Cooperative NHC and Photoredox Catalysis for the Synthesis of β-Trifluoromethylated Alkyl Aryl Ketones

Visible light-mediated organocatalyzed 1,3-aminoacylation of cyclopropane employing N-benzoyl saccharin as bifunctional reagent

The carboamination of unsaturated molecules using bifunctional reagents is considered an attractive approach for the synthesis of nitrogen-containing compounds. However, bifunctional C-N reagents have never been employed in the carboamination of cyclopropane. In this study, we use an N-heterocyclic carbene (NHC), N-benzoyl saccharin, as a bifunctional reagent and a photoredox catalyst for a dual-catalyzed 1,3-aminoacylation of cyclopropane. NHCs play multiple roles, functioning as Lewis base catalysts to activate C-N bonds, promoting the oxidative quenching process of PC*, and acting as efficient acyl radical transfer catalysts for the formation of C-C bonds. The oxidative quenching process between the excited-state PC* and acyl NHC adduct is the key to the photooxidation generality of aryl cyclopropanes.

Photocatalytic Sulfonylation: Innovations and Applications

Photosynthesis, converting sustainable solar energy into chemical energy, has emerged as a promising craft to achieve diverse organic transformations due to its mild reaction conditions, sustainability, and high efficiency. The synthesis of sulfonated compounds has drawn significant attention in the pharmaceuticals, agrochemicals, and materials industries due to the unique structure and electronic properties of the sulfonyl groups. Over the past decades, many photocatalytic sulfonylation reactions have been developed. In this review, the recent advances in photocatalyzed sulfonylation have been reviewed since 2020, with a primary focus on discussing reaction design and mechanism.

Carbene and photocatalyst-catalyzed 3-acylation of indoles for facile access to indole-3-yl aryl ketones

3-Acyl indoles play important roles in both organic synthesis and diverse types of functional molecules. Herein, a facile nitrogen heterocyclic carbene (NHC) and photocatalyst cooperatively-catalyzed 3-acylation of indoles was disclosed. The reaction proceeded via radical cross-coupling of indole-based aryl radical cations with NHC-bound ketyl radical species, which are less explored in radical NHC catalysis. The reaction exhibits mild reaction conditions, broad substrate scope, and good functional group tolerance. Mechanistic studies support our proposed reaction pathway. The synthesis of structurally diverse analogs of an aldose reductase inhibitor and antibacterial activity investigation further demonstrated the utility of the current acylation reaction.

N-Heterocyclic Carbene/Transition Metal Dual Catalysis

N-heterocyclic carbene catalysis has been developed as a versatile method for the enantioselective synthesis of complex organic molecules in organic chemistry. Merging of N-heterocyclic carbene catalysis with transition metal catalysis holds the potential to achieve unprecedented transformations with broad substrate scope and excellent stereoselectivity, which are unfeasible with individual catalyst. Thus, this dual catalysis has attracted increasing attention, and numerous elegant dual catalytic systems have been established. In this review, we summarize the recent achievements of dual NHC/transition metal catalysis, including the reaction design, mechanistic studies and practical applications.

Redox-Paired Reductive Heck Reaction and Oxidative Esterification Catalyzed by Mesoionic Carbenes

Paring a reductive reaction and an oxidative reaction in one reaction could be immensely important in achieving atom economic and environmental advantages. Herein, we report a simple protocol that combines two such reductive Heck reactions and oxidative esterification by using mesoionic carbenes as catalysts to synthesize multiple valuable products under mild conditions.

Synthesis and comparative study of (NHCF)PdCl2Py and (NHCF)Ni(Cp)Cl complexes: investigation of the electronic properties of NHC ligands and complex characteristics

The electron-donating and electron-accepting properties of N-heterocyclic carbene (NHC) ligands play a pivotal role in governing their interactions with transition metals, thereby influencing the selectivity and reactivity in catalytic processes. Herein, we report the synthesis of Pd/NHCF and Ni/NHCF complexes, wherein the electronic parameters of the NHC ligands were systematically varied. By performing a series of controlled structure modifications, we elucidated the influence of the σ-donor and π-acceptor properties of NHC ligands on interactions with the transition metals Pd and Ni and, consequently, the catalytic behavior of Pd and Ni complexes. The present study deepens our understanding of NHC-metal interactions and provides novel information for the rational design of efficient catalysts for organic synthesis.

