Mechanistic Studies on Chiral Zinc Complex-Catalyzed Asymmetric Alkylation

在催化量的(-)-3-外-(二甲氨基)异冰片(DAIB)存在下，二烷基锌与醛的反应被显著地加速，水解后得到相应的高对映体纯度的S醇。在这种对映体选择性烷基化反应中，手性源和产物的光学纯度之间存在非线性关系。例如，苯甲醛和二乙基锌在8mol%的(-)-DAIB存在下在15%ee中反应生成(S)-1-苯基-1-丙醇，在95%ee中。通过动力学测定、烷基杂化实验、单晶X-射线分析、1H-核磁共振研究、某些关键中间体的相对分子质量测定以及DAIB及其立体异构体的系统研究，阐明了手性倍增和放大的机理。这种不寻常的现象被理解为由二烷基锌和DAIB助剂形成的非对映异构体双核络合物的化学性质显著不同的结果，这表明了不对称催化剂的自识别和非自识别的重要性。这一观点得到了模型体系从头算分子轨道计算的支持。

In the presence of a catalytic amount of (-) -3-exo- (dimethylamino) isoborneol (DAIB), reaction of dialkylzincs and aldehydes is accelerated markedly to give, after hydrolysis, the corresponding S alcohols in high enantiomeric purity. In this enantioselective alkylation, nonlinear relationship between optical purities of the chiral source and products. For example, reaction of benzaldehyde and diethylzinc in the presence of 8 mol% of (-)-DAIB in 15% ee leads to (S)-1-phenyl-1-propanol in 95% ee. The mechanisms of chirality multiplication as well as amplification have been elucidated on the basis of the kinetic measurement, alkyl scrambling experiments, single crystal X-ray analysis, 1H-NMR study, molecular-weight determination of certain key intermediates, systematic investigation using DAIB and its stereoisomers. This unusual phenomenon has been understood as a result of a marked difference in chemical properties of the diastereomeric dinuclear complexes formed from dialkylzincs and the DAIB auxiliary, proposing importance of the self and nonself recognition of the asymmetric catalysts. This view is supported the ab initio molecular orbital calculations of the model systems.