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]-3H,4H-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-dihydrocyclohexndienyl 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).