N-Heterocyclic carbene catalytic 1,2-boron migrative acylation accelerated by photocatalysis

The transformation of organoboron compounds plays an important role in synthetic chemistry, and recent advancements in boron-migration reactions have garnered considerable attention. Here, we report an unprecedented 1,2-boron migrative acylation upon photocatalysis-facilitated N-heterocyclic carbene catalysis. The design of a redox-active boronic ester substrate, serving as an excellent β-boron radical precursor, is the linchpin to the success of this chemistry. With the established protocol, a wide spectrum of β-boryl ketones has been rapidly synthesized, which could further undergo various C─B bond transformations to give multifunctionalized products. The robustness of this catalytic strategy is underscored by its successful application in late-stage modification of drug-derived molecules and natural products. Preliminary mechanistic investigations, including several control experiments, photochemistry measurements, and computational studies, shed light on the catalytic radical reaction mechanism.

Stereoselective Fluoroalkylacylation of Alkynes via Cooperative N-Heterocyclic Carbene/Palladium Catalysis

Herein, a cooperative N-heterocyclic carbene- and palladium-catalyzed three-component reaction of alkynes with aldehydes and fluoroalkyl iodides is developed. A series of biologically valuable CF2R-incorporated α-substituted enones was obtained in moderate to good yields. This mild catalytic method exhibits exclusive regio- and stereoselectivity, excellent functional group tolerance, and a broad substrate scope including terminal and internal alkynes. Mechanistic investigations disclose that this alkyne fluoroalkylacylation proceeds via a radical relay process in which vinyl iodides serve as putative reaction intermediates.

Phosphonylacylation of Alkenes Enabled by Visible-Light-Induced N-Heterocyclic Carbene Catalysis

Herein, we report the phosphonylacylation of alkenes via visible-light-induced N-heterocyclic carbene (NHC) catalysis to afford a series of γ-ketophosphonates in moderate to good yields. This protocol features mild conditions, free of photocatalyst, and good compatibility of functional groups. The excited Breslow enolate intermediate was proposed to undergo single-electron transfer with oxime phosphonate to generate the corresponding ketyl radical and phosphonyl radical.

Cooperative Photoredox and N-Heterocyclic Carbene Catalysis Suzuki-Miyaura-Type Reaction: Radical Coupling of Aroyl Fluorides and Alkyl Boronic Acids

An intermolecular Suzuki-Miyaura-type reaction of benzoyl fluorides with alkyl boronic acids to synthetic ketone was revealed by cooperative N-heterocyclic carbene (NHC) and photoredox catalysis. Various alkyl boric acids can be converted into alkyl radicals without external oxidants or activators. Moreover, the catalytic system was feasible for the difunctionalization of styrenes via a radical relay process. Mechanistic experiments suggested that the benzoate anion intermediate might play a unique role in this reaction system.

Photo-Catalyzed Acyl Azolium Promoted Selective α-C(sp3 )-H Acylation of Acetone via HAT: Access to Thermodynamically Less Favoured (Z)-α,β-Unsaturated Ketones

Mono α-acylation of acetone has been achieved for the first time by reacting with bench-stable acyl azolium salts under violet-LED light at room temperature. The intermolecular hydrogen atom transfer (HAT) from acetone to triplet state of azolium salts under violet LED irradiation resulted in thermodynamically less favourable (Z)-α,β-unsaturated ketones with up to 99 : 1 selectivity via C-C bond formation. This compelling protocol access the desired α-C(sp3 )-H acylation product under metal-, ligand- and oxidant-free conditions on a wide range of substrates.

