Mechanistic Insights into the Nickel-Catalyzed Cross-Coupling Reaction of Benzaldehyde with Benzyl Alcohol via C–H Activation: A Theoretical Investigation

Abstract

With the aid of density functional theory (DFT) calculations, mechanistic investigations have been carried out for the nickel-catalyzed dehydrogenative cross-coupling reaction of benzaldehyde with benzyl alcohol in the presence of N-heterocyclic carbene (NHC) ligand. The overall Ni(0)/Ni(II) catalytic cycle consists of four basic steps: ligand exchange, oxidative addition, hydrogen transfer, and reductive elimination. Considerable interests are paid on detecting the transition state of the rate-determining step, with particular emphasis on the structural and electronic properties, together with clarifying the important roles of external oxidant and hydrogen acceptor. The hydrogen transfer process in the oxidative addition step is rate-determining in the whole catalytic cycle, which is accomplished by C-H (active H) activation without generating the high energy nickel hydride intermediate. Such process could be understood as the direct hydrogen transfer, instead of general concerted oxidative addition to low valent transition metal. The analysis of the bond distances, electron distributions, and orbital interactions highlights the direct hydrogen transfer mechanism. Furthermore, by exploring the influences from the electronic effect of different substrates on the reaction energy barriers, the a,a,a-trifluoroacetophenone could accelerate the direct hydrogen transfer with low activate energy.

Insights into the nucleophilic substitution of pyridine at an unsaturated carbon center

“…17,18 Considering that the energy gap between the p* C]O and s* C-Cl molecular orbitals of CH 3 COCl is small (1.3 eV), electron migration from pyridine induces sufficient mixing of these two MOs. 15,[17][18][19]21 Based on the results of charge decomposition analysis (CDA) [38][39][40][41][42] CDA was also performed on the reaction of Cl À and CH 3 -COCl. The results shown in Fig.…”

“…37 As for the orbital interactions and orbital composition analysis, they were studied according to the charge decomposition analysis (CDA) method and Hirshfeld method implemented in the Multiwfn program. [38][39][40][41][42][43][44][45][46] IBO (intrinsic bond orbital) analyses was performed to study the electronic structure changes in the reaction of pyridine with CH 3 COCl along the reaction pathway. 47…”

Enhancement of the Catalytic Activities of Heteronuclear Bimetallic Cations for the C–H Bond Activation of Cyclohexane

“…In order to unveil the electronic processes of C–H bond activation, transition states of the first, third, and fifth C–H bond activations previously described are divided into two fragments such as the metallic cation moiety and the cyclic hydrocarbon moiety. Fragment molecular orbital analysis indicates that charge transfer (CT) − occurs from the interacting orbital of the metallic cation to the σ*-antibonding orbital of the C–H bond, which was frequently applied in diverse metal catalytic reactions. − The orbital interactions in the transition states ts21 and TS21 are shown in Figure . The correlative figures for other transition states are shown in the Supporting Information (Figure S2).…”

Nickel‐Catalyzed Dehydrogenative Couplings

“…Very recently, Tang et al . theoretically studied the mechanism of Ni‐catalyzed dehydrogenative cross‐coupling reactions between benzyl alcohol and benzaldehyde using DFT calculations and they discarded the formation of nickel hydride intermediate, rather a direct hydrogen transfer was proposed . The reaction mechanism involves four steps: ligand exchange, oxidative addition, hydrogen transfer and reductive elimination.…”