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 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(μ-O-i-Pr)2Ti(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.