A photoelectrocatalytic system as a reaction platform for selective radical-radical coupling

The selection of electrode material is a critical factor that determines the selectivity of electrochemical organic reactions. However, the fundamental principles governing this relationship are still largely unexplored. Herein, we demonstrate a photoelectrocatalytic (PEC) system as a promising reaction platform for the selective radical-radical coupling reaction owing to the inherent charge-transfer properties of photoelectrocatalysis. As a model reaction, the radical trifluoromethylation of arenes is shown on hematite photoanodes without employing molecular catalysts. The PEC platform exhibited superior mono- to bis-trifluoromethylated product selectivity compared to conventional electrochemical methods utilizing conducting anodes. Electrochemical and density functional theory (DFT) computational studies revealed that controlling the kinetics of anodic oxidation of aromatic substrates is essential for increasing reaction selectivity. Only the PEC configuration could generate sufficiently high-energy charge carriers with controlled kinetics due to the generation of photovoltage and charge-carrier recombination, which are characteristic features of semiconductor photoelectrodes. This study opens a novel approach towards selective electrochemical organic reactions through understanding the intrinsic physicochemical properties of semiconducting materials.

Recyclable V2O5/g-C3N4 Heterojunction-Catalyzed Visible-Light-Promoted C3-H Trifluoromethylation of Quinoxalin-2-(1H)-ones

A visible-light-initiated C-H trifluoromethylation of quinoxalin-2(1H)-ones was established using a Z-scheme V2O5/g-C3N4 heterojunction as a recyclable photocatalyst in an inert atmosphere at room temperature under additive-free and mild conditions. A variety of trifluoromethylated quinoxalin-2-(1H)-one derivatives were heterogeneously generated in moderate to high yields, exhibiting good functional group tolerance. Remarkably, the recyclable V2O5/g-C3N4 catalyst could be reused five times with a slight loss of catalytic activity.

Electron Donor-Acceptor Complex Driven Photocatalyst-Free Trifluoromethylation of Heterocycles

Heterocyclic trifluoromethylation is efficiently initiated through a photochemical reaction utilizing an electron donor-acceptor (EDA) complex, proceeding smoothly without the use of photocatalysts, transition-metal catalysts, or additional oxidants. This method has been optimized through extensive experimentation, demonstrating its versatility and efficacy across various substrates, including quinoxalinones, coumarins, and indolones. Notably, this approach enables the practical synthesis of trifluoromethylated quinoxalinones on a gram scale. Mechanistic investigations that incorporate radical trapping and ultraviolet/visible spectroscopy, confirmed the formation of the an EDA complex and elucidated the reaction pathways. This study highlights the crucial role of EDA photoactivation in trifluoromethylation, significantly expanding the application scope of EDA complexes in chemical synthesis.

Photoinduced Dehydrogenative Amination of Quinoxalin-2(1H)-ones with Air as an Oxidant

A facile and eco-friendly photoinduced dehydrogenative amination of quinoxalin-2(1H)-ones with aliphatic amines without any metal, strong oxidant, and photocatalyst has been established for the first time. This reaction proceeding efficiently with air as the sole oxidant at room temperature obtains a wide range of 3-aminoquinoxaline-2(1H)-ones in high yields with excellent functional group tolerance. The mechanistic studies show an interesting involvement of quinoxalin-2(1H)-ones as a photosensitizer, which eliminates the requirement for external photocatalysts.

The preparation of difluoromethylated indoles via electrochemical oxidation under catalyst- and oxidant-free conditions

A green and efficient electrochemical method for the preparation of difluoromethylated indoles has been developed. In this work, sodium difluoromethanesulfinate (HCF₂SO₂Na) was used as the fluorinating reagent, and various indole derivatives with difluoromethylation at the C-2 position were obtained in moderate to good yields under catalyst- and oxidant-free conditions. Moreover, this C-2 difluoromethylation protocol is operationally simple, proceeds at room temperature, and can be easily scaled up. Cyclic voltammetry (CV) and control experiments indicated that this transformation may proceed via a radical pathway.

