174. Photochemical α-Cleavage of β, γ-Unsaturated Aryl Ketones: Competition between Recombination/Disproportionation and Dissociation in Geminate Singlet and Triplet Radical Pairs

Article abstract of DOI:10.1002/hlca.19730560534

The photolysis of (R)-(+)-phenyl and (R)-(+)-p-anisyl 1, 2, 3-trimethylcyclopent2-enyl ketone (1, 2) and the corresponding rac-1- and 3-desmethyl analogs (3, 4) led to isomerization due to formal 1,3 aroyl migration and to formation of aryl aldehydes (7, 8), dienes (9, 10) and dimers (5, 6) of the cyclopentenyl radical. Evidence obtained from a chiroptical and mass spectrometric analysis of a crossing experiment and from photolytic CIDNP measurements including the use of CCl₄ as a free radical scavenger, supports the conclusion (1) : that the ketones undergo photochemical α-cleavage predominantly in the triplet state ; (2) : that recombination and disproportionation reactions within the geminate singlet and triplet aroyl/allyl radical pairs (11) compete with the dissociation into free radicals (12) ; (3) : that ketone isomerization by paths not involving polarizable radical intermediates is unimportant ; (4) : that no triplet oxa-di-π-methane type rearrangement products are formed.