Metathesis of terpenes using the second generation Grubbs Ru–alkylidene catalysts: Computational modeling

The initiation step, self metathesis of β-pinene (2) and its cross metathesis with (Z)-3-methylpent-2-ene (16) mediated by the second generation Grubbs catalyst have been modeled at PBE/def2-TZVP level of theory using empirical dispersion correction scheme (D3). It has been found that the initiation step leads to the formation of tertiary not primary metalacarbene which undergoes the nonproductive metathesis reaction with β-pinene. The self-metathesis productive reaction path is inhibited by unfavorable kinetic and thermodynamic factors. The first one is related to the high steric hindrances in transition states and the second one with the low stability of formed primary metalacarbene. The free Gibbs activation energies of cross metathesis of 2 and 16 were found to be around 23 kcal/mol.