Visible Light-Mediated Monofluoromethylation/Acylation of Olefins by Dual Organo-Catalysis

Monofluoromethyl (CH2F) motifs exhibit unique bioactivities and are considered privileged units in drug discovery. The radical monofluoromethylative difunctionalization of alkenes stands out as an appealing approach to access CH2F-containing compounds. However, this strategy remains largely underdeveloped, particularly under metal-free conditions. In this study, we report on visible light-mediated three-component monofluoromethylation/acylation of styrene derivatives employing NHC and organic photocatalyst dual catalysis. A diverse array of α-aryl-β-monofluoromethyl ketones was successfully synthesized with excellent functional group tolerance and selectivity. The mild and metal-free CH2F radical generation strategy from NaSO2CFH2 holds potential for further applications in fluoroalkyl radical chemistry.

Highly Selective 1,4-Diacylation/Cycloisomerization of 1,3-Enynes: De Novo Synthetic Strategy to Polysubstituted Furans

The development of a de novo synthetic strategy for rapid assembly of biologically relevant multisubstituted furans is an appealing but challenging task. Herein, we disclose NHC and organophotocatalysis cocatalyzed three-component radical 1,4-diacylation/cycloisomerization cascade process of readily available 1,3-enynes, which provides an efficient and straightforward entry to a wide range of polysubstituted furans with good yields and excellent regio- and chemoselectivities. The reaction features mild conditions, broad substrate scopes, and good functional group compatibilities.

Photoinduced cerium-catalyzed C-H acylation of unactivated alkanes

Ketones are ubiquitous motifs in the realm of pharmaceuticals and natural products. Traditional approaches to accessing these species involve the addition of metal reagents to carboxyl compounds under harsh conditions. Herein, we report a cerium-catalyzed acylation of unactivated C(sp3)-H bonds using bench-stable acyl azolium reagents under mild and operationally-friendly conditions. This reaction exhibits excellent generality, accommodating a wide range of feedstock chemicals such as cycloalkanes and acyclic compounds as well as facilitating the late-stage functionalization of pharmaceuticals. We demonstrate further applications of our strategy with a three-component radical relay reaction and an enantioselective N-heterocyclic carbene (NHC) and cerium dual-catalyzed reaction.

Nickel/Photo-Cocatalyzed Three-Component Alkyl-Acylation of Aryl-Activated Alkenes

Herein, we disclose a nickel/photo-cocatalyzed three-component alkyl-acylation of aryl-substituted alkenes with aldehydes and electron-withdrawing-group-activated alkyl bromides, providing straightforward access to various ketones under mild and ligand-free conditions. The photocatalyst TBADT plays a dual role in activating the acyl C-H bond of aldehydes via hydrogen atom transfer and reducing the C-Br bond of alkyl bromides via single-electron transfer. While the terminal C-C bond is forged through polarity-matched radical-type addition, nickel is likely involved in the acylation step.

Light-Driven Enantioselective Carbene-Catalyzed Radical-Radical Coupling

An enantioselective carbene-catalyzed radical-radical coupling of acyl imidazoles and racemic Hantzsch esters is disclosed. This method involves the coupling of an N-heterocyclic carbene-derived ketyl radical and a secondary sp3 -carbon radical and allows access to chiral α-aryl aliphatic ketones in moderate-to-good yields and enantioselectivities without any competitive epimerization. The utility of this protocol is highlighted by the late-stage functionalization of various pharmaceutical compounds and is further demonstrated by the transformation of the enantioenriched products to biologically relevant molecules. Computational investigations reveal the N-heterocyclic carbene controls the double-facial selectivity of the ketyl radical and the alkyl radicals, respectively.

Recent advances in combining photo- and N-heterocyclic carbene catalysis

N-Heterocyclic carbenes (NHCs) are unique Lewis basic catalysts that mediate various organic transformations by means of polarity reversal. Although the scope of research on two-electron reactions mediated by NHC catalysts has been expanding, the types of these reactions are limited by the inability of NHCs to engage sp3-electrophiles. However, the revival of photocatalysis has accelerated the development of free-radical chemistry, and combining photoredox catalysis and NHC catalysis to achieve NHC-mediated radical reactions under mild conditions could overcome the above-mentioned limitation. This review summarizes recent advances in combining photoredox and NHC catalysis, focusing on elucidation and exploration of mechanisms, with the aim of identifying challenges and opportunities to develop more types of catalytic models.

