Photocyclization Mechanism of Halopyridinium Salt Tethered to Arene: Flash Photolysis Observation of a Pyridinium σ, Cyclohexadienyl Radicals, and a Dihalide Radical Anion in Aqueous Solution

The photocyclization reactions of 2-halopyridinium salt tethered to arene have been studied. The photocyclization of 2-halophenethylpyridinium salts produced a six-membered heterocyclic product, pyrido[2,1- a ]-3 H,4 H -isoquinolinium salt. The 2-halopyridinium salts tethered to phenyl group with methylene linkage ( 1 and 2a ) are more reactive than the 2-halopyridinium salts with an ethylene group ( 3 and 4a ). As expected, the 2-bromopyridinium salt linked to the phenyl group with ethylene linkage ( 4a ) is more reactive than the 2-chloropyridinium salt with the ethylene group ( 3 ), which is different from the 2-halopyridinium salt with the methylene linkage. The transient intermediates such as pyridinium salt σ, 2,3-dihydrocyclohexadienyl radicals, and a dibromide radical anion are detected with laser flash photolysis facility. The maximum absorption wavelengths of the pyridinium salt σ, 2,3-dihydrocyclohexadienyl radicals, and dibromide radical anion are 250, 310, and 360 nm, respectively. The lifetimes of the pyridinium salt σ, 2,3-dihydrocyclohexadienyl radicals, and dibromide radical anion are 0.93 ms, 0.84 ms, and 25 μs, respectively. Thus, a photohomolytic radical mechanism is proposed for the cyclization reaction. The excited singlet state is mainly involved in the photocyclization of 2-halopyridinium salts tethered to phenyl group with ethylene linkage ( 3 and 4a ), while both the singlet and triplet states are involved in the photocyclization of the 2-halopyridinium salts with methylene linkage ( 1 and 2a ).