Nitrile-forming radical elimination reactions of 1-naphthaldehyde O-(4-substituted benzoyl)oximes activated by triplet benzophenone

It was found that 1-naphthaldehyde O-aroyloximes (1) preferentially adopting the anti-configuration undergo efficient triplet-sensitized reactions to give 1-cyanonaphthalene and monosubstituted benzenes as unimolecular radical elmination products along with syn-1. Negligible formation of 4-substituted benzoic acids strongly suggested the participation of simultaneous N-O and C(O)-Ar bond cleavages in the triplet excited-state anti-isomer. The logarithm of the kr/(kd + ki) ratio (where kr is the rate constant for homolytic bond cleavage in triplet 1, kd the rate constant for deactivation of the triplet anti-isomer, and ki is the rate constant for isomerization of triplet anti-1 into syn-1) used as a measure of the triplet-state reactivity of anti-1 showed a negligible dependence on the substituent constant and solvent polarity. This finding was explained in terms of a very small contribution of the ionic structure to the transition-state for simultaneous N-O and C(O)-Ar bond cleavages.