Photoredox catalysis for oxygenation/deoxygenation between sulfides and sulfoxides by visible-light-responsive polyoxometalates

Abstract

In this paper, we report the unique visible-light-responsive photoredox catalysis of a divacant lacunary silicotungstate TBA₄H₄[γ-SiW₁₀O₃₆] (SiW10) for functional group transformations of sulfur-containing compounds; namely, (i) aerobic oxygenation of sulfides to sulfoxides and (ii) deoxygenation of sulfoxides to sulfides. In the presence of suitable additives, such as Ce³⁺ (electron transfer mediator for oxygenation) and an alcohol (electron and proton donor for deoxygenation), SiW10 shows visible-light-induced charge transfers by using the newly formed highest occupied molecular orbitals derived from the coordinating Ce³⁺ or alcohol at the vacant site of SiW10. Consequently, oxygenation of sulfides and deoxygenation of sulfoxides can selectively proceed by irradiation with visible light (λ > 400 nm) to afford the corresponding desired products in high yields. The SiW10 photocatalysts can readily be recovered and reused for these transformations. Based on evidence from several experiments, the roles of the additives as well as the reaction mechanisms for these transformations are also discussed.