Fraile José M, García José I, Gil María J, Martínez-Merino Víctor, Mayoral José A, Salvatella Luis
Departamento de Química Orgánica, Instituto de Ciencia de Materiales de Aragón, C.S.I.C.-Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain.
Chemistry. 2004 Feb 6;10(3):758-65. doi: 10.1002/chem.200305161.
The effect of a coordinating counteranion on the mechanism of Cu(I)-catalyzed cyclopropanation has been investigated extensively for a medium-sized reaction model by means of theoretical calculations at the B3LYP/6-31G(d) level. The main mechanistic features are similar to those found for the cationic (without a counteranion) mechanism, the rate-limiting step being nitrogen extrusion from a catalyst-diazoester complex to generate a copper-carbene intermediate. The cyclopropanation step takes place through a direct carbene insertion of the metal-carbene species to yield a catalyst-product complex, which can finally regenerate the starting complex. However, the presence of the counteranion has a noticeable influence on the calculated geometries of all the intermediates and transition structures. Furthermore, the existence of a preequilibrium with a dimeric form of the catalyst, together with a higher activation barrier in the insertion step, explains the lower yield of cyclopropane products observed experimentally in the presence of chloride counterion. The stereochemical predictions of a more realistic model (made by considering a chiral bis(oxazoline)-copper(i) catalyst) have been rationalized in terms of the lack of significant steric repulsions, and the model shows good agreement with the low enantioselectivities observed experimentally for these kinds of catalytic systems.
通过在B3LYP/6-31G(d)水平上进行理论计算，针对一个中等规模的反应模型，广泛研究了配位抗衡阴离子对Cu(I)催化环丙烷化反应机理的影响。主要的机理特征与阳离子（无抗衡阴离子）机理相似，限速步骤是从催化剂-重氮酯络合物中挤出氮以生成铜卡宾中间体。环丙烷化步骤通过金属卡宾物种的直接卡宾插入发生，生成催化剂-产物络合物，该络合物最终可以再生起始络合物。然而，抗衡阴离子的存在对所有中间体和过渡结构的计算几何形状有显著影响。此外，催化剂二聚体形式的预平衡的存在，以及插入步骤中较高的活化能垒，解释了在存在氯离子抗衡离子的情况下实验观察到的环丙烷产物较低产率的原因。一个更现实的模型（通过考虑手性双（恶唑啉）-铜（I）催化剂构建）的立体化学预测，已根据缺乏显著的空间排斥作用进行了合理化解释，并且该模型与这类催化体系实验观察到的低对映选择性显示出良好的一致性。
