Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States.
Inorg Chem. 2022 May 2;61(17):6672-6678. doi: 10.1021/acs.inorgchem.2c00839. Epub 2022 Apr 18.
Recent studies showcase reductive concerted proton-electron transfer (CPET) as a powerful strategy for transferring a net hydrogen atom to organic substrates; however, direct application of CPET in the context of C-C bond formation beyond homocoupling is underexplored. We report herein the expansion of electrocatalytic CPET (CPET) using a Brønsted base-appended cobaltocene mediator ([CpCoCp][OTf]) with keto-olefin substrates that undergo cyclization subsequent to ketyl radical generation via CPET. Using acetophenone-derived substrates with tethered acrylates as radical acceptors, in the presence of tosylic acid, we demonstrate that ketyl-olefin cyclization is achieved by characterization of -lactone and alkene products. Mechanistic analysis of this 2 H/2 e process reveals a mixed order in substrate and acid and a Hammett plot with a modest negative slope, highlighting the contribution of sequential CPET and ET/PT steps involved in the overall rate of the reaction and providing support for initial O-H bond formation. The ability to access ketyl radicals at comparatively mild reduction potentials via controlled potential electrolysis enables functional group tolerance across a range of substrates.
最近的研究展示了还原协同质子-电子转移（CPET）作为将净氢原子转移到有机底物的有效策略；然而，CPET 在 C-C 键形成中的直接应用（超出偶联反应）尚未得到充分探索。本文报道了使用 Brønsted 碱附加的二茂钴介体（[CpCoCp][OTf]）扩展电催化 CPET（CPET），该介体可与酮-烯烃底物发生环化反应，随后通过 CPET 生成酮基自由基。使用带有连接的丙烯酸盐的苯乙酮衍生的底物作为自由基受体，在对甲苯磺酸存在下，我们通过 -内酯和烯烃产物的表征证明了酮-烯烃环化的实现。对该 2 H/2 e 过程的机理分析表明，底物和酸呈混合级数，Hammett 图呈适度负斜率，这突出了涉及反应总速率的顺序 CPET 和 ET/PT 步骤的贡献，并为初始 O-H 键形成提供了支持。通过控制电位电解在相对温和的还原电势下获得酮基自由基的能力使各种底物具有官能团耐受性。
