Addition versus oxygenation of alkylbenzenes with 10-methylacridinium ion via photoinduced electron transfer

Addition of alkylbenzenes with 10-methylacridinium ion (AcrH(+)) occurs efficiently under visible light irradiation in deaerated acetonitrile containing H2O to yield 9-alkyl-10-methyl-9,10 -dihydroacridine selectively. On the other hand, the photochemical reaction of AcrH(+) with alkylbenzenes in the presence of perchloric acid in deaerated acetonitrile yields 10-methyl-9, 10-dihydroacridine, accompanied by the oxygenation of alkylbenzenes to the corresponding benzyl alcohols. The photooxygenation of alkylbenzenes occurs also in the presence of oxygen, when AcrH(+) acts as an efficient photocatalyst. The studies on the quantum yields and fluorescence quenching of AcrH(+) by alkylbenzenes as well as the laser flash photolysis have revealed that the photochemical reactions of AcrH(+) with alkylbenzenes in both the absence and presence of oxygen proceed via photoinduced electron transfer from alkylbenzenes to the singlet excited state of AcrH(+) to produce alkylbenzene radical cations and 10-methylacridinyl radical (AcrH(.)). The competition between the deprotonation of alkylbenzene radical cations and the back electron transfer from AcrH(.) to the radical cations determines the limiting quantum yields. In the absence of oxygen, the coupling of the deprotonated radicals with AcrH(.) yields the adducts. The photoinduced hydride reduction of AcrH(+) in the presence of perchloric acid proceeds via the protonation of acridinyl radical produced by the photoinduced electron transfer from alkylbenzenes. In the presence of oxygen, however, the deprotonated radicals are trapped efficiently by oxygen to give the corresponding peroxyl radicals which are reduced by the back electron transfer from AcrH(.) to regenerate AcrH(+), followed by the protonation to yield the corresponding hydroperoxide. The ratios of the deprotonation reactivity from different alkyl groups of alkylbenzene radical cations were determined from both the intra- and intermolecular competitions of the deprotonation from two alkyl groups of alkylbenzene radical cations. The reactivity of the deprotonation from alkylbenzene radical cations increases generally in the order methyl < ethyl < isopropyl. The strong stereoelectronic effects on the deprotonation from isopropyl group of alkylbenzene radical cations appear in the case of the o-methyl isomer.