Mechanism of Alkene Hydrofunctionalization by Oxidative Cobalt(salen) Catalyzed Hydrogen Atom Transfer.
Mechanism of Alkene Hydrofunctionalization by Oxidative Cobalt(salen) Catalyzed Hydrogen Atom Transfer.
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DOI：
10.1021/jacs.3c12329
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发表时间：
2024-01
影响因子：
15
通讯作者：
Conner V Wilson;Patrick L. Holland
中科院分区：
文献类型：
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作者：
Conner V Wilson;Patrick L. Holland
烯烃的氧化 MHAT 加氢官能化提供了一种温和的钴催化途径来形成 C-N 和 C-O 键。在这里，我们表征了相关的萨伦负载钴配合物及其与烯烃、硅烷、氧化剂和溶剂的反应。这些化学计量研究得到了催化反应和催化剂形态的动力学研究的补充。我们描述了难以捉摸的氟化钴(III)络合物的溶液表征，令人惊讶的是，它不是在催化条件下与硅烷反应的物质；相反，钴(III)水合络合物的活性更高。因此，添加水 (0.15 M) 会加速催化反应，动力学研究表明，添加水能够在 -50 °C 的丙酮中 2 小时内形成催化产物。在这些条件下，可以通过紫外可见分光光度法观察钴（III）的静止状态，包括钴（III）-烷基络合物。它来自催化循环的转换限制步骤中形成的瞬态氢化钴(III)络合物。这种氢化物很容易降解，但不会降解为 H2；它通过双金属途径释放 H+，这解释了非循环反应的 [Co]2 依赖性。相反，催化反应速率遵循幂律 kobs[Co]1[硅烷]1。由于催化反应和降解反应对[Co]的依赖性不同，较低的催化剂负载量可以通过降低非生产性硅烷/氧化剂消耗的相对速率来提高催化反应的产率。这些研究阐明了烯烃氧化 MHAT 加氢官能化的机制细节，并为理解氢化钴和烷基钴配合物介导的其他催化反应奠定了基础。
Oxidative MHAT hydrofunctionalization of alkenes provides a mild cobalt-catalyzed route to forming C-N and C-O bonds. Here, we characterize relevant salen-supported cobalt complexes and their reactions with alkenes, silanes, oxidant, and solvent. These stoichiometric investigations are complemented by kinetic studies of the catalytic reaction and catalyst speciation. We describe the solution characterization of an elusive cobalt(III) fluoride complex, which surprisingly is not the species that reacts with silane under catalytic conditions; rather, a cobalt(III) aquo complex is more active. Accordingly, the addition of water (0.15 M) speeds the catalytic reaction, and kinetic studies show that water addition enables catalytic product formation in 2 h at -50 °C in acetone. Under these conditions, cobalt(III) resting states can be observed by UV-vis spectrophotometry, including a cobalt(III)-alkyl complex. It comes from a transient cobalt(III) hydride complex that is formed in the turnover-limiting step of the catalytic cycle. This hydride readily degrades but not to H2; it releases H+ through a bimetallic pathway that explains the [Co]2 dependence of the off-cycle reaction. In contrast, the rate of the catalytic reaction follows the power law kobs[Co]1[silane]1. Because of the different [Co] dependence of the catalytic reaction and the degradation reaction, lower catalyst loading improves the yield of the catalytic reaction by reducing the relative rate of unproductive silane/oxidant consumption. These studies illuminate mechanistic details of oxidative MHAT hydrofunctionalization of alkenes and lay the groundwork for understanding other catalytic reactions mediated by cobalt hydride and cobalt alkyl complexes.