Lee Wan-Chen Cindy, Wang Duo-Sheng, Zhu Yiling, Zhang X Peter
Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, MA, USA.
Nat Chem. 2023 Nov;15(11):1569-1580. doi: 10.1038/s41557-023-01317-8. Epub 2023 Sep 7.
Metalloradical catalysis (MRC) exploits the metal-centred radicals present in open-shell metal complexes as one-electron catalysts for the generation of metal-stabilized organic radicals-key intermediates that control subsequent one-electron homolytic reactions. Cobalt(II) complexes of porphyrins, as stable 15e-metalloradicals with a well-defined low-spin d configuration, have dominated the ongoing development of MRC. Here, to broaden MRC beyond the use of Co(II)-based metalloradical catalysts, we describe systematic studies that establish the operation of Fe(III)-based MRC and demonstrate an initial application for asymmetric radical transformations. Specifically, we report that five-coordinate iron(III) complexes of porphyrins with an axial ligand, which represent another family of stable 15e-metalloradicals with a d configuration, are potent metalloradical catalysts for olefin cyclopropanation with different classes of diazo compounds via a stepwise radical mechanism. This work lays a foundation and mechanistic blueprint for future exploration of Fe(III)-based MRC towards the discovery of diverse stereoselective radical reactions.
金属自由基催化（MRC）利用开壳金属配合物中存在的以金属为中心的自由基作为单电子催化剂，以生成金属稳定的有机自由基——控制后续单电子均裂反应的关键中间体。卟啉的钴（II）配合物作为具有明确低自旋d构型的稳定15e金属自由基，主导着MRC的持续发展。在此，为了将MRC拓展到基于Co（II）的金属自由基催化剂之外的应用，我们描述了一系列系统研究，这些研究确立了基于Fe（III）的MRC的运行，并展示了其在不对称自由基转化中的初步应用。具体而言，我们报道了带有轴向配体的卟啉的五配位铁（III）配合物，它们代表了另一类具有d构型的稳定15e金属自由基，是通过逐步自由基机理与不同类型重氮化合物进行烯烃环丙烷化反应的有效金属自由基催化剂。这项工作为未来探索基于Fe（III）的MRC以发现各种立体选择性自由基反应奠定了基础并提供了机理蓝图。
