Nitroalkene reduction in des promoted by H3NBH3

Ammonia borane is a crystalline white solid, commercially available, moisture and air stable that displays a good thermal stability, with high hydrogen storage capacity, since 19.6% of its molar mass is composed by hydrogen atoms. In the last decade, ammonia borane found application also as reducing agent in organic synthesis, since it could act as a safer replacement for some metal hydrides (e.g. sodium borohydride and derivatives), and as a greener replacement of Hantzsch esters. The use of an atom economical reagent, as ammonia borane, in green reaction media, such as biobased Deep Eutectic Solvents (DESs) could represent a step toward the development of sustainable synthetic methodologies. In this context, we investigated the reduction of nitroalkenes, key precursors in the synthesis of amines, employing ammonia borane as reducing agent in DES. The chemoselective nitroalkene reduction to nitroalkane was achieved in fair to good yield employing either glycerol or choline chloride/glycerol 1:2 DES. The substrate scope included a variety of nitrostyrenes and alkyl-substituted nitroalkenes, including α- and β-substituted nitroolefins. The possibility to isolate the product by liquid-liquid separation, without the employment of any organic solvent and to reuse the DES at least three times in further reactions was also demonstrated.