Bromide-mediated, C2-selective, and oxygenative alkylation of pyridinium salts using alkenes and molecular oxygen

Herein, we report a bromide-mediated, C2-selective, and oxygenative alkylation of pyridinium salts using alkenes and O₂ for the synthesis of important β-2-pyridyl ketones. Notably, a quaternary carbon center was successfully installed at the C2-position of pyridine and the resulting C2-substituents were highly functionalized. The intermediary cycloadduct was isolated and further transformed into the desired product, which indicated that this three-component reaction underwent a reaction cascade including dearomative cycloaddition and rearomative ring-opening oxygenation. Finally, the bromide-mediated mechanism was discussed and active Br(I) species were proposed to be generated in situ and promote the rearomative ring-opening oxygenation by halogen bond-assisted electron transfer.

Catalyst-free electrochemical trifluoromethylation of coumarins using CF3SO2NHNHBoc as the CF3 source

An efficient electrochemical trifluoromethylation of coumarins using CF₃SO₂NHNHBoc as the source of the trifluoromethyl group was developed. Under catalyst-free and external oxidant-free electrolysis conditions, a range of 3-trifluoromethyl coumarins were obtained in moderate to good yields. The method could be easily scaled up with moderate efficiency.

Three-Component Perfluoroalkylvinylation of Alkenes Enabled by Dual DBU/Fe Catalysis

Herein, a simple and efficient strategy that involves dual 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)/iron-catalyzed alkene perfluoroalkylvinylation by using perfluoroalkyl iodides and 2-aminonaphthalene-1,4-diones as coupling partners is demonstrated. In terms of the developed catalytic system, various styrenes and aliphatic alkenes are well-tolerated, leading to the accurate preparation of perfluoroalkyl-containing 2-aminonaphthalene-1,4-diones in excellent regioselectivity. Moreover, the protocol can be readily applied in late-stage modifications of natural products and pharmaceuticals. The title reactions are featured by easily accessible and inexpensive catalysts and substrates, broad substrate applicability, and mild reaction conditions. Mechanistic investigations reveal a tandem C-I cleavable alkylation and C-C vinylation enabled by cooperative DBU/iron catalysis.

Annulation Reaction of Quinoxalin-2(1H)-ones Initiated by Electrochemical Decarboxylation of N-Arylglycines

A new methodology for the synthesis of tetrahydroimidazo[1,5-a]quinoxalin-4(5H)-ones has been accomplished through annulation of quinoxalin-2(1H)-ones initiated by electrochemical decarboxylation of N-arylglycines catalyzed by ferrocene. With a pair of oxidative and reductive processes occurring among the substrates and intermediates instead of on the electrodes, the electricity consumption was decreased.

Electrochemical organic reactions: A tutorial review

Although the core of electrochemistry involves simple oxidation and reduction reactions, it can be complicated in real electrochemical organic reactions. The principles used in electrochemical reactions have been derived using physical organic chemistry, which drives other organic/inorganic reactions. This review mainly comprises two themes: the first discusses the factors that help optimize an electrochemical reaction, including electrodes, supporting electrolytes, and electrochemical cell design, and the second outlines studies conducted in the field over a period of 10 years. Electrochemical reactions can be used as a versatile tool for synthetically important reactions by modifying the constant electrolysis current.

Transition-metal-free electrochemical oxidative C(sp2)-H trifluoromethylation of aryl aldehyde hydrazones

A simple protocol of metal-free C-H trifluoromethylation of hydrazones via electrolysis was developed. This environment-friendly transformation showed high efficiency, good tolerance, and scaled-up functionalization, providing the desired products in moderate to good yields. At the same time, a high yield can be obtained for the substrates either bearing an electron-donating group or an electron-withdrawing group by using different trifluoromethyl reagents. In addition, the radical mechanism was confirmed by the control experiment.

