Optically active, bulky tris(oxazolinyl)borato magnesium and calcium compounds for asymmetric hydroamination/cyclization

The synthesis of the new chiral, pseudo C-3-symmetric, monoanionic ligand tris(4S-tert-butyl-2-oxazolinyl) phenylborate [To(T)](-) is reported. The steric bulk, tridentate coordination, and anionic charge of [To(T)](-) are suitable for formation of complexes of the type To(T)MX, where one valence is available for reactivity. With this point in mind, we prepared magnesium and calcium To(T) complexes that resist redistribution to (To(T))(2)M compounds. Both To(T)MgMe and To(T)CaC(SiHMe2)(3) contain tridentate To(T)-coordination to the metal center, as shown by NMR spectroscopy, infrared spectroscopy, and X-ray crystallography. These compounds are active catalysts for the cyclization of three aminoalkenes to pyrrolidines, and provide non-racemic mixtures of pyrrolidines in enantiomeric excesses up to 36%. (C) 2010 Elsevier B.V. All rights reserved.