Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom.
J Org Chem. 2013 Mar 1;78(5):1874-86. doi: 10.1021/jo3017549. Epub 2012 Nov 28.
The photocycloaddition reaction of benzene with ethylene has been studied at the CASSCF level, including the characterization of an extended conical intersection seam. We show that the chemical selectivity is, in part, controlled by this extended conical intersection seam and that the shape of the conical intersection seam can be understood in terms of simple VB arguments. Further, the shape and energetics of the asynchronous segment of the conical intersection seam suggest that 1,2 (ortho) and 1,3 (meta) will be the preferred chemical products with similar weight. The 1,4 (para) point on the conical intersection is higher in energy and corresponds to a local maximum on the seam. VB analysis shows that the pairs of VB structures along this asynchronous seam are the same and thus the shape will be determined mainly by steric effects. Synchronous structures on the seam are higher in energy and belong to a different branch of the seam separated by a saddle point on the seam. On S1 we have documented three mechanistic pathways corresponding to transition states (with low barriers) between the reactants and the conical intersection seam: a mixed asynchronous/synchronous [1,2] ortho path, an asynchronous [1,3] meta path, and a synchronous [1,3] meta path.
苯与乙烯的光环加成反应已在 CASSCF 水平上进行了研究，包括对扩展的共面交叉 seam 的特征描述。我们表明，化学选择性部分由该扩展的共面交叉 seam 控制，并且共面交叉 seam 的形状可以根据简单的 VB 论点来理解。此外，共面交叉 seam 的异步部分的形状和能态表明，1,2（邻位）和 1,3（间位）将是首选的化学产物，具有相似的权重。在共面交叉 seam 上的 1,4（对位）点的能量较高，对应于 seam 上的局部最大值。VB 分析表明，沿着这条异步 seam 的 VB 结构对是相同的，因此形状将主要由空间位阻效应决定。 seam 上的同步结构的能量较高，属于 seam 上鞍点分离的不同分支。在 S1 上，我们记录了三条对应于反应物与共面交叉 seam 之间的过渡态（具有低能垒）的反应机制途径：混合异步/同步[1,2]邻位途径、异步[1,3]间位途径和同步[1,3]间位途径。
