermolecular C−N bond formation, which initiates a highly selective cascade reaction for the synthesis of pyrrolidine fused β-lactones, is disclosed. The nitrogen-containing bicyclic β-lactone products are obtained with good yields and excellent stereoselectivities. Synthetic transformations of the reaction products into useful functional molecules, such as amino catalysts, can be efficiently realized under mild reaction conditions. Mechanistically, this study provides insights into modulating the reactivities of heteroatoms, such as nitrogen atoms, in challenging carbene-catalyzed asymmetric carbon–heteroatom bond-forming reactions. Fused: The challenging intermolecular addition of a nitrogen nucleophile to a catalytically generated unsaturated acyl azolium intermediate provides a highly efficient method for asymmetric access to pyrrolidine-fused β-lactones. The unique bicyclic β-lactone structure, bearing three contiguous stereogenic centers, is readily transformed into useful functional molecules such as amino catalysts. NHC= N-heterocyclic carbene.
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Noble-Metal-Free Janus-like Structures by Cation Exchange for Z-Scheme Photocatalytic Water Splitting under Broadband Light Irradiation ()
Z-scheme water splitting is a promising approach based on high-performance photocatalysis by harvesting broadband solar energy. Its efficiency depends on the well-defined interfaces between two semiconductors for the charge kinetics and their exposed surfaces for chemical reactions. Herein, we report a facile cation-exchange approach to obtain compounds with both properties without the need for noble metals by forming Janus-like structures consisting of γ-MnS and Cu7S4 with high-quality interfaces. The Janus-like γ-MnS/Cu7S4 structures displayed dramatically enhanced photocatalytic hydrogen production rates of up to 718 μmol g−1 h−1 under full-spectrum irradiation. Upon further integration with an MnOx oxygen-evolution cocatalyst, overall water splitting was accomplished with the Janus structures. This work provides insight into the surface and interface design of hybrid photocatalysts, and offers a noble-metal-free approach to broadband photocatalytic hydrogen production. Janus-like structures consisting of γ-MnS and Cu7S4 with high-quality interfaces were obtained by facile cation exchange. The hybrid structures exhibit broadband light absorption and improved charge separation, which led to a hydrogen production rate of 718 μmol g−1 h−1 under full-spectrum irradiation.
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Hydrophobic Water Probed Experimentally at the Gold Electrode/Aqueous Interface ()
Quantitative description of reaction mechanisms in aqueous phase electrochemistry requires experimental characterization of local water structure at the electrode/aqueous interface and its evolution with changing potential. Gaining such insight experimentally under electrochemical conditions is a formidable task. The potential-dependent structure of a subpopulation of interfacial water with one OH group pointing towards a gold working electrode is characterized using interface specific vibrational spectroscopy in a thin film electrochemical cell. Such free-OH groups are the molecular level observable of an extended hydrophobic interface. This free-OH interacts only weakly with the Au surface at all potentials, has an orientational distribution that narrows approaching the potential of zero charge, and disappears on oxidation of the gold electrode. A population of water molecules with one OH pointing away from bulk liquid (that is, hydrophobic water) are characterized at the gold electrode/aqueous electrolyte interface using interface-specific vibrational sum frequency spectroscopy (VSF). This type of interfacial water disappears on oxidation of the gold electrode. At potentials below gold oxidation, its structure is bias-dependent.
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Regimes of Biomolecular Ultrasmall Nanoparticle Interactions ()
Ultrasmall nanoparticles (USNPs), usually defined as NPs with core in the size range 1–3 nm, are a class of nanomaterials which show unique physicochemical properties, often different from larger NPs of the same material. Moreover, there are also indications that USNPs might have distinct properties in their biological interactions. For example, recent in vivo experiments suggest that some USNPs escape the liver, spleen, and kidney, in contrast to larger NPs that are strongly accumulated in the liver. Here, we present a simple approach to study the biomolecular interactions at the USNPs bio-nanointerface, opening up the possibility to systematically link these observations to microscopic molecular principles. Ultrasmall nanoparticles were synthesized to study size and ligand effects on the formation of the protein corona. The interactions of the ultrasmall nanoparticles in a relevant biological milieu were investigated, opening up the possibility to systematically link these observations to microscopic molecular principles.
