Oxgaard Jonas, Muller Richard P, Goddard William A, Periana Roy A
Materials and Process Simulation Center, Beckman Institute (139-74), Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA.
J Am Chem Soc. 2004 Jan 14;126(1):352-63. doi: 10.1021/ja034126i.
The mechanism of hydroarylation of olefins by a homogeneous Ph-Ir(acac)(2)(L) catalyst is elucidated by first principles quantum mechanical methods (DFT), with particular emphasis on activation of the catalyst, catalytic cycle, and interpretation of experimental observations. On the basis of this mechanism, we suggest new catalysts expected to have improved activity. Initiation of the catalyst from the inert trans-form into the active cis-form occurs through a dissociative pathway with a calculated DeltaH(0 K)() = 35.1 kcal/mol and DeltaG(298 K)() = 26.1 kcal/mol. The catalytic cycle features two key steps, 1,2-olefin insertion and C-H activation via a novel mechanism, oxidative hydrogen migration. The olefin insertion is found to be rate determining, with a calculated DeltaH(0 K)() = 27.0 kcal/mol and DeltaG(298 K)() = 29.3 kcal/mol. The activation energy increases with increased electron density on the coordinating olefin, as well as increased electron-donating character in the ligand system. The regioselectivity is shown to depend on the electronic and steric characteristics of the olefin, with steric bulk and electron withdrawing character favoring linear product formation. Activation of the C-H bond occurs in a concerted fashion through a novel transition structure best described as an oxidative hydrogen migration. The character of the transition structure is seven coordinate Ir(V), with a full bond formed between the migrating hydrogen and iridium. Several experimental observations are investigated and explained: (a) The nature of L influences the rate of the reaction through a ground-state effect. (b) The lack of beta-hydride products is due to kinetic factors, although beta-hydride elimination is calculated to be facile, all further reactions are kinetically inaccessible. (c) Inhibition by excess olefin is caused by competitive binding of olefin and aryl starting materials during the catalytic cycle in a statistical fashion. On the basis of this insertion-oxidative hydrogen transfer mechanism we suggest that electron-withdrawing substituents on the acac ligands, such as trifluoromethyl groups, are good modifications for catalysts with higher activity.
通过第一性原理量子力学方法（DFT）阐明了均相Ph-Ir(acac)(2)(L)催化剂催化烯烃氢芳基化的机理，特别强调了催化剂的活化、催化循环以及对实验观察结果的解释。基于此机理，我们提出了有望具有更高活性的新型催化剂。催化剂从惰性反式转变为活性顺式是通过解离途径发生的，计算得出的ΔH(0 K) = 35.1 kcal/mol，ΔG(298 K) = 26.1 kcal/mol。催化循环具有两个关键步骤，即1,2-烯烃插入和通过一种新颖的机理——氧化氢迁移进行的C-H活化。发现烯烃插入是速率决定步骤，计算得出的ΔH(0 K) = 27.0 kcal/mol，ΔG(298 K) = 29.3 kcal/mol。活化能随着配位烯烃上电子密度的增加以及配体体系中给电子特性的增强而增加。区域选择性显示取决于烯烃的电子和空间特性，空间位阻和吸电子特性有利于线性产物的形成。C-H键的活化通过一种新颖的过渡结构以协同方式发生，该过渡结构最好描述为氧化氢迁移。过渡结构的特征是七配位的Ir(V)，迁移的氢与铱之间形成了一个完整的键。对几个实验观察结果进行了研究和解释：(a)配体L的性质通过基态效应影响反应速率。(b)缺乏β-氢化物产物是由于动力学因素，尽管计算得出β-氢化物消除很容易，但所有进一步的反应在动力学上都是无法进行的。(c)过量烯烃的抑制作用是由于在催化循环中烯烃和芳基起始原料以统计方式竞争性结合所致。基于这种插入-氧化氢转移机理，我们认为acac配体上的吸电子取代基，如三氟甲基，是对具有更高活性的催化剂的良好修饰。
