Benign approaches for the microwave-assisted synthesis of quinoxalines

Two benign, simple, rapid (3 min), solvent-free and versatile microwave energy transfer systems for the synthesis of quinoxalines, i.e. 2,3-diphenylquinoxaline and its derivatives, 2,3-diphenyl-4a,5,6,7,8,8a-hexahydroquinoxaline, and 2-phenylquinoxaline and also pyridine-2,3-diamine and 2,3-dihydro-5,6-diphenylpyrazine have been used. The first approach consists on the use of mineral supports on the reaction of 1,2-dicarbonyl (benzil) or a-hydroxyketone (acyloin) with 1,2-diamines. Among mineral supports, the efficiency of acidic alumina as catalyst was clearly proved (80-86%). Moreover, it has an oxidative role in the tandem oxidation of acyloin to the corresponding 1,2-dicarbonyl. The second approach is the use of polar paste system on the reaction of 1,2-dicarbonyl (benzil) with 1,2-diamines. The excellent products yield (90-97%) and simple washing with water and filtration in the absence of organic solvent makes it an impressive alternative green platform. Although both methods implemented in this study have been designed to circumvent additional purification steps, the latter is cleaner with an easier work-up.