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An Electrophilic Carbene-Anchored Silylene–Phosphinidene ()
The cyclic alkyl(amino) carbene-anchored silylene–phosphinidene was isolated as L−Si−P(:cAAC−Me) (L=benzamidinate) at room temperature, synthesized from the reduction of L−Si(Cl2)−P(:cAAC−Me) (1) using two equivalents of KC8. Compound 1 was prepared by the oxidative addition of a chlorophosphinidene to the benzamidinate substituted silylene center. This is the first molecular example of a silylene–phosphinidene characterized by single-crystal X-ray structural analysis. Moreover, 1H, 31P, and also 29Si NMR spectroscopic data supported the formulation of the products. The theoretical calculations of compound 2 are in good agreement with the experimental results. SiP-ping success: A cyclic alkyl(amino) carbene-anchored silylene–phosphinidene is isolated as L−Si−P(:cAAC−Me) (L=benzamidinate) at room temperature. This is the first molecular example of a silylene–phosphinidene characterized by single-crystal X-ray structural analysis. 1H, 31P, and 29Si NMR spectroscopy support the existence of this molecule. Theoretical calculations provide further insights into the bonding situation.
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A Moldable Nanocomposite Hydrogel Composed of a Mussel-Inspired Polymer and a Nanosilicate as a Fit-to-Shape Tissue Sealant ()
The engineering of bioadhesives to bind and conform to the complex contour of tissue surfaces remains a challenge. We have developed a novel moldable nanocomposite hydrogel by combining dopamine-modified poly(ethylene glycol) and the nanosilicate Laponite, without the use of cytotoxic oxidants. The hydrogel transitioned from a reversibly cross-linked network formed by dopamine–Laponite interfacial interactions to a covalently cross-linked network through the slow autoxidation and cross-linking of catechol moieties. Initially, the hydrogel could be remolded to different shapes, could recover from large strain deformation, and could be injected through a syringe to adhere to the convex contour of a tissue surface. With time, the hydrogel solidified to adopt the new shape and sealed defects on the tissue. This fit-to-shape sealant has potential in sealing tissues with non-flat geometries, such as a sutured anastomosis. Hugs tight, holds tight: A moldable nanocomposite hydrogel was developed by combining a mussel-inspired polymer and a nanosilicate. The hydrogel is remoldable, injectable, and able to adhere to tissue with a convex contour (see picture).
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Manganese-Catalyzed N-Formylation of Amines by Methanol Liberating H2: A Catalytic and Mechanistic Study ()
The first example of a base metal (manganese) catalyzed acceptorless dehydrogenative coupling of methanol and amines to form formamides is reported herein. The novel pincer complex (iPr-PNHP)Mn(H)(CO)2 catalyzes the reaction under mild conditions in the absence of any additives, bases, or hydrogen acceptors. Mechanistic insight based on the observation of an intermediate and DFT calculations is also provided. Back to basics: An acceptorless dehydrogenative coupling of methanol and amines to form formamides that is catalyzed by a well-defined manganese pincer complex (see scheme) is reported. Mechanistic insight based on the observation of an intermediate and density functional calculations is also provided.
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Precise Photoremovable Perturbation of a Virus–Host Interaction ()
Viruses utilize distinct binding interactions with a variety of host factors to gain entry into host cells. A chemical strategy is described to precisely perturb a specific molecular interaction between adeno-associated virus and its host cell, which can be rapidly reversed by light. This strategy enables pausing the virus entry process at a specific stage and then restart it rapidly with a non-invasive stimulus. The ability to synchronize the invading virus population at a discrete step in its entry pathway will be highly valuable for enabling facile experimental characterization of the molecular processes underlying this process. Additionally, adeno-associated virus has demonstrated outstanding potential for human gene therapy. This work further provides a potential approach to create therapeutic vectors that can be photoactivated in vivo with high spatial and temporal control. Precise perturbation of a specific molecular interaction between adeno-associated virus and its host cell using a chemical strategy is described. The perturbation can be rapidly reversed by light. This general strategy can pause the virus entry process and then restart it rapidly with a non-invasive stimulus.
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A Dimeric Bis(melamine)-Substituted Bispidine for Efficient Transmembrane H+/Cl− Cotransport ()
A 3,7-diazabicyclo[3.3.1]nonane linking to two melamines is a unique transmembrane H+/Cl− carrier. In the solid state, the V-shaped compound forms a HCl-bound zig-zag network through cooperative protonation and hydrogen bond interactions. In the lipid membrane, the receptor forms a dimeric self-assembly involving multiple H+ and Cl− leading to the efficient transport of the acid. The pH-dependent Cl− efflux observed for the compound was rationalized based on a gradual protonation model that confers an active transmembrane carrier at physiological pH. A pH-gated dimer of bis(melamine)-substituted bispidine is reported (see picture). It is an efficient cotransporter of H+/Cl− across the lipid bilayer membrane.
