The field of radical carbene catalysis has grown exponentially in recent years (2019–2021) and shows no signs of slowing down in the near future. Single-electron carbene species have seen increased utility in synthesis due to their unique redox properties. N-heterocyclic carbene (NHC) structure has a significant impact on the redox properties of the corresponding Breslow intermediate or acyl azolium and thus influences the reactivity of the resulting radical species. Oxidatively generated radical carbene species have been used for the remote functionalization of unsaturated aldehydes as well as the direct functionalization of aldehydes in the construction of substituted ketones. Reductively generated radical carbene species remain underexplored in comparison to their oxidatively generated counterparts. Inspired by the role of N-heterocyclic carbenes (NHCs) in natural enzymatic processes, chemists have harnessed the umpolung (polarity reversal) reactivity of these Lewis basic species over the past few decades to construct key chemical bonds. While NHCs continue to play a role in two-electron transformations, their unique redox properties enable a variety of useful, stabilized radical species to be accessed via single-electron oxidation or reduction. As a result, their utility in synthesis has grown rapidly concurrent with the revival of radical chemistry, highlighted by their extensive use as reactive single-electron species in recent years. Inspired by the role of N-heterocyclic carbenes (NHCs) in natural enzymatic processes, chemists have harnessed the umpolung (polarity reversal) reactivity of these Lewis basic species over the past few decades to construct key chemical bonds. While NHCs continue to play a role in two-electron transformations, their unique redox properties enable a variety of useful, stabilized radical species to be accessed via single-electron oxidation or reduction. As a result, their utility in synthesis has grown rapidly concurrent with the revival of radical chemistry, highlighted by their extensive use as reactive single-electron species in recent years. a bioactive molecule used to deliver an acetyl group to the citric acid cycle (i.e., Krebs cycle) for the production of energy. Hydrolysis of the key thioester oxidizes the acetyl group to carbon dioxide and water, releasing energy. a computational method based on quantum mechanics that is used to study electronic structure. a technique used to detect unpaired electrons and provide structural information. the concerted translocation of a hydrogen atom (one electron and one proton) from a donor molecule to an acceptor molecule via homolytic cleavage of a C–H bond. heterocyclic species that contain at least one nitrogen atom and feature a stabilized carbene carbon center. proteolysis targeting chimeras are small molecules that consist of a ‘head’ domain to selectively bind target proteins and a ‘tail’ domain used to recruit a cellular enzyme. These two domains are connected via a linker. reference electrodes derived from the reaction between HgCl and Hg0 and are used to standardize electrochemical measurements. the transfer of one electron from one species to another species to produce a radical intermediate. a high energy light (315–400 nm) used in various photoexcitation strategies. 

Single-electron Carbene Catalysis in Redox Processes.

We report the synthesis of acyl azolium salts stemming from thiazolylidenes CNS , triazolylidenes CTN, mesoionic carbenes CMIC 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 CNS . 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 CMIC are not the previously claimed super electron donors, although enolate derivatives of CNS and CMIC are powerful reducing agents.

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

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. 

/pdf/carbene-and-photocatalyst-catalyzed-decarboxylative-radical-3k7x27ul.pdf

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

The Suzuki–Miyaura cross-coupling of C(sp3)-hybridised boronic compounds still remains a challenging task, thereby hindering the broad application of alkyl boron substrates in carbon–carbon bond-forming reactions. Herein, we developed an NHC/photoredox dual catalytic cross-coupling of alkyl trifluoroborates with acid fluorides, providing an alternative solution to the classical acylative Suzuki coupling chemistry. With this protocol, various ketones could be rapidly synthesised from readily available materials under mild conditions. Preliminary mechanistic studies shed light on the unique radical reaction mechanism.

Suzuki-type cross-coupling of alkyl trifluoroborates with acid fluoride enabled by NHC/photoredox dual catalysis

Three-component carboacylation of simple alkenes with readily available reagents is challenging. Transition metal-catalysed intermolecular carboacylation works for alkenes with strained ring or directing groups. Herein, we develop a photoredox cooperative N-heterocyclic carbene/Pd-catalysed alkylacylation of simple alkenes with aldehydes and unactivated alkyl halides to provide ketones in good yields. This multicomponent coupling reaction features a wide scope of alkenes, broad functional group compatibility and free of exogenous photosensitizer or external reductant. In addition, a series of chlorinated cyclopropanes with one or two vicinal quaternary carbons is obtained when chloroform or carbon tetrachloride is used as the alkyl halide. The reaction involves the alkyl radicals from halides and the ketyl radicals from aldehydes under photoredox cooperative N-heterocyclic carbene/Pd catalysis. 

/pdf/photoredox-cooperative-n-heterocyclic-carbene-palladium-2kr11w06.pdf

Photoredox cooperative N-heterocyclic carbene/palladium-catalysed alkylacylation of alkenes

N-Heterocyclic Carbene-Catalyzed Four-Component Reaction: Chemoselective C radical -C radical Relay Coupling Involving the Homoenolate Intermediate

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N-Heterocyclic Carbene-Catalyzed Four-Component Reaction: Chemoselective C radical -C radical Relay Coupling Involving the Homoenolate Intermediate