Photocatalytic oxygenation of pivalic acid with molecular oxygen via photoinduced electron transfer using 10-methylacridinium ions

Photoirradiation of the absorption band of the 10-methylacridinium ion (AcrH⁺) with visible light in deaerated CH₃CN/H ₂O (1:1 v/v) containing pivalic acid (Bu^tCOOH) and less than 1 equiv of NaOH results in the selective formation of 9-tert-buty 1-9,10-dihydro-10-methylacridine (AcrHBu^t). The same product is obtained in O₂-saturated CH₃CN/H₂O under visible light irradiation. Photoirradiation of the absorption band of AcrHBu^t with UV light in deaerated CH₃CN/H₂O (1:1 v/v) results in the formation of tert-butyl hydroperoxide (Bu ^tOOH), accompanied by regeneration of AcrH⁺. This cycle can be repeated several times. When AcrH⁺ is replaced by the 9-phenyl derivative (AcrPh⁺), AcrPh⁺ acts as an effective photocatalyst for the one-pot photooxygenation of Bu^tCOOH in the presence of less than 1 equiv of NaOH relative to Bu^tCOOH with O ₂ to yield Bu^tOOH and Bu^tOH. The photocatalytic oxygenation mechanism is discussed based on the detection of radical intermediates by laser flash photolysis and ESR measurements as well as quantum yield determination.