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.