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 13C-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...
Published Version
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