Flash photolysis observation of aryl, 2,3-dihydrocyclohexadienyl, and halogen anion radicals in aqueous solution: Photohomolytic radical cyclization of aryl halide

A thorough investigation of the transient species from the laser flash photolysis of haloarene tethered arenes has been performed. Some transient species in the laser photolysis of N-benzyl-2-halopyridinium and N-(2-halobenzyl)pyridinium bromides have been investigated as an effort to understand the photocyclized reaction mechanism as well as to know the properties of the transients themselves. Pyridinium sigma, 2,3-dihydrocyclohexadienyl pi, and halogen anion radicals are detected in the photolysis of a N-benzyl-2-halopyridinium salt (1 or 2) in water: the absorption maxima are 250, 310, and 355 (Br-Cl-.-) nm, respectively. The Lifetimes of 2-pyridinium sigma radical, 2,3-dihydrocyclohexadienyl pi, and bromine chloride (in the case of 1) anion radicals are 0.72 ms, 1.2 ms, and 9.3 mu s, respectively. Phenyl sigma 2,3-dihydropyridinium pi, and halogen anion radicals are also detected in the laser photolysis of an aqueous solution of a N-(2-halobenzyl)pyridinium salt (3 or 4): the absorption maxima are 250, 310, and 360 nm (Br-2(.-), in the case of 4), respectively. The lifetimes of the transient intermediates are 1.4 ms, 1.2 ms, and 10 mu s, respectively. On the basis of the observation of these intermediates, their properties, and the reactant reactivities, a mechanism of the photohomolytic bond cleavage of carbon-halogen followed by radical arylation and hydrogen ejection is proposed for the intramolecular photocyclization of haloarene tethered arenes in water.