Cyclooctene Epoxidation by Hydrogen Peroxide in the Presence of Vanadium-Substituted Lindqvist-Type Polyoxotungstate [VW5O19]3–

Abstract

The vanadium-substituted, Lindqvist-type polyoxotungstate [VW5O19]3– significantly promotes cyclooctene epoxidation using H2O2 in CH3CN at 30 °C. The catalytic processes discussed are based on the results of the UV/Vis, ESI–MS, 51V NMR, and 183W NMR spectra of [VW5O19]3– in a H2O2/cyclooctene/CH3CN system. ESI–MS spectral analysis revealed that the [VW5O19]3– anion retained a Lindqvist-type structure and that multiple peroxidation occurred at both the V and W sites during epoxidation. Together with the synergistic effects provided by the W(peroxo) sites, the V(peroxo) site plays a catalytically active role in both epoxidation and H2O2 decomposition. Enhancement between each metal's sites in Lindqvist-type polyoxometalates is strong, as all metal atoms are directly linked to each other via the central oxygen atom. The small negative H2O2 dependence (–0.63) indicates that a catalytically active species is immediately formed with H2O2 and that the transfer of the O atom from the peroxo ligand to cyclooctene proceeds rapidly, which was supported by the phenomenon wherein ESI–MS and 51V NMR measurements for a sample solution become increasingly difficult to obtain when the epoxidation reaction proceeds favorably. This is because structural changes around the V site are frequentative. This small negative H2O2 dependence value also shows that [VW5O19]3– catalyzes H2O2 decomposition considerably, not only epoxidation. The generation of a significant amount of O2 was observed.