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

Abstract

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.

The roles of Lewis acidic additives in organotransition metal catalysis

“…The BDEs of the C-H and C-F bonds in the 26:BPh 3 Lewis acid-base adduct are determined to be similar in free 26; however, the BDE of the C-CN bond is significantly reduced (ΔH = +136.2 kcal mol −1 , free 26; ΔH = +112.7 kcal mol −1 26:BPh 3 ) upon adduct formation. The results affirm findings in other studies, 39 where Lewis acids reduce activation barriers of oxidative addition of cyanoester and alkylnitrile to Ni(0). Lewis acids can also reduce the barrier to migratory insertion of the alkyne into Ni-C bonds.…”

“…In the direct, regioselective In these reactions, the Lewis acid influences selectivity of C-C and C-H bond activation. [37][38][39][40][41] Sakaki and coworkers 37 investigated the mechanism of the aforementioned dual C-C cleavage reaction of 12 to form the coumarin 14 by computation. An abridged mechanism determined by DFT calculations is depicted in Fig.…”