Qin Jingyi, Yin Yiyan, Guan Xiaowen, Ge Xiyang, Cao Mengyu, Ouyang Jin, Na Na
Key Laboratory of Radiopharmaceuticals, Ministry of Education, College of Chemistry, Beijing Normal University Beijing 100875 China
Department of Chemistry, Faculty of Arts and Sciences, Beijing Normal University Zhuhai 519087 China.
Chem Sci. 2025 May 29. doi: 10.1039/d5sc02194d.
Radicals are highly reactive for coupling reactions while the applications are normally limited by the uncontrollable initiation and chaotic conversions. Although transition metal-based single electron transfer (SET) shows potential for controllable radical initiation, the detailed mechanism is still insufficient, especially for the roles of spin state transition in SET-based radical initiation. Herein, with an Fe(iii)-catalyzed thiol-ene click (TEC) reaction as an example, the axial-ligand-induced switching of transition metals' spin states was revealed to facilitate controllable SET-based radical initiation and the subsequent coupling reactions. Given the advantages of online monitoring by ambient mass spectrometry (AMS), the short-lived radical intermediates and their dynamic changes were explored. As demonstrated, initiated by the axial coordination of sulfhydryl with Fe(iii)-porphyrin, the selective generation of a thiyl radical (RS˙) SET was achieved. Besides, as another axial-ligand, O in air was coordinated to Fe(iii)-porphyrin, inducing the conversion of Fe(iii) from a high spin ( = 5/2) to a low spin state ( = 1/2). This lowered the energy barrier for SET-based radical initiation, further facilitating the final selective coupling with the vinyl reactant. Upon revealing the axial-ligand-induced switching of the spin states by AMS and other examinations, rational design of transition metal catalysts would be promoted for efficient and highly selective radical reactions.
自由基对于偶联反应具有高反应活性，但其应用通常受限于不可控的引发过程和混乱的转化过程。尽管基于过渡金属的单电子转移（SET）显示出可控自由基引发的潜力，但其详细机制仍不充分，特别是自旋态转变在基于SET的自由基引发中的作用。在此，以铁（III）催化的硫醇-烯点击（TEC）反应为例，揭示了轴向配体诱导的过渡金属自旋态切换有助于基于SET的可控自由基引发及随后的偶联反应。鉴于常压质谱（AMS）在线监测的优势，对短寿命自由基中间体及其动态变化进行了探索。结果表明，通过巯基与铁（III）-卟啉的轴向配位引发，实现了硫自由基（RS˙）SET的选择性生成。此外，作为另一种轴向配体，空气中的O与铁（III）-卟啉配位，诱导铁（III）从高自旋（S = 5/2）转变为低自旋态（S = 1/2）。这降低了基于SET的自由基引发的能垒，进一步促进了与乙烯基反应物的最终选择性偶联。通过AMS和其他检测揭示轴向配体诱导的自旋态切换后，将推动过渡金属催化剂的合理设计，以实现高效且高选择性的自由基反应。