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Successive Charge Transitions of Unusually High-Valence Fe3.5+: Charge Disproportionation and Intermetallic Charge Transfer ()
A perovskite-structure oxide containing unusually high-valence Fe3.5+ was obtained by high-pressure synthesis. Instability of the Fe3.5+ in Ca0.5Bi0.5FeO3 is relieved first by charge disproportionation at 250 K and then by intermetallic charge transfer between A-site Bi and B-site Fe at 200 K. These previously unobserved successive charge transitions are due to competing intermetallic and disproportionation charge instabilities. Both transitions change magnetic and structural properties significantly, indicating strong coupling of charge, spin, and lattice in the present system. A high achiever under pressure: A perovskite-structure oxide containing unusually high-valence Fe3.5+ was obtained by high-pressure synthesis. Instability of the Fe3.5+ in Ca0.5Bi0.5FeO3 is relieved first by charge disproportionation at 250 K and then by intermetallic charge transfer between A-site Bi and B-site Fe at 200 K.
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A Nickel Dithiolate Water Reduction Catalyst Providing Ligand-Based Proton-Coupled Electron-Transfer Pathways ()
A nickel pyrazinedithiolate ([Ni(dcpdt)2]2−; dcpdt=5,6-dicyanopyrazine-2,3-dithiolate), bearing a NiS4 core similar to the active center of [NiFe] hydrogenase, is shown to serve as an efficient molecular catalyst for the hydrogen evolution reaction (HER). This catalyst shows effectively low overpotentials for HER (330–400 mV at pH 4–6). Moreover, the turnover number of catalysis reaches 20 000 over the 24 h electrolysis with a high Faradaic efficiency, 92–100 %. The electrochemical and DFT studies reveal that diprotonated one-electron-reduced species (i.e., [NiII(dcpdt)(dcpdtH2)]− or [NiII(dcpdtH)2]−) forms at pH<6.4 via ligand-based proton-coupled electron-transfer (PCET) pathways, leading to electrocatalytic HER without applying the highly negative potential required to generate low-valent nickel intermediates. This is the first example of catalysts exhibiting such behavior. Out on a ligand: A nickel pyrazinedithiolate complex is an efficient molecular catalyst for H2 evolution from water. Using it H2-evolution proceeds via the ligand-based proton-coupled electron-transfer (PCET) pathways without forming low-valent nickel species.
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Ultrathin Two-Dimensional Organic–Inorganic Hybrid Perovskite Nanosheets with Bright, Tunable Photoluminescence and High Stability ()
Two-dimensional (2D) organic–inorganic hybrid perovskite nanosheets (NSs) are attracting increasing research interest due to their unique properties and promising applications. Here, for the first time, we report the facile synthesis of single- and few-layer free-standing phenylethylammonium lead halide perovskite NSs, that is, (PEA)2PbX4 (PEA=C8H9NH3, X=Cl, Br, I). Importantly, their lateral size can be tuned by changing solvents. Moreover, these ultrathin 2D perovskite NSs exhibit highly efficient and tunable photoluminescence, as well as superior stability. Our study provides a simple and general method for the controlled synthesis of 2D perovskite NSs, which may offer a new avenue for their fundamental studies and optoelectronic applications. Ultrathin perovskite nanosheets: Single- and few-layer free-standing phenylethylammonium lead halide perovskite nanosheets with controlled lateral sizes were synthesized by a facile and fast crystallization method. These as-prepared nanosheets show bright, tunable light emission (see picture) as well as enhanced stability.
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Iterative C−H Functionalization Leading to Multiple Amidations of Anilides ()
Polyaminobenzenes were synthesized by the ruthenium-catalyzed iterative C−H amidation of anilides using dioxazolones as an amino source. This strategy could be implemented by the sequential activation of C−H bonds of formerly generated compounds by cascade chelation assistance of newly installed amide groups. Computational studies provided a rationale. Repeat performance: A ruthenium-catalyzed cascade amidation of multiple C−H bonds of anilide has been developed. By using dioxazolone as amidating reagents, newly installed amide groups serve as an additional directing groups to drive subsequent C−H activation. This reaction provides a simple and mild approach to the preparation of polyaminobenzenes.
