Abstract
Epoxidation of a range of alkenes with aqueous H2O2 easily proceeds in the presence of the dititanium-containing 19-tungstodiarsenate(III) (Ti-2(OH)(2)As2W19O67(H2O)]8(-) (1), which contains five-coordinate Ti atoms. The results of product and kinetics studies support a mechanism that involves a reversible interaction between H2O2 and the Ti OH group of 1 to produce a titanium hydroperoxo complex followed by electrophilic oxygen atom transfer from the hydroperoxo ligand to the alkene substrate in the rate-limiting step. The effect of the alkene substrate nature has been investigated at the DFT level. A clear correlation between the energy of pi(C=C) orbitals in the alkene and the turnover frequency values has been found, thereby indicating that the higher nucleophilicity of the alkene, the higher the reactivity. ONIOM (ONIOM = our own n-layered integrated molecular orbital and molecular mechanics) calculations have been employed to evaluate the role of steric effects of alkene substituents. The calculations show that the steric bulk can play a secondary role and tunes the activity in specific cases such as trans-stilbene.