Operation of the Boomerang Mechanism in Olefin Metathesis Reactions Promoted by the Second-Generation Hoveyda Catalyst

Abstract

A long-standing question in olefin metathesis centers on whether the “release–return” (boomerang) mechanism contributes to the productivity of Hoveyda-class catalysts. According to this mechanism, a molecule of o -isopropoxystyrene ( A ) is liberated during catalyst initiation, but recaptures the active catalyst following metathesis. The relevance of this pathway for the second-generation Hoveyda catalyst HII was assessed in metathesis of 1,1- and 1,2-disubstituted olefins. Crossover studies with 13 C-labeled A *, as well as competition experiments involving ring-closing or cross metathesis (RCM, CM) in the presence of A (equimolar with HII ) indicated rapid reuptake of styrenyl ether. The crossover studies indicated highly efficient catalyst initiation, with the entire catalyst charge being activated before metathesis was complete. In a comparative study involving CM of anethole with methyl acrylate, sustained activity was shown for HII, whereas the second-generation Grubbs catalyst GII was rapidly deactivated. These data demonstrate that the release–return mechanism is indeed operative for HII in these demanding metathesis reactions, and that facile shuttling from a protected recapture cycle into the productive metathesis cycle contributes to the superior performance of HII relative to GII .