Effects of methylene chains on photoreactions of diphenylalkanediones and phenylalkenones

Abstract

Upon conventional UV-irradiation of 1,ω-diphenylalkane-1,ω-dione (PhC(O)–(CH₂)_n–C(O)Ph, n = 5, 6, 7, 8, and 10) in acetonitrile using a medium-pressure Hg lamp, an intramolecular γ-hydrogen abstraction of the triplet ketone (type II photoreaction) occurred to give acetophenone and 1-phenylalk-n-en-1-one (PhC(O)–(CH₂)_n − 3–CHCH₂), respectively, in a 1∶1 product ratio. Type II photoreaction occurred in all the 1,ω-diphenylalkane-1,ω-diones. The product 1-phenylalk-n-en-1-one also underwent a photoreaction to give secondary products under prolonged photoirradiation. The secondary products obtained from 1-phenylalk-n-en-1-one varied with the number of the methylene group n. For instance, no reaction occurred on the photolysis of 1-phenylbut-4-en-1-one obtained from type II photoreaction of 1,7-diphenylheptane-1,7-dione (n = 5). On the other hand, 1-phenylhex-5-en-1-one from the photoreaction of 1,8-diphenyloctane-1,8-dione (n = 6) gave acetophenone, buta-1,3-diene, 1-phenylhex-4-en-1-one, 2-vinyl-1-phenylcyclobutan-1-ol, and 1-phenylcyclohex-3-en-1-ol. Photolyses of other 1,ω-diphenylalkane-1,ω-dione (n = 7, 8, and 10) gave similar secondary products. Consequently, type II photoreaction of 1,ω-diphenylalkane-1,ω-dione occurs in which the number of methylenes between the two benzoyl groups does not affect the products. On the other hand, the length of the methylene group influences the photoreaction of 1-phenylalk-n-en-1-ones.