Cooperative Photoredox and N-Heterocyclic Carbene Catalyzed Fluoroaroylation for the Synthesis of α-Trifluoromethyl-Substituted Ketones

α-Trifluoromethylated ketones have attracted significant attention as valuable building blocks in organic synthesis. Such compounds are generally accessed through trifluoromethylation of ketones. Here we report an alternative disconnection approach for the construction of α-CF3 carbonyl compounds by using aroyl fluorides as bifunctional reagents for fluoroaroylation of gem-difluoroalkenes through cooperative photoredox and N-heterocyclic carbene (NHC) catalysis. This strategy bypasses the use of expensive or sensitive trifluoromethylation reagents and/or the requirement for ketone pre-functionalization, thus enabling an efficient and general synthetic method to access α-CF3 -substituted ketones. A wide variety of gem-difluoroalkenes and aroyl fluorides bearing a diverse set of functional groups are eligible substrates. Notably, the developed methodology also provides rapid access to mono- or difluoroalkyl ketones. Mechanistic studies reveal that merging photoredox catalysis with NHC catalysis is essential for the reaction.

Visible-Light-Promoted Intermolecular β-Acyl Difunctionalization of Alkenes via Oxidative Radical-Polar Crossover

A visible-light-induced β-acyl difunctionalization of alkenes with acyl oxime esters and various nucleophiles was developed to achieve molecular complexity from readily available raw materials via oxidative radical-polar crossover. A variety of nucleophiles, including NH-sulfoximines, indoles, indazole, and trimethoxybenzene, were all effectively applicable to the sustainable reaction system. The novel synthetic strategy features mild reaction conditions, a broad substrate scope (39 examples), easy scale-up, and excellent regioselectivity.

Photoredox-Catalyzed Radical-Radical Coupling of Potassium Trifluoroborates with Acyl Azoliums

Potassium trifluoroborates have gained significant utility as coupling partners in organic synthesis, particularly in the Suzuki-Miyaura coupling reaction. Recently, they have also been used as radical precursors under oxidative conditions to generate carbon-centered radicals. These versatile reagents have found new applications in photoredox catalysis, including radical substitution, conjugate addition reactions, and transition metal dual catalysis. In addition, this photomediated redox neutral process has enabled radical-radical coupling with persistent radicals in the absence of a metal, and this process remains to be fully explored. In this study, we report the radical-radical coupling of benzylic potassium trifluoroborate salts with isolated acyl azolium triflates, which are persistent radical precursors. The reaction is catalyzed by an organic photocatalyst and forms isolable tertiary alcohol species. These compounds can be transformed into a range of substituted ketone products by simple treatment with a mild base.

Cooperative Carbene Photocatalysis for β-Amino Ester Synthesis

β-Amino acids are useful building blocks of bioactive molecules, including peptidomimetics and pharmaceutical compounds. The current limited accessibility to β2,2-type amino acids which bear an α-quaternary center has limited their use in chemical synthesis and biological investigations. Disclosed herein is the development of a new N-heterocyclic carbene/photocatalyzed aminocarboxylation of olefins, affording β2,2-amino esters with high regioselectivity. The generation of nitrogen-centered radicals derived from simple imides via a sequence of deprotonation and single-electron oxidation allows for the subsequent addition to geminal-disubstituted olefins regioselectively. The intermediate tertiary radicals then cross-couple with a stabilized azolium-based radical generated in situ to efficiently construct the quaternary centers. Mechanistic studies, including Stern-Volmer fluorescence quenching experiments, support the proposed catalytic cycle.

