Efficient asymmetric addition of diethylzinc to aldehydes using C2-novel chiral pyridine β-amino alcohols as chiral ligands

摘要

A series of novel C2-symmetric chiral pyridine p-amino alcohol ligands have been synthesized from 2,6-pyridine dicarboxaldehyde, m-phthalaldehyde and chiral p-amino alcohols through a two-step reaction. All their structures were characterized by ~1H NMR, ~(13)C NMR and IR. Their enantioselective induction behaviors were examined under different conditions such as the structure of the ligands, reaction temperature, solvent, reaction time and catalytic amount. The results show that the corresponding chiral secondary alcohols can be obtained with high yields and moderate to good enantiomeric excess. The best result, up to 89% ee, was obtained when the ligand 3c (2S,2'R)-2,2'-((pyridine-2,6-diylbis(methyIene)) bisazanediyl))bis(4-methyl-1,1-diphenylpentan-1-ol) was used in toluene at room temperature. The ligand 3g (2S,2'R)-2,2'-((1,3-phenylenebis(methylene))bis(azanediyl))bis(4-methyl-1,1-diphenylpentan-1-ol) was prepared in which the pyridine ring was replaced by the benzene ring compared to 3c in order to illustrate the unique role of the N atom in the pyridine ring in the inductive reaction. The results indicate that the coordination of the N atom of the pyridine ring is essential in the asymmetric induction reaction.