The College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, Henan Province 450001, P. R. China.
J Org Chem. 2012 Dec 7;77(23):10729-37. doi: 10.1021/jo302044n. Epub 2012 Nov 27.
Density functional theory (DFT) calculations have been performed to provide the first detailed computational study on the mechanism and enantioselectivity for the [4 + 2] cycloaddition reaction of ketenes with N-benzoyldiazenes catalyzed by N-heterocyclic carbenes (NHCs). Two possible mechanisms have been studied: first is the "ketene-first" mechanism (mechanism A), and second is the novel "diazene-first" mechanism (mechanism B). The calculated results reveal that mechanism B is more favorable than mechanism A because it is not only of lower energy barrier but also more consistent with the provided general experimental procedure (Huang, X.-L.; He, L.; Shao, P.-L.; Ye, S. Angew. Chem., Int. Ed.2009, 48, 192-195). The enantioselectivity-determining step is demonstrated to present during the first process of cycloaddition, and the main product configuration is verified to agree with the experimental ee values very well. This study should be of some worth on forecasting how different substituent groups of catalysts and/or reactants affect the enantioselectivity of products. The obtained novel mechanistic insights should be valuable for not only rational design of more efficient NHC catalysts but also understanding the general reaction mechanism of [4 + 2] cycloaddition of ketenes.
密度泛函理论（DFT）计算已被用于提供酮烯与 N-苯甲酰重氮化合物的[4+2]环加成反应的机制和对映选择性的第一个详细的计算研究，该反应由 N-杂环卡宾（NHCs）催化。研究了两种可能的机制：第一种是“酮烯优先”机制（机制 A），第二种是新颖的“重氮优先”机制（机制 B）。计算结果表明，机制 B 比机制 A 更有利，因为它不仅能量势垒更低，而且更符合提供的一般实验程序（Huang, X.-L.; He, L.; Shao, P.-L.; Ye, S. Angew. Chem., Int. Ed.2009, 48, 192-195）。证明对映选择性决定步骤出现在环加成的第一个过程中，并且主要产物构型与实验 ee 值非常吻合。这项研究对于预测不同催化剂和/或反应物的取代基如何影响产物的对映选择性应该具有一定的价值。所获得的新的机理见解不仅对更有效的 NHC 催化剂的合理设计有价值，而且对理解酮烯的[4+2]环加成的一般反应机理也有价值。
