Lin Qiao, Spielvogel Ethan H, Diao Tianning
Department of Chemistry, New York University, 100 Washington Square East, New York, NY 10003, USA.
Lead contact.
Chem. 2023 May 11;9(5):1295-1308. doi: 10.1016/j.chempr.2023.02.010. Epub 2023 Mar 13.
The capture of carbon-centered radicals at a nickel(II) center is commonly featured in recent cross-coupling and metallaphotoredox catalytic reactions. Despite its widespread application in catalysis, this fundamental step lacks experimental characterization. This report portrays radical capture at catalytically relevant nickel(II) centers from several aspects, including the structure-activity relationships of the ligands, the mechanism, the kinetics, and the stereoselectivity. Spectroscopic data provide evidence for the formation of a nickel(III) intermediate. Strikingly different reactivity between nickel-aryl and nickel-alkyl complexes implies different rate-determining steps for C(sp)-C(sp) and C(sp)-C(sp) bond formation. Kinetic data benchmark the capture rates on the scale of 10 Ms and 10 Ms for primary and secondary radicals, respectively. Overall, the activation energy is higher than that of previous computational estimations. Finally, stoichiometric experiments with well-defined chiral nickel complexes demonstrate that the radical trapping step can confer diastereoselectivity and enantioselectivity with a drastic ligand effect.
镍（II）中心捕获碳中心自由基是近期交叉偶联和金属光氧化还原催化反应中的常见特征。尽管其在催化领域有着广泛应用，但这一基本步骤缺乏实验表征。本报告从多个方面描述了在具有催化活性的镍（II）中心的自由基捕获过程，包括配体的结构-活性关系、机理、动力学和立体选择性。光谱数据为镍（III）中间体的形成提供了证据。镍-芳基和镍-烷基配合物之间显著不同的反应性意味着C（sp）-C（sp）和C（sp）-C（sp）键形成存在不同的速率决定步骤。动力学数据表明，伯自由基和仲自由基的捕获速率分别在10 M⁻¹s⁻¹和10² M⁻¹s⁻¹的范围内。总体而言，活化能高于先前的计算估计值。最后，使用结构明确的手性镍配合物进行的化学计量实验表明，自由基捕获步骤可通过显著的配体效应赋予非对映选择性和对映选择性。
