Bentonite clay: an efficient catalyst for the synthesis of 2-substituted benzimidazoles

Benzimidazoles have been reported to have a wide range of biological and therapeutic properties. For this reason a variety of methods for their synthesis has been described, following one of the two general routes: the coupling of o-phenylenediamine and carboxylic acids or their derivatives using a strong acid and high temperature, or a two-step sequence that involves oxidative cyclodehydrogenation of Schiff's bases, obtained by the reaction of o-phenylenediamines and aromatic aldehydes. A simple, efficient, and environmentally friendly procedure for the synthesis of substituted 2- and 2,5(6)-substituted benzimidazoles is herein described. The procedure is carried out by treatment of o-phenylenediamine or 4-chloro-o-phenylenediamine with aryl or heteroaryl aldehydes. Bentonite clay is used as catalyst in dry acetonitrile at room temperature. This procedure has several important advantages, including short reaction times, large-scale preparations, easy isolation of the products, and good to excellent yields.