Abstract:
A mechanistic study has been carried out to explore the structural and energetic features leading to the decomposition pathways of a Grubbs second-generation olefin metathesis catalyst using density functional theory. The active form of the catalyst 2 has an inherent tendency to undergo intramolecular reactions, as the highly electron-deficient ruthenium center is in close proximity to the C-H bonds of the N-substituents. The theoretical results strongly suggest that the deactivation pathway initiates with the C-H activation rather than pericyclic cyclization suggested for the related Grubbs-Hoveyda catalyst system by Blechert et al. Complex 2 passes through five transition states, viz., (i) formation of an agostic complex through the activation of a C-H bond of the N-heterocyclic carbene (NHC)-phenyl ring; (ii) C-H a-bond metathesis with a carbene moiety to form a benzyl complex; (iii) two-step rotational transformations of the benzy] unit; and (iv) carbene-arene bond formation to yield the first product, 3. The last step is the rate-determining step, with the highest activation barrier of 28.6 kcal/mol, while the activation energy for steps (i), (ii), and (iii) are 13.6, 26.7, and 18.8 kcal/mol, respectively. The transformation of the rigid carbene unit to a flexible benzy] unit facilitates the rotational transformations in step (iii) and the subsequent C-C bond formation in step (iv). The eta(6)-coordination of phenyl ring in 3 changes to eta(2) to produce a less strained complex, and the C-H activation of the second NHC-phenyl ring occurs easily with this transformation, leading to a C-H agostic complex through a transition state with the activation barrier of 28.3 kcal/mol. The agostic interaction breaks up in the next step, leading to the ruthenium-carbon bond formation and the reductive elimination of HCl to the second product, 4. The flexibility of all three phenyl rings through their single bond connectivity plays a major role in the deactivation process of 2, as it leads to C-H agostic interactions with the ruthenium center. Therefore, the deactivation can be controlled by designing NHCs with rigid substituents, which may not undergo agostic interactions.
