Olefin epoxidation catalyzed by η⁵-cyclopentadienyl molybdenum compounds: A computational study

η⁵-Cyclopentadienyl(tricarbonyl)methylmolybdenum(II) (1) is oxidized by tert-butylhydroperoxide (TBHP) to yield η⁵- cyclopentadienyl(dioxo)methylmolybdenum(VI) (2) and η⁵- cyclopentadienyl(oxo)-(peroxo)methylmolybdenum(VI) (3). These two complexes are active catalysts for olefin epoxidation in the presence of TBHP, but inactive in its absence. DFT calculations of the possible pathways, with MP2 single-point energies, indicate that TBHP can react with 2 to form the active intermediate CpMo(O)(OH)(OOCH₃)CH₃ (B) or the peroxo complex 3. Formation of B and epoxidation from B have relatively low activation barriers (gas-phase MP2 ΔE 21 kcal mol^-1), while formation of complex 3 from 2 exhibits a higher barrier (38 kcal mol^-1). The peroxo complex 3 can be further activated by excess TBHP to yield a second active intermediate, CPMO(O₂)(OH)(OOCH₃)CH₃ (C) (24 kcal mol ^-1), that undergoes reaction with olefin, forming epoxide and the intermediate B. Both intermediates B and C display end-on-bound alkyl peroxo ligands. Barriers are comparable for several pathways, suggesting that more than one may take place. The role of solvent (PCM, dichloromethane) is negligible, since the deviations between gas-phase and solvent free energies are ∼1-3 kcal mol^-1.