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

The synthesis of the new chiral, pseudo C3-symmetric, monoanionic ligand tris(4S-tert-butyl-2-oxazolinyl)phenylborate [ToT]− is reported. The steric bulk, tridentate coordination, and anionic charge of [ToT]− are suitable for formation of complexes of the type ToTMX, where one valence is available for reactivity. With this point in mind, we prepared magnesium and calcium ToT complexes that resist redistribution to (ToT)2M compounds. Both ToTMgMe and ToTCaC(SiHMe2)3 contain tridentate ToT-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%.