Isolated Organometallic Nickel(III) and Nickel(IV) Complexes Relevant to Carbon–Carbon Bond Formation Reactions

Nickel-catalyzed cross-coupling reactions are experiencing a dramatic resurgence in recent years given their ability to employ a wider range of electrophiles as well as promote stereospecific or stereoselective transformations. In contrast to the extensively studied Pd catalysts that generally employ diamagnetic intermediates, Ni systems can more easily access various oxidation states including odd-electron configurations. For example, organometallic NiIII intermediates with aryl and/or alkyl ligands are commonly proposed as the active intermediates in cross-coupling reactions. Herein, we report the first isolated NiIII–dialkyl complex and show that this species is involved in stoichiometric and catalytic C–C bond formation reactions. Interestingly, the rate of C–C bond formation from a NiIII center is enhanced in the presence of an oxidant, suggesting the involvement of transient NiIV species. Indeed, such a NiIV species was observed and characterized spectroscopically for a nickelacycle system. Overall, these studies suggest that both NiIII and NiIV species could play an important role in a range of Ni-catalyzed cross-coupling reactions, especially those involving alkyl substrates.

近年来，镍催化交叉偶联反应凭借可使用更广泛的亲电试剂，并能实现立体专一性或立体选择性转化，正迎来迅猛复苏。与被广泛研究的钯(Pd)催化剂（通常采用反磁性中间体）不同，镍基催化体系更易达到包括单电子构型在内的多种氧化态。例如，带有芳基和/或烷基配体的有机金属Ni(III)中间体，常被视作交叉偶联反应中的活性中间体。本文报道首例分离得到的Ni(III)-二烷基配合物，并证实该物种参与了化学计量及催化型C-C键形成反应。值得注意的是，在氧化剂存在时，Ni(III)中心参与的C-C键形成反应速率会得到提升，这暗示体系中存在瞬态Ni(IV)物种。事实上，我们已在镍环体系中通过光谱学手段观测到此类Ni(IV)物种并完成表征。综上，本研究表明Ni(III)与Ni(IV)物种均可在多种镍催化交叉偶联反应中发挥关键作用，尤其是涉及烷基底物的相关反应。