PHOTOCHEMICAL GENERATION OF RADICAL-ANIONS OF PHOTOLABILE ARYL KETONES

The radical anions of a series of substituted alpha-(aryloxy)acetophen ones have been generated by trapping the solvated electron produced by 355-nm laser-induced photoionization of 4,4'-dimethoxystilbene in eit her acetonitrile or dimethylformamide (DMF). The radical anion of the parent ketone, alpha-phenoxyacetophenone, has lambda(max) at 500 nm an d decays with a rate constant 7 x 10(5) s(-1). This rate constant refl ects the rate of beta-cleavage to generate phenoxide ion and phenacyl radical. The rate constant for beta-cleavage decreases for ketones wit h electron donating substituents in the alpha-aryloxy ring. Substituen t effects on the aceotphenone ring are in the opposite direction, and many of the 4-substituted (aryloxy)acetophenones have half-lives in ex cess of 10 mu s that reflect a combination of second order processes a nd are not limited by beta-cleavage. The generality of the photoioniza tion/electron trapping method for the generation of radical anions of ketones with short-lived excited states that preclude direct excitatio n is illustrated by its application to other aryl ketones that undergo alpha-cleavage, beta-phenyl quenching, and intramolecular hydrogen ab straction reactions.