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

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.