Olefin Epoxidation by H2O2/MeCN Catalysed by Cyclopentadienyloxidotungsten(VI) and Molybdenum(VI) Complexes: Experiments and Computations

Compounds [Cp*2M2O5] (M=Mo, 1; W, 2) are efficient pre-catalysts for cyclooctene (COE) epoxidation by aqueous H2O2 in acetonitrile/toluene. The reaction is quantitative, selective and takes place approximately 50 times faster for the W system (k(obs) = 4.32(9) x 10(-4) s(-1) at 55 degrees C and 3 x 10(-3) M concentration for the dinuclear complex, vs. 1.06(7) x 10(-5) s(-1) for the Mo system). The rate law is first order in catalyst and CUE substrate (k= 0.138(7) M-1 s(-1) for the W system at 55 degrees C), whereas increasing the concentration of H2O2 slows down the reaction because of an inhibiting effect of the greater amount of water. The activation parameters for the more active W systems (Delta H-not equal=10.2(6) kcal mol(-1); Delta S-not equal = -32(2) cal mol(-1) K-1) were obtained from an Eyring study in the 25-55 degrees C temperature range. The H2O2 center dot urea adduct was less efficient as an oxidant than the aqueous H2O2 solution. Replacement of toluene with diethyl ether did not significantly affect the catalyst efficiency, whereas replacement with THF slowed down the process. The epoxidation of ethylene as a model olefin, catalysed by the [Cp*MO2Cl] systems (M=W, Mo) in the presence of H2O2 as oxidant and acetonitrile as solvent, has been investigated by DFT calculations with the use of the conductor-like polarisable continuum model (CPCM). For both metal systems, the rate-limiting step is the transfer of the hydroperoxido O-alpha atom to the olefin, in accordance with the first-order dependence on the substrate and the zero-order dependence on H2O2 found experimentally in the catalytic data. The activation barrier corresponding to the rate-limiting step is 4 kcal lower for the W complex than for the corresponding Mo analogue (32.3 vs. 28.3 kcal mol(-1)). This result reproduces well the higher catalytic activity of the W species. The different catalytic behaviour between the two systems is rationalised by a natural bond orbital (NBO) study and natural population analyses (NPA). Compared to Mo, the W-VI centre withdraws more electron density from the sigma bonding [O-O] orbital and favours, as a consequence, the nucleophilic attack of the external olefin on the sigma*[O-O] orbital.