Wang Yang, Wei Donghui, Li Zhenyu, Zhu Yanyan, Tang Mingsheng
The College of Chemistry and Molecular Engineering, Center of Computational Chemistry, Zhengzhou University , Zhengzhou, Henan Province 450001, P.R. China.
J Phys Chem A. 2014 Jun 19;118(24):4288-300. doi: 10.1021/jp500358m. Epub 2014 Jun 6.
The detailed mechanisms and diastereoselectivities of Lewis acid-promoted ketene-alkene [2 + 2] cycloaddition reactions have been studied by density functional theory (DFT). Four possible reaction channels, including two noncatalyzed diastereomeric reaction channels (channels A and B) and two Lewis acid (LA) ethylaluminum dichloride (EtAlCl2) catalyzed diastereomeric reaction channels (channels C and D), have been investigated in this work. The calculated results indicate that channel A (associated with product R-configurational cycloputanone) is more energy favorable than channel B (associated with the other product S-configurational cyclobutanone) under noncatalyzed condition, but channel D leading to S-configurational cyclobutanone is more energy-favorable than channel C, leading to R-configurational cycloputanone under a LA-promoted condition, which is consistent with the experimental results. And Lewis acid can make the energy barrier of ketene-alkene [2 + 2] cycloaddition much lower. In order to explore the role of LA in ketene and C = X (X = O, CH2, and NH) [2 + 2] cycloadditions, we have tracked and compared the interaction modes of frontier molecular orbitals (FMOs) along the intrinsic reaction coordinate (IRC) under the two different conditions. Besides by reducing the energy gap between the FMOs of the reactants, our computational results demonstrate that Lewis acid lowers the energy barrier of the ketene and C = X [2 + 2] cycloadditions by changing the overlap modes of the FMOs, which is remarkably different from the traditional FMO theory. Furthermore, analysis of global reactivity indexes has also been performed to explain the role of LA catalyst in the ketene-alkene [2 + 2] cycloaddition reaction.
通过密度泛函理论（DFT）研究了路易斯酸促进的乙烯酮 - 烯烃[2 + 2]环加成反应的详细机理和非对映选择性。本工作研究了四个可能的反应通道，包括两个非催化的非对映异构反应通道（通道A和B）以及两个路易斯酸（LA）二氯乙基铝（EtAlCl2）催化的非对映异构反应通道（通道C和D）。计算结果表明，在非催化条件下，通道A（与产物R - 构型环丁酮相关）比通道B（与另一种产物S - 构型环丁酮相关）在能量上更有利，但在LA促进的条件下，生成S - 构型环丁酮的通道D比生成R - 构型环丁酮的通道C在能量上更有利，这与实验结果一致。并且路易斯酸可以使乙烯酮 - 烯烃[2 + 2]环加成的能垒大大降低。为了探究LA在乙烯酮与C = X（X = O、CH2和NH）[2 + 2]环加成反应中的作用，我们追踪并比较了两种不同条件下沿内禀反应坐标（IRC）的前沿分子轨道（FMO）的相互作用模式。除了通过减小反应物FMO之间的能隙外，我们的计算结果表明路易斯酸通过改变FMO的重叠模式降低了乙烯酮与C = X [2 + 2]环加成反应的能垒，这与传统的FMO理论有显著不同。此外，还进行了全局反应性指数分析以解释LA催化剂在乙烯酮 - 烯烃[2 + 2]环加成反应中的作用。
