Synthesis of Condensed Benzo[N,N]-Heterocycles by Microwave-Assisted Solid Acid Catalysis

The synthesis of several types of condensed benzo[N,N]-heterocycles such as benzimidazoles, benzodiazepines, quinoxalinones by a microwave-assisted solvent-free solid acid catalyzed method is described. The commercially available, inexpensive K-10 montmorillonite is an excellent catalyst for the synthesis of the target compounds. Our approach is based on the reactions of a wide variety of o-phenylenediamines, with ketones, aldehydes and bifunctional reagents, respectively. The cyclization reactions were initiated by microwave irradiation. The products, in most cases, were obtained in very high yields (up to 98%) and excellent selectivities in very short reaction times. The effective combination of solid acid catalysis, solvent-free conditions and microwave irradiation provides an attractive and highly ecofriendly approach for the synthesis of these important heterocycles.