Photochemistry of acylic βγ-unsaturated ketones. Part 4. The effect of α-methyl substitution with some hex-4-en-2-ones and 1-phenylpent-3-en-1-ones

Abstract

The photochemistry of the two hex-4-en-2-ones (1a and b), and the three 1-phenylpent-3-en-1-ones (1c–e) have been studied. Upon direct irradiation (1a) shows exclusively (E)–(Z) isomerization, whereas (1b) yields only α-cleavage products, illustrating competition between (E)–(Z) isomerization and α-cleavage for the hex-4-en-2-ones. Upon direct irradiation of (1c) no photochemical reaction is observed, but with (1d and e) products resulting from the α-cleavage reaction are obtained. Upon triplet sensitized irradiation (1a and b) yield the corresponding (Z)-isomers, but (1d) gives α-cleavage products. The difference in α-cleavage ability between (1a and c) on the one hand and (1b, d, and e) on the other upon direct irradiation is ascribed to the effect of α-methyl substitution. The difference in the photochemical behaviour of the hex-4-en-2-ones (1a and b) compared with that of the 1-phenylpent-3-en-1-ones (1c–e) is rationalized in terms of differences in the triplet state energy levels of the two types of βγ-enones.