Theoretical Analysis of Competing Pathways for Carbon–Hydrogen Activation of Cyclopentadienyl–Triphenylphosphine–Iridium in Benzene
Theoretical Analysis of Competing Pathways for Carbon–Hydrogen Activation of Cyclopentadienyl–Triphenylphosphine–Iridium in Benzene
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苯中环戊二烯基-三苯基膦-铱碳-氢活化竞争途径的理论分析
DOI：
10.1021/acs.inorgchem.9b02580
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发表时间：
2019
影响因子：
4.6
通讯作者：
Hall, Michael B.
中科院分区：
文献类型：
--
作者：
Yang, Xin;Hall, Michael B.
利用密度泛函理论（DFT）计算了光化学反应生成的（η5-C5 Me 5）IrPPh 3氧化加成苯溶剂分子或膦配体苯基上的C-H键时的反应机理.在实验上，由分子内C-H活化产生的邻位金属化络合物和由分子间C-H活化产生的邻苯基络合物以53：47的比例形成（Janowicz，A. H.的;伯格曼河，巴西-地G.J. Am. 1982，104，352-354）。预计这两种产物都是化学稳定的，因此预计逆反应还原消除是非常不利的。因此，产物比率必须在动力学控制下。DFT计算预测了π-键合中间体η2-C6 H5 X（X = H或PPh 2）的初始形成，其中分子间π-中间体2.0 kcal/mol的自由能比分子内π-中间体更稳定。从这些中间体中，两个竞争反应的自由能势垒略有不同。苯基C-H键的分子内活化生成邻位金属配合物的势垒为13.4 kcal/mol，溶剂分子C6 H6的分子间氧化加成生成（η5-C5 Me 5）Ir（PPh 3）（Ph）H的势垒为15.9 kcal/mol。我们认为，π-中间体之间的交换比氧化加成快，所以占主导地位的早期中间体是分子间π-中间体。因此，从这个中间体的两个反应路径有15.4（分子内）和15.9（分子间）千卡/摩尔的自由能势垒。因此，在DFT的准确度内，分子内途径和分子间途径的自由能势垒与53：47的产物比非常相容。然而，计算不能完全排除更高的交换势垒，这意味着最终产物的比例必须来自两个π-中间体的几乎相等的分布（53：47），然后继续向自己的产物发展。对空间位阻较小的（η5-C5 H5）IrPPh 3的进一步计算表明，分子间产物应占主导地位。
Density functional theory (DFT) calculations are used to evaluate alternative reaction mechanisms when the photochemically produced (η5-C5Me5)IrPPh3oxidatively adds a C–H bond from either a benzene solvent molecule or a phenyl group of the phosphine ligand. Experimentally, the ortho-metalated complexes produced from intramolecular C–H activation and the hydridophenyl complexes produced from intermolecular C–H activation form in a ratio of 53:47 (Janowicz, A. H.; Bergman, R. G.J. Am. Chem. Soc.1982,104, 352–354). Both products are predicted to be thermodynamically stable such that the back reaction, reductive elimination, is predicted to be exceedingly unfavorable. Thus, the product ratio must be under kinetic control. The DFT calculations predict the initial formation of π-bound intermediates, η2-C6H5X (X = H or PPh2), with the intermolecular π-intermediate 2.0 kcal/mol more stable in free energy than the intramolecular π-intermediate. From these intermediates, the two competing reactions have slightly different free-energy barriers. The intramolecular activation of a phenyl C–H bond to yield ortho-metalated complexes has a barrier of 13.4 kcal/mol, and the intermolecular oxidative addition of solvent molecule C6H6to form (η5-C5Me5)Ir(PPh3)(Ph)H has a barrier of 15.9 kcal/mol. We propose that exchange between the π-intermediates is faster than the oxidative additions, so the dominant early intermediate is the intermolecular π-intermediate. Hence, from this intermediate the two reaction paths have free-energy barriers of 15.4 (intramolecular) and 15.9 (intermolecular) kcal/mol. Thus, within the accuracy of DFT, the free-energy barriers for the intramolecular pathway and the intermolecular pathway are very compatible with the 53:47 product ratio. However, the calculations cannot completely exclude a higher interchange barrier, which would mean that the final product ratio must result from the nearly equal distribution (53:47) of the two π-intermediates that then proceed toward their own products. Further calculations on the less sterically crowded (η5-C5H5)IrPPh3predict that the intermolecular product should dominate the ratio.