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 ext ensively photoreduced products, N-pyridinylbenzamides (1c, 10, and 11), wit h minor photocyclized product. Since the photocyclization reaction of 2-chl oro-N-pyridinylbenzamide is retarded by the presence of oxygen and sensitiz ed by the presence of a triplet sensitizer, acetone or acetophenone, a trip let state of the chloro analogue is involved in the reaction. Since several radical intermediates, particularly n-complexes of chlorine radical, are i dentified in the laser flash photolysis of 2-chloro-N-pyridinylbenzamide, a n intramolecular cyclization mechanism of phenyl radical assisted with n-co mplexation of chlorine radical for the cyclization reaction is proposed: th e triplet state (78 kcal/mol) of the chloro analogue (1a), which is populat ed by the excitation of 16 undergoes a homolytic cleavage of the C-Cl bond to give phenyl and chlorine radicals; while chlorine radical holds the neig hbor pyridinyl ring with its n-complexation, the intramolecular arylation o f the phenyl radical with the pyridinyl ring proceeds to produce a conjugat ed 2,3-dihydropyridinyl radical and then the conjugated radical aromatizes to afford a cyclized product, benzo[c]naphthyridinone by ejecting a hydroge n. The photoreduction product can be formed by hydrogen atom abstraction of the phenyl sigma radical from the environment.