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 (O2) 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 O2 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•–Mes•+). The Mes•+ moiety oxidizes Ph3P and PhCH2NH2 to produce the radical cations (Ph3P•+ and PhCH2NH2•+, respectively), whereas the Acr• moiety reduces O2 to O2•−. The produced Ph3P•+ binds with O2•− as well as O2, leading to the oxygenated product (Ph3P=O). On the other hand, proton transfer from PhCH2NH2•+ to O2•− 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.