Mechanistic Characterization of (Xantphos)Ni(I)-Mediated Alkyl Bromide Activation: Oxidative Addition, Electron Transfer, or Halogen-Atom Abstraction
Mechanistic Characterization of (Xantphos)Ni(I)-Mediated Alkyl Bromide Activation: Oxidative Addition, Electron Transfer, or Halogen-Atom Abstraction
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DOI：
10.1021/jacs.8b13499
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发表时间：
2019-01-30
影响因子：
15
通讯作者：
Diao, Tianning
中科院分区：
文献类型：
--
作者：
Diccianni, Justin B.;Katigbak, Joseph;Diao, Tianning
Ni(I)介导单电子氧化活化卤代烷基化合物已被广泛认为是Ni催化的交叉偶联反应生成自由基中间体的关键步骤。这一过程可以通过四种机制进行：氧化加成、外球电子转移、内球电子转移和卤素原子协同抽提。尽管有大量的计算研究，但还没有实验研究来评估所有四种Ni(I)介导的烷基形成途径。在这里，我们报道了一系列(Xantphos)Ni(I)-Ar络合物的分离，这些络合物选择性地激活了芳基卤化物上的烷基卤化物以排出自由基并形成Ni(II)络合物。这一观察结果允许应用对空间、电子和溶剂效应的动力学研究，结合密度泛函计算，系统地评估四种可能的途径。我们的数据表明，(Xantphos)Ni(I)介导的烷基卤化物活化是通过协调的卤素原子抽象机制进行的。这一结果证实了(Terpy)Ni(I)-和(Py)Ni(I)介导的烷基生成的密度泛函理论研究，并与(PPh_3)(4)Ni(O)介导的芳基卤化物活化的外球电子转移途径形成对比。这一模型体系的研究揭示了镍催化交叉偶联反应的总体机理，并为区分交叉亲电偶联反应中的亲电体提供了基础。
Ni(I)-mediated single-electron oxidative activation of alkyl halides has been extensively proposed as a key step in Ni-catalyzed cross-coupling reactions to generate radical intermediates. There are four mechanisms through which this step could take place: oxidative addition, outer-sphere electron transfer, inner-sphere electron transfer, and concerted halogen atom abstraction. Despite considerable computational studies, there is no experimental study to evaluate all four pathways for Ni(I)-mediated alkyl radical formation. Herein, we report the isolation of a series of (Xantphos)Ni(I)-Ar complexes that selectively activate alkyl halides over aryl halides to eject radicals and form Ni(II) complexes. This observation allows the application of kinetic studies on the steric, electronic, and solvent effects, in combination with DFT calculations, to systematically assess the four possible pathways. Our data reveal that (Xantphos)Ni(I)-mediated alkyl halide activation proceeds via a concerted halogen-atom abstraction mechanism. This result corroborates previous DFT studies on (terpy)Ni(I)- and (py)Ni(I)-mediated alkyl radical formation, and contrasts with the outer-sphere electron transfer pathway observed for (PPh3)(4)Ni(O)-mediated aryl halide activation. This study of a model system provides insight into the overall mechanism of Ni-catalyzed cross-coupling reactions and offers a basis for differentiating electrophiles in cross-electrophile coupling reactions.