N-Heterocyclic Carbene/Magnesium Cocatalyzed Radical Relay Assembly of Aliphatic Keto Nitriles

Switching mesoionic carbene-organocatalysis from radical to ionic pathway through base-controlled formation of Breslow intermediates versus Breslow enolates

N-heterocyclic carbene (NHC) organocatalysis has experienced significant advancements. Two distinct reaction pathways have been developed, ionic and radical, through Breslow intermediates (BIs) and Breslow enolates (BI-s), respectively. The ability to selectively generate these intermediates is crucial for optimizing reaction outcomes. In this paper we show that with mesoionic carbenes (MICs) it is possible to control the formation of BIs versus BI-s, through the use of weak bases and strong bases, respectively. Of particular interest is the coupling of aldehydes and alkyl halides to yield ketones via an ionic pathway.

Enantioselective Cascade Annulation of 1,2,3-Triazoles and Enals Enabled by Sequential Rhodium and Oxidative NHC Catalysis Involving Cleavage, Migration, and Cyclization

The in situ-generated pyrrolin-3-ones serve as novel and versatile synthons, being employed as intermediates for the efficient production of pyrrole-fused lactones with high yield and excellent enantioselectivity. Herein, we introduce emerging rhodium and oxidative N-heterocyclic carbene relay catalysis that enables a highly enantioselective cascade annulation between easily available 1,2,3-triazoles and enals. In this proof-of-concept study, the in situ-generated pyrrolin-3-ones engage α,β-unsaturated acylazolium intermediates generated from enals via oxidative N-heterocyclic carbene catalysis.

Visible-light-driven EDA complex-promoted cascade cyclization to construct 4-cyanoalkyl isoquinoline-1,3-diones

Visible-light-induced EDA complex-promoted ring-opening of cycloketone oxime esters to synthesise various cyanoalkylated products with N-methacryloyl benzamides was developed. Various radical receptors were compatible with the current reaction system to furnish diverse heterocyclic compounds. Mechanistic analysis shows that the formation of an EDA complex was crucial to the photocatalytic strategy. Importantly, 4-cyanoalkyl isoquinoline-1,3-diones were obtained in high yields by using a catalytic amount of 1,4-diazabicyclo[2.2.2]octane (DABCO) through prolonging the reaction time, which provided a practical approach to give a variety of isoquinoline-1,3-dione derivatives.

Interrupted N-Heterocyclic Carbene-Catalyzed Radical Coupling Strategy: A Versatile Platform for Alkylation and Arylation of [60]Fullerene

An interrupted N-heterocyclic carbene-catalyzed radical coupling strategy is disclosed for efficient alkylation and arylation of [60]fullerene. This novel and general strategy bridges the gap between organocatalytic radical cross-coupling and functionalization of fullerenes. Readily available feedstocks, remarkably broad substrate scope and functional group compatibility, and convenient late-stage nanomodification of complex molecules make this strategy with incomparable diversity and practicality in the synthesis of monoalkylated and -arylated fullerenes.

Photoredox-Catalyzed Three-Component 1,2-Cyanoalkylpyridylation of Styrenes with Nonredox-Active Cyclic Oximes

Three-component alkene 1,2-difunctionalizations have emerged as a powerful strategy for rapid buildup of diverse and complex alkylpyridines, but the distal functionalized alkyl radicals for the alkene 1,2-alkylpyridylations were still rare. Herein, we report an example of regioselective three-component 1,2-cyanoalkylpyridylation of feedstock styrenes with accessible nonredox-active cyclic oximes through visible-light photoredox catalysis, providing a series of structurally diverse β-cyanoalkylated alkylpyridines. This protocol proceeds through a radical relay pathway including the generation of iminyl radicals enabled by phosphoranyl radical-mediated β-scission, radical transposition through C-C bond cleavage, highly selective radical addition, and precise radical-radical cross-coupling sequence, thus facilitating the regioselective formation of two distinct C-C single bonds in a single-pot operation. This synthetic strategy features mild conditions, broad compatibility of functional groups and substrate scope, diverse product derivatization, and late-stage modification.

