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. 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.
Original language | English (US) |
---|---|
Pages (from-to) | 2387-2394 |
Number of pages | 8 |
Journal | ACS Catalysis |
Volume | 4 |
Issue number | 7 |
DOIs | |
State | Published - Jul 3 2014 |
Externally published | Yes |
Keywords
- Hoveyda catalysts
- boomerang mechanism
- homogeneous catalysis
- olefin metathesis