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

摘要

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~TMX, 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) _2M compounds. Both To~TMgMe and To ~TCaC(SiHMe_2)_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%.