The Effect of Added Ligands on the Reactions of [Ni(COD)(dppf)] with Alkyl Halides: Halide Abstraction May Be Reversible

The reactions of dppf-nickel(0) with alkyl halides proceed via three-coordinate nickel(0) intermediates of the form [Ni­(dppf)­(L)]. The effects of the identity of the added ligand (L) on catalyst speciation and the rates of reactions of [Ni­(COD)­(dppf)] with alkyl halides have been investigated using kinetic experiments and density functional theory calculations. A series of monodentate ligands have been investigated in attempts to identify trends in reactivity. Sterically bulky and electron-donating ligands are found to decrease the reaction rate. It was found that (i) the halide abstraction step is not always irreversible and the subsequent recombination of a nickel­(I) complex with an alkyl halide can have a significant effect on the overall rate of the reaction and (ii) some ligands lead to very stable [Ni­(dppf)­(L)2] species. The yields of prototypical (dppf)­nickel-catalyzed Kumada cross-coupling reactions of alkyl halides are significantly improved by the addition of free ligands, which provides another important variable to consider when optimizing nickel-catalyzed reactions of alkyl halides.双(二苯基膦)二茂铁（dppf）修饰的零价镍与卤代烷烃的反应，经由形式为[Ni(dppf)(L)]的三配位零价镍中间体进行。本研究借助动力学实验与密度泛函理论（Density Functional Theory）计算，考察了外加配体（L）的种类对催化剂物种分布以及[Ni(1,5-环辛二烯（COD）)(dppf)]与卤代烷烃反应速率的影响。为探明反应活性的变化趋势，本研究对一系列单齿配体开展了系统考察。研究发现，空间位阻较大的给电子配体可降低反应速率。结果表明：(i) 卤原子夺取步骤并非始终不可逆，一价镍配合物与卤代烷烃的后续重组过程会对反应总速率产生显著影响；(ii) 部分配体可生成稳定性优异的[Ni(dppf)(L)₂]配合物种。通过添加游离配体，典型的dppf-镍催化卤代烷烃熊田交叉偶联（Kumada cross-coupling）反应的收率得到显著提升，这为优化镍催化卤代烷烃反应提供了另一项关键考量变量。