Computational Modeling of Renewable Molecules. Ruthenium Alkylidene-Mediated Metathesis of Trialkyl-Substituted Olefins

2010-04-12T00:00:00Z (GMT) by Serguei Fomine Mikhail A. Tlenkopatchev
The metathesis reaction of (Z)-3-methyl-2-pentene (2) mediated by the first- and second-generation Grubbs catalysts has been studied at the PBE0/LACV3P**// PBE0/LACVP* level of theory using 1,2-dichloroethane as a solvent. Calculations established direct correlation between the activation energy of the metathesis and the number of substituents at the double bond of an olefin. The relative stabilities of secondary and tertiary metallacarbenes depend on the interplay between steric and electronic factors. Thus, for the first-generation Grubbs catalyst the contribution of steric factor dominates, while for the second-generation Grubbs catalyst both the stabilizing electronic factors and the destabilizing steric effects are equally important. Secondary metallacarbene was found to be more active in the metathesis reaction due to less sterically hindered transition states, and therefore, secondary metallacarbene 8b is the principal reactive intermediate in the reaction under kinetic control. The higher activity of the second-generation Grubbs catalyst was confirmed, which is reflected in lower activation energies of the metathesis for the latter. Calculations also demonstrated the low stereoselectivity of Ru-alkylidene catalysts in the metathesis reaction of trialkyl-substituted olefins.