One-step synthesis and spectral study of some 1-methylbenzimidazoles, including use of a lanthanide shift reagent

Abstract

Substituted o-phenylenediamines (1) or their monoacetyl derivatives react with formaldehyde in boiling ethanol containing hydrochloric acid to give moderate yields of the corresponding 1-methylbenzimidazoles in one step. 4-Nitro-o-phenylenediamine gives the 6-nitro-derivative (2a; R = NO₂); 4-chloro-o-phenylenediamine produces the 5-chlorobenzimidazole (2b; R = Cl), but a mixture of the isomers (2a) and (2b) is obtained when R is another substituent. The synthesis of several new 1-methylbenzimidazoles is described and a comparison is made of the spectra of several isomeric pairs. They are most easily distinguished by their n.m.r. spectra: it is usually necessary to use a lanthanide shift reagent in order to analyse the spectra. The latter show that tris(pivaloylmethanato)europium(III) complexes at N-3 in the presence of various substituents on the benzene ring, e.g., methyl, halogeno, cyano, nitro, ethoxycarbonyl, or amino.