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A Stable, Soluble, and Crystalline Supramolecular System with a Triplet Ground State ()
A supramolecular complex was constructed by encapsulation of a 3O2 molecule inside an open-cage C60 derivative. Its single-crystal X-ray diffraction analysis revealed the presence of the 3O2 at the center of the fullerene cage. The CV measurements suggested that unprecedented dehydrogenation was promoted by the encapsulated 3O2 after two-electron reduction. The ESR measurements displayed the triplet character as well as the anisotropy of the 3O2. Additionally, the SQUID measurements also demonstrated the paramagnetic behavior above 3 K without an antiferromagnetic transition. Upon photoirradiation with visible light, three phosphorescent bands at the NIR region were observed, arising from the exited 1O2 generated by self-sensitization with the outer cage, whose lifetimes were not affected by the environments. These studies confirmed that the complex is a crystalline triplet system with incompatible “high spin density” but “small interspin interaction” properties. Triplet character: A supramolecular complex was synthesized by the encapsulation of a 3O2 molecule inside an open-cage C60 derivative. The electronic, magnetic, and photophysical measurements revealed that the supramolecular complex is a stable, soluble, and crystalline triplet system with incompatible “high spin density” but “small interspin interaction” properties.
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Divalent Thulium Triflate: A Structural and Spectroscopic Study ()
The first molecular TmII luminescence measurements are reported along with rare magnetic, X and Q bands EPR studies. Access to simple and soluble molecular divalent lanthanide complexes is highly sought for small-molecule activation studies and organic transformations using single-electron transfer processes. However, owing to their low stability and propensity to disproportionate, these complexes are hard to synthetize and their electronic properties are therefore almost unexplored. Herein we present the synthesis of [Tm(μ-OTf)2(dme)2]n, a rare and simple coordination compound of divalent thulium that can be seen as a promising starting material for the synthesis of more elaborated complexes. This reactive complex was structurally characterized by X-ray diffraction analysis and its electronic structure has been compared with that of its halide cousin TmI2(dme)3. Filling the gap: The synthesis of a reactive but yet simple divalent thulium triflate salt allows the first emission studies on molecular divalent thulium that confirm the predicted f13 electronic structure. The first magnetism and X- and Q-band EPR data on a molecular divalent thulium compound show that the ground state is multiconfigurational.
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Asymmetric Grignard Synthesis of Tertiary Alcohols through Rational Ligand Design ()
A simple, general and practical method is reported for highly enantioselective construction of tertiary alcohols through the direct addition of organomagnesium reagents to ketones. Discovered by rational ligand design based on a mechanistic hypothesis, it has an unprecedented broad scope. It utilizes a new type of chiral tridentate diamine/phenol ligand that is easily removed from the reaction mixture. It is exemplified by application to a formal asymmetric synthesis (>95:5 d.r.) of vitamin E. The road less traveled: A simple, general, direct route for asymmetric Grignard synthesis of tertiary alcohols was facilitated by coordinating tridentate ligands to magnesium. The method was found by rational design and has wide scope; it was applied to a formal asymmetric synthesis of vitamin E.
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Synthesis of Indoles and Pyrroles Utilizing Iridium Carbenes Generated from Sulfoxonium Ylides ()
Metal carbenes can undergo a myriad of synthetic transformations. Sulfur ylides are potential safe precursors of metal carbenes. Herein, we report cascade reactions that involve carbenoids derived from sulfoxonium ylides for the efficient and regioselective synthesis of indoles and pyrroles. The tandem action of iridium and Brønsted acid catalysts enables rapid assembly of the heterocycles from unmodified anilines or readily accessible enamines under microwave irradiation. The key mechanistic steps are the catalytic transformation of the sulfoxonium ylide into an iridium–carbene complex, followed by N−H or C−H functionalization of an aniline or enamine, respectively, and a final acid-catalyzed cyclization. The present method was successfully applied to the synthesis of the densely functionalized pyrrole subunit of atorvastatin. Take the shortcut: Substituted indoles and pyrroles, including the densely functionalized pyrrole subunit of atorvastatin, were assembled in one step from sulfoxonium ylides and unmodified anilines or readily accessible enamines through tandem catalysis by [{Ir(cod)Cl}2] and a Brønsted acid under microwave irradiation (see scheme). The transformation involves the formation of an iridium–carbene complex and an unprecedented C−H functionalization step.
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Catalytic Ester and Amide to Amine Interconversion: Nickel-Catalyzed Decarbonylative Amination of Esters and Amides by C−O and C−C Bond Activation ()
An efficient nickel-catalyzed decarbonylative amination reaction of aryl and heteroaryl esters has been achieved for the first time. The new amination protocol allows the direct interconversion of esters and amides into the corresponding amines and represents a good alternative to classical rearrangements as well as cross coupling reactions. An efficient nickel-catalyzed decarbonylative amination reaction of readily available aryl and heteroaryl esters has been developed. This new amination procedure shows high tolerance towards a variety of aryl and heteroaryl esters, thus providing a practical and versatile access to valuable primary amines. cod=1,5-cyclooctadiene.