Spotting Trends in Organocatalyzed and Other Organomediated (De)polymerizations and Polymer Functionalizations

Organocatalysis has evolved into an effective complement to metal- or enzyme-based catalysis in polymerization, polymer functionalization, and depolymerization. The ease of removal and greater sustainability of organocatalysts relative to transition-metal-based ones has spurred development in specialty applications, e.g., medical devices, drug delivery, optoelectronics. Despite this, the use of organocatalysis and other organomediated reactions in polymer chemistry is still rapidly developing, and we envisage their rapidly growing application in nascent areas such as controlled radical polymerization, additive manufacturing, and chemical recycling in the coming years. In this Review, we describe ten trending areas where we anticipate paradigm shifts resulting from novel organocatalysts and other transition-metal-free conditions. We highlight opportunities and challenges and detail how new discoveries could lead to previously inaccessible functional materials and a potentially circular plastics economy.

Sunlight Induced and Recyclable g-C3N4 Catalyzed C-H Sulfenylation of Quinoxalin-2(1H)-Ones

A sunlight-promoted sulfenylation of quinoxalin-2(1H)-ones using recyclable graphitic carbon nitride (g-C₃N₄) as a heterogeneous photocatalyst was developed. Using the method, various 3-sulfenylated quinoxalin-2(1H)-ones were obtained in good to excellent yields under an ambient air atmosphere. Moreover, the heterogeneous catalyst can be recycled at least six times without significant loss of activity.

Progress in the Electrochemical Reactions of Sulfonyl Compounds

Electrosynthesis has recently attracted more and more attention due to its great potential to replace chemical oxidants or reductants in molecule-electrode electron transfer. Sulfonyl compounds such as sulfonyl hydrazides, sulfinic acids (and their salts), sulfonyl halides have been discovered as practical precursors of several radicals. As electrochemical redox reactions can provide green and efficient pathways for the activation of sulfonyl compounds, studies for electrosynthesis have rapidly increased. Several types of radicals can be generated from anodic oxidation or cathodic reduction of sulfonyl compounds and can initiate fluoroalkylation, benzenesulfonylation, cyclization or rearrangement. In this Review, we summarize the electrosynthesis developments involving sulfonyl compounds mainly in the last decade.

Theoretical study for evaluating and discovering organic hydride compounds as novel trifluoromethylation reagents

Trifluoromethylation reaction is one of the significant and practical organic chemical reactions, and the design and discovery of novel trifluoromethylation reagents have been attracting more and more attention. Trifluoromethyl-substituted organic hydride compounds (XH) have the potential to be novel trifluoromethylation reagents in organic synthesis due to the favorable tendency of XH˙⁺ releasing ˙CF₃ to form stable aromatic structures in terms of thermodynamics. The key elementary step of the trifluoromethylation is the radical cation (XH˙⁺) generation by catalysis or single-electron activation releasing ˙CF₃ to form a stable aromatic structure, which also provides the thermodynamic driving force of the chemical process. In this work, 47 new trifluoromethylation reagent candidates of XHs were designed and calculated for the Gibbs free energy and activation free energy [ΔG^‡_RD(XH˙⁺)] of XH˙⁺ releasing ˙CF₃ using the density functional theory (DFT) method, in order to quantitatively measure the reactivity of XHs as trifluoromethylation reagents, and to establish the molecular library as well as reactivity database of novel trifluoromethylation reagents for synthetic chemists. According to the and ΔG^‡_RD(XH˙⁺) values, all the XHs can be reasonably divided into 3 classes, including class 1 (excellent trifluoromethylation reagents), class 2 (potential trifluoromethylation reagents) and class 3 (not trifluoromethylation reagents). To our delight, 15 XHs with a 1,4-dihydropyridine structure and 3 XHs with a 3,4-dihydropyrimidin-2-one structure are identified to be novel excellent and potential trifluoromethylation reagents, respectively, according to their reactivity data. The relationship between the structural features, including methylation, heteroatom, substituents, conjugated structure and so on, and the reactivity of XHs as trifluoromethylation reagents are also discussed in this work. The computation results indicate that trifluoromethyl-substituted 1,4-dihydropyridine compounds and 3,4-dihydropyrimidin-2-one analogues could be possible trifluoromethylation reagents in organic synthesis. This work may provide the theoretical basis and references for discovering organic hydride compounds as novel reagents for trifluoromethylation or other alkylation reactions.

