Lin Zhicong, Zhou Qian, Liu Yan, Chen Chenli, Jie Jialong, Su Hongmei
College of Chemistry, Beijing Normal University Beijing 100875 China
Chem Sci. 2024 Jun 27;15(30):11919-11927. doi: 10.1039/d4sc03183k. eCollection 2024 Jul 31.
Photoredox catalytic radical acylation reactions, utilizing [Ir(dFCFppy)(dtbbpy)] (IrIII) as the photocatalyst and α-keto acids as the starting substrates, have recently emerged as an attractive strategy for preparing ketone derivatives. While there is consensus on the importance of detailed mechanistic insights to maximize the formation of desired products, efforts focused on uncovering the underlying elementary mechanisms of IrIII photocatalytic radical acylation reactions are still lacking. Herein, using time-resolved spectroscopy, we observed the efficient quenching of the triplet state, IrIII*, electron transfer from α-keto acids, resulting in the generatation of the reduced IrII. Subsequently, IrII rapidly transforms into a stable IrH species through protonation, with α-keto acid acting as a proton donor. Upon absorbing additional photon(s), IrH is expected to transform into IrH, involving further hydrogenation/protonation. Emission and Fourier transform infrared (FTIR) spectroscopy, together with global analysis, identify the character of IrH/IrH* and corroborate its contribution to representative radical acylation reactions (decarboxylative 1,4-addition of α-keto acids with Michael acceptors, decarboxylative coupling of α-keto acids with aryl halides, and decarboxylative cyclization of 2-alkenylarylisocyanides with α-keto acids), where IrH/IrH* serves as the key species to trigger the second photoredox cycle. These results elucidate the existence and generality of the tandem photoredox catalysis mechanism for IrIII photocatalytic radical acylation reactions, providing advanced insights into the mechanism of IrIII-based photoredox processes and potentially expanding their application in the design and development of new synthetic methodologies.
利用[Ir(dFCFppy)(dtbbpy)]（IrIII）作为光催化剂，以α-酮酸作为起始底物的光氧化还原催化自由基酰化反应，最近已成为制备酮衍生物的一种有吸引力的策略。虽然人们一致认为详细的机理见解对于最大化所需产物的形成很重要，但目前仍缺乏专注于揭示IrIII光催化自由基酰化反应潜在基本机理的研究。在此，我们使用时间分辨光谱法观察到三线态IrIII的有效猝灭，α-酮酸发生电子转移，从而生成还原态的IrII。随后，IrII通过质子化迅速转化为稳定的IrH物种，α-酮酸作为质子供体。在吸收额外的光子后，IrH有望转化为IrH，涉及进一步的氢化/质子化。发射光谱和傅里叶变换红外（FTIR）光谱以及全局分析确定了IrH/IrH的性质，并证实了其对代表性自由基酰化反应（α-酮酸与迈克尔受体的脱羧1,4-加成、α-酮酸与芳基卤化物的脱羧偶联以及2-烯基芳基异氰化物与α-酮酸的脱羧环化）的贡献，其中IrH/IrH作为触发第二个光氧化还原循环的关键物种。这些结果阐明了IrIII光催化自由基酰化反应串联光氧化还原催化机理的存在及其普遍性，为基于IrIII的光氧化还原过程的机理提供了深入见解，并可能扩大其在新合成方法的设计和开发中的应用。