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Rh/Cu-Catalyzed Cascade [4+2] Vinylic C−H O-Annulation and Ring Contraction of α-Aryl Enones with Alkynes in Air ()
An unprecedented Rh-catalyzed ketone-directed vinylic C−H activation/[4+2] O-annulation of α-aryl enones with internal alkynes followed by a Cu-catalyzed ring contraction in air to provide multiaryl-substituted furan derivatives has been developed. The preliminary mechanism study identifies the active pyrylium salt as the key intermediate. First rhodium, then copper: An unprecedented cascade reaction consisting of a Rh-catalyzed ketone-directed vinylic C−H [4+2] O-annulation of α,β-enones with alkynes and a subsequent Cu-catalyzed ring contraction in air to form multiaryl-substituted furan derivatives is realized. The pyrylium salt is identified as the key intermediate.
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Stereoselective Catalytic Synthesis of Active Pharmaceutical Ingredients in Homemade 3D-Printed Mesoreactors ()
3D-printed flow reactors were designed, fabricated from different materials (PLA, HIPS, nylon), and used for a catalytic stereoselective Henry reaction. The use of readily prepared and tunable 3D-printed reactors enabled the rapid screening of devices with different sizes, shapes, and channel dimensions, aimed at the identification of the best-performing reactor setup. The optimized process afforded the products in high yields, moderate diastereoselectivity, and up to 90 % ee. The method was applied to the continuous-flow synthesis of biologically active chiral 1,2-amino alcohols (norephedrine, metaraminol, and methoxamine) through a two-step sequence combining the nitroaldol reaction with a hydrogenation. To highlight potential industrial applications of this method, a multistep continuous synthesis of norephedrine has been realized. The product was isolated without any intermediate purifications or solvent switches. Homemade reactors: Stereoselective catalytic reactions were conducted in tunable homemade 3D-printed mesoreactors. This method was applied to the continuous-flow synthesis of biologically active chiral 1,2-amino alcohols in a two-step sequencing combining a nitroaldol reaction and hydrogenation.
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Bis-Gadolinium Complexes for Solid Effect and Cross Effect Dynamic Nuclear Polarization ()
High-spin complexes act as polarizing agents (PAs) for dynamic nuclear polarization (DNP) in solid-state NMR spectroscopy and feature promising aspects towards biomolecular DNP. We present a study on bis(Gd-chelate)s which enable cross effect (CE) DNP owing to spatial confinement of two dipolar-coupled electron spins. Their well-defined Gd⋅⋅⋅Gd distances in the range of 1.2–3.4 nm allowed us to elucidate the Gd⋅⋅⋅Gd distance dependence of the DNP mechanism and NMR signal enhancement. We found that Gd⋅⋅⋅Gd distances above 2.1 nm result in solid effect DNP while distances between 1.2 and 2.1 nm enable CE for 1H, 13C, and 15N nuclear spins. We compare 263 GHz electron paramagnetic resonance (EPR) spectra with the obtained DNP field profiles and discuss possible CE matching conditions within the high-spin system and the influence of dipolar broadening of the EPR signal. Our findings foster the understanding of the CE mechanism and the design of high-spin PAs for specific applications of DNP. Poles apart: Bis(Gd-chelate)s are investigated as polarizing agents for dynamic nuclear polarization (DNP). On varying the length of the linker between two chelated GdIII ions, a transition from the solid effect to the cross effect DNP mechanism is observed. The analysis of NMR signal enhancements for 1H, 13C, and 15N sheds light on the distance dependence between two electron spins for the cross effect.
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Activation of SF6 at Platinum Complexes: Formation of SF3 Derivatives and Their Application in Deoxyfluorination Reactions ()
The activation of SF6 at [Pt(PR3)2] R=Cy, iPr complexes in the presence of PR3 led selectively and in an unprecedented reaction route to the generation of the SF3 complexes trans-[Pt(F)(SF3)(PR3)2]. These can also be synthesized from SF4 and the SF2 derivative trans-[Pt(F)(SF2)(PCy3)2][BF4] was characterized by X-ray crystallography. trans-[Pt(F)(SF3)(PR3)2] complexes are useful tools for deoxyfluorination reactions and novel fluorido complexes bearing a SOF ligand are formed. Based on these studies a process for the deoxyfluorination of ketones was developed with SF6 as fluorinating agent. Special eFFects: The activation of SF6 at Pt complexes yields SF3 fluorido complexes which can be used for the fluorination of ketones.