NHC-catalyzed radical acylation of cycloalkyl silyl peroxides to access 1,6-,1,7-, and 1,8-diketones

An unprecedented N-heterocyclic carbene (NHC)-catalyzed radical acylation of cycloalkyl silyl peroxides was developed using readily available aldehydes as the acylating agents. This protocol provides an exceptionally useful method for the efficient and rapid synthesis of long-chain 1,6-/1,7-/1,8-diketones, especially unsymmetrical ones. This strategy also has the advantages of mild conditions, good functional group compatibility, and potential applications in the late-stage functionalization of aldehydes with bioactive fragments and in the construction of long-chain complex bioactive molecules.

Ketones from aldehydes via alkyl C(sp3)-H functionalization under photoredox cooperative NHC/palladium catalysis

Direct synthesis of ketones from aldehydes features high atom- and step-economy. Yet, the coupling of aldehydes with unactivated alkyl C(sp³)-H remains challenging. Herein, we develop the synthesis of ketones from aldehydes via alkyl C(sp³)-H functionalization under photoredox cooperative NHC/Pd catalysis. The two-component reaction of iodomethylsilyl alkyl ether with aldehydes gave a variety of β-, γ- and δ-silyloxylketones via 1,n-HAT (n = 5, 6, 7) of silylmethyl radicals to generate secondary or tertiary alkyl radicals and following coupling with ketyl radicals from aldehydes under photoredox NHC catalysis. The three-component reaction with the addition of styrenes gave the corresponding ε-hydroxylketones via the generation of benzylic radicals by the addition of alkyl radicals to styrenes and following coupling with ketyl radicals. This work demonstrates the generation of ketyl radical and alkyl radical under the photoredox cooperative NHC/Pd catalysis, and provides two and three component reactions for the synthesis of ketones from aldehydes with alkyl C(sp³)-H functionalization. The synthetic potential of this protocol was also further illustrated by the late-stage functionalization of natural products.

On the Redox Properties of the Dimers of Thiazol-2-ylidenes That Are Relevant for Radical Catalysis

We report the isolation and study of dimers stemming from popular thiazol-2-ylidene organocatalysts. The model featuring 2,6-di(isopropyl)phenyl (Dipp) N-substituents was found to be a stronger reducing agent (E_ox = -0.8 V vs SCE) than bis(thiazol-2-ylidenes) previously studied in the literature. In addition, a remarkable potential gap between the first and second oxidation of the dimer also allows for the isolation of the corresponding air-persistent radical cation. The latter is an unexpected efficient promoter of the radical transformation of α-bromoamides into oxindoles.

Synthesis of Thioesters from Aldehydes via N-Heterocyclic Carbene (NHC) Catalyzed Radical Relay

We have developed an efficient N-heterocyclic carbene (NHC)-catalyzed thioesterification of aldehydes using N-thiosuccinimides as the thiolation reagent. This organocatalyzed transition involves the generation of sulfur radicals by single electron transfer of the Breslow enolate (generated from aldehyde and NHC catalyst) with N-thiosuccinimides. This method offers facile access to various highly functionalized thioesters and exhibits good chemical yields and functional group tolerance.

Recent Advances in Molecule Synthesis Involving C-C Bond Cleavage of Ketoxime Esters

The synthetic strategies of oxime derivatives participating in radical-type reactions have been rapidly developed in the last few decades. Among them, the N–O bond cleavage of oxime esters leading to formation of nitrogen-centered radicals triggers adjacent C–C bond cleavage to produce carbon-centered free radicals, which has been virtually used in organic synthesis in recent years. Herein, we summarized the radical reactions involving oxime N–O bond and C–C bond cleavage through this special reaction form, including those from acyl oxime ester derivatives and cyclic ketoxime ester derivatives. These contents were systematically classified according to different reaction types. In this review, the free radical reactions involving acyl oxime esters and cyclic ketoxime esters after 2021 were included, with emphasis on the substrate scope and reaction mechanism.

Iminoacylation of Alkenes via Photoredox N-Heterocyclic Carbene Catalysis

The iminoacylation of alkenes via photoredox N-heterocyclic carbene catalysis is developed with the employment of alkene-tethered α-imino-oxy acids and acyl imidazoles. The corresponding substituted 3,4-dihydro-2H-pyrroles were afforded in moderate to good yields with good to high diastereoselectivities in most cases. The reaction involves the 5-exo-trig radical cyclization of an alkene-tethered iminyl radical and the following coupling with a ketyl radical from acyl imidazole under NHC catalysis.

