Highly reductive photocatalytic systems in organic synthesis

Liao, Li‐Li; Song, Lei; Yan, Si‐Shun; Ye, Jian‐Heng; Yu, Da‐Gang

doi:10.1016/j.trechm.2022.03.008

Cited by 42 publications

(26 citation statements)

References 83 publications

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“…However, no systematic review has been reported on the preparation of aryl radicals by visible light-induced reduction of aryl halides. Due to the extreme redox potentials of aryl halides, their range of applications is limited by the reduction ability of excited photosensitizers and the visible light energy [ 18 , 19 , 20 ]. The general process of visible-light-driven reduction of aryl halides to aryl radicals initiates by visible light-induced electron transfer from excited photocatalyst to aryl halide to generate an aryl halide radical anion, closely followed by the halide negative ion-mediated cleavage of carbon–halide bond, leaving halide ions (X – ) to generate a highly reactive aryl radical intermediate that can be used to construct various classes of aryl compounds ( Figure 1 ).…”

Section: Introductionmentioning

confidence: 99%

Visible-Light Photocatalytic Reduction of Aryl Halides as a Source of Aryl Radicals

et al. 2022

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Aryl- and heteroaryl units are present in a wide variety of natural products, pharmaceuticals, and functional materials. The method for reduction of aryl halides with ubiquitous distribution is highly sought after for late-stage construction of various aromatic compounds. The visible-light-driven reduction of aryl halides to aryl radicals by electron transfer provides an efficient, simple, and environmentally friendly method for the construction of aromatic compounds. This review summarizes the recent progress in the generation of aryl radicals by visible-light-driven reduction of aryl halides with metal complexes, organic compounds, semiconductors as catalysts, and alkali-assisted reaction system. The ability and mechanism of reduction of aromatic halides in various visible light induced systems are summarized, intending to illustrate a comprehensive introduction of this research topic to the readers.

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Section: Introductionmentioning

confidence: 99%

Visible-Light Photocatalytic Reduction of Aryl Halides as a Source of Aryl Radicals

et al. 2022

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“…In contrast, the development of strongly reducing OPCs and their application is less advanced than that of strongly oxidizing ones. ,,− Highly reducing photocatalytic systems have great potential to pave new avenues for organic syntheses based on dehalogenation, − carbanion formation from organic radicals, − and the reduction of highly oxidized stable compounds (e.g., carboxylic acids/esters, − amides, − and phosphine oxides), among other mechanisms. Meanwhile, our research group has recently developed several precious-metal complexes for the reductive transformation of various carboxylic acid derivatives − and CO 2 under thermal conditions or visible light irradiation. − Complexes of earth-abundant Fe(II) were also used as effective CO 2 reduction sites, which were difficult to be realized without photosensitization using fac -[Ir(ppy) 3 ] .…”

Section: Introductionmentioning

confidence: 99%

Arylamines as More Strongly Reducing Organic Photoredox Catalysts than fac-[Ir(ppy)₃]

Noto

Saitō

2022

ACS Catal.

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Organic photoredox catalysts (OPCs) have the potential to replace precious-metal-based photoredox catalysts (PMPCs). Compared with strongly oxidizing OPCs, such as the representative acridinium salts, however, the recent development of strongly reducing OPCs has been relatively sluggish. In this Perspective, strongly reducing OPCs bearing arylamine motifs are introduced. One of the advantages of OPCs is their versatility in catalyst design, which makes it easier to develop catalysts with a reducing capability superior to that of fac-[Ir(ppy)3], which is the strongest reductant among the commonly used PMPCs. Easy access to structural diversity also contributes to the rapid development of appropriate catalysts for various applications, for instance, not only simple organo-radical reactions but also precise control of polymer synthesis and properties through photocatalytic (organocatalyzed) atom-transfer radical polymerization. While light with a shorter wavelength (higher energy), such as near-ultraviolet light, is typically involved in conferring OPCs with their strongly reducing natures, strategies to develop strongly reducing catalytic systems using a longer wavelength (lower energy) of visible light, including consecutive photoinduced electron transfer, are emerging as a defacto standard. These strategies for the design of OPC systems, which allow them to achieve otherwise inaccessible reactions using visible light, are also described.

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“…9 In both processes, the open-shell doublet organic radical PC • (anionic or neutral) is generated in-situ from a closed-shell singlet species (neutral or cationic), followed by photoexcitation generating the radical excited state PC • * that can act as a super photoreducing agent (E1/2 red * = -2.3 to -3.4 V vs SCE). 10 The concept of a two-photon excitation process, commonly purported as a consecutive photoelectron transfer (conPET) pathway, has been reported with numerous notable photocatalysts such as PDI, DCA, anthraquinone, Rhodamine 6G, benzo[ghi]perylene (BPI), 4-DPAIPN, 3-CzEPAIPN, Mes-Acr, and Deazaflavin. [11][12][13][14][15][16][17][18][19] As a common benchmark reaction, photoredox C(sp 2 )-X bond activation in aryl bromides and chlorides, Birch reduction, and sulfonamide cleavage has showcased the extreme photoreducing ability of radical photocatalysts.…”

Section: Introductionmentioning

confidence: 99%

Solvated Electrons! The Missing Link in Highly Reducing Photocatalysis

Shaikh

Hossain

Moutet

et al. 2022

Preprint

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In recent years, in-situ generated organic radicals have been used as highly potent photoinduced electron transfer (PET) agents resulting in catalytic systems as reducing as alkaline metals that can activate remarkably stable bonds. However, the transient nature of these doublet state open-shell species has led to debatable mechanistic studies, hindering adoption and development. Herein, we document the use of an isolated and stable neutral organic radical as a highly photoreducing species with a reduction potential lower than – 3.5 V vs SCE. The isolated radical offers a unique platform to investigate the mechanism behind the photocatalytic activity of organic radicals. Our mechanistic study supports the involvement of solvated electrons formed by single electron transfer (SET) between the short-lived excited state organic radical and the solvent.

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Highly reductive photocatalytic systems in organic synthesis

Cited by 42 publications

References 83 publications

Visible-Light Photocatalytic Reduction of Aryl Halides as a Source of Aryl Radicals

Visible-Light Photocatalytic Reduction of Aryl Halides as a Source of Aryl Radicals

Arylamines as More Strongly Reducing Organic Photoredox Catalysts than fac-[Ir(ppy)₃]

Solvated Electrons! The Missing Link in Highly Reducing Photocatalysis

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Highly reductive photocatalytic systems in organic synthesis

Cited by 42 publications

References 83 publications

Visible-Light Photocatalytic Reduction of Aryl Halides as a Source of Aryl Radicals

Visible-Light Photocatalytic Reduction of Aryl Halides as a Source of Aryl Radicals

Arylamines as More Strongly Reducing Organic Photoredox Catalysts than fac-[Ir(ppy)3]

Solvated Electrons! The Missing Link in Highly Reducing Photocatalysis

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Arylamines as More Strongly Reducing Organic Photoredox Catalysts than fac-[Ir(ppy)₃]