Immobilized piperazine on the surface of graphene oxide as a heterogeneous bifunctional acid-base catalyst for the multicomponent synthesis of 2-amino-3-cyano-4H-chromenes

Immobilized piperazine on the surface of graphene oxide (piperazine-GO) is synthesized and characterized by different methods such as FT-IR, solid-state(29)Si{H-1} and(13)C{H-1} CP/MAS NMR, elemental analysis, TGA, TEM, FE-SEM, XPS, and TPD. Subsequently, it is used as a heterogeneous bifunctional acid-base catalyst for the efficient multicomponent reaction of malononitrile, different active compounds containing enolizable C-H bonds and various aryl/alkyl aldehydes in aqueous ethanol. A wide variety of 2-amino-3-cyano-4H-chromenes are synthesized in the presence of this heterogeneous catalyst in good to high yields and with short reaction times. The catalyst is easily separated and reused for at least six times without significant loss of activity. The acidic nature of GO improves the catalytic activity of the supported piperazine and also provides heterogeneity to the catalyst. Use of aqueous ethanol as a green solvent, high turnover numbers (TON), facile catalyst recovery and reuse, simple work-up and generality of the method make this protocol an environmentally benign procedure for the synthesis of the title heterocycles.