HYDROCARBON ANALOGS OF THE TYPE-II PHOTO-ELIMINATIONS OF KETONES - PHOTOCHEMISTRY OF 1-SUBSTITUTED 4-PHENYL-4-PENTENES

Direct irradiation of 1,4-diphenyl-4-penten-1-ol produces mainly 2-methyl-2,5-diphenyltetrahydrofuran, while benzophenone-sensitized photolysis gives a-methylstyrene, acetophenone, and 1,4-diphenyl-l-pentanone. The direct irradiation is postulated to proceed via the radical anion of the alkene. A mechanism for the inefficient triplet state reaction (Φ = 0.0005) is proposed which involves initial hydrogen abstraction by the methylene carbon of the excited alkene to give a 1,4 biradical, which then produces the observed products. The mechanism is analogous to the accepted mechanism for the type II photofragmentation of ketones. The mechanism is supported by solvent effects and deuterium-labeling studies. Two related alkenes, 4-phenyl-4-penten-1-ol and 1,4-diphenyl-4-pentene, show similar photochemical behavior, although they react even less efficiently than 1,4-diphenyl-4-penten-1-ol. © 1979, American Chemical Society. All rights reserved.