An insight into a base-free Michael addition reaction as catalyzed by a bifunctional nickel N-heterocyclic carbene complex using density functional theory studies

Abstract

A proposed catalytic pathway for a base-free Michael addition reaction mediated by a N/O-functionalized N-heterocyclic carbene (NHC) based bifunctional nickel precatalyst has been probed using density functional theory (DFT) studies. In particular, the base-free Michael addition of a β-dicarbonyl compound namely, 2-acetyl-cyclopentanone (a) with methyl vinyl ketone (b) as catalyzed by a representative bifunctional nickel precatalyst viz. [1-(Me)-3-N-(methylacetamido)imidazol-2-ylidene]2Ni (A) has been investigated. The modeling studies reveal that the nucleophilic attack of a metal bound enolate moiety of a 1,3-dicarbonyl adduct species (B) to the approaching activated olefinic substrate, methyl vinyl ketone (b), is the crucial rate-limiting step of the reaction yielding a Michael addition product adduct species (C). Interestingly, the subsequent intramolecular rearrangement of (C) to a different O-bound intermediate (D) exhibit nearly equal activation barrier.