Carbocatalytic Acetylene Cyclotrimerization: A Key Role of Unpaired Electron Delocalization

Gordeev E.G., Pentsak E.O., Ananikov V.P., J. Am. Chem. Soc., 2020, 142, 3784-3796.
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Development of sustainable catalysts for synthetic transformations is one of the most challenging and demanding goals. The high prices of precious metals and the unavoidable leaching of toxic metal species leading to environmental contamination make the transition metal-free catalytic systems especially important. Here we demonstrate that carbene active centers localized on carbon atoms at the zigzag edge of graphene represent an alternative platform for efficient catalytic carbon–carbon bond formation in the synthesis of benzene. The studied acetylene trimerization reaction is an efficient atom-economic route to build an aromatic ring—a step ubiquitously important in organic synthesis and industrial applications. Computational modeling of the reaction mechanism reveals a principal role of the reversible spin density oscillations that govern the overall catalytic cycle, facilitate the product formation, and regenerate the catalytically active centers. Dynamic π-electron interactions in 2D carbon systems open new opportunities in the field of carbocatalysis, unachievable by means of transition metal-catalyzed transformations. The theoretical findings are confirmed experimentally by generating key moieties of the carbon catalyst and performing the acetylene conversion to benzene.

The Key Role of R-NHC Couplings (R = C, H, Heteroatom) and M-NHC Bond Cleavage in the Evolution of M/NHC Complexes and Formation of Catalytically Active Species

Chernyshev V. M., Denisova E.A., Eremin D. B., Ananikov V. P., Chem. Sci., 2020, ASAP.
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Complexes of metals with N-heterocyclic carbene ligands (M/NHC) are typically considered the systems of choice in homogeneous catalysis due to their stable metal−ligand framework. However, it becomes obvious that even metal species with a strong M-NHC bond can undergo evolution in catalytic systems, and processes of M-NHC bond cleavage are common for different metals and NHC ligands. This review is focused on the main types of the M-NHC bond cleavage reactions and their impact on activity and stability of M/NHC catalytic systems. For the first time, we consider these processes in terms of NHC-connected and NHC-disconnected active species derived from M/NHC precatalysts and classify them as fundamentally different types of catalysts. Problems of rational catalyst design and sustainability issues are discussed in the context of the two different types of M/NHC catalysis mechanisms.

Preventing Pd-NHC Bond Cleavage and Switching from Nano-Scale to Molecular Catalytic Systems: Amines and Temperature as Catalyst Activators

Khazipov O. V., Shevchenko M. A., Pasyukov D. V., Chernenko A. Yu., Astakhov A. V., Tafeenko V. A., Chernyshev V. M., Ananikov V. P., Catal. Sci. Technol., 2020, 10, 1228-1247 .
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Many reactions catalyzed by Pd complexes with N-heterocyclic carbene (NHC) ligands are performed in the presence of amines which usually act as coupling reagents or mild bases. However, amines can react with Pd/NHC complexes in a number of ways: enhancing molecular catalysis, causing the catalyst deactivation or triggering the ligandless modes of catalysis by producing NHC-free active palladium species. This study gains insight into conditions required for the efficient use of amines as activators of molecular Pd/NHC catalysis and preventing the undesirable reductive cleavage of the Pd-NHC bond in catalytic systems. Reactions of Pd/NHC complexes with various amines within a temperature range of 25–140 °C and thermal stability of the resulting amino-complexes are examined. The results indicate the major influence of the amine structure and reaction temperature on the catalyst transformations. In particular, thermal decomposition of Pd/NHC complexes with aliphatic amine ligands predominantly leads to the reductive Pd-NHC bond cleavage, while deprotonation of the complexes with primary and secondary aliphatic amine ligands in the presence of strong bases at 25–60 °C promotes the activation of molecular Pd/NHC catalysis. Efficient Pd-PEPPSI complex – amine systems suitable for the strong-base-promoted C-S cross-coupling reactions between aryl halides and thiols are suggested on the basis of these findings.

Electron Microscopy Dataset for the Recognition of Nanoscale Ordering Effects and Location of Nanoparticles

Boiko D.A., Pentsak E.O., Cherepanova V.A., Ananikov V.P., Sci. Data, 2020, 7, 101.
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A unique ordering effect has been observed in functional catalytic nanoscale materials. Instead of randomly arranged binding to the catalyst surface, metal nanoparticles show spatially ordered behavior resulting in formation of geometrical patterns. Understanding of such nanoscale materials and analysis of corresponding microscopy images will never be comprehensive without appropriate reference datasets. Here we describe the first dataset of electron microscopy images comprising individual nanoparticles which undergo ordering on a surface towards the formation of geometrical patterns. The dataset developed in this study spans three levels of nanoscale organization: (i) individual nanoparticles (1–5 nm) and arrays of nanoparticles (5–20 nm), (ii) ordering effects (20–200 nm) and (iii) complex patterns (from nm to μm scales). The described dataset for the first time provides a possibility for the development of machine learning algorithms to study the unique phenomena of nanoparticles ordering and hierarchical organization.

