A highly versatile synthesis of amine-boranes via carbonyl reduction by sodium borohydride is described.

In situ generated, transient sodium monohydroxyborohydride readily reacts with a present amine to form amine-borane adducts in ethyl acetate at room temperature. Synthesis of heteroaromatic pyridine-, 2-picoline-, and 5-ethyl-2-methylpyridine-boranes, those relevant to large-scale reductive aminations, has been demonstrated at 100 mmol scale, and pyridine-borane at 1.1 mole scale. This first report substituting ethyl acetate for tetrahydrofuran, the traditional solvent for amine-borane synthesis, allows for activation of sodium borohydride by the aqueous portion of the heterogeneous dual-solvent system. These factors make this green protocol more economical than those currently available for the large-scale production of amine-boranes as demonstrated, quantitatively, by the computed green chemistry metrics.