Abstract
2-Arylbenzimidazole and 5-substituted derivatives were synthesized by condensation at p-substituted-o-phenylene nitrogens with 2- or 4-picoline in the presence of sulfur or with a carboxylic acid group in the presence of polyphosphoric acid. Imidazole cyclizations from N'-(o, m or p-substituted-phenyl) arylamidines were also investigated, using sodium hypochlorite to react with N-arylamidines at pH 3 to afford the corresponding imino chlorides in higher yields. The chlorides were heated with an excess of sodium carbonate in situ for cyclization to an imidazole. Their cyclizing efficiencies to benzimidazoles decreased in correlation with the order of pKa values due to the dissociation of the amidino site in alkaline aqueous buffers. The acidities of 7-substituted-2-arylbenzimidazoles formed by ring closure at the position ortho to the substituent on the aniline ring in N'-(m-substituted-phenyl) arylamidines and those of compounds formed by the ring closure of N'-(o-substituted-phenyl)-arylamidines fitted the Taft equation for which steric hindrances are negligible (log K/KCH3=ρ*σ*). On the other hand, the acidities of 5-substituted-2-arylbenzimidazoles formed by ring closure at the position para to the substituent on the aniline ring in N'-(m-substituted-phenyl) arylamidines, by ring closure of N'-(p-substituted-phenyl) arylamidines and by ring closure of p-substituted-o-phenylene diamines with a carboxylic acid group fitted the Hammett equation using σpara (p=1.3).
