Second generation Grubbs and Hoveyda-Grubbs complexes are the most employed metathesis (pre)catalysts. They are believed to generate the same catalytically active species, however they can perform very differently in a given olefin metathesis reaction. Why is that? We don’t have a definitive answer yet, but the elegant mechanistic studies on Hoveyda-Grubbs complexes recently reported by Plenio highlight fundamental differences in the initiation reaction.1
For Grubbs-type complexes, the initiation event involves the initial substitution of the phosphine ligand with the olefinic substrate. Kinetic studies derived from NMR spectroscopy showed that this step proceeds via a dissociative mechanism, and importantly, phosphine dissociation is the rate-limiting step for olefin metathesis. (See Study on the Mechanism of Grubbs Catalysts)
To study the initiation reaction of Hoveyda-Grubbs complexes, Plenio took advantage of their UV/Vis signature. The kinetic data suggest an interchange mechanism, possibly with associative character for the substitution of the ether with the olefinic substrate. This event is the rate limiting step for the initiation reaction as long as the olefin concentration is not too high. At very high olefin concentrations, the rate-limiting step occurs later in the catalytic cycle and is olefin-independent. It has not been identified yet but I think it is only a matter of time.
1 Vorfalt, T.; Wannowius, K.-J.; Plenio, H. Angew. Chem. Int. Ed. 2010, 49, 5533 – 5536.