烯烃亲电加成反应机理的理论探讨

Theoretical Study for the Mechanism of Electrophilic Addition to Alkenes Reactions

采用PM3方法计算了不同亲电试剂与乙烯生成的中间体正离子的势能曲线．结果显示，当亲电中心原子为价电子层没有d轨道的第一、二周期元素时，开放型中间体正离子的能量比桥型中间体正离子的能量低；此类加成经历开放中间体离子机制进行更为有利，表现出立体非专一性和区域选择性；当亲电中心原子为价电子层有d轨道的第三、四、五周期的元素时，桥型中间体正离子的能量比开放型中间体正离子的能量低；此类加成经历桥型中间体离子机制进行更为有利，表现出立体专一性和区域非专一性．以d-π和s-p-π配位键轨道模型，尝试阐述其内在的轨道控制机制，较好地解释了实验事实．

The potential intermediates of a series of electrophilic agents reacting with ethene have been computed with PM3 semiempirical method. The results show that the energy of the bridged struture intermediates is lower than that of the open structhe intermediates as the central atom of electrophilic agents is the third, fourth or fifth periodic element, but higher as the central atom is the first or second periodic element. Thereby, the electrophilic agents with third, fourth or fifth periodic element as central atoms should give stereospecific and non-regionspecific products and those with first or second periodic element as central atoms should give non-stereospecific and regionspecific Products. It is well an accordance with experimental results. Meanwhile, the model of d-π and s-p-π coordination bond has been put forward to try to explain the molecular mechanism.