Kinetic studies on phenylphosphopolyperoxotungstates catalyzed epoxidation of olefins with hydrogen peroxide

Kinetic studies on the catalytic activity of phenylphosphopolyperoxotungstate complexes [C₂₁H₃₇N]₂[(PhPO₃){WO(O₂)₂}₂{WO(O₂)₂H₂O}] (A) and [C₂₁H₃₇N]₂[(p-NO₂Ph(O)PO₃){WO(O₂)₂}₂{WO(O₂)₂H₂O}] (B) in the epoxidation of olefins with hydrogen peroxide as the oxidizing agent were carried out in CH₃CN/H₂O solvent mixtures. The nitro-substituted catalyst, B, was shown to be more active than A. However, the higher acidity of B enhances more epoxide ring-opening to yield 1,2-diol. Under biphasic conditions, the epoxide ring-opening is reduced, and in most cases the 1,2-diol products do not form. Under homogeneous conditions, the reaction is first-order with respect to the catalyst and the olefin, and between zero- and first-order with respect to the H₂O₂ concentration. The rate of the reaction increases with the olefin nucleophilicity. The epoxidation rate constants of para and meta substituted styrenes are correlated with σ⁺ according to the Hammett equation. The reaction constants (ρ ∼ -0.9) and the reaction stereoselectivity suggest a mechanism, which involves an external nucleophilic attack of the π-system of the olefin on the electropositive oxygen of the W-peroxo group. A catalytic cycle involving a fast (pre-equilibrium) reaction between H₂O₂ and the catalyst followed by the oxygen-transfer step (rate-determining) is proposed.