Density functional theory study of the mechanism of zinc carbenoid promoted cyclopropanation of allenamides
作者:Huiying Xu、Xiting Zhang、Zhuo-Feng Ke、Zhi-Feng Li、Xian-Yan Xu、Cheng-Yong Su、David Lee Phillips、Cunyuan Zhao
DOI:10.1039/c3ra42168f
日期:——
The mono- and bis-cyclopropanation of allenamides with the zinc carbenoid Zn(CH2Cl)2 have been studied using density functional theory calculations employing the M06 functional. The monomeric and dimeric precursor complexes were both constructed to model the reaction processes. In the monomeric reaction, the formation of the endo-monocyclopropyl species takes place via a methylene transfer pathway rather than a carbometalation pathway. The formation of the exo-monocyclopropyl species does not readily occur via a methylene transfer pathway due to a high activation barrier. The corresponding carbometalation pathway was not able to be found. Following the monocyclopropanation step, the biscyclopropanation of the endo-monocyclopropyl species is facile to form amidospiro[2.2]pentane. In the aggregation model, the allenamides and the zinc carbenoid form a dimer aggregate that is then followed by two pathways. One pathway takes place via transition states inside the aggregate structure (denoted here as a closed-mode process) while the other pathway introduces another zinc carbenoid molecule from outside the aggregation species (denoted here as an open-mode process). The aggregate mechanisms are not favored because the dimeric reactant of the open-mode process is not stable to coexist with the monomer and the activation barriers of the two aggregate pathways are higher than those of the monomeric pathways. The calculation results show that the key factors in the reaction mechanisms are the co-planarity of the allenic moiety with the oxazolidinone ring, the torsional strain in the butterfly-type transition state, the ring strain in the substrate–carbenoid complexes and the coordination between the carbenoid-Zn and O(CO) atoms and other long-distance interactions.
利用 M06 函数进行密度泛函理论计算,研究了烯酰胺与碳化锌 Zn(CH2Cl)2 的单环和双环丙烷化反应。为了模拟反应过程,我们构建了单体和二聚前体配合物。在单体反应中,内单环丙基的形成是通过亚甲基转移途径而不是碳甲基化途径进行的。由于活化障碍较高,外单环丙基物质不易通过亚甲基转移途径形成。相应的碳甲基化途径也未能找到。在单环丙烷化步骤之后,内单环丙烷的双环丙烷化很容易形成脒螺[2.2]戊烷。在聚合模型中,烯酰胺和类羰基锌形成二聚物聚合体,然后通过两条途径进行聚合。一条途径是通过聚合体结构内部的过渡态进行的(在此表示为闭合模式过程),而另一条途径是从聚合体外部引入另一个类羰基化锌分子(在此表示为开放模式过程)。由于开放模式过程中的二聚反应物不能稳定地与单体共存,而且两种聚集途径的活化势垒高于单体途径,因此聚集机制并不被看好。计算结果表明,影响反应机理的关键因素是异戊烯基与噁唑烷酮环的共平面性、蝶式转变态中的扭转应变、底物-类羰基复合物中的环应变以及类羰基-Zn 原子与 O(CO)原子之间的配位和其他长距离相互作用。