We have developed a highly chemo- and diastereoselective alkylation of N-tert-butanesulfinyl aldimines with diborylmethane. Whereas the addition of diborylmethane under metal-free conditions shows poor diastereoselectivity, the use of a copper catalyst and a bidentate phosphine ligand significantly enhances the diastereoselectivity, providing chiral β-aminoboronates in good yields. On the basis of the stereochemical outcome, we propose that the reaction likely proceeds via a boron-chelating six-membered chairlike transition state.
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