RECENT ADVANCES IN THE PHOTOCHEMISTRY OF THE CARBON-NffROGEN DOUBLE BOND

The excited states of imines were shown not to be reactive intermediates in the photoreduction of a series of benzaldehyde N-alkylimines. Instead, ketyl radicals were clearly implicated as the active reducing agent. These ketyl radicals were derived from carbonyl compounds present in the reaction mixture as an impurity, an added sensitizer, or as a photogenerated species. The lack of reactivity of the excited state of simple imines suggests that the excited states undergo a very efficient non-radiative decay to ground state thereby precluding hydrogen abstraction. This facile energy deactivation is a conse-quence of rotation about the C—N double bond (i.e. syn-anti photoisomeriza-tion). In rigid systems, this mode of energy dissipation would not be available and these molecules would have maximum opportunity to undergo reaction from an electronically excited state. In this regard, the photocycloaddition of arylazir-enes with electron deficient olefins to give A'-pyrrolines has been examined in mechanistic detail. The formation of the adduct was interpreted as proceeding by way of irreversible opening of the azirene ring to form a nitrile ylide inter-mediate which is then trapped by a suitable dipolarophile. Irradiation of a number of substituted arylazirenes in an inert solvent gives 1,3-diazabicyclo-[3.1.O]hex-3-enes as primary photoproducts. The formation of these dimers can be rationalized by 1,3-dipolar addition of the initially generated nitrile ylide on to a ground state azirene molecule. Support for this conclusion was obtained by a study of the variation of the quantum yield for adduct forma-tion as a function of the concentration of added dipolarophile. The study shows that the amount of adduct formed is dependent on the initial concentra-tion of arylazirene and on the activity of the dipolarophile