Photocatalytic Oxygenation Reactions Using Water and Dioxygen.

Water (H2 O) 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 H2 O as an oxygen source and O2 as an oxidant. Metal complexes can be oxidized by two molecules of one-electron oxidants with H2 O 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)MIII ]+ produces a substrate radical cation (S.+ ), accompanied by the regeneration of [(L)MII ]. S.+ then reacts with H2 O to produce an OH adduct radical that is oxidized by [(L)MIII ]+ to yield an oxygenated product (SO), in which the oxygen atom originates from H2 O, accompanied by regeneration of [(L)MII ]. Photocatalytic oxidation of H2 O by O2 to produce H2 O2 is combined with the catalytic oxygenation of substrates with H2 O2 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 H2 O as an oxygen source and O2 as the greenest oxidant.