Switchable Product Selectivity in Diazoalkane Coupling Catalyzed by a Two-Coordinate Cobalt Complex
Switchable Product Selectivity in Diazoalkane Coupling Catalyzed by a Two-Coordinate Cobalt Complex
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DOI：
10.1021/acscatal.1c02926
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发表时间：
2021-09
期刊：
影响因子：
12.9
通讯作者：
Yuyang Dong;Michael I. Lipschutz;Ryan J. Witzke;J. Panetier;T. Tilley
中科院分区：
文献类型：
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作者：
Yuyang Dong;Michael I. Lipschutz;Ryan J. Witzke;J. Panetier;T. Tilley
本文报道了一价钴配合物（IPr）Co[N（SiMe_3）DIPP] [2，IPr = 1，3-双（2，6-二异丙基苯基）咪唑-2-亚基; DIPP = 2，6-二异丙基苯基]，由大体积酰胺和N-杂环卡宾（NHC）配体支撑，及其9-重氮芴（FluN_2）加合物（IPr）Co[N（SiMe_3）DIPP]（FluN_2）（3）。配合物3具有高自旋的二价钴中心与配体基自由基反铁磁耦合，形成三重态自旋基态（S= 1）. 2和3都是室温下FluN 2在苯中偶联生成1，2-二（9 H-芴-9-亚基）肼（8）和9，9 ′-联芴亚基（9）的催化剂前体，二者的比例为1：8. 1。在极性溶剂四氢呋喃（THF）中或在外源性良好L型配体如叔丁腈存在下，观察到反应产物选择性的转变，生成相应的肼8作为主要产物。进行了一项机制研究，以合理化所观察到的产品分布。该反应表现出一级速率依赖于FluN 2和钴催化剂（2）的浓度（通过1H NMR光谱监测），和3被确定为催化静止状态。进行理论计算来模拟肼8和烯烃9的产生。计算结果表明，8和9在苯中的转化率分别为4.6 × 10- 7和2.3 × 10-6s-1，与实验结果吻合较好.模拟在更高极性介质如THF中的反应，由于更极性的C-N键形成过渡态的稳定性，肼8形成的动力学势垒更有利（8，TOF = 2.6 × 10-5s-1 vs 9，TOF = 6.4 × 10-6s-1，在THF中）。此外，模拟的势能面与协调的L-型供体，如乙腈，表明选择性开关也可能导致从一个修改的配体字段，使重氮烷加合物3更亲核和降低的障碍限速C-N键形成给肼8。
The low-coordinate monovalent cobalt complex (IPr)Co[N(SiMe3)DIPP] [2, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; DIPP = 2,6-diisopropylphenyl], supported by bulky amide and N-heterocyclic carbene (NHC) ligands and its 9-diazofluorene (FluN2) adduct (IPr)Co[N(SiMe3)DIPP](FluN2) (3) are described. Complex3was characterized as possessing a high-spin divalent cobalt center antiferromagnetically coupled to a ligand-based radical, resulting in an overall triplet spin ground state (S= 1). Both2and3are catalyst precursors for the homocoupling of FluN2in benzene under ambient conditions to produce 1,2-di(9H-fluoren-9-ylidene)hydrazine (8) and 9,9′-bifluorenylidene (9) in a ratio of 1:8.1. A switch in product selectivity was observed for the reaction in the polar solvent tetrahydrofuran (THF), or in the presence of exogenous good L-type ligands such astert-butylnitrile, to generate the corresponding hydrazine8as the major product. A mechanistic study was carried out to rationalize the observed product distributions. The reaction exhibits first-order rate dependence on both the FluN2and cobalt catalyst (2) concentrations (monitored by1H NMR spectroscopy), and3was identified as the catalytic resting state. Theoretical calculations were carried out to simulate the production of hydrazine8and olefin9. The result predicted turnover frequencies (TOFs) of 4.6 × 10–7and 2.3 × 10–6s–1for the generation of8and9in benzene, respectively, in good agreement with the experimentally observed product ratio. Modeling the reaction in media with higher polarity such as THF resulted in a more favorable kinetic barrier toward the formation of hydrazine8due to the stabilization of the more polar C–N bond-forming transition state (8, TOF = 2.6 × 10–5s–1vs9, TOF = 6.4 × 10–6s–1, in THF). Moreover, simulation of the potential energy surface with a coordinated L-type donor, such as acetonitrile, suggests that the selectivity switch could also result from a modified ligand field, rendering diazoalkane adduct3more nucleophilic and lowering the barrier of rate-limiting C–N bond formation to give hydrazine8.