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Redox Self-Adaptation of a Nitrene Transfer Catalyst to the Substrate Needs ()
The development of iron catalysts for carbon–heteroatom bond formation, which has attracted strong interest in the context of green chemistry and nitrene transfer, has emerged as the most promising way to versatile amine synthetic processes. A diiron system was previously developed that proved efficient in catalytic sulfimidations and aziridinations thanks to an FeIIIFeIV active species. To deal with more demanding benzylic and aliphatic substrates, the catalyst was found to activate itself to a FeIIIFeIVL. active species able to catalyze aliphatic amination. Extensive DFT calculations show that this activation event drastically enhances the electron affinity of the active species to match the substrates requirements. Overall this process consists in a redox self-adaptation of the catalyst to the substrate needs. Self-adaptive catalyst: An efficient diiron catalyst mediates nitrene transfer to sulfides through an FeIV active state but self-activates to FeV when facing aliphatic substrates that are harder to oxidize.
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Alkenyl Isocyanide Conjugate Additions: A Rapid Route to γ-Carbolines ()
Isocyanides are exceptional building blocks, the wide deployment of which in multicomponent and metal-insertion reactions belies their limited availability. The first conjugate addition/alkylation to alkenyl isocyanides is described, which addresses this deficiency. An array of organolithiums, magnesiates, enolates, and metalated nitriles add conjugately to β- and β,β-disubstituted arylsulfonyl alkenyl isocyanides to rapidly assemble diverse isocyanide scaffolds. The intermediate metalated isocyanides are efficiently trapped with electrophiles to generate substituted isocyanides incorporating contiguous tri- and tetra-substituted centers. The substituted isocyanides are ideally functionalized for elaboration into synthetic targets as illustrated by the three-step synthesis of γ-carboline N-methyl ingenine B. Conjugate addition of diverse organometallic compounds to sulfonyl-substituted alkenyl isocyanides overcomes the historical challenge of rapidly assembling complex isocyanides that retain the isocyanide functionality. The strategy affords complex isocyanides that are poised for elaboration into heterocycles, as illustrated by the three-step synthesis of N-methyl ingenine B.
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Interactions of Renal-Clearable Gold Nanoparticles with Tumor Microenvironments: Vasculature and Acidity Effects ()
The success of nanomedicines in the clinic depends on our comprehensive understanding of nano–bio interactions in tumor microenvironments, which are characterized by dense leaky microvasculature and acidic extracellular pH (pHe) values. Herein, we investigated the accumulation of ultrasmall renal-clearable gold NPs (AuNPs) with and without acidity targeting in xenograft mouse models of two prostate cancer types, PC-3 and LNCaP, with distinct microenvironments. Our results show that both sets of AuNPs could easily penetrate into the tumors but their uptake and retention were mainly dictated by the tumor microvasculature and the enhanced permeability and retention effect over the entire targeting process. On the other hand, increased tumor acidity indeed enhanced the uptake of AuNPs with acidity targeting, but only for a limited period of time. By making use of simple surface chemistry, these two effects can be synchronized in time for high tumor targeting, opening new possibilities to further improve the targeting efficiencies of nanomedicines. The accumulation of ultrasmall renal-clearable gold nanoparticles (AuNPs) with and without acidity targeting was investigated in xenograft mouse models of two prostate cancer types with distinct microenvironments. Both sets of AuNPs can easily penetrate into the tumors, but their uptake and retention are mainly dictated by the tumor microvasculature.
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Tandem Coupling of Azide with Isonitrile and Boronic Acid: Facile Access to Functionalized Amidines ()
Amidine is a notable nitrogen-containing structural motif found in bioactive natural products and pharmaceuticals. Herein, a novel rhodium(I)-catalyzed tandem reaction of readily accessible azides with isonitriles and boronic acids via a carbodiimide intermediate is achieved. This protocol offers an alternative approach toward N-sulfonyl-, N-acyl-, and N- phosphoryl-functionalized, as well as general N-aryl and N-alkyl amidines with broad substrate scope. In addition, functionalized guanidines can also been synthesized when amines are used instead. The accomplishment of estrone-derived amidine and glibenclamide bioisosteres further reveals the practical utility of this strategy. In tandem: A rhodium(I)-catalyzed tandem reaction of readily accessible azides with isonitriles and boronic acids via a carbodiimide intermediate offers an alternative approach towards N-sulfonyl-, N-acyl-, and N-phosphoryl-functionalized as well as general N-aryl and N-alkyl amidines with broad substrate scope. The synthesis of estrone-derived amidine and glibenclamide bioisosteres reveals the practical utility of this strategy.
