Cobalt-catalysed desymmetrization of malononitriles via enantioselective borohydride reduction.

Abstract

The high nitrogen content and diverse reactivity of malononitrile are widely harnessed to access nitrogen-rich fine chemicals. Although the facile substitutions of malononitrile can give structurally diverse quaternary carbons, their access to enantioenriched molecules, particularly chiral amines that are prevalent in bioactive compounds, remains rare. Here we report a cobalt-catalysed desymmetric reduction of disubstituted malononitriles to give highly functionalized β-quaternary amines. The pair of cobalt salt and sodium borohydride is proposed to generate a cobalt-hydride intermediate and initiate the reduction. Meanwhile, the enantiocontrol of the dinitrile is achieved through a tailored bisoxazoline ligand with two large flanks that create a narrow gap to host the bystanding nitrile and thus restrict the C(ipso)-C(α) bond rotation of the complexed one. Combined with the extensive derivatization possibilities of all substituents on the quaternary carbon, this asymmetric reduction unlocks pathways from malononitrile as a bulk chemical feedstock to intricate, chiral nitrogen-containing molecules.