Predicting the origin of selectivity in NHC-catalyzed ring opening of formylcyclopropane: a theoretical investigation

Abstract

Using density functional theory, we investigated the origin of selectivity in the N-heterocyclic carbene (NHC)-catalyzed transformation of formylcyclopropane with an alkylidene oxindole. Our proposed mechanism includes the formation of a Breslow intermediate, ring opening of cyclopropane, formation of an enolate intermediate, formal [4 + 2] cycloaddition and disassociation of the catalyst. The transformation from an enolate intermediate into an alkylidene oxindole determines the chemoselectivity and stereoselectivity. Local reactivity analysis indicates that the chemoselectivity in the controlled transition state of the [4 + 2] cycloaddition is intrinsically favored by the higher reactivity of the CC moiety of the alkylidene oxindole. Electron localization function analysis indicates that the NHC alleviates the electronic density between Cα and Cγ, and thus facilitates opening of the cyclopropane ring. This knowledge of NHC organocatalysis helps predict the chemoselectivity of divergent reactions involving cyclopropane rings.