Kinetics and mechanisms of nucleophilic displacements with heterocycles as leaving groups. Part 14. The preparation and reactions of some further α-heteroaryl-pyridinium salts

Abstract

Novel pyryliums containing fused dihydrocarbazole and dihydrobenzothiophene rings and α-benzothiazole substituents are described, together with their conversion to a variety of derived pyridiniums. The N-(2-pyridyl) group rearranges thermally at 60°C from the pyridinium nitrogen to the nitrogen atom of a 2-(2-benzimidazolyl) group [in (19)]; similar rearrangement in the carbazole (7) is complicated by dehydrogenation. Compared with the corresponding 2,4,6-triphenylpyridinium, replacement of α-phenyl by α-2-benzothiazolyl has a large rate-enhancing effect for N-benzyl, but much smaller for N-n-alkyl. Replacement of α-phenyl by a fused dihydrobenzothiophene system shows rate enhancement for N-benzyl; ring fission occurs for N-n-alkyl. In the single cases studied carbazole is as effective as dibenzothiophene, but benzimidazole less so than benzothiazole.