Electrochemical Oxidative C-H Arylation of Quinoxalin(on)es with Arylhydrazine Hydrochlorides under Mild Conditions

A practical and scalable protocol for electrochemical arylation of quinoxalin(on)es with arylhydrazine hydrochlorides under mild conditions has been developed. This method exhibits high efficiency, easy scalability, and broad functional group tolerance. Various quinoxalin(on)es and arylhydrazines underwent this transformation smoothly in an undivided cell, providing the corresponding aryl-substituted quinoxalin(on)es in moderate to good yields. A radical mechanism is involved in this arylation reaction.

Electrochemical-Oxidation-Promoted Direct N-ortho-Selective Difluoromethylation of Heterocyclic N-Oxides

An efficient and green electrochemical N-ortho-selective difluoromethylation method of various quinoline and isoquinoline N-oxides has been developed. In this method, sodium difluoromethanesulfinate (HCF₂SO₂Na) was used as the source of the difluoromethyl moiety, and various N-ortho-selective difluoromethylation quinoline and isoquinoline N-oxides were obtained in good to excellent yields under a constant current. In addition, the reaction was easy to scale up and maintained a good yield. Preliminary mechanism studies suggested that the reaction undergoes a free-radical addition and hydrogen elimination pathway.

Renewable Electricity Enables Green Routes to Fine Chemicals and Pharmaceuticals

Syntheses of chemicals using renewable electricity and when generating high atom economies are considered green and sustainable processes. In the present state of affairs, electrochemical manufacturing of fine chemicals and pharmaceuticals is not as common place as it could be and therefore, merits more attention. There is also a need to turn attention toward the electrochemical synthesis of valuable chemicals from recyclable greenhouse gases that can accelerate the process of circular economy. CO₂ emissions are the major contributor to human-induced global warming. CO₂ conversion into chemicals is a valuable application of its utilisation and will contribute to circular economy while maintaining environmental sustainability. Herein, we present an overview of electro-carboxylation, including mechanistic aspects, which forms carboxylic acids using molecular carbon dioxide. We also discuss atom economies of electrochemical fluorination, methoxylation and amide formation reactions.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

Recent Advances in Transition-Metal Mediated Trifluoromethylation Reactions

Fluorine compounds are known for their abundance in more than 20% of pharmaceutical and agrochemical products mainly due to the enhanced lipophilicity, metabolic stability and pharmacokinetic properties of organofluorides. Consequently,...

Electrochemical benzylic C-H arylation of xanthenes and thioxanthenes without catalyst and oxidant

A catalyst-free and oxidant-free C-H arylation of xanthenes and thioxanthenes using electrochemistry has been developed, which affords a number of cross-coupling products in moderate to good yields. This method is...

Graphene oxide-catalyzed trifluoromethylation of alkynes with quinoxalinones and Langlois' reagent

The direct C–H trifluoromethylation of alkynes and quinoxalinones has been achieved using a graphene oxide/Langlois' reagent system. This multi-component tandem reaction using graphene oxide as the catalyst and Langlois' reagent as the robust CF₃ radical source results in the formation of olefinic C–CF₃ to access a series of 3-trifluoroalkylated quinoxalin-2(1H)-ones.

The direct C–H trifluoromethylation of alkynes and quinoxalinones using a graphene oxide/Langlois' reagent system.

