Summary2-Cyclopentenyl and 3-phenyl-2-cyclopentenyl methyl ketones (15-18, 30, 31) undergo a 1,3-acetyl shift on direct irradiation, and the oxa-di-7c-methane rearrangement to photochemically non-interconverting endo and exo bicyclo-[2.l.O]pentyl methyl ketones on triplet sensitization. Exceptions include the 2-methyl-3-phenyl-2-cyclopentenyl methyl ketone 32 and the I-phenyl-2-cyclopentenyl methyl ketone 44 which are unreactive on direct irradiation and on triplet sensitization, respectively, and the 2-phenyl-2-cyclopentenyl methyl ketones 42 and 43 which do not react under either condition. The reactive triplet of the 3-phenyl-2-cyclopentenyl methyl ketone 30 has been identified as the localized styrene 7c, 7c*-state of ET= 59 kcaVmol by comparison of its phosphorescence at 77 K in rigid glasses with that of l-phenyl-cyclopentene, and by the independence of the quantum yield on sensitizer energy in the range of 61-74 kcaVmol.

No abstract

Summary. The photolysis of (I?)-( + )-phenyl and (R)-(+ )-p-anisyl 1,2,3-triinethylcyclopent-2-enyl ketone (1, 2) and the corresponding rac-1-and 3-desmethyi analogs (3, 4) led t o isomerization due to formal 1 , 3 aroyl migration and to formation of aryl aldehydes (7, 8), dienes (9,lO) 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, as a free radical scavenger, supports the conclusion ( I ) : that the ketones undergo photochemical cc-cleavage predominantly in the triplet state ; (2) : that recombination and disproportionation reactions within the geminate singlct and triplet aroyllallyl 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-nmethane type rearrangement products are formed.We have previously shown that cyclopent-2-enyl methyl ketones undergo 1,3 acetyl shifts and a-cleavage to radicals in the excited singlet state faster than intersystem crossing, and oxa-di-n-methane type rearrangements to 5-acetyl bicyclopentanes (endo > exo) in the triplet state [2]2). Recent CIDNP investigations [4] revealed, furthermore, that radical recombinations are very minor paths for these reactions.We now report (7): that the corresponding aryl ketones, e.g. (R)-(+)-l, undergo photochemical a-cleavage predominantly in the triplet state; (2) : that recombination and disproportionation reactions within the geminate singlet and triplet aroyl/allyl radical pairs compete with the dissociation into free radicals; (3): that ketone isomerization by paths not involving polarizable radical intermediates is unimportant, and (4) : that no triplet oxa-di-%-methane type rearrangement products are formed. Part of these results were presented at the IVth International IUPAC Conference on Photochemistry, Baden-Baden, July 1972 [l]. Such divergent behavior of excited singlets and triplets is often observed with /3,y-unsaturated ketones; see [3] for references. Irradiations were carried out in iso-octane, benzene, or acetone at wavelengths 254 nm and > 340 nm.

Dedicated to Professor V . Prelog on his 70th anniversary (16. 111. 76)Sunznzury. 1-Phenyl-3-acet.ylcyclopentene1) (3) undergoes triplet-sensitized oxa-di-n-methane rearrangement to a cu. 3 : 1 mixture of e d o -and exo-l-phenyl-5-acctylbicycio~.l.0;pentane2) (4). The reactive triplet of 3 has been identified as the localized styrene n.,n*-state of ET = 59 kcal/rnol by comparison of its phosphorescence at 77K in a rigid glass with that of I-phenyicyclopentene, and by sensitization quantum yields at room temperature in liquid solution. These quantum yields are independent of sensitizer energy in the range of 61-74 kcal/mol and reach zero for product formation with a sensitizer of Er = 59 kcal/mol. Although S1-t TI intersystem crossing is appreciable under phosphorescence conditions (fDD = 0.05), direct irradiation of 3 at 20" does not lead to triplet rearrangement, and the typical singlet 1,3-acetyl shift to 3-acctyl-3-phcnylcyclopentenes) (8) is observed only.