Mechanism and the Origins of Stereospecificity in Copper-Catalyzed Ring Expansion of Vinyl Oxiranes: A Traceless Dual Transition-Metal-Mediated Process

Abstract

Density functional theory computations of the Cu-catalyzed ring expansion of vinyloxiranes is mediated by a traceless dual Cu­(I)-catalyst mechanism. Overall, the reaction involves a monomeric Cu­(I)-catalyst, but a single key step, the Cu migration, requires two Cu­(I)-catalysts for the transformation. This dual-Cu step is found to be a true double Cu­(I) transition state rather than a single Cu­(I) transition state in the presence of an adventitious, spectator Cu­(I). Both Cu­(I) catalysts are involved in the bond forming and breaking process. The single Cu­(I) transition state is not a stationary point on the potential energy surface. Interestingly, the reductive elimination is rate-determining for the major diastereomeric product, while the Cu­(I) migration step is rate-determining for the minor. Thus, while the reaction requires dual Cu­(I) activation to proceed, kinetically, the presence of the dual-Cu­(I) step is untraceable. The diastereospecificity of this reaction is controlled by the Cu migration step. Suprafacial migration is favored over antarafacial migration due to the distorted Cu π-allyl in the latter