An ab initio study on the mechanism of the ketene-imine cycloaddition reaction
An ab initio study on the mechanism of the ketene-imine cycloaddition reaction
复制标题
DOI：
10.1021/ja00041a055
复制
发表时间：
1992-07
影响因子：
15
通讯作者：
J. Sordo;Javier R González;T. Sordo
中科院分区：
文献类型：
--
作者：
J. Sordo;Javier R González;T. Sordo
用6-31G*基组与实验结果吻合较好。据此，在RHF/6-31G*和RMP2/6-32G*理论水平上对Staudinger反应进行了研究。在这两个水平上都发现了一种不协调的机制。我们的计算预测反应将通过第一个过渡结构(TS1，图2)进行，从而得到与实验一致的两性离子中间体(I，图2)，7它通过第二个过渡结构(TS2，图2)转变为产物。这些系统的能量如表1所示。我们在这里介绍了这些驻点在RMP2/6-31G*水平上最有趣的特征；为了比较，RHF/6-31G*值在括号中给出。第一个过渡结构TS1(见图2)对应于亚胺与亲电性烯酮的亲核加成，得到两性离子中间体1(见图2)。在第一个过渡结构中形成的键(烯酮的亚胺N-中心C)的长度为1.751 A(1.745 A)，几乎是一个完全形成的单键；此外，Mulliken布居分析表明，在过渡结构Ts1中，从亚胺到烯酮部分有显著的电荷转移，0.216电子(0.211电子)。第一个跃迁结构的活化能为3.7kcal/mol(12.5kcal/mol)，并导致中间体I，其稳定性仅比Ts1高0.2kcal/mol(0.4kcal/mol)。中间体I具有平面构型，其氮和烯酮中心碳之间的键长为1.587 A(1.579 A)。I的两性离子性质表现在其重要的电荷分离上：在Cl、C3、O和N上的Mulliken电荷分别为-0.291、0.505、-0.724和-0.215电子(-0.304、0.495、-0.712和-0.205)；在分离的反应物中，相同原子上的电荷分别为-0.110、0.225、-0.470和-0.225电子
Satisfactory agreement with experiment is obtained by using the 6-31G* basis set. Accordingly the Staudinger reaction was studied at RHF/6-31G* and RMP2/6-32G* theory levels. At both levels a nonconcerted mechanism was found. Our calculations predict the reaction to proceed through a first transition structure (TS1, Figure 2) leading to a zwitterionic intermediate (I, Figure 2) in agreement with the experiment, 7 which transforms into the product through a second transition structure (TS2, Figure 2). The energies of these systems are given in TableI. We present here the most interesting features of these stationary points at the RMP2/6-31G* level; for comparison the RHF/6-31G* values are given in parentheses.The first transition structure, TS1 (see Figure 2), corresponds to the nucleophilic addition of the imine to the electrophilic ketene, to give a zwitterionic intermediate, 1 (see Figure 2). The length of the bond being formed (iminic N-central C of the ketene) in the first transition structure is 1.751 A (1.745 A), almost a fully formed single bond; also, the Mulliken population analysis shows a significant charge transfer, 0.216 electrons, from the imine to the ketene moiety in the transition structure TS1 (0.211 electrons). This first transition structure has an activation energy of 3.7 kcal/mol (12.5 kcal/mol) and leads to an intermediate, I, which is just 0.2 kcal/mol (0.4 kcal/mol) more stable than TS1. The intermediate I has a planar geometry, with a bond length between the nitrogen and the central carbon of the ketene of 1.587 A (1.579 A). The zwitterionic character of I is shown in its important charge separation: the Mulliken charges on Cl, C3, O, and N (see Figure 2 for numbering scheme) are-0.291, 0.505,-0.724, and-0.215 electrons, respectively (-0.304, 0.495,-0.712, and-0.205); in the separated reactants, the charges on the same atoms are-0.110, 0.225,-0.470, and-0.225 electrons, respectively