Showing papers in "Chemistry: A European Journal in 2017"

Neural-Symbolic Machine Learning for Retrosynthesis and Reaction Prediction

Abstract: Reaction prediction and retrosynthesis are the cornerstones of organic chemistry. Rule-based expert systems have been the most widespread approach to computationally solve these two related challenges to date. However, reaction rules often fail because they ignore the molecular context, which leads to reactivity conflicts. Herein, we report that deep neural networks can learn to resolve reactivity conflicts and to prioritize the most suitable transformation rules. We show that by training our model on 3.5 million reactions taken from the collective published knowledge of the entire discipline of chemistry, our model exhibits a top10-accuracy of 95 % in retrosynthesis and 97 % for reaction prediction on a validation set of almost 1 million reactions.

Difluoromethylation Reactions of Organic Compounds.

Abstract: Fil: Yerien, Damian Emilio. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina. Universidad de Buenos Aires. Facultad de Farmacia y Bioquimica. Departamento de Quimica Organica; Argentina

Decarboxylation as the Key Step in C−C Bond‐Forming Reactions

Abstract: C−C bond forming reactions incarnate the core of organic synthesis because of their fundamental applications to molecular diversity and complexity. In recent years, use of carboxylic acid as one of the coupling partners in place of conventional organometallic reagents has seen an upsurge due to its potency to generate similar organometallic intermediates after decarboxylation. This Review provides an overview on the most recent progress in the field of C−C bond formation involving decarboxylation as a key step. Different important developments, which are not included in earlier Reviews in this area, have been summarized with representative examples and discussions on their reaction mechanisms.

Molybdenum carbide based electrocatalysts for hydrogen evolution reaction

Abstract: Electrocatalytic water splitting is a promising approach for clean and sustainable hydrogen production. Its large-scale application relies on the availability of low cost and efficient electrocatalysts. Earth-abundant transition-metal carbides, especially molybdenum carbides (Mox C), are regarded as potential candidates to replace state-of-art but expensive platinum-group electrocatalysts. In this Review, we summarize recent progress in Mox C electrocatalysts for hydrogen evolution reaction (HER). Nanostructure engineering on the design and preparation of highly efficient electrocatalysts based on Mox C is presented, followed by the comparison and discussion of HER performance on Mox C-based electrocatalysts. Finally, we offer a perspective on the future development of Mox C-based electrocatalysts towards HER.

Twisted Amides: From Obscurity to Broadly Useful Transition-Metal Catalyzed Reactions by N-C Amide Bond Activation.

Abstract: The concept of using amide bond distortion to modulate amidic resonance has been known for more than 75 years. Two classic twisted amides (bridged lactams) ingeniously designed and synthesized by Kirby and Stoltz to feature fully perpendicular amide bonds, and as a consequence emanate amino-ketone-like reactivity, are now routinely recognized in all organic chemistry textbooks. However, only recently the use of amide bond twist (distortion) has advanced to the general organic chemistry mainstream enabling a host of highly attractive N−C amide bond cross-coupling reactions of broad synthetic relevance. In this Minireview, we discuss recent progress in this area and present a detailed overview of the prominent role of amide bond destabilization as a driving force in the development of transition-metal-catalyzed cross-coupling reactions by N−C bond activation.

Layered Structural Co‐Based MOF with Conductive Network Frames as a New Supercapacitor Electrode

Abstract: Layered structural Co-MOF nanosheets were synthesized and then used as an electrode material for supercapacitors for the first time This material exhibited a high specific capacitance, a good rate capability, and an excellent cycling stability A maximum capacitance of 2564 F g-1 can be achieved at a current density of 1 Ag-1 Moreover, the capacitance retention can be kept at 958 % respectively of its initial value after 3000 cycles To the best of our knowledge, both the specific capacitance and the capacitance retention were the highest values reported for MOF materials as supercapacitor electrodes until now Such a high supercapacitive performance might be attributed to the intrinsic characteristics of this kind of Co-MOF material, including its layered structure, conductive network frame, and thin nanosheet

Ab Initio Crystal Field for Lanthanides.

