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Structural Design for Molecular Catalysts
Published in Qingmin Ji, Harald Fuchs, Soft Matters for Catalysts, 2019
Qingmin Ji, Qin Tang, Jonathan P. Hill, Katsuhiko Ariga
The successful isolation and characterization of an N-heterocyclic carbene in 1991 opened up a new class of organic compounds for investigation. N-heterocyclic carbenes (NHCs) today rank among the most powerful tools in organic chemistry, with numerous applications in commercially important processes [105]. Defined as neutral compounds containing a divalent carbon atom with a six-electron valence shell, carbenes are an intriguing class of carbon-containing compounds. Their incomplete electron octet and coordinative unsaturation, render free carbenes inherently unstable and can form highly reactive transient intermediates in organic transformations. As excellent ligands for transition metals, NHCs have found multiple applications in some of the most important catalytic reactions in the chemical industry. With intensive exploration on the rich chemistry of these compounds, they have become new major class of organocatalysts for research.
Reactions of thiocarbonyl compounds with electrophilic and nucleophilic carbenes as well as with their metal complexes
Published in Journal of Sulfur Chemistry, 2020
Grzegorz Mlostoń, Heinz Heimgartner
Another method to generate reactive thiocarbonyl ylides comprises the electrophilic attack of a two-valent carbene 8 or carbenoid onto the electron-rich sulfur atom of the C = S group. Diazomethane derivatives are known as the most important source of carbenes/carbenoids, which are formed upon heating, photolysis or by metal-assisted decomposition. It is well established that diazomethanes, functionalized at the α-position with electron withdrawing groups, display reduced reactivity as 1,3-dipoles, and in most cases their reactions with thiocarbonyl groups require the involvement of a catalyst to change the reaction type from the [3 + 2]-cycloaddition to the electrophilic approach of the carbene. The formation of thiocarbonyl ylides can also be achieved when reactive carbenes are generated from other precursors, e.g. dichlorocarbene from chloroform in a two-phase system or by thermal decomposition of Seyfert's reagent (PhHgCCl3).
Intermolecular interactions involving the N-heterocyclic carbene and its heavier analogues C2H4N2X: (X = C, Si, Ge, Sn, Pb) with YH3F (Y = Si, Ge, Sn, Pb)
Published in Molecular Physics, 2023
Yishan Chen, Lifeng Yao, Fan Wang
Carbenes are generally too reactive to be isolated, but N-heterocyclic carbenes (NHC) such as imidazol-2-ylidene (C2H4N2C:) is stable [31–35]. NHC and its related complexes are often used effectively for capturing the greenhouse gases such as CO2, OCS and CS2 [21–25]. The simplest carbene is methylene (H2C:). Unlike H2C: which has a triplet ground state, the ground state is a singlet state for C2H4N2C: Figure 1 gives the structures of the singlet C2H4N2C: and H2C:. As shown in Figure 1, the singlet H2C: possesses two binding sites, namely, the lone pair electrons and the vacant p-orbital on the carbene C atom. The lone pair can act as an electron donor, and the vacant p-orbital can serve as an electron acceptor. In contrast with H2C:, C2H4N2C: has two electron-rich moieties, one of which is the lone pair on the carbene C atom and the other is the 6π-electron system around the five-membered ring. The stability of C2H4N2C: is partially attributed to the π-electron delocalisation within the aromatic ring with two N and three C atoms [36]. Our recent study clearly indicates that both the lone pair and π-system of C2H4N2C: can be employed as electron donors in HB interactions [37]. These two electron-rich moieties of C2H4N2C: can also be expected to serve as electron donors in TB interactions. When the carbene C atom in NHC is displaced by the other heavier tetrel atoms (Si, Ge, Sn, Pb), we obtain the heavy-atom analogues of NHC [38–50]. It should be noted that the lone pairs of the heavier analogues of NHC are generally expected to be relatively inert because their lone pairs exhibit higher s-character compared with that of NHC [39]. The previous theoretical studies of TB interactions involving NHC explore the electron-donating ability of the carbene C atom but have not investigated the π-system of NHC [20–25], and the theoretical studies of TB interactions involving the heavier analogues of NHC are even absent. Considering that NHC and its heavier congeners possess multiple binding sites for TB interactions, a comprehensive study of TB interactions involving NHC and its heavier congeners should be interesting and can be expected to provide some new insights into TB interactions.