Recent advances in difunctionalization of alkenes using pyridinium salts as radical precursors

Wang, Yanan; Bao, Yanyang; Tang, Meifang; Ye, Zhegao; Yuan, Zheliang; Zhu, Gangguo

doi:10.1039/d2cc00369d

Cited by 92 publications

(28 citation statements)

References 65 publications

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“…Pyridinium salts are versatile pyridine surrogates that are significantly more reactive than pyridines, which can undergo various organic transformations under transition metal or photoredox catalysis, and they have emerged as promising radical precursors for the generation of a series of synthetically useful radicals. − Indeed, through reductive single-electron transfer (SET), various carbon/nitrogen/oxygen-centered and trifluoromethyl radicals can be generated from N -functionalized pyridinium salts upon excitation by visible light. Strategies for site-selective functionalization using N -substituted pyridinium salts have also been developed (Figure ).…”

Section: Introductionmentioning

confidence: 99%

N-Functionalized Pyridinium Salts: A New Chapter for Site-Selective Pyridine C–H Functionalization via Radical-Based Processes under Visible Light Irradiation

2022

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Conspectus The radical-mediated C–H functionalization of pyridines has attracted considerable attention as a powerful tool in synthetic chemistry for the direct functionalization of the C–H bonds of the pyridine scaffold. Classically, the synthetic methods for functionalized pyridines often involve radical-mediated Minisci-type reactions under strongly acidic conditions. However, the site-selective functionalization of pyridines in unbiased systems has been a long-standing challenge because the pyridine scaffold contains multiple competing reaction sites (C2 vs C4) to intercept free radicals. Therefore, prefunctionalization of the pyridine is required to avoid issues observed with the formation of a mixture of regioisomers and overalkylated side products. Recently, N-functionalized pyridinium salts have been attracting considerable attention in organic chemistry as promising radical precursors and pyridine surrogates. The notable advantage of N-functionalized pyridinium salts lies in their ability to enhance the reactivity and selectivity for synthetically useful reactions under acid-free conditions. This approach enables exquisite regiocontrol for nonclassical Minisci-type reactions at the C2 and C4 positions under mild reaction conditions, which are suitable for the late-stage functionalization of bioactive molecules with greater complexity and diversity. Over the past five years, a variety of fascinating synthetic applications have been developed using various types of pyridinium salts under visible light conditions. In addition, a new platform for alkene difunctionalization using appropriately designed N-substituted pyridinium salts as bifunctional reagents has been reported, offering an innovative assembly process for complex organic architectures. Intriguingly, strategies involving light-absorbing electron donor–acceptor (EDA) complexes between pyridinium salts and suitable electron-rich donors further open up new reactivity under photocatalyst-free conditions. Furthermore, we developed enantioselective reactions using pyridinium salts to afford enantioenriched molecules bearing pyridines through single-electron N-heterocyclic carbene (NHC) catalysis. Herein, we provide a broad overview of our recent contributions to the development of N-functionalized pyridinium salts and summarize the cornerstones of organic reactions that successfully employ these pyridinium salts under visible light conditions. The major advances in the field are systematically categorized on the basis of the pyridines’ N-substituent, N–X (X = O, N, C, and SO2CF3), and its reactivity patterns. Furthermore, the identification of new activation modes and their mechanistic aspects are discussed by providing representative contributions to each paradigm. We hope that this Account will inspire broad interest in the continued innovation of N-functionalized pyridinium salts in the exploration of new transformations.

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Section: Introductionmentioning

confidence: 99%

N-Functionalized Pyridinium Salts: A New Chapter for Site-Selective Pyridine C–H Functionalization via Radical-Based Processes under Visible Light Irradiation

2022

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“…5 Recently, radical difluoromethylation of alkenes has been developed into a practical approach for introducing a difluoromethyl group with high chemo- and regioselectivity. 6…”

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confidence: 99%

Photocatalytic difluoromethylarylation of unactivated alkenes via a (hetero)aryl neophyl-like radical migration

et al. 2022

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“…With a resurgence of interest in the field of synthetic radical chemistry, 6 radical-mediated transformation of 1,3-dienes has developed into a powerful tool for the construction of various diversely substituted allylic compounds. 7 Therefore, we hypothesized that the exploration of suitable radical precursors and catalytic activation methods might offer more oppor- tunities to realize three-component 1,2-aminooxygenation reactions of 1,3-dienes, in spite of the only report developed by Wang. 5 The readily available N-substituted pyridinium salts have recently found wide applications in radical synthetic chemistry owing to their inherent redox property.…”

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confidence: 99%

Selective Three-Component 1,2-Aminoalkoxylation of 1-Aryl-1,3-dienes by Dual Photoredox and Copper Catalysis

Jiang

Rao

et al. 2022

Org. Lett.

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A three-component 1,2-aminooxygenation reaction of 1,3-dienes by dual photoredox and copper catalysis is described. This protocol uses N-aminopyridinium salts as N-centered radical precursors and nucleophilic alcohols as oxygen sources, providing modular and practical access to 1,2-aminoalkoxylation products with good yields and regioselectivity. Preliminary mechanistic studies support the radical property of the reaction and the involvement of N-centered radical intermediates.

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Recent advances in difunctionalization of alkenes using pyridinium salts as radical precursors

Abstract: In this review, we summarise the recent applications of pyridinium salts in radical-mediated difunctionalization of alkenes. Pyridinium salts are a privileged class of compounds that show great utility in natural...

Cited by 92 publications

References 65 publications

N-Functionalized Pyridinium Salts: A New Chapter for Site-Selective Pyridine C–H Functionalization via Radical-Based Processes under Visible Light Irradiation

N-Functionalized Pyridinium Salts: A New Chapter for Site-Selective Pyridine C–H Functionalization via Radical-Based Processes under Visible Light Irradiation

Photocatalytic difluoromethylarylation of unactivated alkenes via a (hetero)aryl neophyl-like radical migration

Selective Three-Component 1,2-Aminoalkoxylation of 1-Aryl-1,3-dienes by Dual Photoredox and Copper Catalysis

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