Abstract: An ab initio methodology for the first-principle derivation of crystal-field (CF) parameters for lanthanides is described. The methodology is applied to the analysis of CF parameters in [Tb(Pc)2 ]- (Pc=phthalocyanine) and Dy4 K2 ([Dy4 K2 O(OtBu)12 ]) complexes, and compared with often used approximate and model descriptions. It is found that the application of geometry symmetrization, and the use of electrostatic point-charge and phenomenological CF models, lead to unacceptably large deviations from predictions based on ab initio calculations for experimental geometry. It is shown how the predictions of standard CASSCF (Complete Active Space Self-Consistent Field) calculations (with 4f orbitals in the active space) can be systematically improved by including effects of dynamical electronic correlation (CASPT2 step) and by admixing electronic configurations of the 5d shell. This is exemplified for the well-studied Er-trensal complex (H3 trensal=2,2',2"-tris(salicylideneimido)trimethylamine). The electrostatic contributions to CF parameters in this complex, calculated with true charge distributions in the ligands, yield less than half of the total CF splitting, thus pointing to the dominant role of covalent effects. This analysis allows the conclusion that ab initio crystal field is an essential tool for the decent description of lanthanides.

Inorganic Colloidal Perovskite Quantum Dots for Robust Solar CO2 Reduction

Abstract: Inorganic perovskite quantum dots as optoelectronic materials have attracted enormous attention in light-harvesting and emitting devices However, photocatalytic conversion based on inorganic perovskite halides has not been reported Here, we have synthesized colloidal quantum dots (QDs, 3-12 nm) of cesium lead halide perovskites (CsPbBr3 ) as a new type of photocatalytic material The band gap energies and photoluminescence (PL) spectra are tunable over the visible spectral region according to quantum size effects on an atomic scale The increased carrier lifetime revealed by time-resolved PL spectra, indicates the efficient electron-hole separation and transfer As expected, the CsPbBr3 QDs with high selectivity of greater than 99 % achieve an efficient yield of 209 μmol g-1 towards solar CO2 reduction This work has opened a new avenue for inorganic colloidal perovskite materials as efficient photocatalysts to convert CO2 into valuable fuels

Immobilizing Molecular Metal Dithiolene-Diamine Complexes on 2D Metal-Organic Frameworks for Electrocatalytic H2 Production.

Abstract: Carbon electrocatalysts consisting of metal complexes such as MNx or MSx are promising alternatives to high-cost Pt catalysts for the hydrogen evolution reaction (HER). However, the exact HER active sites remain elusive. Here, molecular metal dithiolene-diamine (MS2 N2 , M=Co and Ni), metal bis(dithiolene) (MS4 ), and metal bis(diamine) (MN4 ) complexes were selectively incorporated into carbon-rich 2D metal-organic frameworks (2D MOFs) as model carbon electrocatalysts. The 2D MOF single layers, powders, and composites with graphene were thus prepared and showed definite active sites for H2 generation. The electrocatalytic HER activity of the 2D MOF-based catalysts with different metal complexes follow the order of MS2 N2 >MN4 >MS4 . Moreover, the protonation preferentially occurred on the metal atoms, and the concomitant heterolytic elimination of H2 was favored on the M-N units in the MS2 N2 active centers. The results provide an in-depth understanding of the catalytic active sites, thus making way for the future development of metal complexes in carbon-rich electrode materials for energy generation.

Viologens and Their Application as Functional Materials

Abstract: Organic materials have recently gained considerable attention for electronic applications, improving performance and sustainability to current technologies. Commercialized metal-based systems are generally expensive, toxic and difficult to recycle, however organic materials offer promising solutions. Viologens, N,N' di-quaternized bipyridyl salts, are a well-studied species exhibiting three reversible redox states, possessing valuable electrochromic and electron-accepting properties. These properties can be fine-tuned through synthesis by altering the nitrogen substituents and various counteranions. Currently, viologens have become of great interest as functional materials in a wide array of applications; a few to name include electrochromic devices, molecular machines, and organic batteries. This review highlights representative recent work and advances towards utilizing viologens in practical applications that currently compete with metal-based technologies. Additionally, modified viologens that can be further fine-tuned will be discussed.

Manganese-Catalyzed Hydrogenation of Esters to Alcohols.

