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 π bond in the final R1R2CCR1R2 olefin during the second turnover.