手性过渡金属(Mn,Co,Ni)-Salen配合物催化NaOCl不对称环氧化苯乙烯的反应研究

Studies of Asymmtric Epoxidation of Styrene Catalyzed by Chiral Metal-Salen Complexes

本文论述了由手性1,2-二苯基乙二胺与适当取代的邻羟基苯甲醛生成的希佛碱作为过渡金属Co(Ⅱ)、Ni(Ⅲ)、Mn(Ⅲ)的配体合成的6种过渡金属-Salen配合物在NaOCl下均相催化苯乙烯不对称环氧化反应的效果。对环氧化反应进行控制性研究,我们发现环氧化反应与中心金属离子的相关性可能主要源于配体结构、金属离子的选择及二者结合而形成的配合物的空间构型。好的不对称环氧化反应催化剂要求中心金属离子具有适当的氧化还原电位,对底物烯烃因其取代基的大小,催化剂结构的空间构型应有好坏之分,我们得到的循环伏安数据及实验结果部分说明这一点。同时对手性镍(Ⅱ)-Salen及手性锰(Ⅲ)-Salen配合物的催化反应机理配合实验现象分别作出假设,都经过自由基历程,但却是截然不同的活性氧化物种,同时,由于起主要作用的不对称诱导因素不同,不对称诱导方向有异。对同一催化剂在不同pH值下起作用的活性氧化物种类别也可能不同。

Chiral Mn(III), Co (II) and Ni(II) - Salen complexes which prepared from the schiffbase of(+)or(-)1,2- diamino- 1,2- diphenylethane and substituted salialdehyde catalyzed the asymmetric epoxidation of styrene with sodium hypochlorite. The relevance about epoxidation and central metal ion is maybe the structure of ligand ,the choice of metal atom and the configuration of the complexes. Good asymmetric epoxidation catalyst has suitable half - wave potential. The configuration of catalyst must be adjust to make the need of enantioselectivity according to the bulkiness of olefins substituent. The data of cyclic voltammograms study about catalysts and experimental facts give rational explain. At the same time, the mechanism catalyzed by chiral Ni(II) - Salen complex and that catalyzed by chiral Mn(III) - Salen complexes may be different. Both involve free radical mechanism but different active bxidant. The sign of enentioselectivity is changed with the main asymmetric induced factor, the steric hindrance or n - II interaction. For the same catalyst, the kind of active species may be different under different pH.