2-氧代-4-苯基丁酸乙酯不对称加氢反应的动力学和机理探讨

Reaction Kinetics and Mechanism of Enantioselective Hydrogenation of Ethyl 2-Oxo-4-Phenylbutyrate

采用10,11-二氢辛可尼定修饰的Pt/-γAl2O3催化2-氧代-4-苯基丁酸乙酯不对称加氢合成(R)-2-羟基-4-苯基丁酸乙酯,考察了修饰剂用量、底物浓度、氢压和反应温度对加氢反应速率和光学选择性的影响.结果表明,修饰剂用量和底物浓度在适中范围内对反应速率和光学选择性均较为有利;提高氢压和温度均能显著加快反应速率,但氢压对光学选择性影响不大,温度升高则不利于目标产物的生成.根据反应动力学特征,认为修饰剂喹啉环平行吸附于Pt表面,被质子化后与以si面吸附在催化剂表面的底物作用,经过立体选择生成单一对映体.用提出的反应机理推导出不对称加氢反应速率方程,可以较好地描述实验结果.

Ethyl （R）-2-hydroxy-4-phenylbutyrate was synthesized from ethyl 2-oxo-4-phenylbutyrate by enantioselective hydrogenation on Pt/γ-Al2O3 modified by 10,11-dihydrocinchonidine. The effects of modifier amount, substrate concentration, hydrogen pressure and reaction temperature on hydrogenation rate and enantiomeric excess were investigated. It was found that the initial rate and enantioselectivity showed a maximum in a range of modifier amounts and in a range of substrate concentration. The reaction rate increased with the increase of hydrogen pressure, which almost had no effect on the enantioselectivity. Higher temperature could enhance the reaction rate remarkably, but it had adverse influence on enantiomeric excess. On the basis of the previous conclusions and the kinetic results in this paper, it was suggested that the modifier quinoline was adsorbed with its ring parallel to Pt surface. After protonated, the modifier interacted with the adsorbed substrate via si face, which led to （R ）-product. Being different from the enantioselective hydrogenation of ethyl pyruvate, the competitive adsorption between modifier and substrate was obvious in this reaction. According to the proposed mechanism, the kinetic equation of enantioselective hydrogenation was obtained, it could depict the experimental results well.