Complete Reaction Pathway of Ruthenium-Catalyzed Olefin Metathesis of Ethyl Vinyl Ether: Kinetics and Mechanistic Insight from DFT

Abstract

Density functional theory calculations accounting for dispersion, thermochemical, and continuum solvent effects have been applied to study the metathesis of ethyl vinyl ether (EVE) as mediated by ruthenium catalysts L(PCy3)(X)2Ru═CHPh (L = PCy3, IMes (1,3-dimesitylimidazol-2-ylidene), H2IMes (1,3-dimesityl-4,5-dihydroimidazol-2-ylidene); X = Cl, Br, I) in toluene. The computational approach has been validated against experimental data for ruthenium-based olefin metathesis catalysts and is found to give acceptable accuracy even for weakly bound transition states of phosphine and olefin dissociation and association. All relevant stationary points of the EVE metathesis reaction have been included in the study, allowing comparison with experimental kinetic data (Sanford, M. S.; Love, J. A; Grubbs, R. H.J. Am. Chem. Soc. 2001, 123, 6543). From the active 14-electron complex, the barriers to both phosphine association (at a rate proportional to k–1) and olefin binding (k2) involve contributions from entropy and solute–solvent interactions. The overall barrier to EVE cycloaddition (k2*) is higher than that to binding only (k2). The thus obtained ratios k–1/k2* compare nicely with those originally measured and interpreted as k–1/k2 by Sanford et al., suggesting that the experimental obtained ratios are better understood as k–1/k2*. Complementing the theoretical rate constants with concentrations, the thus obtained “computational kinetics” reproduces known trends among the various catalysts and also offers mechanistic insight.