Abstract: Homogeneous catalytic hydrogenation of esters to alcohols is an industrially important, environmentally benign reaction While precious metal-based catalysts for this reaction are now well known, only very few catalysts based on first-row metal complexes were reported Here we present the hydrogenation of esters catalyzed by a complex of earth-abundant manganese The reaction proceeds under mild conditions and insight into the mechanism is provided based on an NMR study and the synthesis of novel Mn complexes postulated as intermediates

Visible Light Induced Organic Transformations Using Metal‐Organic‐Frameworks (MOFs)

Abstract: With the aim of developing renewable energy based processes, researchers are paying increasing interest to light induced organic transformations. Metal-organic frameworks (MOFs), a class of micro-/mesoporous hybrid materials, are recently emerging as a new type of photoactive materials for organic syntheses due to their unique structural characteristics. In this Review, we summarized the recent applications of MOFs as photocatalysts for light induced organic transformations, including (1) oxidation of alcohols, amines, alkene, alkanes and sulfides; (2) hydroxylation of aromatic compounds like benzene; (3) activation of the C-H bonds to construct new C-C or C-X bonds; (4) atom-transfer radical polymerization (ATRP). This Review starts with general background information of using MOFs in photocatalysis, followed by a description of light induced organic transformations promoted by photoactive inorganic nodes and photocatalytic active ligands in MOFs, respectively. Thereafter, the use of MOFs as multifunctional catalysts for light induced organic transformations via an efficient merge of the metal/ligand/guest based catalysis where the photocatalytic activity of MOFs plays a key role are discussed. Finally, the limitations, challenges and the future perspective of the application of MOFs for light induced organic transformations were addressed. The objective of this Review is to serve as a starting point for other researchers to get into this largely unexplored field. It is also our goal to stimulate intensive research in this field for rational designing of MOF materials to overcome their current limitations in photocatalysis, which can lead to more creative visible-light-induced organic transformations.

Gold‐Catalyzed Enantioselective Annulations

Abstract: In the past decade, there have been many extraordinary advances in the development of gold-catalyzed enantioselective annulations, such as cycloadditions, cyclizations, cycloisomerizations, and tandem annulations, which are of particular interest owing to their potential for rapid construction of optically active hetero- and carbocyclic molecules. This Review summarizes the methods to construct chiral cyclic compounds by gold-catalyzed enantioselective annulations reported since 2005. The Review is organized according to the general annulation types catalyzed by chiral gold complexes or chiral gold salts, which have four main types (cycloadditions, cyclizations of C−C multiple bonds with tethered nucleophiles, cycloisomerization or cyclization of enynes, and tandem annulations), as well as some other strategies. The general reaction mechanisms of each subcategory, key intermediates for some unusual transformations, and the application of several novel ligands and chiral gold salts are also discussed.

Direct Functionalization of C−H Bonds by Iron, Nickel, and Cobalt Catalysis

Abstract: Non-precious-metal-catalyzed reactions are of increasing importance in chemistry due to the outstanding ecological and economic properties of these metals. In the subfield of metal-catalyzed direct C−H functionalization reactions, recent years have shown an increasing number of publications dedicated to this topic. Nickel, cobalt, and last but not least iron, have started to enter a field which was long dominated by precious metals such as palladium, rhodium, ruthenium, and iridium. The present review article summarizes the development of iron-, nickel-, and cobalt-catalyzed C−H functionalization reactions until the end of 2016, and discusses the scope and limitations of these transformations.

Feasibility of N2 Binding and Reduction to Ammonia on Fe-Deposited MoS2 2D Sheets: A DFT Study

Abstract: LMA and LC acknowledge King Abdullah University of Science and Technology (KAUST) for support, and CS and DRM thank the Australian Research Council (ARC) for a Future Fellowship and Laureate Fellowship, respectively, as well as support through the ARC Centre of Excellence for Electromaterials Science Gratitude is also due to the KAUST Supercomputing Laboratory using the supercomputer Shaheen II and the National Computational Infrastructure (NCI) for providing the computational resources

Laying Waste to Mercury: Inexpensive Sorbents Made from Sulfur and Recycled Cooking Oils

