Monoradical Rearrangements of the 1,4-Biradicals Involved in Norrish Type II Photoreactions

Article abstract of DOI:10.1021/ja00403a035

The photochemistry of α-allylbutyrophenone (α-AB) and that of γ-cyclopropylbutyrophenone (γ-CB) both reveal that the 1,4-biradicals generated by triplet-state γ-hydrogen abstraction undergo typical radical rearrangements in competition with their more normal type II reactions.From α-AB, 2-phenyl-2-norbornanol is formed in 1/24th the combined yield of 1-phenyl-4-penten-1-one and 1-phenyl-2-allylcyclobutanol.Its formation is explained by a 5-hexenyl-to-cyclopentylmethyl rearrangement of the 1,4-biradical, with a rate constant of 5 x 10⁵ s^-1.From γ-CB, 1-phenyl-4-hepten-1-one and 1-phenyl-4-cycloheptenol together are formed in double the yield of acetophenone.Their formation is explained by a cyclopropylcarbinyl-to-allylcarbinyl rearrangement of the 1,4-biradical, with a rate constant of 2 x 10⁷ s^-1.In both cases, the rearrangement percentages are what would be predicted if the biradicals have the same 35 - 50-ns lifetime measured for other ketones and if they rearrange with the same rate constants characteristic of monoradicals.Triplet γ-CB decays only 1.5 times faster than triplet γ-isopropylbutyrophenone, indicating that there is at most a small enhancement of the γ-hydrogen abstraction rate by cyclopropyl cojugation.Triplet α-AB decays 100 times faster than triplet butyrophenone, 99percent representing internal quenching by the β-vinyl group ( k = 8 x 10⁸ s^-1).Bicyclic oxetanes are formed in quantum yields of only 0.01.No oxetanes were isolated, only rearranged unsaturated alcohols and aldehydes.