The initiation step in olefin metathesis catalyzed by Hoveyda-Grubbs complexes involves the substitution of the benzylidene ether ligand and the release of isopropoxy styrene. After the metathesis reaction, the released isoproxy styrene can cross back onto the ruthenium and reform the benzylidene (pre)catalyst. That is what is called the release-return (or boomerang) mechanism.
Various labeling studies provide evidence that this mechanism is operative under high catalyst loading conditions (5 mol%).1,2 In a 2008 publication, Grela et al. argued that during the metathesis reaction, the whole amount of Hoveyda (pre)catalyst was involved in the catalytic cycle and was then regenerated by the release-return mechanism (allowing for its re-isolation at the end of the reaction).
However, since the experimental conditions used high catalyst loadings, the relevance of the boomerang mechanism in truly catalytic reactions has always been somewhat questioned. Recently, Plenio et al. investigated this matter using a fluorophore-tagged Hoveyda complex. 3 Using 0.2 mol% of their (pre)catalyst, and following the fluorescence intensity over the course of the ring closing metathesis (RCM) reaction they could determine if the tagged ligand was dissociated or not. They did not find any “evidence supportive of a significant contribution of a release-return mechanism in RCM reactions in the Grubbs-Hoveyda-type complexes”. They actually believe that the re-isolation is due to incomplete activation (initiation) of the initial (pre)catalyst.
So, who should we believe? Personally, I want to see more facts. The only thing I know for sure is that they use way too much catalyst since you need less than 0.005 mol% of Hoveyda-Grubbs catalyst to effect the RCM reaction they studied!
1 Kingsburry, J. S.; Hoveyda, A. H. J. Am. Chem. Soc. 2005, 127, 4510 – 4517.
2 Bieniek, M.; Michrowska, A.; Usanov, D. L.; Grela, K. Chem. Eur. J. 2008, 14, 806 – 818.
3 Vorfalt, T.; Wannowius, K.-J.; Thiel, V.; Plenio. H. Chem. Eur. J. 2010, 16, 12312 – 12315.