Photochemistry of selenoketones

Abstract

The photochemistry of di-tert-butylselenoketone 1 and selenofenchone 2 has been examined. Their photochemical properties resemble those of thioketones rather than ketones in that hydrogen abstraction occurs from an upper excited state and not from the lowest (n → π*) excited state. Di-tert-butylselenoketone, 1, on irradiation into the S2 band forms the diselenide 3. The quantum yield for this process was found to be 0.0055 and was increased in the presence of a good hydrogen atom donor. No reaction was observed following excitation into the S1 state unless a good hydrogen atom donor was present, and even then the photoreaction was very inefficient. Quenching experiments with biacetyl demonstrated the lifetime of the abstracting state to be in the order of 10−11 s. The formation of the diselenide 3 has been rationalized in terms of hydrogen abstraction from solvent by photoexcited 1 and trapping of the radicals produced by ground state 1, leading to the radical 6. The mechanism is supported by esr evidence for the existence of 6. Selenofenchone, 2, also, did not react following excitation into the S1 state, but underwent intramolecular cyclization in a similar manner to bicyclic thioketones following excitation into the S2 state.