Photochemical α-Cleavage of β, γ-Unsaturated Aryl Ketones: Competition between recombination/disproportionation and dissociation in geminate singlet and triplet radical pairs
✍ Scribed by Hans-Ulrich Gonzenbach; Kurt Schaffner; Bernd Blank; Hanns Fischer
- Publisher
- John Wiley and Sons
- Year
- 1973
- Tongue
- German
- Weight
- 632 KB
- Volume
- 56
- Category
- Article
- ISSN
- 0018-019X
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✦ Synopsis
Abstract
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.