Photocatalytic electron-transfer oxidation of triphenylphosphine and benzylamine with molecular oxygen via formation of radical cations and superoxide ion

Photooxygenation of triphenylphosphine (Ph3P) to triphenylphosphine oxide (Ph3P=O) with molecular oxygen (O-2) occurs under photoirradiation of 9-mesityl-10-methylacridinium perchlorate ([Acr(+)-Mes]ClO4-) which acts as an efficient electron-transfer photocatalyst. Photooxidation of benzylamine (PhCH2NH2) with O-2 also occurs efficiently under photoirradiation of Acr(+)-Mes to yield PhCH2N=CHPh and hydrogen peroxide (H2O2). Each photocatalytic reaction is initiated by intramolecular photoinduced electron transfer from the Mes moiety to the singlet excited state of the Acr(+) moiety to produce the electron-transfer state (Acr(center dot)-Mes(center dot+)). The Mes(center dot+) moiety oxidizes Ph3P and PhCH2NH2 to produce the radical cations (Ph3P center dot+ and PhCH2NH2 center dot+, respectively), whereas the Acr(center dot) moiety reduces O-2 to L-2(center dot-). The produced Ph3P center dot+ binds with O-2(center dot-) as well as O-2, leading to the oxygenated product (Ph3P=O). On the other hand, proton transfer from PhCH3NH2 center dot+ to O-2(center dot-) occurs, followed by hydrogen transfer, leading to the dehydrogenated dimer product. PhCH2N=CHPh. In each case, the radical intermediates were detected by laser flash photolysis and ESR measurements to clarify the photocatalytic mechanism.