Kinetics and Mechanism of Isocyanide-Promoted Carbene Insertion into the Aryl Substituent of an N-Heterocyclic Carbene Ligand in Ruthenium-Based Metathesis Catalysts

Abstract

In situ IR spectroscopy was used to study the kinetics of addition of L = alkyl and aryl isocyanides to the Grubbs second-generation carbene complex Ru(H2IMes)(CHPh)(PCy3)Cl2 (H2IMes = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene), which triggers carbene insertion into an aromatic ring of the N-heterocyclic carbene supporting ligand, forming Ru{1-mesityl-3-(7′-Ph-2′,4′,6′-trimethylcycloheptatrienyl)-4,5-dihydroimidazol-2-ylidene}L2(PCy3)Cl2. The rate law was determined to be first order in isocyanide concentration and first order in carbene complex concentration. For various isocyanides CNR the rate increases as R = tert-butyl ≪ cyclohexyl < n-octyl < CH2Ph ≈ CH2CO2Me ≈ CH2SO2C6H4-4-Me < C6H4-4-OMe < C6H4-4-Cl. The proposed mechanism involves reversible addition of isocyanide followed by rate-determining, irreversible carbene insertion and subsequent, rapid addition of the second isocyanide. The carbene insertion is accelerated by the electrophilicity of the carbene, which is enhanced due to ligand binding...