Synergistic Mechanistic Study of Nickel(0)/Lewis Acid Catalyzed Cyanoesterification: Effect of Lewis Acid

The
highly selective nickel(0)-catalyzed cyanoesterification of
cyanoformates with alkynes is greatly facilitated by Lewis acid (LA).
We report density functional theory (DFT) investigations on the mechanistic
details and provide a rational explanation of the “LA effect”.
Our calculations disclose that an unusual biphosphine cycle including
rate-determining oxidative addition of a C–CN bond, alkyne
migratory insertion, and reductive elimination steps is more favorable
than the generally accepted monophosphine cycle. The LA has a non-negligible
impact on the mechanistic origin of selective C–CN bond activation.
Furthermore, the presence of LA dramatically accelerates alkyne migratory
insertion because the strong electron-withdrawing property of LA strengthens
the coordination ability of the Ni center with alkyne to form the
thermodynamically favorable five-coordinate nickel­(II) cyanide carboxylate
species, which then avoids the large deformation of the transition
state of alkyne migration insertion to give a moderate activation
energy. Thus, the proposed biphosphine cycle successfully rationalizes
the experimental observation that the presence of LA is comparatively
crucial to improve this reaction with high regioselectivity.