Photochemical Generation of Radical Anions of Photolabile Aryl Ketones

Article abstract of DOI:10.1021/j100020a048

The radical anions of a series of substituted α-(aryloxy)acetophenones have been generated by trapping the solvated electron produced by 355-nm laser-induced photoionization of 4,4'-dimethoxystilbene in either acetonitrile or dimethylformamide (DMF).The radical anion of the parent ketone, α-phenoxyacetophenone, has λ_max at 500 nm and decays with a rate constant 7 x 1E5 s^-1.This rate constant reflects the rate of β-cleavage to generate phenoxide ion and phenylacyl radical.The rate constant for β-cleavage decreases for ketones with electron donating substituents in the α-aryloxy ring.Substituent effects on the acetophenone ring are in the opposite direction, and many of the 4-substituted (aryloxy)acetophenones have half-lives in excess of 10 μs that reflect a combination of second order processes and are not limited by β-cleavage.The generality of the photoionization/electron trapping method for the generation of radical anions of ketones with short-lived excited states that preclude direct excitation is illustrated by its application to other aryl ketones that undergo α-cleavage, β-phenyl quenching, and intramolecular hydrogen abstraction reactions.