Epoxidation of α,β-unsaturated acids catalyzed by tungstate (VI) or molybdate (VI) in aqueous solvents: a specific direct oxygen transfer mechanism

The epoxidation mechanism of α,β-unsaturated acids catalyzed by Na2WO4 or Na2MoO4 in aqueous solvents has been studied by means of kinetic experiments, MS, 31P NMR and 13C NMR. The results show Na2WO4 or Na2MoO4 and H2O2 can rapidly form the diperoxocomplex irreversibly in aqueous solvent. 31P NMR and kinetics study reveal that –Pdouble bond; length as m-dashO in cis-1-propenylphosphonic acid (CPPA) can fast coordinate to the metal center (Mo or W) of the diperoxocomplex irreversibly to form a stable complex. The subsequent direct oxygen transfer from peroxo bond to double bond is a monomolecular process, thus the epoxidation is zero-order on CPPA. However, 13C NMR shows the –Cdouble bond; length as m-dashO in α,β-unsaturated carboxylic acids cannot coordinate to metal center of the diperoxocomplex. Therefore, the oxygen transfer is a bimolecular process and the epoxidation is a first-order reaction on the acids. In the mechanism, the nucleophilic attack of double bond towards the peroxo bond is regarded as through a spiro-transition structure.