The intramolecular photocyclization and photoreduction of 2-halo-N-pyridinylbenzamide

The intramolecular photocyclization of aryl halide has been investigated in relation to the synthetic method and the reaction mechanism. It is reported that heterocyclic molecular systems are formed from the photocyclization of haloarene tethered to arene by several linkages and a photohomolytic radical mechanism is involved in the photocyclization of N-phenylalkyl-2-halopyridinium and N-(2-halophenyl)alkyl pyridinium salts. In conjunction with the study of the heterocyclic ring formation, we observed that photoreaction of N-(2-halophenyl)pyridinecarboxamide in a basic medium produced an intramolecular photosubstitution product, 2-pyridinylbenzoxazole, instead of a photocyclization product, whereas photoreaction of 2-chloro-N-pyridinylbenzamide in a weak basic or neutral medium afforded an intramolecular photocyclization product, benzo[c]naphthyridinones. These reactions are straightforward and valuable for the syntheses of heterocyclic compounds such as benzoxazole and benzonaphthyridinone derivatives. Thus, it is desirable to develop the synthetic methods of the heterocyclic compounds and to clarify their reaction mechanisms. In this work, we have studied the photochemical reaction mechanism of 2-halo-N-pyridinylbenzamide by using a nanosecond laser flash photolysis technique.