The last two decades have experienced a dramatic development of nickel-catalyzed cross-coupling reactions involving alkyl substrates, including a wide range of stereoselective transformations. In these reactions, nickel complexes supported by N-donor ligands have been proposed to involve paramagnetic Ni(I) and Ni(III) species as the active intermediates during catalysis. Herein, we report the use of a bulky 1,4,7-triisopropyl-1,4,7-triazacyclononane (iPr3TACN) ligand that allowed the detection and isolation of uncommon organometallic Ni(I) and Ni(III) complexes, which are involved in well-defined oxidative addition, transmetallation, and reductive elimination steps of the proposed catalytic cycle. Moreover, (iPr3TACN)Ni(II) complexes were shown to be efficient catalysts for the alkyl-alkyl Kumada cross-coupling, while a beneficial effect of acetonitrile on the yield of the desired cross-coupled products was observed, likely due to the stabilization of transient Ni(I) species. Overall, these studies provide unambiguous evidence for the involvement of both Ni(III) and Ni(I) organometallic species in Ni-catalyzed alkyl-alkyl cross-coupling reactions.

中文翻译：

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三异丙基-1,4,7-三氮杂环壬烷的神奇之处：探究 Ni(III) 和 Ni(I) 中心在交叉偶联催化中的作用

在过去的二十年里，涉及烷基底物的镍催化交叉偶联反应取得了显着发展，包括范围广泛的立体选择性转化。在这些反应中，已提出由 N-供体配体支持的镍络合物涉及顺磁性 Ni(I) 和 Ni(III) 物种作为催化过程中的活性中间体。在此，我们报告了使用体积庞大的 1,4,7-三异丙基-1,4,7-三氮杂环壬烷 (iPr3TACN) 配体，该配体允许检测和分离不常见的有机金属 Ni(I) 和 Ni(III) 配合物，它们是参与所提出的催化循环的明确定义的氧化加成、金属转移和还原消除步骤。此外，(iPr3TACN)Ni(II) 络合物被证明是烷基-烷基 Kumada 交叉偶联的有效催化剂，同时观察到乙腈对所需交叉偶联产物产率的有益影响，这可能是由于瞬态 Ni(I) 物种的稳定作用。总体而言，这些研究为 Ni(III) 和 Ni(I) 有机金属物种参与 Ni 催化的烷基-烷基交叉偶联反应提供了明确的证据。