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 1 H 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) 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.
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You et al. (2000) studied this question.