Deactivation of Ru-benzylidene Grubbs catalysts active in olefin metathesis

Abstract

In this work we explore the reactivity induced by coordination of a CO molecule trans to the Ru-benzylidene bond of a prototype Ru-olefin metathesis catalyst bearing the N-heterocyclic carbene (NHC) ligand SIMes. Static DFT calculations indicate that CO binding to the Ru center promotes a cascade of reactions with very low energy barriers that lead to the final crystallographically characterized product, in which the original benzylidene group has attacked the proximal aromatic ring of the SIMes ligand leading to a cycloheptatriene through a Buchner ring expansion. In conclusion, it consists of a carbene insertion into a mesityl group of the second generation Grubbs carbene complex. Instead of simply occupying open coordination sites, the ligand pushes the ruthenium-bound carbene into a position-selective cyclopropanation and rearrangement reaction manifold giving a cycloheptatriene ring, i.e. a Buchner reaction. The insertion may prove useful to identify carbene intermediates in enyne metathesis. Both carbon monoxide and isocyanides promote the cyclopropanation experimentally, that a computational analysis of the relevant molecular orbitals illuminates the key role of the π-acid CO coordinated trans to the Ru-benzylidene bond to promote this reactivity. Results clearly indicate that a π-acidic group, as CO, that can approach the Ru center in the sterically crowded position trans to the Ru-benzylidene bond can promote this deactivation route.