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%.

Magnesium and calcium organometallic compounds containing a new chiral, pseudo C3-symmetric, monoanionic ligand tris(4S-tert-butyl-2-oxazolinyl)phenylborate [ToT]− are catalysts for the stereoselective hydroamination/cyclization of three aminopentenes to optically active α-methyl-pyrrolidines with enantiomeric excesses up to 36%..Figure optionsDownload as PowerPoint slideResearch highlights
► Synthesis of new chiral tris(4S-tert-butyl-2-oxazolinyl)borate ligand.
► Chiral tris(oxazolinyl)borates support monomeric magnesium and calcium compounds.
► X-ray structure of four-coordinate chiral magnesium compound.
► Enantioselective hydroamination with magnesium and calcium catalysts.