Exploring the Versatility of the Amidation of Aryl Acid Fluorides using the Germylamines R3 GeNMe2

The formation of amide bonds is an important process since this linkage is an essential component in proteins, pharmaceuticals, and other medicinally and biologically significant molecules. Recently, it was demonstrated that germylamines R3 GeNR'2 were useful reagents for the conversion of acid fluorides to amides. This transformation occurs readily at room temperature and has a low activation energy. In the present study, the versatility of this amidation reaction with aryl acid fluorides is investigated. A series of thirteen acid fluorides with various substituents on the aromatic ring were reacted with the germylamine Ph3 GeNMe2 and twelve of these were converted to the corresponding amides in high yields, the exception being 1,4-benzenedicarbonyl difluoride. The germylamines Bun 3 GeNMe2 and Pri 3 GeNMe2 also could be used for this interconversion, and both of these species successfully converted 1,4-benzenedicarbonyl difluoride to the corresponding amide. In addition, the crystal structure of Ph3 GeNMe2 is reported. This represents one of only three crystallographically characterized germylamines. The synthesis and 19 F NMR characterization of three fluorogermanes R3 GeF (R=Bun , Pri , and Mes) are also reported herein.

Ketone Synthesis via Irradiation-Induced Generation of a Persistent Ketyl Radical from Acyl Azolium Salts

A novel three-component α-acylated difunctionalization of alkenes strategy has been developed on the basis of a direct hydrogen atom transfer (HAT) process of photoinduced acyl azolium salts. With simple irradiation without the catalyst, a variety of olefins can be directly converted into ketone derivatives, including 1,4-dione, β-silyl ketone, 1,5-dione, etc. Mechanistic investigations indicated that the unique reactivity of the acyl azonium triplet excited state is crucial to the strategy's success.

Recent Advances in Visible Light-Mediated Radical Fluoro-alkylation, -alkoxylation, -alkylthiolation, -alkylselenolation, and -alkylamination

In the last few years, many reagents and protocols have been developed to allow for the efficient fluorofunctionalization of a diverse set of scaffolds ranging from alkanes, alkenes, alkynes, and (hetero)arenes. The concomitant rise of organofluorine chemistry and visible light-mediated synthesis have synergistically expanded the fields and have mutually benefitted from developments in both fields. In this context, visible light driven formations of radicals containing fluorine have been a major focus for the discovery of new bioactive compounds. This review details the recent advances and progress made in visible light-mediated fluoroalkylation and heteroatom centered radical generation.

Turning sulfonyl and sulfonimidoyl fluoride electrophiles into sulfur(VI) radicals for alkene ligation

Sulfonyl and sulfonimidoyl fluorides are versatile substrates in organic synthesis and medicinal chemistry. However, they have been exclusively used as S(VI)+ electrophiles for defluorinative ligations. Converting sulfonyl and sulfonimidoyl fluorides to S(VI) radicals is challenging and underexplored due to the strong bond dissociation energy of SVI-F and high reduction potentials, but once achieved would enable dramatically expanded synthetic utility and downstream applications. In this report, we disclose a general platform to address this issue through cooperative organosuperbase activation and photoredox catalysis. Vinyl sulfones and sulfoximines are obtained with excellent E selectivity under mild conditions by coupling reactions with alkenes. The synthetic utility of this method in the preparation of functional polymers and dyes is also demonstrated.

Silylacylation of Alkenes through N-Heterocyclic Carbene Catalysis

The construction of silicon-containing molecules has received increasing attention in recent years. Herein, we report the generation of silyl radicals through NHC catalysis under mild reaction conditions. This methodology offers a novel and convenient route to a diverse range of β-silyl ketones with a broad substrate scope and good functional group compatibility. Both the radical clock and electrochemical studies are consistent with the hypothesis of ground-state SET, and a plausible mechanism for the organocatalytic transformation is proposed.

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.

Acyl Azolium-Photoredox-Enabled Synthesis of β-Keto Sulfides

α-Heteroatom functionalization is a key strategy for C-C bond formation in organic synthesis, as exemplified by the addition of a nucleophile to electrophilic functional groups, such as iminium ions; oxocarbenium ions; and their sulfur analogues, sulfenium ions. We envisioned a photoredox-enabled radical Pummerer-type reaction realized through the single-electron oxidation of a sulfide. Following this oxidative event, α-deprotonation would afford α-thio radicals that participate in radical-radical coupling reactions with azolium-bound ketyl radicals, thereby accessing a commonly proposed mechanistic intermediate of the radical-radical coupling en route to functionalized additive Pummerer products. This system provides a complementary synthetic approach to highly functionalized sulfurous products, including modification of methionine residues in peptides, and beckons further exploration in C-C bond formations previously limited in the standard two-electron process.

