Photoreaction of 2-halo-N-pyridinylbenzamide: intramolecular cyclization mechanism of phenyl radical assisted with n-complexation of chlorine radical

The photochemical behavior of 2-halo-N-pyridinylbenzamide (1-4 in Chart 1) was studied. The photoreaction of 2-chloro-N-pyridinylbenzamides 1a, 2a, 3a, and 4 afforded photocyclized products, benzo[c]naphthyridinones (6-9 and 16), in high yield, whereas the bromo analogues 1b, 2b, and 3b produced extensively photoreduced products, N-pyridinylbenzamides (1c, 10, and 11), with minor photocyclized product. Since the photocyclization reaction of 2-chloro-N-pyridinylbenzamide is retarded by the presence of oxygen and sensitized by the presence of a triplet sensitizer, acetone or acetophenone, a triplet state of the chloro analogue is involved in the reaction. Since several radical intermediates, particularly n-complexes of chlorine radical, are identified in the laser flash photolysis of 2-chloro-N-pyridinylbenzamide, an intramolecular cyclization mechanism of phenyl radical assisted with n-complexation of chlorine radical for the cyclization reaction is proposed: the triplet state (78 kcal/mol) of the chloro analogue (1a), which is populated by the excitation of 1a undergoes a homolytic cleavage of the C-Cl bond to give phenyl and chlorine radicals; while chlorine radical holds the neighbor pyridinyl ring with its n-complexation, the intramolecular arylation of the phenyl radical with the pyridinyl ring proceeds to produce a conjugated 2,3-dihydropyridinyl radical and then the conjugated radical aromatizes to afford a cyclized product, benzo[c]naphthyridinone by ejecting a hydrogen. The photoreduction product can be formed by hydrogen atom abstraction of the phenyl sigma radical from the environment.