Mechanisms for photochemical hydration of N-substituted pyridinium ions

Abstract

Photoinduced hydration of some pyridinium ions has been studied. Several pathways in the photohydration of quaternary pyridinium ions were observed. The substituents, especially at the nitrogen atom of the ring profoundly affect the reaction pathway, i.e. N-benzyl- and N-phenyl-pyridinium ions gave ring-cleaved ω-aminoaldehydes [e.g.(3)]via 2-hydroxy-1,2-dihydropyridines [e.g.(2)](Type A), while N-methyl isomers afforded azabicyclo[3.1.0]hex-3-en-2-ols (14) as reported by Kaplan (Type B). But exceptionally, N-methyl-3-carbamoylpyridinium ion (11h), which has an electron-withdrawing group, undergoes the Type A reaction. The Type A reaction clearly predominates over Type B, since nucleophiles can add in the dark to a pyridinium ion possessing an appropriate electron-withdrawing group in the ring.