Origin of the Electronic Effects on Regioselectivity in Ni-Catalyzed Indenone Synthesis

Abstract

The migratory insertion of an alkyne into a metal–carbon bond is a key elementary step in many reactions. For transformations that utilize unsymmetrical alkynes, reliable control over the regioselectivity of this process is required. Several factors that control alkyne migratory insertion selectivity are well-known. Other factors remain poorly understood. While substituents on an alkene or alkyne undergoing migratory insertion are known to produce electronic effects that alter the selectivity of this process, our group recently reported a Ni-catalyzed indenone synthesis in which the electronic character of the migrating aryl ligands exerted a substantial influence over regioselectivity. Herein we provide several hypothetical causes for these unusual electronic effects, and experimental evidence to support them. We determined that the previously observed electronic effects on regioselectivity are the result of unfavorable dipole–dipole interactions that develop between the migrating aryl ligands and the silyl-substituted alkynes undergoing insertion. These regioselectivity effects are anticipated to be somewhat general for alkyne migratory insertion processes at other metals, as a related Co-catalyzed indenone synthesis displays a similar electronic relationship.