Abstract: Mercury pollution threatens the environment and human health across the globe. This neurotoxic substance is encountered in artisanal gold mining, coal combustion, oil and gas refining, waste incineration, chloralkali plant operation, metallurgy, and areas of agriculture in which mercury-rich fungicides are used. Thousands of tonnes of mercury are emitted annually through these activities. With the Minamata Convention on Mercury entering force this year, increasing regulation of mercury pollution is imminent. It is therefore critical to provide inexpensive and scalable mercury sorbents. The research herein addresses this need by introducing low-cost mercury sorbents made solely from sulfur and unsaturated cooking oils. A porous version of the polymer was prepared by simply synthesising the polymer in the presence of a sodium chloride porogen. The resulting material is a rubber that captures liquid mercury metal, mercury vapour, inorganic mercury bound to organic matter, and highly toxic alkylmercury compounds. Mercury removal from air, water and soil was demonstrated. Because sulfur is a by-product of petroleum refining and spent cooking oils from the food industry are suitable starting materials, these mercury-capturing polymers can be synthesised entirely from waste and supplied on multi-kilogram scales. This study is therefore an advance in waste valorisation and environmental chemistry.

Ring-Expansion Reactions in the Synthesis of Macrocycles and Medium-Sized Rings.

Abstract: Functionalised macrocycles and medium-sized rings have applications in a number of scientific fields, ranging from medicinal chemistry and supramolecular chemistry, to catalysis and nanotechnology. However, their value in these areas can be undermined by a simple, but important limitation: large ring systems are very often difficult to make. Traditional end-to-end cyclisation reactions of long linear precursors are typically unpredictable and impractical processes, mainly due to unfavourable enthalpic and entropic factors. Most published methods to make large rings focus on minimising the damage inflicted by performing the difficult cyclisation step; in contrast, ring-expansion reactions enable it to be avoided altogether. In this Review article, it is highlighted how “growing” rings from existing cyclic systems via ring expansion can expedite the efficient, practical and scalable synthesis of macrocycles and medium-sized rings.

Improved and General Manganese‐Catalyzed N‐Methylation of Aromatic Amines Using Methanol

Abstract: A novel lutidine-based manganese PNP-pincer complex has been synthesized for the selective N-methylation of aromatic amines with methanol. Using borrowing hydrogen methodology, a selection of differently functionalized aniline derivatives is selectively methylated in good yields.

Confined Sulfur in 3 D MXene/Reduced Graphene Oxide Hybrid Nanosheets for Lithium–Sulfur Battery

Abstract: Three-dimensional metal carbide MXene/reduced graphene oxide hybrid nanosheets are prepared and applied as a cathode host material for lithium–sulfur batteries. The composite cathodes are obtained through a facile and effective two-step liquid-phase impregnation method. Owing to the unique 3 D layer structure and functional 2 D surfaces of MXene and reduced graphene oxide nanosheets for effective trapping of sulfur and lithium polysulfides, the MXene/reduced graphene oxide/sulfur composite cathodes deliver a high initial capacity of 1144.2 mAh g−1 at 0.5 C and a high level of capacity retention of 878.4 mAh g−1 after 300 cycles. It is demonstrated that hybrid metal carbide MXene/reduced graphene oxide nanosheets could be a promising cathode host material for lithium–sulfur batteries.

Photoredox-Catalysed Decarboxylative Alkylation of N-Heteroarenes with N-(Acyloxy)phthalimides

Abstract: An iridium photoredox catalyst in combination with either a stoichiometric amount of Bronsted acid or a catalytic amount of Lewis acid is capable of catalyzing regioselective alkylation of N-heteroarenes with N-(acyloxy)phthalimides at room temperature under irradiation. A broad range of N-heteroarenes can be alkylated using a variety of secondary, tertiary, and quaternary carboxylates. Mechanistic studies suggest that an IrII /IrIII redox catalytic cycle is responsible for the observed reactivity.

Strongly Reducing, Visible-Light Organic Photoredox Catalysts as Sustainable Alternatives to Precious Metals.

Abstract: Photoredox catalysis is a versatile approach for the construction of challenging covalent bonds under mild reaction conditions, commonly using photoredox catalysts (PCs) derived from precious metals. As such, there is need to develop organic analogues as sustainable replacements. Although several organic PCs have been introduced, there remains a lack of strongly reducing, visible-light organic PCs. Herein, we establish the critical photophysical and electrochemical characteristics of both a dihydrophenazine and a phenoxazine system that enables their success as strongly reducing, visible-light PCs for trifluoromethylation reactions and dual photoredox/nickel-catalyzed C-N and C-S cross-coupling reactions, both of which have been historically exclusive to precious metal PCs.

