Intramolecular photocyclization of N‐[(2‐haloaryl)methyl]pyridinium and n‐(arylmethyl)‐2‐halopyridinium salts

Abstract Various N ‐[(2‐haloaryl)methyl]pyridinium, N ‐(arylmethyl)‐2‐halopyridinium and N ‐(2‐halobenzyl)iso‐quinolinium salts have been synthesized and their intramolecular photocyclization reactions studied. Upon irradiation the aqueous solution of N ‐[(2‐haloaryl)methyl]pyridinium, and N ‐arylmethyl‐2‐halopyridinium salts 1, 2 were cyclized to give isoindolium salts. In contrast to the pyridinium salts 1, 2 , the aqueous solution of N ‐(2‐halobenzyl)isoquinolinium salts 3 appear not to undergo photocyclization. N ‐Benzyl‐2‐chloropyridinium salts 1c is more reactive than N ‐(2‐chlorobenzyl)pyridinium salt 1a in the photocyclization. N ‐(2‐Chlorobenzyl)‐2‐chloropyridinium salt 1d is three times more reactive than 1c . A mechanism of π‐complex formation of the halogen moiety of the pyridinium ring with the phenyl ring is suggested for the reactive pyridinium salt. The triplet energy of the isoquinolinium salts 3 is tool low to photocyclize.