A Computational Study of the Olefin Epoxidation Mechanism Catalyzed by Cyclopentadienyloxidomolybdenum(VI) Complexes
Résumé
A DFT analysis of the epoxidation of C2H4 by H2O2 and MeOOH (as models of tert‐butylhydroperoxide, TBHP) catalyzed by [Cp*MoO2Cl] (1) in CHCl3 and by [Cp*MoO2(H2O)]+ in water is presented (Cp*=pentamethylcyclopentadienyl). The calculations were performed both in the gas phase and in solution with the use of the conductor‐like polarizable continuum model (CPCM). A low‐energy pathway has been identified, which starts with the activation of ROOH (R=H or Me) to form a hydro/alkylperoxido derivative, [Cp*MoO(OH)(OOR)Cl] or [Cp*MoO(OH)(OOR)]+ with barriers of 24.9 (26.5) and 28.7 (29.2) kcal mol−1 for H2O2 (MeOOH), respectively, in solution. The latter barrier, however, is reduced to only 1.0 (1.6) kcal mol−1 when one additional water molecule is explicitly included in the calculations. The hydro/alkylperoxido ligand in these intermediates is η2‐coordinated, with a significant interaction between the Mo center and the Oβ atom. The subsequent step is a nucleophilic attack of the ethylene molecule on the activated Oα atom, requiring 13.9 (17.8) and 16.1 (17.7) kcal mol−1 in solution, respectively. The corresponding transformation, catalyzed by the peroxido complex [Cp*MoO(O2)Cl] in CHCl3, requires higher barriers for both steps (ROOH activation: 34.3 (35.2) kcal mol−1; O atom transfer: 28.5 (30.3) kcal mol−1), which is attributed to both greater steric crowding and to the greater electron density on the metal atom.
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