Stereoselective control in the Staudinger reactions involving monosubstituted ketenes with electron acceptor substituents: experimental investigation and theoretical rationalization
作者:Hengzhen Qi、Xinyao Li、Jiaxi Xu
DOI:10.1039/c0ob00783h
日期:——
The stereoselectivity of the Staudinger reactions involving monosubstituted ketenes with electron acceptor substituents was investigated experimentally by determination of the product stereochemistry and theoretically via DFT calculations. The results indicate that imines preferentially attack the less sterically hindered exo-side of the ketenes to generate zwitterionic intermediates. Subsequently, for cyclic imines, the intermediates undergo a conrotatory ring closure directly to produce β-lactams, while for linear imines, the imine moiety of the intermediates isomerizes to more stable intermediates, which further undergo a conrotatory ring closure to afford trans-β-lactams. The steric hindrance and the isomerization, rather than the torquoelectronic effect, play crucial roles in controlling the stereoselectivity in the practical Staudinger reactions involving monosubstituted ketenes with electron acceptor substituents, although the unaccessible borylketene with a powerful electron acceptor group controls the stereoselectivity torquoelectronically, in theory.
通过确定产物立体化学和基于密度泛函理论(DFT)的计算,实验上研究了涉及带有电子受体取代基的单取代乙烯酮的斯陶丁格反应的立体选择性。结果表明,亚胺优先攻击乙烯酮上立体障碍较小的外侧,生成内鎓离子中间体。随后,对于环状亚胺,中间体直接进行协同环合反应生成β-内酰胺;而对于线性亚胺,中间体的亚胺部分异构化为更稳定的中间体,后者进一步进行协同环合反应,产生反式β-内酰胺。在实际涉及带有电子受体取代基的单取代乙烯酮的斯陶丁格反应中,立体障碍和异构化而不是扭曲电子效应在控制立体选择性中起着关键作用,尽管在理论上,具有强电子受体基团的不可获得的硼乙烯酮通过扭曲电子效应控制立体选择性。