Oxygenation of α-methylstyrene with molecular oxygen, catalyzed by 10-methylacridinium ion via photoinduced electron transfer

Photooxygenation of α-methylstyrene with oxygen occurs efficiently in the presence of 10-methylacridinium perchlorate (AcrH⁺ClO₄^-) under visible light irradiation in oxygen-saturated acetonitrile (MeCN) to yield acetophenone as the main oxygenated product. No photoinduced oxygenation of α-methylstyrene occurs in the absence of AcrH⁺ under otherwise the same experimental conditions. Little photodegradation of AcrH^- occurs in the present photocatalytic system, which provides a clean method of photoinduced oxygenation reaction with molecular oxygen, alternate to the ene reaction of singlet oxygen. The photocatalytic oxygenation of α-methylstyrene with oxygen is shown to proceed via photoinduced electron transfer from α-methylstyrene to the singlet excited state of AcrH^- (¹AcrH⁺*) on the basis of the fluorescence quenching of ¹AcrH⁺* by α-methylstyrene, the quantum yield determination, and the detection of radical intermediates by laser flash photolysis and ESR measurements.