Visible light-mediated NHC and photoredox co-catalyzed 1,2-sulfonylacylation of allenes via acyl and allyl radical cross-coupling

Visible light-mediated NHC and photoredox co-catalyzed radical 1,2-sulfonylacylation of allenes via cross-coupling between an allyl radical and an NHC-stabilized acyl radical.

CF2Br2 as a Source for Difluoroolefination of 1,3-Enynes via N-Heterocyclic Carbene Catalysis

Commercially available CF₂Br₂ has been used as a convenient source for the rapid and reliable incorporation of the gem-difluorovinyl motif into an allene framework via an N-heterocyclic carbene catalyzed difluoroolefination of 1,3-enynes. The reaction proceeds through a cascade three-component radical relay/elimination process. This protocol is distinguished by its mild conditions, readily accessible starting materials, wide substrate scope, and ease of late-stage functionalization, thus unlocking an untraditional strategy to construct a new class of functionalized gem-difluorovinyl allenes.

Synthesis of β-Thiolated-α-arylated Ketones Enabled by Photoredox and N-Heterocyclic Carbene-Catalyzed Radical Relay of Alkenes with Disulfides and Aldehydes

β-Thiolated-α-arylated ketones are perversive in bioactive molecules and serve as potential bidentate ligands for catalysis. Herein, a straightforward protocol to access β-thiolated ketones from aldehydes, alkenes, and disulfides enabled by the combination of photocatalysis and N-heterocyclic carbene catalysis is reported. The sequential radical addition to alkenes and subsequent radical-radical coupling cascade process simultaneously forge C-S and C-C bonds. The mild conditions allow for radical relay coupling with a broad functional group tolerance.

Photoinduced radical cascade cyclization of acetylenic acid esters with oxime esters to access cyanalkylated coumarins

A photoinduced radical cascade cyclization of acetylenic acid esters with oxime esters is described, providing cyanalkylated coumarins in superior yields under mild conditions. Radical capture and luminescence quenching experiments showed that this transformation was accomplished via a radical addition/5-exo spirocyclization/1,2-ester migration process.

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

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.

The combination of carbene- and photocatalysis has enabled unorthodox routes to ketone syntheses, but usually requires engineered or activated substrates. Herein the authors present a carbene- and photocatalytic decarboxylative radical coupling of carboxylic acids and acyl imidazoles, in which the carboxylic acids are directly used as radical precursors.

Enantioselective Cascade Annulation of α-Amino-ynones and Enals Enabled by Gold and Oxidative NHC Relay Catalysis

We report herein an unprecedented gold and oxidative NHC relay catalysis that enables highly enantioselective cascade annulation between readily available α-amino-ynones with enals. This method utilizes the in situ-generated pyrrolin-4-ones as a novel and versatile synthon, which engage with α,β-unsaturated acylazolium intermediates generated from enals by oxidative NHC catalysis to produce pyrrole-fused lactones in high yield and excellent enantioselectivity. Synthetic utility of the lactone products is also demonstrated by facile conversion to densely functionalized pyrroles and pyrrolin-4-ones in high yields with excellent stereopurity.

Single-electron carbene catalysis in redox processes

Inspired by the role of N-heterocyclic carbenes (NHCs) in natural enzymatic processes, chemists have harnessed the umpolung (polarity reversal) reactivity of these reactive, 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.

Combined Photoredox and Carbene Catalysis for the Synthesis of γ-Aryloxy Ketones

N-heterocyclic carbenes (NHCs) have emerged as catalysts for the construction of C-C bonds in the synthesis of substituted ketones under single-electron processes. Despite these recent reports, there still remains a need to increase the utility and practicality of these reactions by exploring new radical coupling partners. Herein, we report the synthesis of γ-aryloxyketones via combined NHC/photoredox catalysis. In this reaction, an α-aryloxymethyl radical is generated via oxidation of an aryloxymethyl potassium trifluoroborate salt, which is then added into styrene derivatives to provide a stabilized benzylic radical. Subsequent radical-radical coupling reaction with an azolium radical affords the γ-aryloxy ketone products.