Electrochemical Trifluoromethylation of Glycals

Carbohydrates play essential roles in various physiological and pathological processes. Trifluoromethylated compounds have wide applications in the field of medicinal chemistry. Herein, we report a practical and efficient trifluoromethylation of glycals by an electrochemical approach using CF₃SO₂Na as the trifluoromethyl source and MnBr₂ as the redox mediator. A variety of trifluoromethylated glycals bearing different protective groups are obtained in 60-90% yields with high regioselectivity. The successful capture of a CF₃ radical indicates that a radical mechanism is involved in this reaction.

Electrochemical Trifluoromethylation of Thiophenols with Sodium Trifluoromethanesulfinate

We developed an electrochemical trifluoromethylation of thiophenols without the use of metal catalysts and oxidants. This reaction features mild reaction conditions, readily available substrate, as well as moderate to good yields. In addition, this protocol can be easily scaled up with moderate efficiency.

Alkylation of quinoxalin-2(1H)-ones using phosphonium ylides as alkylating reagents

A practical and efficient methodology for the construction of 3-alkylquinoxalinones through base promoted direct alkylation of quinoxalin-2(1H)-ones with phosphonium ylides as alkylating reagents under metal- and oxidant-free conditions was developed. Various 3-alkylquinoxalin-2(1H)-ones were easily obtained in good to excellent yields. Tentative mechanistic studies suggest that this reaction is likely to involve a nucleophilic addition-elimination process.

Electrochemical-Promoted Nickel-Catalyzed Oxidative Fluoroalkylation of Aryl Iodides

This work describes a general strategy for metal-catalyzed cross-coupling of fluoroalkyl radicals with aryl halides under electrochemical conditions. The contradiction between anodic oxidation of fluoroalkyl sulfinates and cathodic reduction of low-valent nickel catalysts can be well addressed by paired electrolysis, allowing for direct introduction of fluorinated functionalities into aromatic systems.

Radical Perfluoroalkylation of Arenes via Carbanion Intermediates

The use of sodium dithionite with perfluoroalkyl iodides under basic conditions facilitates the direct perfluoroalkylation of arenes with pendant benzylic electron-withdrawing groups. This occurs via attack of the arene on the electrophilic perfluoroalkyl radical, through the donation of electron density from a benzylic anion. The substrate scope was expanded beyond benzylic nitriles with cyclic substrates bearing electron-withdrawing groups at the benzylic position-enforcing donation of electron density to the aromatic ring and enabling attack on the perfluoroalkyl radical.

Electrochemical trifluoromethylation/cyclization for the synthesis of isoquinoline-1,3-diones and oxindoles

Herein, we describe a protocol for electrochemical cathode reduction to generate trifluoromethyl radicals. The trifluoromethylation reagent (IMDN-SO₂CF₃) used in this strategy is inexpensive and easy to obtain, and the reaction can be conducted efficiently without the addition of additional redox reagents. Using this strategy, we achieved electrochemical trifluoromethylation/cyclization for the synthesis of isoquinoline-1,3-diones and oxindoles. This protocol has good functional group tolerance and a broad substrate scope.

Electrochemical Installation of CFH2 -, CF2 H-, CF3 -, and Perfluoroalkyl Groups into Small Organic Molecules

Electrosynthesis can be considered a powerful and sustainable methodology for the synthesis of small organic molecules. Due to its intrinsic ability to generate highly reactive species under mild conditions by anodic oxidation or cathodic reduction, electrosynthesis is particularly interesting for otherwise challenging transformations. One such challenge is the installation of fluorinated alkyl groups, which has gained significant attention in medicinal chemistry and material science due to their unique physicochemical features. Unsurprisingly, several electrochemical fluoroalkylation methods have been established. In this review, we survey recent developments and established methods in the field of electrochemical mono-, di-, and trifluoromethylation, and perfluoroalkylation of small organic molecules.