Transition states of epoxidations: Diradical character, spiro geometries, transition state flexibility, and the origins of stereoselectivity
Transition states of epoxidations: Diradical character, spiro geometries, transition state flexibility, and the origins of stereoselectivity
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DOI：
10.1021/ja963847x
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发表时间：
1997-10-22
影响因子：
15
通讯作者：
Condroski, KR
中科院分区：
文献类型：
--
作者：
Houk, KN;Liu, J;Condroski, KR
采用密度泛函理论方法（使用Becke3LYP泛函和6 - 31G*基组）确定了过甲酸、二氧杂环丙烷、氧杂氮丙啶和过氧亚硝酸对乙烯环氧化的过渡态结构。所有的环氧化反应都具有螺环过渡态；过甲酸和二氧杂环丙烷参与的反应过渡态出现较早，涉及同步的氧转移，而氧杂氮丙啶和过氧亚硝酸参与的反应过渡态出现较晚，涉及异步的氧转移。将Becke3LYP/6 - 31G*理论的结果与文献中的MP2/6 - 31G*值进行了比较。乙烯上被甲氧基、甲基、乙烯基和氰基取代会使过渡态几何结构朝着异步螺环结构转变。在过甲酸和二氧杂环丙烷的反应中，除氰基外，所有取代基都会降低活化能。利用基于这些过渡态结构的过渡态结构模型对实验立体选择性进行了合理的解释。
The transition structures for the epoxidations of ethylene by performic acid, dioxirane, oxaziridine, and peroxynitrous acid have been located with density functional theory methods using the Becke3LYP functional and 6-31G* basis set. All of the epoxidations have spiro transition states; those with performic acid and dioxirane are early and involve synchronous oxygen transfer, while those with oxaziridine and peroxynitrous acid are later with asynchronous oxygen transfer. The results from Becke3LYP/6-31G* theory are compared with MP2/6-31G* literature values. Substitution on ethylene by methoxy, methyl, vinyl, and cyano groups changes the transition state geometries toward asynchronous spiro structures. The activation energies are lowered by all substituents except the cyano group in reactions of performic acid and dioxirane. Experimental stereoselectivities are rationalized by using transition structure models based upon these transition structures.