Topics in Catalysis ( IF 2.8 ) Pub Date : 2021-08-05 , DOI: 10.1007/s11244-021-01468-3 Immanuel Reim 1 , Giovanni Occhipinti 1 , Karl W. Törnroos 1 , Deryn E. Fogg 1, 2 , Vidar R. Jensen 1
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The selective transformation of 1-alkenes into E-olefins is a long-standing challenge in olefin metathesis. Density functional theory (DFT) calculations predict high E-selectivity for catalysts incorporating a bidentate, dianionic thio-indolate ligand within a RuXX’(NHC)(py)(= CHR) platform (NHC = N-heterocyclic carbene; py = pyridine). Such complexes are predicted to yield E-olefins by favoring anti-disposed substituents in the transition state expected to be rate-determining: specifically, that for cycloreversion of the metallacyclobutane intermediate. Three pyridine-stabilized catalysts Ru21a-c were synthesized, in which the thio-indolate ligand bears a H, Me, or Ph substituent at the C2 position, and the NHC ligand is the unsaturated imidazoline-2-ylidene Me2IMes (which bears N-mesityl groups and methyl groups on the C4,5 backbone). Single-crystal X-ray diffraction analysis of Ru21c confirms the ligand orientation required for E-selective metathesis, with the thio-indolate sulfur atom binding cis to the NHC, and the indolate nitrogen atom trans to the NHC. However, whereas the new complexes mediated metathetic exchange of their 2-thienylmethylidene ligand in the presence of the common metathesis substrates styrene and allylbenzene, no corresponding self-metathesis products were obtained. Only small amounts of 2-butene (73% (Z)-2-butene) were obtained in self-metathesis of propene using Ru21a. Detailed DFT analysis of this process revealed that product release is surprisingly slow, limiting the reaction rate and explaining the low metathesis activity. With the barrier to dissociation of (Z)-2-butene being lower than that of (E)-2-butene, the calculations also account for the observed Z-selectivity of Ru21a. These findings provide guidelines for catalyst redesign in pursuit of the ambitious goal of E-selective 1-alkene metathesis.
Graphic abstract
中文翻译：
走向 E 选择性烯烃复分解：钌硫代吲哚催化剂的计算设计和实验实现
1-烯烃选择性转化为 E-烯烃是烯烃复分解中长期存在的挑战。密度泛函理论 (DFT) 计算预测了在 RuXX'(NHC)(py)(= CHR) 平台（NHC = N-杂环卡宾；py = 吡啶）中加入双齿双阴离子硫代吲哚配体的催化剂的高 E 选择性. 预计此类配合物将通过有利于预期决定速率的过渡态中的反处置取代基产生 E-烯烃：具体而言，用于金属环丁烷中间体的环回复。合成了三种吡啶稳定催化剂Ru21a-c，其中硫代吲哚酸配体在C2位带有H、Me或Ph取代基，NHC配体为不饱和咪唑啉-2-亚基Me 2IMes（在 C4,5 主链上带有 N-甲基和甲基）。Ru21c的单晶 X 射线衍射分析证实了 E 选择性复分解所需的配体取向，硫代吲哚硫原子与 NHC 顺式结合，而吲哚氮原子与 NHC 反式结合。然而，虽然新的复合物介导了其 2-噻吩基亚甲基配体在常见复分解底物苯乙烯和烯丙基苯的存在下的复分解交换，但没有获得相应的自复分解产物。在使用Ru21a的丙烯自复分解中仅获得少量 2-丁烯（73% (Z)-2-丁烯）. 该过程的详细 DFT 分析表明，产物释放出奇地缓慢，限制了反应速率并解释了低复分解活性。由于(Z)-2-丁烯的解离势垒低于(E)-2-丁烯的解离势垒，计算还解释了观察到的Ru21a的Z-选择性。这些发现为催化剂重新设计提供了指导方针，以实现 E 选择性 1-烯烃复分解的雄心勃勃的目标。
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