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