Photocatalytic Oxygenation Reactions Using Water and Dioxygen

Water (H₂O) is the most environmentally benign reductant and is oxidized to evolve dioxygen (O₂)—the greenest oxidant—in photosystem II. This Minireview focuses on photocatalytic oxygenation of substrates with H₂O as an oxygen source and O₂ as an oxidant. Metal complexes can be oxidized by two molecules of one-electron oxidants with H₂O to produce high-valent metal–oxo complexes, which act as active oxidants for oxygenating organic substrates. When an appropriate oxidant is employed for the substrate oxidation, the reduced oxidant can be oxidized by dioxygen to regenerate the oxidant when water and dioxygen are used as an oxygen source and an oxidant, respectively. Photoinduced electron transfer from a substrate (S) to the excited state of complex [(L)M^III]⁺ produces a substrate radical cation (S^.+), accompanied by the regeneration of [(L)M^II]. S^.+ then reacts with H₂O to produce an OH adduct radical that is oxidized by [(L)M^III]⁺ to yield an oxygenated product (SO), in which the oxygen atom originates from H₂O, accompanied by regeneration of [(L)M^II]. Photocatalytic oxidation of H₂O by O₂ to produce H₂O₂ is combined with the catalytic oxygenation of substrates with H₂O₂ to produce the oxygenated products, in which the oxygen atom originates from O₂ at the beginning but later from water. This Minireview provides a promising strategy for oxygenation of substrates by using H₂O as an oxygen source and O₂ as the greenest oxidant.