Direct allylic acylation via cross-coupling involving cooperative N‑heterocyclic carbene, hydrogen atom transfer, and photoredox catalysis

Herein, we report a mild, operationally simple, multicatalytic method for the synthesis of β,γ-unsaturated ketones via allylic acylation of alkenes. Specifically, the method combines N‑heterocyclic carbene catalysis, hydrogen atom transfer catalysis, and photoredox catalysis for cross-coupling reactions between a wide range of feedstock carboxylic acids and readily available olefins to afford structurally diverse β,γ-unsaturated ketones without olefin transposition. The method could be used to install acyl groups on highly functionalized natural-product-derived compounds with no need for substrate pre-activation, and C-H functionalization proceed with excellent site selectivity. To demonstrate the potential applications of the method, we convert a representative coupling product into various useful olefin synthons.

Synthesis of β-Ketonitriles via N-Heterocyclic-Carbene-Catalyzed Radical Coupling of Aldehydes and Azobis(isobutyronitrile)

Herein, an NHC (N-heterocyclic carbene)-catalyzed radical coupling reaction between aldehydes and azobis(isobutyronitrile) (AIBN) has been developed. This method provides an efficient and convenient approach for the synthesis of β-ketonitriles containing a quaternary carbon center (31 examples, up to >99% yield) utilizing commercially available substrates. This protocol features broad substrate scope, good functional group tolerance, and high efficiency under metal-free and mild reaction conditions.

A Mechanistically Deceiving Formation of Aryl(1-indanyl)ketones via Acid-Catalyzed Cyclization of ortho-Alkynylarylmethanols

The generation of reactive carbocation intermediates from ortho-alkynylarylmethanol substrates was utilized as a means for the synthesis of aryl(1-indanyl)ketones . Substrates with a tertiary carbon at the β-position to the arene generated a carbocation intermediate via dehydration/protonation, followed by cyclization and hydration to give indanylketone products. For substrates with a quaternary carbon at that position, a carbocation intermediate was generated by protonation/elimination of water, followed by a 1,2-shift and a subsequent cyclization/hydration to give highly substituted indanylketones.

General Method for Selective Three-Component Carboacylation of Alkenes via Visible-Light Dual Photoredox/Nickel Catalysis

A photoredox/nickel dual catalytic protocol for the regioselective three-component carboacylation of alkenes with tertiary and secondary alkyltrifluoroborates as well as acyl chlorides is described. This redox-neutral protocol can be applied to the rapid synthesis of ketones with high diversity and complexity via a radical relay process. Many functional groups, allowing for various commercially available acyl chlorides, alkyltrifluoroborates, and alkenes, are tolerated under these mild conditions.

Recent advances in the application of Langlois' reagent in olefin difunctionalization

In this review, we summarise the recent applications of Langlois' reagent in the radical-mediated difunctionalization of alkenes. Among the various trifluoromethylation reagents, Langlois' reagent is an exceptional compound, and many important organic transformations have been realized by employing such reagents. Various organic transformations of Langlois' reagent, especially in radical chemistry, have been developed in recent years. This review describes several key activation methods for Langlois' reagent in the difunctionalization of alkenes by showcasing selected cornerstone research areas and related mechanisms to stimulate the interest of readers in promoting the wider development and application of Langlois' reagent.

