Photoreactivity of α-Fluorinated Phenyl Alkyl Ketones

Article abstract of DOI:10.1021/ja00284a042

The photoreactivities of the mono-, di-, and tri-α-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,?^* to ?,?^* with two or three fluorines and ?,?^*triplets are unreactive in simple hydrogen atom abstraction.In contrast, α-fluorination of valerophenone does not significantly increase the rate of triplet γ-hydrogen abstraction.The inductive effect on reactivity apparently is offset by a conformational effect.Th α-fluorinated phenones give predominantly cyclobuthanols instead of Norrish type II elimination. α-fluoroacetophenone forms predominantly acetophenone and HF when irradiated with 2-propanol, in what appears 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 and additives.It is suggested that radical solvation can affect radical coupling rates sufficiently to prevent statistical ratios of cross-coupling and homo-coupling.