Effect of ring size on the photochemical behavior of .beta.,.gamma.-unsaturated ketones

Systematic structural modifications in the photochemistry of .beta.,.gamma.-unsaturated ketones. II. Acyclic olefins and acetylenes

“…Since 11,15, and 16 are acyclic, they all meet the flexibility requirement; however, their IPv's are 10.26, 9.46, and 9.26 eV, respectively,47 suggesting that the oxetane yield should increase in this sequence. In fact, we were unable to detect oxetane from 11 whereas it was the only product found from 16.…”

“…Four energy-wasting mechanisms which might operate in these ketones are (a) degenerate cis-trans isomerization of the double bonds, (b) noncage recombination of acyl-dimethylallyl pairs to regenerate starting material, (c) cage recombination of these pairs, and (d) radiationless decay of the excited singlet state. Our study11 of stereospecifically deuterated 11 showed that the quantum yield for cis-trans isomerization under direct irradiation was only 0.02, which does not account for much of the energy lost in 11, and by analogy, in 13. Explanation (b) has already been ruled out for 11 since free acetyl and dimethylallyl radicals show little propensity for head recombination. The very short lifetime of 33 mitigates against noncage recombination in 13. Consideration of the two remaining alternatives, cage recombination and radiationless decay, leads to a rather detailed picture of the energy-wasting process.…”

“…Acetone Sensitization. As mentioned in a preliminary report,11 irradiation of 3,3-dimethyl-4-penten-2-one (11) in acetone did not produce the ODPM product 17 (cf. Scheme I).…”

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The photochemical reactions of six acyclic 18,7-unsaturated ketones have been investigated. Direct irradiation of those with a simple olefin (11,15,16) leads to an -cleavage, cyclobutanol, or oxetane, with the quantum yield of ketone disappearance decreasing when the olefin is more heavily alkylated. Although d,7-acetylenic ketone 12 also exhibits a-cleavage, an overwhelming preference for turning around of the allylic moiety is found in ketone 14, in which the two modes of cleavage are allowed to compete.

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Photoisomerization of 4-pyridones to 2-pyridones

“…While the nature of the substituent effect upon photoisomerization of hindered 4-pyrones was not fully understood, it was anticipated that introduction of the phenyl substituent into the C-3 and C-5 positions of 4-pyridones might increase the photoreactivity of 4-pyridones. In this respect l,2,6-trimethyl-3,5-diphenyl-4-pyridone (1), l-ethyl-2,6-dimethyl-3,5-diphenyl-4-pyridone (2), l-npropyl-2,6-dimethyl-3,5-diphenyl-4-pyridone (3), 1methyl-2,3,5,6-tetraphenyl-4-pyridone (4), and 1,2,6trimethyl-3-phenyl-4-pyridone (5) were prepared as the hindered 4-pyridones and their photochemistry was studied. The present paper14 deals with the photoisomerization of these hindered 4-pyridones to 2-pyridones as well as comparison of the photoreactivity of these compounds with that of hindered 4-pyrones.12…”