Oxidation of aryl-substituted cycloheptatrienes by photoinduced electron transfer

The photooxidation of the aryl-substituted cycloheptatrienes 7-(p-methoxyph enyl)cycloheptatriene (1a), 7-, 1- and 3-(p-dimethylaminophenyl)cycloheptat riene (1b, 1c and 1d) in deaerated acetonitrile solution at room temperatur e to the corresponding radical cations is accomplished by electron transfer to the electronically excited acceptors 9,10-dicyanoanthracene (A2), N-met hylquinolinium perchlorate (A3), N-methylacridinium perchlorate (A4), 2,4,6 -triphenylpyrylium tetrafluoroborate (A5) and 1,1'-dimethyl-4,4'-bipyridini um dichloride (A6). In the case of 1 back electron transfer is unimportant. The primarily formed radical cation deprotonates and the resultant cyclohe ptatrienyl radical undergoes a self reaction thus forming bitropyl. By cont rast, 1b, 1c and, 1d decompose only very slightly, bitropyl is not formed a nd the acceptors are not consumed. Notably, the photooxidation of 1a in air-saturated acetonitrile solution co ntaining HBF4 (3.2 x 10(-2) M) and one of the acceptors results in the form ation of tropylium salt. A tentative mechanism for this process is postulat ed.