Visualization of Catalyst Dynamics and Development of a Practical Procedure to Study Complex “Cocktail”-Type Catalytic Systems

Galushko A.S., Gordeev E.G., Kashin A.S., Zubavichus Y.V., Ananikov V.P., Faraday Discuss., 2020, ASAP.
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The ability to distinguish molecular catalysis from nanoscale catalysis provides a key to success in the field of catalyst development, particularly for the transition to sustainable economies. Complex evolution of catalyst precursors, facilitated by dynamic interconversions and leaching, makes the identification of catalytically active forms an independent task, sometimes very difficult. We propose a simple method for in situ capturing of nanoparticles with carbon-coated grids directly from reaction mixtures. Application of this method to Mizoroki-Heck reaction allowed visualization of dynamic changes of the dominant form of palladium particles in reaction mixtures with homogeneous and heterogeneous catalyst precursors. Changes in the size and shape of palladium particles reflecting the progress of the catalytic chemical reaction were demonstrated. Detailed computational modeling was carried out to confirm the generality of this approach and its feasibility for different catalytic systems. The computational models revealed strong binding of metal particles to the carbon coating comprising efficient binding sites. The approach was tested for trapping Cr, Co, Ag, Ni, Cu, Pd, Cd, Ir, Ru and Rh nanoparticles from solutions containing micromolar starting concentrations of the metal precursors. The developed approach provides a unique tool for studying intrinsic properties of catalytic systems.

Ambident Reactivity of Imidazolium Cations as Evidence of the Dynamic Nature of N‐Heterocyclic Carbene‐Mediated Organocatalysis

Galkin K.I., Karlinskii B.Ya., Kostyukovich A.Yu., Gordeev E.G., Ananikov V.P., Chem. Eur. J., 2020, ASAP.
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This work reveals ambident nucleophilic reactivity of imidazolium cations towards carbonyl compounds at the C2 or C4 carbene centers depending on the steric properties of the substrates and reaction conditions. Such an adaptive behavior indicates the dynamic nature of organocatalysis proceeding via a covalent interaction of imidazolium carbenes with carbonyl substrates and can be explained by generation of the H‐bonded ditopic carbanionic carbenes.

Examining the Vinyl Moiety as a Protecting Group for Hydroxyl (-OH) Functionality under Basic Conditions

Voronin V.V., Ledovskaya M.S., Rodygin K.S., Ananikov V.P., Org. Chem. Front., 2020, ASAP.
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A method for protection of alcohols with vinyl groups is suggested and studied in detail. The procedures of protection and deprotection via vinylation and devinylation reactions are evaluated. Vinylation reaction is performed using cheap and convenient calcium carbide reagent. Stability of the vinyl group under various conditions is examined. The vinyl group is found to be stable under basic conditions and labile under acidic conditions. The vinyl protecting group shows high tolerance to functional groups and good compatibility with common synthetic reagents. Applicability of the procedure in the Suzuki and Sonogashira catalytic reactions and its flexible utilization in the reaction with Grignard reagent are demonstrated.

Efficient Labeling of Organic Molecules Using 13C Elemental Carbon: Universal Access to 13C2-Labeled Synthetic Building Blocks, Polymers and Pharmaceuticals

Ledovskaya M.S., Voronin V.V., Rodygin K.S., Ananikov V.P., Org. Chem. Front., 2020, 7, 638-647.
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Among different types of labeling, 13C-labeled compounds are the most demanding in organic chemistry, life sciences and materials design. However, 13C-labeled organic molecules are very difficult to employ in practice due to extreme cost. The rather narrow range of labeled organic starting materials and the absence of universal synthetic building units further complicate the problem and make utilization of 13C-labeled molecules hardly possible in many cases. Here we report a versatile approach for 13C2-labeling of organic molecules starting with 13C elemental carbon: 13C carbon is applied for the synthesis of calcium carbide (Ca13C2), which is subsequently used to generate acetylene – a universal 13C2 unit for atom-economic organic transformations. Syntheses of labeled alkynes, O,S,N-functionalized vinyl derivatives, polymers and pharmaceutical substances were demonstrated. Elemental 13C carbon, as the chemically most simple source for 13C2-labeling, here was successfully combined with universal synthetic applicability of alkynes.

Primary Vinyl Ethers as Acetylene Surrogate: a Flexible Tool for Deuterium‐Labeled Pyrazole Synthesis

Ledovskaya M.S., Voronin V.V., Polynski M.V., Lebedev A.N., Ananikov V.P., Eur. J. Org. Chem, 2020, ASAP.
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A novel synthetic path to 1,3‐disubstituted pyrazoles and their deuterated derivatives was developed. It is based on the reaction of vinyl ethers with hydrazonoyl chlorides in the presence of triethylamine. The reaction mechanism, clarified by the joint experimental and computational study, involves 1,3‐dipolar cycloaddition of the in situ generated nitrile imines to vinyl ethers and subsequent cleavage of alcohol from the formed alkoxypyrazoline. The results highlight the possibility of using vinyl ethers as acetylene surrogate and provide a novel access to pyrazoles, 4,5‐dideuteropyrazoles and their regioselectively labeled derivatives, 5‐deuteropyrazoles.

Assessing Possible Influence of Structuring Effects in Solution on Cytotoxicity of Ionic Liquid Systems

Egorova K.S., Posvyatenko A.V., Fakhrutdinov A.N., Kashin A.S., Ananikov V.P., J. Mol. Liquid., 2020, 297, 111751.
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During the last decades, micro-structuring phenomena, formation of polar domains and assembling of nano-scale heterogeneities in ionic liquids (ILs) have been discovered. Here we assess the influence of structuring effects in solution on biological activity of ionic systems. In the present work, we studied cytotoxicity of aqueous solutions of binary mixtures of common ILs and showed that it mostly did not comply with the concentration addition model thus suggesting the occurrence of toxicity-affecting interactions in the media. In most cases, antagonistic effects were observed in the studied systems. Micro-heterogeneous water structures were experimentally detected in the binary IL mixtures for the first time. By using a cytotoxicity assay, NMR spectroscopy and scanning electron microscopy, a novel research direction was explored dealing with a relationship between dynamic structuring effects in ILs and their cytotoxicity.