Highly Efficient and Facile Method for Synthesis of 2-Substituted Benzimidazoles via Reductive Cyclization of O-Nitroaniline and Aryl Aldehydes

A versatile and convenient synthesis of 2-substituted benzimidazoles, using o-nitroaniline as starting material with several aryl aldehydes, has been accomplished by using a small amount of a reluctant agent. The reaction was carried out under very mild conditions at room temperature. The yields obtained are very good in reasonably short reaction times. 1. Introduction Benzimidazoles are very useful intermediates/subunits for the development of molecules of pharmaceutical or biological interest. Substituted benzimidazole derivatives have found applications in diverse therapeutic areas including antihypertensives, antivirals, antifungals, anticancers, and antihistaminics [1]. There are two general methods for the synthesis of 2-substituted benzimidazole. One is coupling of o-phenylenediamines and carboxylic acids [2] or their derivatives (nitriles, imidates, or orthoesters) [3], which often require strong acidic conditions, and sometimes combine with very high temperature or the use of microwave irradiation [4]. The other way involves a two-step procedure that is oxidative cyclodehydrogenation of aniline schiff’s bases, which are often generated in situ from the condensation of o-phenylenediamines and aldehydes. Various oxidative reagents such as tetracyano ethylene [5], nitrobenzene [6], 1,4-benzoquinone [7], DDQ [8], benzofuroxan [9], NaHSO3 [10], MnO2 [11], oxone [12], DMP [13], Pb(OAc)4 [14], and NH4VO3 [15] have been employed. However, all of these methods have problems, including drastic reaction conditions, expensive catalyst, low yields, and severe side-reactions. Therefore, the development of a cost-effective, safe, and inexpensive reagent system is desirable. In this research, we report a one-pot, high-yield, facile, and inexpensive synthesis of 2-substituted benzimidazoles directly from o-nitroanilines and aryl aldehydes via reductive condensation of o-nitroanilines at room temperature under mild conditions. 2. Experimental Section 2.1. Materials All the materials were of commercial reagent grade. The aromatic aldehydes and o-nitroaniline were purified by standard procedures and purity determined by thin layer chromatography (TLC). 2.2. Apparatus IR spectra were recorded as KBr pellets on a Perkin-Elmer 781 spectrophotometer and an Impact 400 Nicolet FT-IR spectrophotometer. 1H NMR and 13C NMR were recorded in DMSO solvent on a Bruker DRX-400 spectrometer with tetramethylsilane as internal reference. Melting points obtained with a Yanagimoto micromelting point apparatus are uncorrected. The purity determination of the substrates and reaction