Modelling Chemical Reasoning to Predict and Invent Reactions

Abstract: The ability to reason beyond established knowledge allows organic chemists to solve synthetic problems and invent novel transformations. Herein, we propose a model that mimics chemical reasoning, and formalises reaction prediction as finding missing links in a knowledge graph. We have constructed a knowledge graph containing 14.4 million molecules and 8.2 million binary reactions, which represents the bulk of all chemical reactions ever published in the scientific literature. Our model outperforms a rule-based expert system in the reaction prediction task for 180 000 randomly selected binary reactions. The data-driven model generalises even beyond known reaction types, and is thus capable of effectively (re-)discovering novel transformations (even including transition metal-catalysed reactions). Our model enables computers to infer hypotheses about reactivity and reactions by only considering the intrinsic local structure of the graph and because each single reaction prediction is typically achieved in a sub-second time frame, the model can be used as a high-throughput generator of reaction hypotheses for reaction discovery.

Three-Dimensional MoS2@CNT/RGO Network Composites for High-Performance Flexible Supercapacitors

Abstract: Two-dimensional atomically thick materials, reduced graphene oxide (RGO), and layered molybdenum disulfide (MoS2) have been investigated as potential novel energy storage materials because of their distinct physicochemical properties. These materials suffer, however, from rapid capacity decay and low rate capability. This study describes a facile, binder-free approach to fabricate large-scale, 3D network structured MoS2@carbon nanotube (CNT)/RGO composites for application in flexible supercapacitor devices. The as-obtained composites possess a hierarchical porosity, and an interconnected framework. The electrochemical supercapacitive measurements of the MoS2@CNT/RGO electrode show a high specific capacitance of 129 mF cm−2 at 0.1 mA cm−2. The symmetric supercapacitor devices based on the as-obtained composites exhibit a long lifetime (94.7 % capacitance retention after 10 000 cycles), and a high electrochemical performance (29.7 mF cm−2). The present experimental findings will lead to scalable, binder-free synthesis of MoS2@CNT/RGO hybrid electrodes, with enhanced, flexible, supercapacitive performance, in portable and wearable energy storage devices.

Divergent Iron-Catalyzed Coupling of O-Acyloximes with Silyl Enol Ethers.

Abstract: An iron-catalyzed coupling reaction of O-acyloximes and O-benzoyl amidoximes with silyl enol ethers is reported. The protocol provides access to functionalized pyrroles, 1,6-ketonitriles, pyrrolines and imidazolines via carbon-centered radicals generated from an initially formed iminyl radical. The intramolecular cyclization and ring-opening processes of the iminyl radical take place preferentially over reactions that proceed through a 1,3-hydrogen transfer, providing insights into iron-catalyzed reactions with oxime derivatives. The cheap and environmentally friendly iron catalyst, the broad substrate scope and the functional group compatibility make this protocol useful for synthesis of valuable nitrogen-containing products.

Catalytic Strategies for Diastereodivergent Synthesis.

Abstract: Chiral molecules with multiple stereocenters are widely distributed in nature and drugs. Asymmeric catalysis has allowed the formation of a chiral molecule's enantiomers with equal ease by applying the enantiomeric pair of a chiral catalyst. However, the diastereodivergent synthesis is a great challenge because the formation of one of the diastereomers is inherently preferred in most reactions. Under the efforts of chemists, elegant strategies have been developed to full control of absolute and relative stereochemical configurations. In this Minireview, we highlight the significant advances achieved in this field, and present the diastereo-control mechanism in the appropriate place on the expectation that new diastereodivergent strategies can be inspired and more types of enantioselective diastereodivergent reactions can be realized.

Surface Immobilization of Molecular Electrocatalysts for Energy Conversion.

Abstract: Electrocatalysts are critically important for a secure energy future, as they facilitate the conversion between electrical and chemical energy. Molecular catalysts offer precise control of structure that enables understanding of structure–reactivity relationships, which can be difficult to achieve with heterogeneous catalysts. Molecular electrocatalysts can be immobilized on surfaces by covalent bonds or through non-covalent interactions. Advantages of surface immobilization include the need for less catalyst, avoidance of bimolecular decomposition pathways, and easier determination of catalyst lifetime. This Minireview highlights surface immobilization of molecular electrocatalysts for reduction of O2, oxidation of H2O, production of H2, and reduction of CO2.

