Enantioselective addition of diethylzinc to aldehydes catalyzed by titanium(IV) complexes of diol derivatives of D-mannitol and the spectroscopic study of the catalytic system

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 H-1 NMR study shows that the catalytic system in solution contains unreacted Ti(O-i-Pr)(4), a dimeric Ti(TV) complex [Ti(O-i-Pr)(2)(mu-diolate)](2) (4) containing two bridging diolate Ligands, and a dimeric Ti(IV) complex [(diolate)(O-i-Pr)Ti-(mu-O-i-Pr)(2)Ti(O-i-Pr)(3)] (5) containing one, diolate ligand attached to only one Ti(TV) metal center. The complex 5 with the more electropositive Ti(TV) 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