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 a N-heterocyclic carbene (NHC) ligand. 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 ligand leading to a cycloheptatriene ring through a Buchner ring expansion. In conclusion, the overall mechanism is best described as a carbene insertion into a C-C bond of the aromatic N-substituent of the NHC ligand, forming a cyclopropane ring. This cyclopropanation step is followed by a Buchner ring expansion reaction, leading to the experimentally observed product presenting a cycloheptatriene ring

The use of the sterically demanding IPr* and related ligands in catalysis

“…6,7 A bulkier ligand can possibly stabilize the active species during the catalytic reaction, disfavoring bimolecular decomposition pathways or C-H activation and cyclometallation. [8][9][10][11] However, a much larger ligand can disfavor the substrate approach to the metal and thereby reduce catalytic activity. Therefore a careful balance of the two aspects proves critical.…”

A Molecular modeling study of the changes of some steric properties of the precatalysts during the olefin metathesis reaction

“…During the last two decades, since the first ruthenium Grubbs‐type carbene precatalyst was reported, the field of olefin metathesis has grown tremendously . Computational studies have been used over the last decade to try to elucidate and gain a deeper insight into the effect of ligand coordination and the mechanism of the catalytic reaction . Several attempts have also been made to use theoretical studies to elucidate various steric effects that might play a role during the course of the metathesis reaction , .…”

N,N′‐bridged binuclear NHC palladium complexes: A combined experimental catalytic and computational study for the Suzuki reaction

“…[ 19 ] Bulky ligands stabilize the active species and disfavor bimolecular decomposition and other deactivation events. [ 20 ] However, steric bulk hinders the approach of the substrate, which might diminish catalytic activity. Although electron richness and steric bulk are important, a delicate balance with the degree of ligand flexibility seems to be the key to successful enhancement of catalytic efficacy.…”