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–Ha (active Ha) 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.