CpRu（PN）催化剂在烯烃异构化反应中高动力学（<i>E</i>）选择性的起源：结合实验和计算方法,ACS Catalysis

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CpRu（PN）催化剂在烯烃异构化反应中高动力学（E）选择性的起源：结合实验和计算方法
ACS Catalysis ( IF 11.3 ) Pub Date : 2020-12-10 , DOI: 10.1021/acscatal.0c03497
Thomas C. Cao ₁ , Andrew L. Cooksy ₁ , Douglas B. Grotjahn ₁

Affiliation

烯烃异构化催化剂CPRU（κ ² -PN）（CH ₃ CN）] ⁺ [PF ₆ ] ^-（茹（ACN） ）的价值在于它的高活性和高动力学选择性用于形成和与反应（ë） -链烯烃，但是需要针对这种选择性的机理研究。在这里，依赖于时间和温度的NMR光谱研究表明，当使用较低的底物/催化剂比例时，烯烃络合物会丢失。为帮助与理论进行严格的比较，使用丁-1-烯和异构体作为最简单的可异构化烯烃，并且丁-1-烯快速反应生成（E）-丁-2-烯和（Z的反应非常慢进行）-丁-2-烯。从与时间有关的浓度曲线拟合得到与温度有关的速率常数。产生的Arrhenius激活能量可以与计算模型进行比较。丁烯异构化机理的密度泛函理论（DFT）计算考虑了几种途径。预计丁烯以外向取向与催化剂结合是形成丁-2-烯的优选途径的第一步，其中E异构体优于Z由于与催化剂结合时较低的空间相互作用。对丁烯-催化剂配合物的实验观察表明，速率确定步骤先于丁-2-烯-催化剂配合物的形成，而DFT计算确定了Ru-N键的断裂与较弱的烯丙基C的较弱的相互作用形成。 -H和关键的烯丙基中间体中Ru-P键的扭转是速率确定步骤的紧密竞争者。还获得了丁-1-烯异构化的选定热力学参数的实验值。

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Origins of High Kinetic (E)-Selectivity in Alkene Isomerization by a CpRu(PN) Catalyst: a Combined Experimental and Computational Approach

The alkene isomerization catalyst CpRu(κ²-PN)(CH₃CN)]⁺[PF₆]⁻ (Ru(acn)) is valued for its high activity and high kinetic selectivity for forming and reacting with (E)-alkenes, but a mechanistic study addressing such selectivity is needed. Here, time- and temperature-dependent NMR spectroscopic studies reveal alkene complexes that were missed when lower substrate-to-catalyst ratios were used. To aid in rigorous comparison of the experiment with theory, but-1-ene and isomers were used as the simplest isomerizable alkenes, and both the fast reaction of but-1-ene to form (E)-but-2-ene and the very slow reaction of (Z)-but-2-ene were performed. Temperature-dependent rate constants were obtained from fits to the time-dependent concentration profiles. The resulting Arrhenius activation energies allow comparison to a computational model. Density functional theory (DFT) calculations of the butene isomerization mechanism considered several pathways. Binding of the butene to the catalyst in an exo-orientation is predicted to be the initial step of the preferred pathway for forming but-2-ene, with the E isomer favored over the Z by virtue of lower steric interactions while bound to the catalyst. Experimental observation of the butene–catalyst complex establishes that the rate-determining step precedes formation of the but-2-ene–catalyst complex, and DFT calculations identify breaking of the Ru–N bond with formation of a weaker agostic interaction of an allylic C–H and torsion about the Ru–P bond in a key allylic intermediate as close competitors for the rate-determining step. Experimental values for selected thermodynamic parameters of the but-1-ene isomerization were also obtained.

更新日期：2020-12-18

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