Activation of Hydrogen Peroxide by Polyoxometalates

Numerous catalytic H₂O₂-based oxidation reactions by polyoxometalates (POMs) have been developed. POMs for H₂O₂-based oxidations can be classified into three groups from the standpoints of their structures and H₂O₂ activation properties: di- and tetranuclear small peroxotungstates, lacunary polyoxotungstates, and transition-metal-substituted POMs. This chapter focuses on the activation of H₂O₂ on the basis of groups small peroxotungstates and lacunary polyoxotungstates including reaction mechanisms. In recent studies on the catalyst design of POMs showed that various POMs and peroxotungstate could be used as effective catalysts for selective oxygen transfer reactions, such as epoxidation, sulfoxidation, and Baeyer-Villiger oxidation. The high efficiency of H₂O₂ utilization, which means that negligible decomposition of H₂O₂ occurs to form molecular oxygen, reduces by-product formation and the risk of building an explosive atmosphere, and leads to the simple, efficient, and safe oxidation processes. For the present epoxidation and sulfoxidation with H₂O₂, the catalytically active sites play an important role in the activation of H₂O₂. The reaction of the catalyst with H₂O₂ results in the generation of the active species which may be hydroperoxo or bridging peroxo species. These electrophilic active oxygen species attack the C=C double bond of olefins to give the corresponding epoxide with high selectivity and efficient H₂O₂ utilization. In addition, unique stereospecificity, diastereoselectivity, and regioselectivity are observed due to the sterically hindered active oxygen species embedded in the POM frameworks.