Photoreactivity of .alpha.-fluorinated phenyl alkyl ketones

Abstract

The photoreactivities of the mono-, di-, and tri-..cap alpha..-fluorinated acetophenones have been compared to that of acetophenone itself. All four ketones have similar triplet excitation energies; the three fluorinated ketones have reduction potentials 0.5-0.7 eV lower than that of acetophenone. Triplet reactivity toward alkylbenzenes keeps increasing with fluorine substitution, since the rate-determining step becomes charge-transfer complexation as the ketone reduction potential decreases. The primary/tertiary C-H selectivity toward p-cymene increases with the number of fluorines. Triplet reactivity toward cyclopentane also is increased by fluorination but peaks at two fluorines, since the lowest triplet switches from n,..pi..* to ..pi..,..pi..* with two or three fluorines and ..pi..,..pi..* triplets are unreactive in simple hydrogen atom abstraction. In contrast, ..cap alpha..-fluorination of valerophenone does not significantly increase the rate of triplet ..gamma..-hydrogen abstraction. The inductive effect on reactivity apparently is offset by a conformational effect. The ..cap alpha..-fluorinated phenones give predominantly cyclobutanols instead of Norrish type II elimination. ..cap alpha..-Fluoroacetophenone forms predominantly acetophenone and HF when irradiated with 2-propanol, in what appears to be a short chain process involving electron transfer to ketone followed by fluoride ion loss. Finally, the radical coupling products in these reactions are formed in varying yields, depending on solvent andmore » additives.« less