pubs.acs.org/Organometallics Published on Web 03/11/2010 r 2010 American Chemical Society 1580 Organometallics 2010, 29, 1580–1587 DOI: 10.1021/om900848q Computational Modeling of Renewable Molecules. Ruthenium Alkylidene-Mediated Metathesis of Trialkyl-Substituted Olefins Serguei Fomine* and Mikhail A. Tlenkopatchev Instituto de Investigaciones en Materiales, Universidad Nacional Aut  onoma de M exico, Apartado Postal 70-360, CU, Coyoac  an, M exico DF 04510, M exico Received September 30, 2009 The metathesis reaction of (Z)-3-methyl-2-pentene (2) mediated by the first- and second-genera- tion 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. Introduction Trisubstituted olefins are essential components of many biobased products. Thus, various plant cis-trans (Z,E)- polyprenols and dolichols, 1 tocotrienols, 2 vitamin K 1 , 3 and natural rubber 4 are some of the examples of trisubstituted olefinic carbon-carbon bond containing compounds. Trialkyl-substituted olefins are also very important key intermediates for the synthesis of many natural products, in particular for the preparation of vitamin E molecules. It is worth noting that the metathesis of these molecules is challenging 5 due to the presence of an alkyl group directly linked to the double bond, which can impede the coordina- tion with a metal active center. 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