Oxidation of aryl-substituted cycloheptatrienes by photoinduced electron transfer

Abstract

The photooxidation of the aryl-substituted cycloheptatrienes 7-(p-methoxyphenyl)cycloheptatriene (1a), 7-, 1- and 3-(p-dimethylaminophenyl)cycloheptatriene (1b, 1c and 1d) in deaerated acetonitrile solution at room temperature to the corresponding radical cations is accomplished by electron transfer to the electronically excited acceptors 9,10-dicyanoanthracene (A2), N-methylquinolinium perchlorate (A3), N-methylacridinium perchlorate (A4), 2,4,6-triphenylpyrylium tetrafluoroborate (A5) and 1,1′-dimethyl-4,4′-bipyridinium dichloride (A6). In the case of 1 back electron transfer is unimportant. The primarily formed radical cation deprotonates and the resultant cycloheptatrienyl radical undergoes a self reaction thus forming bitropyl. By contrast, 1b, 1c and, 1d decompose only very slightly, bitropyl is not formed and the acceptors are not consumed.

Notably, the photooxidation of 1a in air-saturated acetonitrile solution containing HBF₄ (3.2 × 10^–2 M) and one of the acceptors results in the formation of tropylium salt. A tentative mechanism for this process is postulated.