Photochemistry of α-substituted cyclohexanones. Chemistry of the intermediate type II biradicals

Abstract

U.v. irradiation of α-alkyl-substituted cyclohexanones bearing γ-hydrogen atoms leads to the generation of 1,4-biradicals via the Norrish Type II reaction. Triplet derived biradicals are produced with rather low yields reflecting the occurrence of singlet state processes as well as competition with the Norrish Type I reaction. The biradicals are good electron donors and have lifetimes in the neighbourhood of 1–2 µs in wet acetonitrile. Intramolecular γ-hydrogen abstraction in substituted cycloalkanones is considerably faster than in aliphatic ketones, reflecting lower entropy requirements in the former. A comparison of triplet and Type II biradical yields reveals that the main reaction path of the former is the formation of Type I biradicals.