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 CCl 4 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.