Green synthesis of 1,3-dihydrobenzimidazol-2-ones from aromatic diamines by microwave in a tetrabutylammonium bromide–ethanol molten salt paste

Abstract

The replacement of hazardous solvents by greener alternatives is a major goal in green chemistry. Benzimidazol-2-ones are heterocycles exhibiting a wide range of biological activities. Conventionally, their synthesis from urea and arene-1,2-diamines requires high-boiling-point, harmful organic solvents, high reaction temperatures, long reaction times and the formation of unwanted biuret as by-product. Here, we designed microwave-induced, rapid synthesis of 1,3-dihydrobenzimidazol-2-ones in the tetrabutylammonium bromide (TBAB)–ethanol biphasic system. The reaction was tested using aromatic 1,2-diamines with various functionalities. Reaction conditions were first optimized under TBAB-catalyzed reaction of urea with benzene-1,2-diamine in the presence of TBAB and gave 78% yield of product. The yield was further increased to 97% by using biphasic systems in TBAB–ethanol. The reaction is simple to perform, gives in general high to excellent yields within 15 min irradiation and is isolated by simple addition of hot water and filtration with a negative biuret test.