N-heterocyclic carbene-catalyzed radical reactions
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heterocyclic carbene-catalyzed radical reactions 1†
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Kun-Quan Chen , He Sheng , Qiang Liu , Pan-Lin Shao & Xiang-Yu Chen 1
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School of Chemical Sciences, University of the Chinese Academy of Sciences, Beijing 100049, China; College of Innovation and Entrepreneurship, Southern University of Science and Technology, Shenzhen 518055, China Received July 9, 2020; accepted August 14, 2020; published online September 29, 2020
While N-heterocyclic carbene (NHC) catalyzed electron-pair-transfer processes have been developed into an important tool for synthetically important bond formations during the past decades, the corresponding radical reactions via NHC catalysis have only received growing attention in the past six years. Taking into account the advantages NHC-catalyzed radical reactions might bring, such as creating new activation modes that were previously unobtainable, it is worthwhile to provide a conceptual understanding of this emerging area. Therefore, herein we give an overview of NHC-catalyzed radical reactions via different synthetic techniques. N-heterocyclic carbene, radical, one-electron oxidant, photochemistry, electrochemistry Citation:
Chen KQ, Sheng H, Liu Q, Shao PL, Chen XY. N-heterocyclic carbene-catalyzed radical reactions. Sci China Chem, 2020, 63, https://doi.org/10.1007/ s11426-020-9851-8
1 Introduction Nature has always been regarded as a source of inspiration for synthesis chemists to develop efficient catalysts by mimicking nature’s enzyme machinery. Inspired by nature, the last two decades have witnessed the breathtaking speed development of organocatalysis in organic synthesis [1], and quite a number of new transformations have been developed by using several types of small organic molecules, such as amines, thioureas, ketones, and N-heterocyclic carbenes (NHCs) [2]. Among them, NHCs, which can inverse the inherent reactivity of aldehydes, have become one of the most powerful synthetic tools in organic synthesis. A wide range of reactions associated with NHC catalysis via electron-pair-transfer processes have been developed (Scheme 1 (a, b)) [3]. The early applications of NHC catalysis were the benzoin and Stetter reactions via Breslow intermediates [4]. In 2004, the research groups of Glorius and Bode [5] independently extended this concept to homoenolate equiva†These authors contributed equally to this work. *Corresponding authors (email: [email protected]; [email protected])
lents. The ability of NHCs for the generation of azolium enolates was demonstrated by the research groups of Rovis, Bode, Ye and Smith [6]. The [4+2] cycloadditions via azolium dienolates were independently reported by Ye et al. and Chi et al. [7]. The research groups of Castells, Zeitler, Scheidt, Lupton and Studer [31t,3,8] made important contributions to the development of (α,β-unsaturated) acyl azolium intermediates. Despite its tremendous success, nowadays the development of NHC encounters a ‘‘bottleneck’’ possibly as a result of the limitation of electron-pairtransfer processes; the
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