Photochemical α‐Cleavage of β, γ‐Unsaturated Aryl Ketones: Competition between recombination/disproportionation and dissociation in geminate singlet and triplet radical pairs

The photolysis of (R)‐(+)‐phenyl and (R)‐(+)‐p‐anisyl 1, 2, 3‐trimethylcyclopent‐2‐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.