Use of Ligand Steric Properties to Control the Thermodynamics and Kinetics of Oxidative Addition and Reductive Elimination with Pincer-Ligated Rh Complexes
Use of Ligand Steric Properties to Control the Thermodynamics and Kinetics of Oxidative Addition and Reductive Elimination with Pincer-Ligated Rh Complexes
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DOI：
10.1021/acs.organomet.0c00122
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发表时间：
2020-05-26
期刊：
影响因子：
2.8
通讯作者：
Gunnoe, T. Brent
中科院分区：
文献类型：
--
作者：
Gu, Shunyan;Nielsen, Robert J.;Gunnoe, T. Brent
氧化加成和还原消除反应是许多催化过程的核心步骤，控制反应中间体的能量学是实现高效催化的关键。研究了（(PNP)-P-R）RhX配合物（R =叔丁基、异丙基、甲酰基、苯基；X = Cl， I）的一系列氧化加成和还原消除反应，以推断磷化氢取代基大小的影响。以（(PNP)-P-R）RhCI为起始原料，观察到MeI的氧化添加生成（(PNP)-P-R）Rh(Me)(I)Cl，随后MeCl的还原消除生成（(PNP)-P-R）RhI。热力学和动力学随PNP配体中磷取代基R的不同而变化。相对于两个较小的取代基（例如，R =异丙基，苯基），在膦上存在较大的空间体（例如，R =叔丁基，甲酰基）有利于Rh(I)。在the -d(8)中，MeI与（(PNP)-P-tBu）RhCl氧化加成的Eyring图符合极性两步反应途径，[((PNP)-P-tBu)Rh(Me)I]I的形成也符合这一机制。DFT计算表明，位阻体对生成六坐标配合物的加成反应的反应能有几十千卡摩尔（-1）的影响。根据计算，配体的空间体积对S-N2加成势垒的影响减小（几千卡摩尔（-1）），而S-N2加成势垒只需要进入正方形平面的一侧。
Oxidative addition and reductive elimination reactions are central steps in many catalytic processes, and controlling the energetics of reaction intermediates is key to enabling efficient catalysis. A series of oxidative addition and reductive elimination reactions using ((PNP)-P-R)RhX complexes (R = tert-butyl, isopropyl, mesityl, phenyl; X = Cl, I) was studied to deduce the effect of the size of the phosphine substituents. Using ((PNP)-P-R)RhCI as the starting material, oxidative addition of MeI was observed to produce ((PNP)-P-R)Rh(Me)(I)Cl, which was followed by reductive elimination of MeCl to form ((PNP)-P-R)RhI. The thermodynamics and kinetics vary with the identity of the substituent R on phosphorus of the PNP ligand. The presence of large steric bulk (e.g., R = tert-butyl, mesityl) on the phosphine favors Rh(I) in comparison to the presence of two smaller substituents (e.g., R = isopropyl, phenyl). An Eyring plot for the oxidative addition of MeI to ((PNP)-P-tBu)RhCl in THE-d(8) is consistent with a polar two-step reaction pathway, and the formation of [((PNP)-P-tBu)Rh(Me)I]I is also consistent with this mechanism. DFT calculations show that the steric bulk affects the reaction energies of addition reactions which generate six-coordinate complexes by tens of kcal mol(-1). The ligand steric bulk is calculated to have a reduced effect (a few kcal mol(-1)) on S-N2 addition barriers, which only require access to one side of the square plane.