Triphenylpyrylium-salt-sensitized electron transfer oxygenation of adamantylideneadamantane. Product, fluorescence quenching, and laser flash photolysis studies

Abstract

Triphenylpyrylium tetrafluoroborate (TPP⁺BF₄^–) has been found to sensitize oxygenation of adamantylideneadamantane to its dioxetane in good yield. Product studies, measurements of quantum yields for dioxetane formation, laser flash photolysis, and quenching experiments of TPP⁺ fluorescence by various electron donors have been carried out to elucidate the mechanism for dioxetane formation. Cyclic voltammetry was also used to examine the reactivity of pyryl radicals (TPP˙) with molecular oxygen. The results are consistent with a mechanism involving electron transfer from the alkene to the excited states of TPP⁺ to give radical pairs consisting of the alkene radical cations and TPP˙. The alkene radical cation reacts with molecular oxygen to afford the dioxetane. A singlet oxygen mechanism is not likely since the quenching rate of the excited triplet state of TPP⁺ by the alkene is 400 times faster than by dissolved oxygen under the photooxygenation conditions.