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Expansion of the ADOR Strategy for the Synthesis of Zeolites: The Synthesis of IPC-12 from Zeolite UOV ()
The assembly–disassembly–organization–reassembly (ADOR) process has been used to disassemble a parent zeolite with the UOV structure type and then reassemble the resulting layers into a novel structure, IPC-12. The structure of the material has previously been predicted computationally and confirmed in our experiments using X-ray diffraction and atomic resolution STEM-HAADF electron microscopy. This is the first successful application of the ADOR process to a material with porous layers. A germane assembly: The synthesis of a new zeolite IPC-12 using the assembly–disassembly–organization–reassembly (ADOR) transformation of a germanosilicate zeolite with the UOV topology is reported.
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Cobalt-Catalyzed Z-Selective Hydrosilylation of Terminal Alkynes ()
A cobalt-catalyzed Z-selective hydrosilylation of alkynes has been developed relying on catalysts generated from bench-stable Co(OAc)2 and pyridine-2,6-diimine (PDI) ligands. A variety of functionalized aromatic and aliphatic alkynes undergo this transformation, yielding Z-vinylsilanes in high yields with excellent selectivities (Z/E ratio ranges from 90:10 to >99:1). The addition of a catalytic amount of phenol effectively suppressed the Z/E-isomerization of the Z-vinylsilanes that formed under catalytic conditions. Catching some Z’s: A cobalt catalyst, which was generated from bench-stable Co(OAc)2 and a pyridine-2,6-diimine derivative and activated in situ by PhSiH3, promotes the highly regio- and Z-selective hydrosilylation of terminal aromatic and aliphatic alkynes. This reaction tolerates a variety of functional groups such as halide, aldehyde, ketone, ester, amide, cyano, and free hydroxy and amine functionalities.
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Unexpected Photodegradation of a Phosphaketenyl-Substituted Germyliumylidene Borate Complex ()
The first zwitterionic borata-bis(NHC)-stabilized phosphaketenyl germyliumylidene [(L2(O=C=P)Ge:] 2 (L2=(p-tolyl)2B[1-(1-adamantyl)-3-yl-2-ylidene]2) has been synthesized by salt-metathesis reaction of [L2(Cl)Ge:] 1 with sodium phosphaethynolate [(dioxane)nNaOCP]. Unexpectedly, its exposure to UV light affords, after reductive elimination of the entire PCO group, the unprecedented [L2Ge-GeL2] complex 3 in 54 % yields bearing the Ge22+ ion with Ge in the oxidation state +1. In addition, the 1,3-digermylium-2,4-diphosphacyclobutadiene [L2Ge(μ-P)2GeL2] 4 and bis(germyliumylidenyl)-substituted diphosphene [(L2Ge-P=P-GeL2)] 5 could also be obtained in moderate yields. The formation of 3–5 and their electronic structures have been elucidated with DFT calculations. Three birds with one stone: UV irradiation of the PCO-functionalized germyliumylidene 1 supported by a bis-NHC borate (L2−) affords the first digermyliumylidene dication complex 2 as the main product through reductive elimination of the PCO group in 1 and subsequent GeI–GeI bond formation. In addition, the unusual 1,3-digermylium-2,4-diphosphacyclobutadiene diborate 3 and the unprecedented bis(germyliumylidenyl)diphosphene diborate 4 are also isolated in low yields.
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Brønsted Acid Catalysis in Visible-Light-Induced [2+2] Photocycloaddition Reactions of Enone Dithianes ()
1,3-Dithiane-protected enones (enone dithianes) were found to undergo an intramolecular [2+2] photocycloaddition under visible-light irradiation (λ=405 nm) in the presence of a Brønsted acid (7.5–10 mol %). Key to the success of the reaction is presumably the formation of colored thionium ions, which are intermediates of the catalytic cycle. Cyclobutanes were thus obtained in very good yields (78–90 %). It is also shown that the dithiane moiety can be reductively or oxidatively removed without affecting the photochemically constructed ring skeleton. Acidic metamorphosis: The photochemically inert enone dithianes 1 are transformed into yellow thionium ions in the presence of Brønsted acid 3. They can consequently be converted into the [2+2] photocycloaddition products 2 in high yields (78–90 %) in an acid-catalyzed process under visible-light irradiation.
