A computational study of the olefin epoxidation mechanism catalyzed by cyclopentadienyloxidomolybdenum(VI) complexes

A DFT analysis of the epoxidation of C2H4 by H 2O2 and MeOOH (as models of tert-butylhydroperoxide, TBHP) catalyzed by [Cp MoO2Cl] (1) in CHCl3 and by [CpMoO 2(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) kcalmol-1 when one additional water molecule is explicitly included in the calculations. The hydro/alkylperoxido ligand in these intermediates is if-coordinated, with a significant interaction between the Mo center and the O13 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 [CpMoO(O 2)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. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA,.