Role of Substrate Substituents in Alkene Metathesis Mediated by a Ru Alkylidene Catalyst

Quantum mechanical calculations on the mechanism of olefin metathesis with a variety of substituents mediated by a Ru alkylidene catalyst reveal multistep processes along the general reactants → adduct → coordination complex → metallacycle → decoordination complex → products pathway for two consecutive turnovers. Net energy barriers in solution do not exceed 12 kcal mol-1 during the [Ru]=CHPh + R1R2C=CH2 → [Ru] =CR1R2 + H2C=CHPh first turnover and 20 kcal mol-1 during the [Ru] =CR1R2 + R1R2C=CH2 → [Ru]=CH2 + R1R2C=CR1R2 second turnover. The complex series of steps is initially driven by the evolution of the Ru(catalyst)···C(olefin) contact. Dissection of bonding interactions using the tools provided by the natural bond orbitals and by the quantum theory of atoms in molecules methods indicate that each contact in the Ru(catalyst)···C(catalyst)···C(olefin)···C(olefin)···Ru(catalyst) cyclic reactive center undergoes the following series of transformations in different orders: no interaction → long range → σ → → π. Every single contact in this reactive center gains/loses an entire σ bond during the ···TS → metallacycle → TS··· interval. The lowest point in the potential energy surface is usually the metallacycle. For the first turnover, cycloreversion and final elimination of the products exhibit late transition states leading to higher relative energy barriers. Conversely, for the second turnover, it is the metallacycle to decoordination complex transformation step which leads to the highest barriers, constituting the rate-determining step for the entire process. Each step of the reaction is best described as a highly asynchronous process. Electron-withdrawing groups exhibit the largest overall barriers by virtue of destabilizing the emerging πbond in the final R1R2C=CR1R2 olefin during the second turnover.