Kinetic studies on the cat
alytic activity of phenylphosphopolyperoxotungstate complexes [C
21H
37N]
2[(PhPO
3){WO(O
2)
2}
2{WO(O
2)
2H
2O}] (
A) and [C
21H
37N]
2[(
p-NO
2Ph(O)PO
3){WO(O
2)
2}
2{WO(O
2)
2H
2O}] (
B) in the epoxidation of olefins with hydrogen peroxide as the oxidizing agent were carried out in CH
3CN/H
2O solvent mixtures. The nitro-substituted cat
alyst,
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 cat
alyst and the olefin, and between zero- and first-order with respect to the H
2O
2 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 ![](http://www.sciencedirect.com/scidirimg/entities/223c.gif)
alt="not, vert, similar" title="not, vert, similar" border="0"> −0.9) and the reaction stereoselectivity suggest a mechanism, which involves an extern
al nucleophilic attack of the π-system of the olefin on the electropositive oxygen of the W-peroxo group. A cat
alytic cycle involving a fast (pre-equilibrium) reaction between H
2O
2 and the cat
alyst followed by the oxygen-transfer step (rate-determining) is proposed.