Bates, J. M.; Lummiss, J. A. M.; C.; Bailey; G. A., and Fogg, D. E. “Operation of the Boomerang Mechanism in Olefin Metathesis Reactions Promoted by the Second-Generation Hoveyda Catalyst” ACS Catal., 2014, 4 (7), 2387-94.
No consensus has been reached about the validity of the release-return or Boomerang mechanism involved in the metathesis reactions with the 2nd gen Hoveyda-Grubbs type catalysts despite the numerous studies on the subject. Basically, the views are divided between the two extremes:
- Only a fraction of the precatalyst initiates and the uninitiated catalyst can be recovered after the reaction.
- All the precatalyst initiates during the reaction and then is regenerated by reuptake by the Hoveyda ligand allowing for almost quantitative recovery.
Trying to resolve this long standing debate in the metathesis community, Deryn Fogg’s group undertook recently a new study revisiting previous experiments and addressing their deficiencies.
A prerequisite for the operation of the release-return mechanism is the ability of the Hoveyda ligand to compete with a large excess of the substrate and recapture the active catalyst during the catalytic cycle. Competition experiments showed that the styrenyl ether can inhibit the metathesis reaction even when present in small amounts under cross-metathesis or ring closing metathesis conditions indicating rapid uptake by the active catalyst in both early and late stages of the reactions.
Crossover studies with the 13C-labeled Hoveyda ligand (HL*) showed very efficient initiation as well as very rapid uptake of the styrenyl ether during cross metathesis reactions or RCM. Near equilibrium amount of the 13C-labeled HII* (47%) were reached.1 In contrast, the background reaction in the absence of metathesis substrates was much slower and reaches only 5% even at much higher catalyst concentrations.
These new results are pretty convincing in favor of the release-return mechanism. But no matter what, the fact is that the second gen Hoveyda can outperform the Grubbs catalyst in some challenging RCM reactions. The release-return mechanism can be one of the contributors to the higher productivity of this class of catalysts. Due to the ease of initiation and regeneration the Hoveyda pre-catalyst can be viewed as a reactive resting state species which protects the active catalyst from decomposition and readily re-enters the catalytic cycle.
1Uptake of HL* was measured without work up to exclude the possibility for accelerated regeneration during concentration.