Aerobic Oxidative EDA Catalysis: Synthesis of Tetrahydroquinolines Using an Organocatalytic EDA Active Acceptor

A catalytic electron donor-acceptor (EDA) complex for the visible-light-driven annulation reaction between activated alkenes and N,N-substituted dialkyl anilines is reported. The key photoactive complex is formed in situ between dialkylated anilines as donors and 1,2-dibenzoylethylene as a catalytic acceptor. The catalytic acceptor is regenerated by aerobic oxidation. Investigations into the mechanism are provided, revealing a rare example of a catalytic acceptor in photoactive EDA complexes that can give access to selective functionalization of aromatic amines under mild photochemical conditions.

N-Heterocyclic-Carbene-Catalyzed C-H Acylation via Radical Relay

A method of N-fluorocarboxamide-directed N-heterocyclic-carbene (NHC)-catalyzed benzylic C-H acylation with aldehydes via the hydrogen atom transfer strategy is disclosed. This transformation involves a sequence of single-electron transfer, 1,5-hydrogen atom transfer, and radical cross-coupling steps. This method offers facile access to various highly functionalized ketones and exhibits good chemical yields and functional group tolerance.

Exploring the regioselectivity of the cyanoalkylation of 3-aza-1,5-dienes: photoinduced synthesis of 3-cyanoalkyl-4-pyrrolin-2-ones

Regioselective cyanoalkylalkenylation of 3-aza-1,5-dienes with oxime esters induced by visible light.

Three-component carboacylation of alkenes via cooperative nickelaphotoredox catalysis

Various commercially available acyl chlorides, aldehydes, and alkanes were exploited for versatile three-component 1,2-carboacylations of alkenes to forge two vicinal C–C bonds through the cooperative action of nickel and sodium decatungstate catalysis. A wealth of ketones with high levels of structural complexity was rapidly obtained via direct functionalization of C(sp²)/C(sp³)–H bonds in a modular manner. Furthermore, a regioselective late-stage modification of natural products showcased the practical utility of the strategy, generally featuring high resource economy and ample substrate scope.

Various commercially available acyl chlorides, aldehydes, and alkanes were exploited for versatile three-component 1,2-carboacylations of alkenes to forge two vicinal C–C bonds through the cooperative action of nickel and sodium decatungstate catalysis.

Three-component 1,2-dicarbofunctionalization of alkenes involving alkyl radicals

1,2-Dicarbofunctionalization of alkenes represents an appealing strategy for chemical bond formation in organic synthesis, which could enable the rapid construction of molecular complexity from simple and readily available starting materials by incorporating two functional groups onto a carbon-carbon double bond in one step. In this field, the dicarbofunctionalization of alkenes with different alkyl radicals in a controlled manner represents an elegant and versatile strategy to access structurally diverse functionalized alkanes, which have witnessed significant progress over the last five years. Due to the importance of alkyl radicals in organic synthesis and medicinal chemistry, this review provides a comprehensive perspective on the development of alkyl radical precursors including electrophilic precursors such as alkyl halides, alkyl peroxides, alkyl NHP esters, cycloketone oxime esters, and Katritzky pyridinium salts, and nucleophilic precursors such as alkyl acids, alkyl oxalates, alkylborates, alkylsilicates, and unactivated hydrocarbons, which generate alkyl radicals by photocatalysis or transition metal catalysis to engage in dicarbofunctionalization under oxidative reaction conditions, redox-neutral conditions, or reductive conditions. The mechanisms of these dicarbofunctionalization reactions have also been discussed in detail.

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

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.

N-Heterocyclic Carbene-Catalyzed 1,4-Alkylacylation of 1,3-Enynes

The radical relay coupling reaction recently emerged as a powerful synthetic strategy for producing tetrasubstituted allenes. However, bond-forming processes involving the allenyl radical intermediate are mostly limited to those promoted by transition metals. In this report, we describe that a ketyl radical generated from single-electron oxidation of the Breslow intermediate is an excellent coupling partner of allenyl radicals. An organocatalytic 1,4-alkylacylation of 1,3-enynes occurred smoothly in the presence of an aldehyde, a radical precursor, and an N-heterocyclic carbene catalyst. This transformation showed remarkable tolerance to both aromatic and aliphatic aldehydes, enyne substitution, and diversified radical precursors.

N-Heterocyclic carbene-catalyzed radical ring-opening acylation of oxime esters with aldehydes

We report a radical cross-coupling of cycloketone oxime esters with aldehydes by N-heterocyclic carbene (NHC) organocatalysis. This protocol features easy operation, with no need for external redox reagent, and a broad functional group compatibility.