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The mechanisms of N‐heterocyclic carbene (NHC)‐catalyzed dearomatization reaction of alkyl pyridinium have been intensively studied using the density functional theory (DFT) method. The chemoselectivity and stereoselectivity were both analyzed based on the established mechanisms. The computational results showed that the chemoselective C4‐addition occurred before the C2‐addition, which is mainly because the C4 atom was more reactive according to the local reactivity analysis. The stereoselectivity was also investigated, and the C−C bond formation step was identified as the stereoselectivity‐determining step. The C−H⋅⋅⋅F and O−H⋅⋅⋅N interactions are identified as the main factors that govern the stereoselectivity by NCI analysis, and the preferred product was the RS‐configuration. This theoretical investigation would provide a case for understanding and predicting the selectivity of the organocatalytic dearomatization reactions of alkyl pyridinium. [ABSTRACT FROM AUTHOR]

Graphical abstract Theoretical advances on the mechanistic study of Ag-catalyzed carbene/nitrene/silylene transfer reactions were summarized in this review, which could be considered to involve two major steps: Ag–carbene/nitrene/silylene complex formation and the subsequent transformation of these complexes. Highlights • DFT studies on the mechanism of Ag-catalyzed group transfer reactions are summarized. • The mechanism consists of Ag–carbene/nitrene/silylene formation and transformation. • The mechanism is related to the electronic behaviors of Ag–carbene/nitrene/silylene. Abstract Carbene and its isoelectronic species nitrene and silylene, have long been considered reactive intermediates in organic synthesis. Ag-catalyzed carbene/nitrene/silylene transfer reactions have emerged as an important topic because they have made remarkable progress in organic synthesis. A series of theoretical investigations on the mechanism of these reactions is important to ensure their thorough understanding and aid design of new synthetic methodologies. Here, a mechanistic overview of Ag-catalyzed carbene/nitrene/silylene transfer reactions is given, which can be separated into two major steps including Ag–carbene/nitrene/silylene complex formation and the subsequent transformation of these complexes. Theoretical studies revealed that Ag–carbene and Ag–silylene complexes can be considered as typical Fischer-type complexes, in which the carbene/silylene moiety behaves as both a sp σ donor and p π acceptor. Conversely, the triplet Ag–nitrene complexes are usually more stable than the singlet ones because of the higher electronegativity of nitrogen than those of carbon and silicon. Density functional theory calculations have indicated that the triplet state of an Ag–nitrene complex has two unpaired electrons that are mainly localized on nitrogen. In this review, theoretical studies on the generation of Ag–carbene/nitrene/silylene complexes are systematically summarized. Moreover, the nucleophilic functionalization of Ag–carbene/nitrene/silylene complexes with halogen, hydroxyl, carbonyl, alkane, or arene groups are also considered theoretically. We believe that the mechanistic details discussed here will be of great importance for rational design of desired catalytic cycles and reactions. [ABSTRACT FROM AUTHOR]

Ir-catalysis has been widely adopted in hydrogenation reactions for the transformation of unsaturated molecules to the corresponding saturated ones. We reviewed herein the DFT studies on mechanism of Ir-catalyzed homogeneous hydrogenations with respect to various hydrogen sources. Moreover, the stereochemistry and substituent effects in the Ir-catalyzed hydrogenations are considered. • Mechanistic studies towards Ir-mediated hydrogenation were summarized in this review. • General catalytic cycles given by theoretical studies are involved in each example. • Mechanism discussions are classified according to the substrates. • The understanding of mechanism would be helpful for further design new Ir-catalysis. Ir catalysis is widely used in hydrogenation reactions to transform unsaturated molecules to the corresponding saturated molecules. Understanding the reaction mechanism is helpful for design of new Ir-catalyzed hydrogenation reactions, as well as for controlling the regio/stereoselectivity. Density functional theory is a powerful tool for mechanistic study of organometallic catalysis, and it has been widely used to reveal the reaction pathways in this research area. With the development of computational methods, much progress has recently been made in mechanistic study of Ir-catalyzed hydrogenation reactions. Herein, we present a review of theoretical studies of the mechanism of Ir-catalyzed homogeneous hydrogenation. A redox pathway is commonly proposed for hydrogenation of non-polar unsaturated bonds, which involves oxidative addition of a hydrogen molecule to afford a high valence Ir hydride complex, insertion of an unsaturated bond into the Ir–H bond, and reductive elimination. Alternatively, the dihydrogen molecule can undergo a heterolysis reaction to provide a formal hydride ion and a proton. Subsequent nucleophilic and electrophilic attack can then also achieve hydrogenation of the polar unsaturated bond. In this review, the studies of the mechanism of Ir-catalyzed hydrogenation are classified according to the type of substrate: olefins, carbonyls, and imines. In each category, the reactions are discussed with respect to the various hydrogen sources. The stereochemistry and substituent effect in Ir-catalyzed hydrogenation are also considered. [ABSTRACT FROM AUTHOR]

[Display omitted] • The mechanism and origin of selectivities of NHC-catalyzed hydroacylation-Stetter reaction has been systematically studied. • The role of NHC has been disclosed by using global reactivity index analysis. • ELF and IRC analyses are performed to clarify the bonding modes of hydroacylation and Stetter reaction. The regioselectivity of hydroacylation mediated by transition metal and organocatalyst has been and continues to be one of the most challenging questions in the synthesis field. To explore the possible mechanisms and origin of the regio-/enantioselectivity, the density functional theory (DFT) calculations were performed for studying intramolecular hydroacylation-Stetter reaction of alkynyl bisbenzaldehyde catalyzed by N-heterocyclic carbene (NHC). Computational results show that the hydroacylation process was the rate-determining step and irreversible. Four different stereoisomers (RR, RS, SR, and SS) were considered and the lowest energy mode agreed with the reported enantioselectivity in experiment. Global reaction index (GRI) analysis showed that NHC catalyst can convert alkynyl bisbenzaldehyde electrophilic carbonyl carbon into nucleophilicity to initiate the reaction. Moreover, electron localization function (ELF) and intrinsic reaction coordinate (IRC) analyses were performed to characterize the manner difference between the process of hydroacylation and Stetter reaction. [ABSTRACT FROM AUTHOR]