om7b00342_si_001.pdf (2.11 MB)
Kinetics and Mechanism of Isocyanide-Promoted Carbene Insertion into the Aryl Substituent of an N‑Heterocyclic Carbene Ligand in Ruthenium-Based Metathesis Catalysts
journal contribution
posted on 2017-08-16, 13:21 authored by Justin R. Griffiths, Elan J. Hofman, Jerome B. Keister, Steven T. DiverIn 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.