ADDITION VERSUS OXYGENATION OF ALKYLBENZENES WITH 10-METHYLACRIDINIUMION VIA PHOTOINDUCED ELECTRON-TRANSFER

Addition of alkylbenzenes with 10-methylacridinium ion (AcrH(+)) occur s efficiently under visible light irradiation in deaerated acetonitril e containing H2O to yield 9-alkyl-10-methyl-9,10 -dihydroacridine sele ctively. On the other hand, the photochemical reaction of AcrH(+) with alkylbenzenes in the presence of perchloric acid in deaerated acetoni trile yields 10-methyl-9, 10-dihydroacridine, accompanied by the oxyge nation of alkylbenzenes to the corresponding benzyl alcohols. The phot ooxygenation of alkylbenzenes occurs also in the presence of oxygen, w hen AcrH(+) acts as an efficient photocatalyst. The studies on the qua ntum yields and fluorescence quenching of AcrH(+) by alkylbenzenes as well as the laser flash photolysis have revealed that the photochemica l reactions of AcrH(+) with alkylbenzenes in both the absence and pres ence of oxygen proceed via photoinduced electron transfer from alkylbe nzenes to the singlet excited state of AcrH(+) to produce alkylbenzene radical cations and 10-methylacridinyl radical (AcrH(.)). The competi tion 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 o f the deprotonated radicals with AcrH(.) yields the adducts. The photo induced hydride reduction of AcrH(+) in the presence of perchloric aci d proceeds via the protonation of acridinyl radical produced by the ph otoinduced electron transfer from alkylbenzenes. In the presence of ox ygen, however, the deprotonated radicals are trapped efficiently by ox ygen to give the corresponding peroxyl radicals which are reduced by t he back electron transfer from AcrH(.) to regenerate AcrH(+), followed by the protonation to yield the corresponding hydroperoxide. The rati os of the deprotonation reactivity from different alkyl groups of alky lbenzene radical cations were determined from both the intra- and inte rmolecular 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 deprot onation from isopropyl group of alkylbenzene radical cations appear in the case of the o-methyl isomer.