Mesoionic carbene-Breslow intermediates as super electron donors: Application to the metal-free arylacylation of alkenes

Liu, Wei; Vianna, Adam; Zhang, Zengyu; Huang, Shiqing; Huang, Liang; Melaïmi, Mohand; Bertrand, Guy; Yan, Xiaoyu

doi:10.1016/j.checat.2021.03.004

Cited by 83 publications

(58 citation statements)

References 51 publications

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“…In summary, this achievement expanded the scope of radical NHC catalysis, enabling the generation and utilization of aryl radicals 42 . The enolate form of the Breslow intermediate derived from an aldehyde and a thiazolium-type NHC in the presence of a base undergoes SET to the aryl iodide, producing an aryl radical.…”

Section: Entrymentioning

confidence: 90%

Aryl radical-mediated N-heterocyclic carbene catalysis

et al. 2021

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There have been significant advancements in radical reactions using organocatalysts in modern organic synthesis. Recently, NHC-catalyzed radical reactions initiated by single electron transfer processes have been actively studied. However, the reported examples have been limited to catalysis mediated by alkyl radicals. In this article, the NHC organocatalysis mediated by aryl radicals has been achieved. The enolate form of the Breslow intermediate derived from an aldehyde and thiazolium-type NHC in the presence of a base undergoes single electron transfer to an aryl iodide, providing an aryl radical. The catalytically generated aryl radical could be exploited as an arylating reagent for radical relay-type arylacylation of styrenes and as a hydrogen atom abstraction reagent for α-amino C(sp3)–H acylation of secondary amides.

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Section: Entrymentioning

confidence: 90%

Aryl radical-mediated N-heterocyclic carbene catalysis

et al. 2021

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“…Multi-component radical relay reactions provide a powerful tool for the synthesis of complex skeletons from simple and readily accessible starting materials 80 – 82 . While significant progress has been achieved in NHC and photocatalyst co-catalyzed ketone synthesis 57 – 64 , 83 , three-component radical relay coupling that allows various carbon radicals involved is still limited 72 .…”

Section: Resultsmentioning

confidence: 99%

Carbene and photocatalyst-catalyzed decarboxylative radical coupling of carboxylic acids and acyl imidazoles to form ketones

et al. 2022

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The carbene and photocatalyst co-catalyzed radical coupling of acyl electrophile and a radical precursor is emerging as attractive method for ketone synthesis. However, previous reports mainly limited to prefunctionalized radical precursors and two-component coupling. Herein, an N-heterocyclic carbene and photocatalyst catalyzed decarboxylative radical coupling of carboxylic acids and acyl imidazoles is disclosed, in which the carboxylic acids are directly used as radical precursors. The acyl imidazoles could also be generated in situ by reaction of a carboxylic acid with CDI thus furnishing a formally decarboxylative coupling of two carboxylic acids. In addition, the reaction is successfully extended to three-component coupling by using alkene as a third coupling partner via a radical relay process. The mild conditions, operational simplicity, and use of carboxylic acids as the reacting partners make our method a powerful strategy for construction of complex ketones from readily available starting materials, and late-stage modification of natural products and medicines.

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“…It was recently claimed that corresponding Breslow-type enols, such as 8a•H, were super electron donors and were able to reduce iodoaryls. [18] However, reported cyclic voltammetry experiments were inconsistent. Surprisingly, voltammograms of the proposed reducing agent 8a•H featured no oxidation wave.…”

Section: Resultsmentioning

confidence: 99%

“…Finally, we reinvestigated derivatives of mesoionic carbenes C MIC . It was recently claimed that corresponding Breslow‐type enols, such as 8 a⋅H , were super electron donors and were able to reduce iodoaryls [18] . However, reported cyclic voltammetry experiments were inconsistent.…”

Section: Resultsmentioning

confidence: 99%

“…For instance, the substitution of aldehydes to afford ketones under thiazolylidene ( C NS ) catalysis was achieved with N ‐(acyloxy)phthalimides [5] or oxime ethers, [6] which are known sources of alkyl radicals upon reduction below −1.4 V [15] and −1.6 V, [16] respectively. Even more, the groups of Ohmiya [17] and Bertrand with Yan [18] independently reported the activation of challenging iodoaryls in presence of styrenes and aldehydes, with thiazolylidenes ( C NS ) or mesoionic carbenes ( C MIC ), respectively. For C MIC catalysts, the authors proposed that the mesoionic structure results in an electron‐rich enaminol A⋅H with an oxidation potential as low as −1.93 V. Yet, this unusual value was supported by ambiguous electrochemical data (see further below).…”

Section: Introductionmentioning

confidence: 99%

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Critical Assessment of the Reducing Ability of Breslow‐type Derivatives and Implications for Carbene‐Catalyzed Radical Reactions**

et al. 2021

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We report the synthesis of acyl azolium salts stemming from thiazolylidenes C NS , triazolylidenes C TN, mesoionic carbenes C MIC and the generation of their corresponding radicals and enolates, covering about 60 Breslow-type derivatives. This study highlights the role of additives in the redox behavior of these compounds and unveils several critical misconceptions about radical transformations of aldehyde derivatives under N-heterocyclic carbene catalysis. In particular, the reducing ability of enolates has been dramatically underestimated in the case of biomimetic C NS . In contrast with previous electrochemical studies, we show that these catalytic intermediates can transfer electrons to iodobenzene within minutes at room temperature. Enols derived from C MIC are not the previously claimed super electron donors, although enolate derivatives of C NS and C MIC are powerful reducing agents.

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Mesoionic carbene-Breslow intermediates as super electron donors: Application to the metal-free arylacylation of alkenes

Cited by 83 publications

References 51 publications

Aryl radical-mediated N-heterocyclic carbene catalysis

Aryl radical-mediated N-heterocyclic carbene catalysis

Carbene and photocatalyst-catalyzed decarboxylative radical coupling of carboxylic acids and acyl imidazoles to form ketones

Critical Assessment of the Reducing Ability of Breslow‐type Derivatives and Implications for Carbene‐Catalyzed Radical Reactions**

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