Photochemistry of Methyl-Substituted Butyrophenones

The effects of methyl substituents upon the photochemical behavior of butyrophenone have been investigated. ier?-Alkyl phenyl ketones undergo competitive a- (type I) cleavage and type II photoelimination and cyclization. The rate constant for a-cleavage (~1 X 107 sec-1) is considerably slower than for a-cleavage of triplet feri-butyl alkyl ketones owing to a difference in the energetics of the reactions. The reactivity of the aromatic carbonyl triplet state toward 7-hydrogen abstraction is determined primarily by 7-substitution and is less affected by a and β substituents, a substituents increase the percentage of cyclization products whereas β substituents have the opposite effect. The effects of substituents on the behavior of the 1, 4-biradical intermediates are explained in terms of the transition states for biradical cyclization and elimination.