Yan Wenhao, Poore Andrew T, Yin Lingfeng, Carter Samantha, Ho Yeu-Shiuan, Wang Chao, Yachuw Stephen C, Cheng Yu-Ho, Krause Jeanette A, Cheng Mu-Jeng, Zhang Shiyu, Tian Shiliang, Liu Wei
Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221, United States.
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States.
J Am Chem Soc. 2024 Jun 5;146(22):15176-15185. doi: 10.1021/jacs.4c01668. Epub 2024 May 21.
Stepwise oxidative addition of copper(I) complexes to form copper(III) species via single electron transfer (SET) events has been widely proposed in copper catalysis. However, direct observation and detailed investigation of these fundamental steps remain elusive owing largely to the typically slow oxidative addition rate of copper(I) complexes and the instability of the copper(III) species. We report herein a novel aryl-radical-enabled stepwise oxidative addition pathway that allows for the formation of well-defined alkyl-Cu species from Cu complexes. The process is enabled by the SET from a Cu species to an aryl diazonium salt to form a Cu species and an aryl radical. Subsequent iodine abstraction from an alkyl iodide by the aryl radical affords an alkyl radical, which then reacts with the Cu species to form the alkyl-Cu complex. The structure of resultant [(bpy)Cu(CF)(alkyl)] complexes has been characterized by NMR spectroscopy and X-ray crystallography. Competition experiments have revealed that the rate at which different alkyl iodides undergo oxidative addition is consistent with the rate of iodine abstraction by carbon-centered radicals. The Cu intermediate formed during the SET process has been identified as a four-coordinate complex, [Cu(CHCN)(CF)], through electronic paramagnetic resonance (EPR) studies. The catalytic relevance of the high-valent organo-Cu has been demonstrated by the C-C bond-forming reductive elimination reactivity. Finally, localized orbital bonding analysis of these formal Cu complexes indicates inverted ligand fields in σ(Cu-CH) bonds. These results demonstrate the stepwise oxidative addition in copper catalysis and provide a general strategy to investigate the elusive formal Cu complexes.
在铜催化中，人们广泛提出通过单电子转移（SET）事件将铜（I）配合物逐步进行氧化加成以形成铜（III）物种。然而，由于铜（I）配合物通常较慢的氧化加成速率以及铜（III）物种的不稳定性，对这些基本步骤的直接观察和详细研究仍然难以实现。我们在此报告了一种新型的由芳基自由基引发的逐步氧化加成途径，该途径能够从铜配合物形成结构明确的烷基 - 铜物种。该过程是通过从铜物种到芳基重氮盐的单电子转移形成一个铜物种和一个芳基自由基来实现的。随后，芳基自由基从烷基碘中夺取碘生成一个烷基自由基，该烷基自由基再与铜物种反应形成烷基 - 铜配合物。所得的[(bpy)Cu(CF)(烷基)]配合物的结构已通过核磁共振光谱和X射线晶体学进行了表征。竞争实验表明，不同烷基碘进行氧化加成的速率与以碳为中心的自由基夺取碘的速率一致。通过电子顺磁共振（EPR）研究，在单电子转移过程中形成的铜中间体被鉴定为一种四配位配合物[Cu(CHCN)(CF)]。高价有机铜的催化相关性已通过形成碳 - 碳键的还原消除反应性得到证明。最后，对这些形式上的铜配合物进行的定域轨道键合分析表明，σ（Cu - CH）键中存在反转的配体场。这些结果证明了铜催化中的逐步氧化加成，并提供了一种研究难以捉摸的形式上的铜配合物的通用策略。
