Olefin Metathesis by Grubbs–Hoveyda Complexes: Computational and Experimental Studies of the Mechanism and Substrate-Dependent Kinetics

The potential energy surfaces for the activation of Grubbs–Hoveyda-type precatalysts with the substrates ethene, propene, 1-hexene, and ethyl vinyl ether (EVE) have been probed at the density functional theory (DFT) (M06-L) level. The energetically favored pathway of the reaction leading to a 14e Fischer carbene and styrene starts with an initiation step in which the incoming substrate and outgoing alkene ligand are both clearly associated with the ruthenium center. For these substrates, with the exception of ethene, the rate determining step is predicted to be the formation of the metallocyclobutane (MCB). We have taken the initial reactant to be a weak van der Waals complex between substrate and precatalyst. This model yields good agreement between the computed activation parameters for both the parent Grubbs–Hoveyda and Grela complex with EVE substrate, and the experimental values, reported here. The alternative model which takes the initial reactant to be two isolated molecules requires an estimate of the entropy loss on formation of the initial complex in solution which is difficult to evaluate. Our estimate of this quantity yields a barrier for the rate determining step for the interchange mechanism which is close to the value we find for the alternative mechanism in which the rate determining step is the initial dissociation of the precatalyst. The relative energetics of these two mechanisms involving different initiation steps but with similar activation barriers, could well be dependent upon the precatalyst and substrate in line with the recent experimental findings of Plenio and co-workers.