Enantioselective addition of diethylzinc to aldehydes catalyzed by Ti (IV) complexes of diol derivatives of D-mannitol was carried out. Good enantioselectivities were obtained with ee values up to 86% of R-configuration. A 1H NMR study shows that the catalytic system in solution contains unreacted Ti(O-i-Pr)4, a dimeric Ti (IV) complex [Ti (O-i-Pr) 2 (μ-diolate)] 2 (4) containing two bridging diolate ligands, and a dimeric Ti (IV) complex [(diolate) (O-i-Pr)Ti-(Ì-O-i-Pr) 2 Ti (O-i-Pr) 3] (5) containing one diolate ligand attached to only one Ti (IV) metal center. The complex 5 with the more electropositive Ti (IV) metal center is considered the most active catalyst in solution. The structure of 4 was determined by X-ray analysis, and the FAB-mass spectroscopic study confirms the existence of complex 5. The role of excess Ti (O-i-Pr) 4 in the catalytic system is that Ti (O-i-Pr) 4 further reacts with the complex 4 to give the most active catalyst 5 in solution.

This contribution gives an overview of the synthesis of chiral β-amino acids via asymmetric hydrogenation of the corresponding dehydroamino derivatives. Literature results are discussed regarding substrate synthesis and catalyst performance and how it is affected by substrate and catalyst structure as well as experimental parameters. A tentative mechanistic concept for the hydrogenation step is also presented. Scheme 1. Biologically active compounds with β-amino acid unitsa