Acidic properties of benzimidazoles and substituent effects. V. Protection of benzimidazoles by N-alkyl bond formation using vinylpyridines.

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Vinylpyridines were utilized for protection of the benzimidazole N-H bond to give 1-(2-pyridylethyl)-benzimidazoles. The reaction was found to progress smoothly when glacial acetic acid was used as a catalyst. In the alkylation of 5- or 7-substituted-2-arylbenzimidazoles with vinylpyridines, the yield decreased with increasing electronattracting effect of the substituent groups in the benzimidazole ring. On the other hand, the removal of pyridylethyl groups by the use of aluminum chloride as a catalyst was kinetically examined ; a large excess of sodium hydroxide was used for decomposition of the intermediate adduct of aluminum chloride. The rate increased somewhat when electron-releasing substituent groups were present in the benzimidazole ring. 1-[2-(2-Pyridyl) ethyl]-2-arylbenzimidazoles were resistant to removal of their (2-pyridyl) ethyl groups. 4-Vinylpyridine can be used more efficiently as a protecting agent.

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