Improved Synthesis of β-Aminoboronate Esters via Copper-Catalyzed Diastereo- and Enantioselective Addition of 1,1-Diborylalkanes to Acyclic Arylaldimines

We describe herein an improved and scalable procedure to achieve the copper-catalyzed diastereo- and enantioselective 1,2-addition of 1,1-diborylalkanes to N-protected acyclic arylaldimines. We find that the installation of an N,N-dimethylsulfamoyl-group as N-protecting group in acyclic arylaldimines allows the 1,2-addition reaction to proceed with good-to-excellent diastereoselectivity and enantioselectivity, thus providing β-aminoboronate esters bearing vicinal chiral secondary amines and secondary boronate esters in good yields. The reaction can be easily scaled up to gram scale (3.25 g), highlighting the usefulness of the developed process. Synthetic applications whereby the obtained β-aminoboronate ester is converted to other enantioenriched compounds are also demonstrated.