PHOTOCHEMISTRY OF beta , gamma -ENONES. 7. 1 INTRAMOLECULAR COMPETITION BETWEEN DI- pi -METHANE AND OXA-DI- lambda -METHANE REARRANGEMENTS. ON THE INTERMEDIACY OF CHARGE- TRANSFER COMPLEXES AND ZWITTERIONS IN THE DI- pi -METHANE REARRANGEMENTS.

Article abstract of DOI:10.1021/ja00351a040

The intramolecular competiton between the di- pi -methane (DPM) rearrangement and the oxa-di- pi -methane (ODPM) rearrangement of 3-(3,4-dihydro-2-naphthyl)-3-methylpent-4-en-2-one (1), (E)-3-methyl-3-vinyl-5-phenylpent-4-en-2-one (2), and 2-cyclopent-1enyl-2-vinylcyclopentanone (3) has been examined. Dienone 1 in benzene upon triplet photosenitization with 4-benzoylbiphenyl leads to the formation of one DPM isomer and two ODPM isomers in ratio of 35:17:10. The results are consistent with a stepwide mechanism via 1,4- and 1,3-biradicals as the subsequent intermediates. The occurrence of only one instead of the expected two DPM isomers is explained in terms of the specific charge-transfer complexation in the 1,3-biradical rotamer intermediate in which the phenylene and acetyl group are 'cisoid' (CTC).