Polyoxometalate Photocatalysis for Liquid-Phase Selective Organic Functional Group Transformations

Polyoxometalates (POMs), which are anionic metal oxide clusters, have recently attracted considerable attention as photocatalysts because of their unique photoinduced charge-transfer properties, redox properties, acid–base properties, and reactivities. In this Review, we present a summary of recent developments in POM photocatalysis for organic synthesis. Various organic functional group transformations can be selectively induced by photoirradiation in the presence of catalytic amounts of suitably designed POMs. In particular, many liquid-phase functional group transformations based on the activation of substrates by decatungstate have been reported. However, decatungstate photocatalysis requires irradiation with UV light because of the large energy gaps between the O2–-based highest occupied molecular orbitals (HOMOs) and the W6+-based lowest unoccupied molecular orbitals (LUMOs) therein. Various strategies have been developed in efforts to utilize visible light, including hybridization with photosensitizers, metal substitution, and coordination of ligands (substrates) at the vacant sites of lacunary POMs. We also present here an overview of our recent work on the development of visible-light-responsive POM catalysts by HOMO- and LUMO-engineering strategies for both the oxidation and reduction of organic substrates, including amines, alcohols, nitroarenes, sulfides, sulfoxides, and pyridine N-oxides.