含邻手性碳原子双键亲电加成反应的立体选择性模型

Stereoselective Models for the Electrophilic Addition on the Double Bond Adjacent to A Chiral Centre

本文主要介绍了预测邻位具有手性中心π体系亲电加成反应的立体选择性模型,尤其是α,β-不饱和羰基化合物亲核共轭加成形成的烯醇负离子及其类似物和硝基烯烃亲核加成形成的氮酸根类中间体的烷基化和动力学控制质子化反应的立体选择性模型。立体位阻效应控制的Zimmerman前过渡态模型主要适用于底物构象受限的环状或联烯型烯醇负离子的动力学质子化。对于构象转化相对灵活的直链底物,Houk基于量子化学计算研究提出了亲电试剂的进攻角度或接触角在经过环状过渡态时是锐角还是钝角决定了亲电加成反应的立体选择性控制(Houk模型)。而Fleming模型首次将烯丙基A-1,3张力引入各类烯醇负离子烷基化和质子化的过渡态。Mohrig模型在考虑A-1,3张力的基础上,将吸电子取代基与即将形成的σ键反式共平面,主要阐述了立体电子效应对酯的烯醇负离子动力学控制质子化立体选择性的影响,后来又提出了金属离子参与的立体电子效应控制的六元环状半椅式模型。本文希望对理解亲电加成反应的立体化学控制提供一些有用的信息。

The development of paradigms for diastereoselective control in electrophilic attack on trigonal carbon adjacent to a chiral centre,especially for the alkylation and protonation of enolate anions and equivalents,generated from nucleophilic conjugative additions of α,β-unsaturated carbonyl compounds,and nitrates yielded from nucleophilic additions of nitroolefins,is introduced and review ed. The diastereoselectivity in the kinetic protonation of conformationally restricted cyclic or allenic enol and enolate derivatives can be rationalized by Zimmerman＇s early transition state model,which is considered to be governed by steric factors exculsively.When acyclic substrates bearing conformational flexibility are employed,Houk＇s argument which is based on ab initio M O calculation has placed great importance on the approach angle,acute or obtuse which determines the sense of the diastereoselectivity. Subsequently,Fleming ＇s successive refinements incline to avoid destabilizingallylic 1,3-interactions in the reaction of enolate anions with electrophiles. Excepting rationally tuning A-1,3strain,electronegative heteroatom substituents are considered to occupy an antiperiplanar position to the formingσ bond via the stereoelectronic interaction in Mohrig ＇s general rule for controlling the diastereoselectivity of electrophilic attack on enolate anions.