A Retrosynthesis Approach for Biocatalysis in Organic Synthesis.

Abstract: For the planning of an organic synthesis route, the disconnection approach guided by retrosynthetic analysis of possible intermediates and the chemical reactions involved, back to ready available starting materials, is well established In contrast, such concepts just get developed for biocatalytic routes In this Review we highlight functional group interconversions catalyzed by enzymes The article is organized rather by chemical bonds formed-exemplified for C-N, C-O- and C-C-bonds-and not by enzyme classes, covering a broad range of reactions to incorporate the desired functionality in the target molecule Furthermore, the successful use of biocatalysts, also in combination with chemical steps, is exemplified for the synthesis of various drugs and advanced pharmaceutical intermediates such as Crispine A, Sitagliptin and Atorvastatin This Review also provides some basic guidelines to choose the most appropriate enzyme for a targeted reaction keeping in mind aspects like commercial availability, cofactor-requirement, solvent tolerance, use of isolated enzymes or whole cell recombinant microorganisms aiming to assist organic chemists in the use of enzymes for synthetic applications

Frontispiece: A Lanthanide MOF Thin‐Film Fixed with Co3O4 Nano‐Anchors as a Highly Efficient Luminescent Sensor for Nitrofuran Antibiotics

Abstract: Nitrofurans are a group of widely used veterinary antibiotics which have been forbidden due to antibiotics pollution. Development of a rapid and effective method for the detection of nitrofuran antibiotics (NFAs) are very important but challenging. Herein, we designed a chemical sensor based on a lanthanide metal-organic framework (Ln-MOF) thin film of {[Eu2(BCA)3(H2O)(DMF)3]·0.5DMF·H2O}n (Eu-BCA, where BCA is 2,2′-biquinoline-4,4′-dicarboxylate) fabricated on the cost-effective stainless steel wire mesh (SSWM) by Co3O4 nano-anchors fixation method. The MOF coatings were well adhere to every stainless steel wire of SSWM resulting in a three-dimensional (3D) porous, flexible and processable sensor. The structure of as-prepared MOF thin film is confirmed by powder X-ray diffraction (PXRD) and the surface morphology is examined by scanning electron microscopy (SEM). Significantly, the Eu-BCA thin film exhibited highly selective and sensitive to NFAs, and yet remain unaffected by other common antibiotics that may coexist. The limits of detection for nitrofurantoin (NFT) and nitrofurazone (NFZ) are 0.21 and 0.16 μM, respectively. It was also successfully employed to detect NFAs in original water of Pearl River and bovine serum samples. Hence, the reported Ln-MOFs thin film sensor has a promising application in the determination of NFAs for protecting human being from all manner of hazards that arise from the abuse of antibiotics in livestock breeding.

Nitrene Radical Intermediates in Catalytic Synthesis

Abstract: Nitrene radical complexes are reactive intermediates with discrete spin density at the nitrogen-atom of the nitrene moiety. These species have become important intermediates for organic synthesis, being invoked in a broad range of C-H functionalization and aziridination reactions. Nitrene radical complexes have intriguing electronic structures, and are best described as one-electron reduced Fischer-type nitrenes. They can be generated by intramolecular single electron transfer to the 'redox non-innocent' nitrene moiety at the metal. Nitrene radicals generated at open-shell cobalt(II) have thus far received most attention in terms of spectroscopic characterization, reactivity screening, catalytic application and (computational and experimental) mechanistic studies, but some interesting iron and precious metal catalysts have also been employed in related reactions involving nitrene radicals. In some cases, redox-active ligands are used to facilitate intramolecular single electron transfer from the complex to the nitrene moiety. Organic azides are among the most attractive nitrene precursors in this field, typically requiring pre-activated organic azides (e.g. RSO2N3, (RO)2P(=O)N3, ROC(=O)N3 and alike) to achieve efficient and selective catalysis. Challenging, non-activated aliphatic organic azides were recently added to the palette of synthetically useful reactions proceeding via nitrene radical intermediates. This concept article describes the electronic structure of nitrene radical complexes, emphasizes on their usefulness in the catalytic synthesis of various organic products, and highlights the important developments in the field.