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Long-Chain Alkyl Cyanides: Unprecedented Volatile Compounds Released by Pseudomonas and Micromonospora Bacteria ()
The analysis of volatiles from bacterial cultures revealed long-chain aliphatic nitriles, a new class of natural products. Such nitriles are produced by both Gram-positive Micromonospora echinospora and Gram-negative Pseudomonas veronii bacteria, although the structures differ. A variable sequence of chain elongation and dehydration in the fatty acid biosynthesis leads to either unbranched saturated or unsaturated nitriles with an ω−7 double bond, such as (Z)-11-octadecenenitrile, or methyl-branched unsaturated nitriles with the double bond located at C-3, such as (Z)-13-methyltetradec-3-enenitrile. The nitrile biosynthesis starts from fatty acids, which are converted into their amides and finally dehydrated. The structures and biosyntheses of the 19 naturally occurring compounds were elucidated by mass spectrometry, synthesis, and feeding experiments with deuterium-labeled precursors. Some of the nitriles showed antimicrobial activity, for example, against multiresistant Staphylococcus aureus strains. A new class of natural products featuring long-chain aliphatic nitriles is produced by Gram-positive and Gram-negative bacteria. Variable chain lengths and degrees of hydration in the fatty acid biosynthesis lead to either unbranched or methyl-branched unsaturated nitriles with an ω−7 or C-3 double bond. Some of these nitriles showed antimicrobial activity, for example, against multiresistant Staphylococcus aureus strains.
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An Efficient Chemoenzymatic Synthesis of Dihydroartemisinic Aldehyde ()
Artemisinin from the plant Artemisia annua is the most potent pharmaceutical for the treatment of malaria. In the plant, the sesquiterpene cyclase amorphadiene synthase, a cytochrome-dependent CYP450, and an aldehyde reductase convert farnesyl diphosphate (FDP) into dihydroartemisinic aldehyde (DHAAl), which is a key intermediate in the biosynthesis of artemisinin and a semisynthetic precursor for its chemical synthesis. Here, we report a chemoenzymatic process that is able to deliver DHAAl using only the sesquiterpene synthase from a carefully designed hydroxylated FDP derivative. This process, which reverses the natural order of cyclization of FDP and oxidation of the sesquiterpene hydrocarbon, provides a significant improvement in the synthesis of DHAAl and demonstrates the potential of substrate engineering in the terpene synthase mediated synthesis of high-value natural products. An offering to Artemis: Amorphadiene synthase (ADS) from Artemisia annua can be used to convert 12-hydroxyfarnesyl diphosphate into dihydroartemisinic aldehyde, a direct biosynthetic precursor of the high-value antimalarial drug artemisinin.
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Biocatalytic Total Synthesis of Ikarugamycin ()
Nature provides an inexhaustible diversity of small organic molecules with beautiful molecular architectures that have strong and selective inhibitory activities. However, this tremendous biomedical potential often remains inaccessible, as the structural complexity of natural products can render their synthetic preparation extremely challenging. This problem is addressable by harnessing the biocatalytic procedures evolved by nature. In this work, we present an enzymatic total synthesis of ikarugamycin. The use of an iterative PKS/NRPS machinery and two reductases has allowed the construction of 15 carbon–carbon and 2 carbon–nitrogen bonds in a biocatalytic one-pot reaction. By scaling-up this method we demonstrate the applicability of biocatalytic approaches for the ex vivo synthesis of complex natural products. Three enzymes in one pot: The total synthesis of the structurally challenging natural product ikarugamycin has been achieved by a biocatalytic approach. The compound is synthesized in a one-pot reaction with only three recombinant enzymes, which install 17 individual bonds and set 8 stereogenic centers highly selectively.
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Stabilization of High Oxidation State Upconversion Nanoparticles by N-Heterocyclic Carbenes ()
The stabilization of high oxidation state nanoparticles by N-heterocyclic carbenes is reported. Such nanoparticles represent an important subset in the field of nanoparticles, with different and more challenging requirements for suitable ligands compared to elemental metal nanoparticles. N-Heterocyclic carbene coated NaYF4:Yb,Tm upconversion nanoparticles were synthesized by a ligand-exchange reaction from a well-defined precursor. This new photoactive material was characterized in detail and employed in the activation of photoresponsive molecules by low-intensity near-infrared light (λ=980 nm). Up, up, and carbene: High oxidation state NaYF4:Yb,Tm upconversion nanoparticles (UCNPs) were stabilized by N-heterocyclic carbenes (NHCs) through a ligand-exchange reaction from a well-defined precursor and characterized in detail. The ligands for such nanoparticles require different properties compared to elemental metal nanoparticles. This new photoactive material was employed in reactions of photoresponsive molecules.
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