Identifying the Imperative Role of Metal-Olefin Interactions in Catalytic C-O Reductive Elimination from Nickel(II).
Identifying the Imperative Role of Metal-Olefin Interactions in Catalytic C-O Reductive Elimination from Nickel(II).
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DOI：
10.1021/acscatal.1c02790
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发表时间：
2021-08-20
期刊：
影响因子：
12.9
通讯作者：
Stoltz BM
中科院分区：
文献类型：
--
作者：
Lohrey TD;Cusumano AQ;Goddard WA 3rd;Stoltz BM
我们提出了一系列的实验和计算机制的调查一个异常容易的例子，镍催化的C-O键的形成。我们的方法最初于2016年报道，涉及从带有侧醇基团的乙烯基碘形成环状烯醇醚。我们的研究结果表明，所观察到的反应性是由乙烯基碘原料和烯醇醚产物中的烯烃与Ni（0）中间体的配位引起的。密度泛函理论计算揭示了一个合理的催化机制，涉及Ni（II）/Ni（0）的氧化还原循环，具有两个电子的C-I氧化加成和C-O还原消除步骤。从关键的Ni（II）醇盐中间体直接形成η2-烯醇醚Ni（0）络合物显著地改变了乙烯基C-O还原消除步骤相对于在Ni（II）处的C-O还原消除的其他实例的自由能（ΔG）。此外，乙烯基C-O还原消除过程中有效的σ-π混合导致了较低的计算动力学势垒（ΔG）.从这些计算模型得出的结论得到支持的合成有机金属实验，其中乙烯基-镍（II）碘化物中间体被分离，其特征在于，并证明产生烯醇醚，暴露于三乙胺。我们进行了进一步的实验和计算，这表明Ni（0）对乙烯基碘的两电子氧化加成取决于η2-乙烯基碘预络合物的形成，这是基于在空间位阻配体（例如，三环己基膦）。
We present a series of experimental and computational mechanistic investigations of an unusually facile example of Ni-catalyzed C–O bond formation. Our method, originally reported in 2016, involves the formation of cyclic enol ethers from vinyl iodides bearing pendant alcohol groups. Our findings suggest that the observed reactivity arises from the coordination of the olefin in the vinyl iodide starting material and the enol ether product with Ni(0) intermediates. Density functional theory calculations reveal a plausible catalytic mechanism involving a Ni(II)/Ni(0) redox cycle featuring two-electron C–I oxidative addition and C–O reductive elimination steps. The direct formation of a η2-enol ether Ni(0) complex from a key Ni(II) alkoxide intermediate dramatically alters the free energy (ΔG) for the vinyl C–O reductive elimination step relative to other examples of C–O reductive elimination at Ni(II). Furthermore, efficient σ-π mixing in the course of vinyl C–O reductive elimination leads to lower computed kinetic barriers (ΔG‡) relative to those of aryl C–O reductive elimination. The conclusions drawn from these computational models are supported by synthetic organometallic experiments, whereby a vinyl–Ni(II) iodide intermediate was isolated, characterized, and proved to yield enol ether, following exposure to triethylamine. We conducted further experiments and computations, which indicated that the two-electron oxidative addition of vinyl iodides by Ni(0) depends on the formation of an η2-vinyl iodide precomplex, based on the observation of one-electron activation of the same vinyl iodide in the presence of sterically encumbering ligands (e.g., tricyclohexylphosphine).
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