Origin of stereofacial selectivity in electrophilic additions to methylenecyclohexanes and methylenedioxanes. A theoretical and experimental study
作者:John Hudec、Jerry Huke、John W. Liebeschuetz
DOI:10.1039/a800999f
日期:——
Addition reaction studies and ab initio calculations on methylenecyclohexane and 5-methylene-1,3-dioxane systems suggest that two electronic factors contribute to the stereoselectivity of epoxidation and diimide reduction. These are respectively the spatial anisotropy of the HOMO with respect to the two faces of the double bond, common to both molecules, which is likely to be responsible for the overall axial stereofacial selectivity exhibited, and a similar anisotropy in the electrostatic potential field of the methylenedioxane caused by the oxygens; which also favours attack from an axial direction by polarisable electrophilic species. The anisotropy of the HOMO arises from the important topological difference between the contributions made to the HOMO by the periplanar β C–H σ bonds and opposing β C–O or C–C σ bonds. Catalytic reduction proceeds with equatorial face selectivity for both the cyclohexane and the dioxane systems and appears to be governed largely by steric effects.
对亚甲基环己烷和 5-亚甲基-1,3-二氧六环体系进行的加成反应研究和 ab initio 计算表明,两个电子因素有助于环氧化和二亚胺还原的立体选择性。这两个因素分别是:HOMO 相对于双键两个面的空间各向异性(这是两个分子共有的),这可能是表现出的整体轴向立体选择性的原因;由氧原子引起的亚甲基二氧杂环己烷静电势场的类似各向异性;这也有利于可极化亲电物种从轴向进行攻击。HOMO 的各向异性源于周面 β C-H σ 键和对立的 β C-O 或 C-C σ 键对 HOMO 的贡献存在重要的拓扑差异。环己烷和二氧杂环烷体系的催化还原过程具有赤道面选择性,似乎主要受立体效应的影响。