Theoretical Evidence for Bond Stretch Isomerism in Grubbs Olefin Metathesis

Abstract

A comprehensive density functional theory study on the dissociative and associative mechanisms of Grubbs first and second generation olefin metathesis catalysis reveals that ruthenacyclobutane intermediate (RuCB) observed in the Chauvin mechanism is not unique as it can change to a non-metathetic ruthenacyclobutane (RuCB0) via the phenomenon of bond stretch isomerism (BSI). RuCB and RuCB0 differ mainly in RuCa, RuCb, and CaCb bond lengths of the metallacycle. RuCB is metathesis active due to the agostic type bonding-assisted simultaneous activation of both CaCb bonds, giving hypercoordinate character to Cb whereas an absence of such bonding interactions in RuCB0 leads to typical CC single bond distances and metathesis inactivity. RuCB and RuCB0 are connected by a transition state showing moderate activation barrier. The new mechanistic insights invoking BSI explains the non-preference of associative mechanism and the requirement of bulky ligands in the Grubbs catalyst design. The present study lifts the status of BSI from a concept of largely theoretical interest to a phenomenon of intense importance to describe an eminent catalytic reaction.