Reduction of aryl tropylium ions by thermal hydride transfer or by photochemical reactions

The reduction of aryl-substituted tropylium cations in deaerated acetonitri le solution at room temperature can be achieved by two methods: (a) thermal hydride transfer from the heterocyclic compounds 9,10-dihydro-10-methylacr idine, AcrH(2), and 2,4,6-triphenyl-4H-pyran, TPPH, to p-methoxyphenyl trop ylium perchlorate, 1, and p-dimethylaminophenyl tropylium perchlorate, 2, t hat affords regioselectively aryl-substituted cycloheptatrienes and (b) pho toinduced electron transfer from AcrH(2), TPPH, 10,10'-dimethyl-9,9'-bisacr idane, (AcrH)(2), or 2,2',4,4',6,6'-hexamethyl-4,4'-bi-(4H-pyran), (TMP)(2) to 1 and 2. The light-induced reduction was studied with the aid of steady state photol ysis and laser flash spectroscopy. A mechanism involving the photoionizatio n of (AcrH)(2) is proposed. (C) 1999 Elsevier Science S.A. All rights reser ved.