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

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 by isocyanides with lower σ-donor/π-acceptor ratios.