Photocyclization of substituted alpha-(O-ethylphenyl)-acetophenones and alpha-(O-tolyl)acetophenones

The photochemistry of several substituted alpha-arylacetophenones was investigated. These ketones undergo intramolecular delta-hydrogen abstraction to form 1, 5 biradicals, which upon cyclization form indanols as photoproducts.; The intramolecular OH-to-benzene ring hydrogen bonding in the preferred conformation of such 1,5 biradical intermediate effects the photochemical behavior of these ketones. Tuning up the strength of such intramolecular hydrogen bonding is achieved by putting different ring substituents on the benzene ring. It turns out that an electron-donating methoxy group increases the product diastereoselectivity of the substituted alpha-(o-ethylphenyl) acetophenone by strengthening the above-mentioned hydrogen-bonding in reaction intermediate, while an electron-withdrawing cyanide group has exactly the opposite effect on diastereoselectivity. Temperature also effects product diastereoselectivity of these ketones.; Substituent also has an effect on steady-state photokinetics of alpha-arylacetophenones. The electron-donating methoxy group tends to increase the quantum yield and fasten the hydrogen abstraction while electron-withdrawing cyanide group has the opposite effects.