Enantioselective N-Heterocyclic Carbene Catalyzed α-Oxidative Coupling of Enals with Carboxylic Acids Using an Iodine(III) Reagent

The enantioselective α-oxidative coupling of enals with carboxylic acids was developed via the umpolung of an NHC-bound enolate with an iodine(III) reagent. The corresponding α-acyloxyl-β,γ-unsaturated esters were afforded in good yields, with high regio- and enantioselectivities. The key step of the reaction involves the formation of enol iodine(III) intermediate from the enolate with iodosobenzene, which changes the polarity of α-carbon of the enal from nucleophilic to electrophilic, and thus facilitates the subsequent addition of carboxylate.

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.

Photoinduced Acylations Via Azolium-Promoted Intermolecular Hydrogen Atom Transfer

Photoinduced hydrogen atom transfer (HAT) has been developed as a powerful tool to generate synthetically valuable radical species. The direct photoexcitation of ketones has been known to promote HAT or to generate acyl radicals through Norrish-type pathways, but these modalities remain severely limited by radical side reactions. We report herein a catalyst- and transition metal-free method for the acylation of C-H bonds that leverages the unique properties of stable, isolable acyl azolium species. Specifically, acyl azolium salts are shown to undergo an intermolecular and regioselective HAT upon LED irradiation with a range of substrates bearing active C-H bonds followed by C-C bond formation to afford ketones. Experimental and computational studies support photoexcitation of the acyl azolium followed by facile intersystem crossing to access triplet diradical species that promote selective HAT and radical-radical cross-coupling.

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.

α-Acylation of Alkenes by a Single Photocatalyst

A direct strategy for the difunctionalization of alkenes, with acylation occurring at the more substituted alkene position, would be attractive for complex ketone synthesis. We report herein a reaction driven by a single photocatalyst that enables α-acylation in this way with the introduction of a fluoromethyl, alkyl, sulfonyl or thioether group at the β-position of the alkene with high chemo- and regioselectivity under extremely mild conditions. Crucial to the success of this method are rate differences in the kinetics of radical generation through single-electron transfer (SET) between different radical precursors and the excited photocatalyst (PC*). Thus, the β-position of the alkene is first occupied by the group derived from the radical precursor that can be generated most readily, and α-keto acids could be used as an electrophilic reagent for the α-acylation of alkenes.

Diastereoselective Radical Aminoacylation of Olefins through N-Heterocyclic Carbene Catalysis

There have been significant advancements in radical-mediated reactions through covalent-based organocatalysis. Here, we present the generation of iminyl and amidyl radicals via N-heterocyclic carbene (NHC) catalysis, enabling diastereoselective aminoacylation of trisubstituted alkenes. Different from photoredox catalysis, single electron transfer from the deprotonated Breslow intermediate to O-aryl hydroxylamine generates an NHC-bound ketyl radical, which undergoes diastereocontrolled cross-coupling with the prochiral C-centered radical. This operationally simple method provides a straightforward access to a variety of pyrroline and oxazolidinone heterocycles with vicinal stereocenters (77 examples, up to >19:1 d.r.). Electrochemical studies of the acyl thiazolium salts support our reaction design and highlight the reducing ability of Breslow-type derivatives. A detailed computational analysis of this organocatalytic system suggests that radical-radical coupling is the rate-determining step, in which π-π stacking interaction between the radical intermediates subtly controls the diastereoselectivity.

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

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.

Three-component carboacylation of simple alkenes often requires directing groups and strained substrates. Here, the authors report a photoredox N-heteroyclic carbene/Pd-catalysed alkylacylation of alkenes with aldehydes and unactivated alkyl halides; exogenous photosensitizer or external reductant are not required.

Cooperative NHC and Photoredox Catalysis for the Synthesis of 1,4-Dicarbonyl Compounds via Diacylation of Alkenes

An intermolecular 1,2-diacylation of alkenes is disclosed via cooperative N-heterocyclic carbene and photoredox catalysis under the mediation of PPh₃ and Cs₂CO₃. This protocol provides a practical approach for construction of 1,4-dicarbonyl compounds toward novel diketone and pharmaceutical derivatives. Furthermore, the regioselective dicarbonyl compounds can be synthesized by adding acyl azolium salt. Mechanistic investigations suggest that the process was a critical radical/radical cross coupling of ketyl radicals with benzylic C-radicals.