Electrochemically C–H/S–H Oxidative Cross-Coupling between Quinoxalin-2(1H)-ones and Thiols for the Synthesis of 3-Thioquinoxalinones

Iodine-mediated thio-arylation under electrochemical conditions

An efficient iodine-catalyzed thio-arylation reaction of aniline was developed under electrochemical conditions. A variety of diaryl sulfide compounds can be obtained under metal-free and chemical oxidant-free conditions. The reaction features a broad substrate scope, regulation of product distribution, and scalable preparation.

Advances in cross-coupling and oxidative coupling reactions of NH-sulfoximines - a review

Due to the special structure and physicochemical properties of sulfoximines, research on sulfoximines has achieved great progress in recent decades, especially in chemical and medicinal fields. This review highlights recent advancements in the N-functionalization of NH-sulfoximines, focusing on classical cross-coupling reactions with electrophilic agents and oxidative coupling reactions with extensive organic compounds, including specific (hetero)arenes, alkenes (1,4-naphthoquinones), alkanes (cyclohexanes), nucleophiles (thiols, disulfides, sulfinates, diarylphosphine oxides), organyl boronic acids, and arylhydrazines. Transition metal-catalyzed, metal-free, electrochemical and radical oxidative coupling reactions are discussed. This review also reports and discusses the mechanistic pathways of some typical reactions.

Visible light induced hydroxyfluoroalkylation of quinoxalin-2(1H)-ones with N-trifluoroethoxyphthalimide under catalyst-free conditions

For the first time, we achieved visible light-induced direct C3-hydroxyfluoroalkylation of quinoxalin-2(1H)-ones using N-trifluoroethoxyphthalimide as the trifluoroethanol radical precursor, without the need for a photocatalyst. The metal-free and catalyst-free nature of this method makes it an efficient and environmentally friendly approach for synthesizing C3-hydroxyfluoroalkylated quinoxalin-2(1H)-ones.

Acid-Promoted Self-Photocatalyzed Regioselective Oxidation: A Novel Strategy for Accessing Quinoxaline-2,3-diones

Disclosed here is an efficient approach for the preparation of quinoxaline-2,3-diones using air (O2) as a green oxidant via acid-promoted self-photocatalyzed regioselective oxidation of quinoxalin-2(1H)-ones at C-3 position. This protocol presents a novel synthetic route for the preparation of quinoxaline-2,3-dione derivatives, featuring mild reaction conditions, simple operation, and a wide range of substrates, without the need for external photocatalysts, metal reagents, and strong oxidants.

Recent Advances of C-S Coupling Reaction of (Hetero)Arenes by C-H Functionalization

Organic sulfur compounds encompass a vast and diverse variety of species that possess unique biological activity due to the presence of sulfur atoms or sulfur-containing functional groups. These compounds are widely present in natural products, pharmaceuticals, agricultural chemicals, and functional materials. In recent years, numerous sulfur-containing compounds such as thiols, thioethers, disulfides, thiourea, dimethyl sulfoxide, sulfonates and their derivatives, as well as sulfur-containing inorganic compounds, have been utilized as coupling agents to synthesize (hetero)aryl sulfides via C-H Functionalization. These novel transformations provide effective methods for constructing C-S bond of (hetero)arenes, while also expanding the scope of (hetero)aryl sulfides with the potential biological activity. Therefore, the synthesis of aryl sulfides through C-H bond functionalization has attracted widespread attention. This review mainly focuses on the construction of (hetero)aryl sulfides via C-H bond functionalization since 2015. We hope this review offers a useful conceptual overview and inspires further advancements in the efficient construction of C-S bonds.

A General Radical Functionalization of Quinoxalin-2(1H)-ones via a Donor-Acceptor Inversion Strategy

The radical donor-acceptor inversion strategy represents a highly promising approach in the field of radical chemistry. The present study initially describes a metal-free, versatile, and modular approach for the radical functionalization of quinoxalin-2(1H)-ones via a strategy of radical donor-acceptor inversion under simple reaction conditions. More than 66 examples were provided in moderate yields. The mechanistic study has confirmed that the driving force behind this radical reaction is the in situ formation of a salt through the interaction between quinoxalin-2(1H)-ones and acid/HFIP, which exhibits potent oxidation properties. Additionally, it has been observed that the evident hydrogen bonding between quinoxalin-2(1H)-ones and HFIP can effectively mitigate the oxidation potential.

Visible-light-induced tandem reaction of quinoxalin-2(1H)-ones, alkenes, and sulfonyl chlorides

A visible-light-induced tandem reaction involving quinoxalin-2(1H)-ones, alkenes, and sulfonyl chlorides, catalyzed by 4CzIPN, was developed. The utilization of easily accessible sulfonyl chlorides, metal-free conditions, and a wide substrate scope established this protocol as an efficient and alternative method for obtaining sulfonated quinoxalin-2(1H)-ones.

Electrochemically promoted thio-Michael addition of N-substituted maleimides to thiols in an aqueous medium

A stable and practical electrochemical method was developed to promote the thio-Michael addition of N-substituted maleimides to various thiols in an aqueous medium. This protocol was found to be excellent in terms of facile scale-up, oxidant- and catalyst-free conditions, broad substrate scopes, good functional group tolerance, and easily available substrates. Notably, a plausible reaction mechanism was derived from the results of a series of control experiments and CV studies, which indicated that a radical pathway might speed up the thio-Michael addition under constant current.

Electrochemically Enable N-Sulfenylation/Phosphinylation of Sulfoximines via Oxidative Dehydrocoupling Reaction

An electrochemical oxidative cross-coupling strategy for the synthesis of N-sulfenylsulfoximines from sulfoximines and thiols was accomplished, giving diverse N-sulfenylsulfoximines in moderate to good yields. Moreover, this strategy can be extended to construct the N-P bond of N-phosphinylated sulfoximines. With electrons as reagents, the oxidative dehydrogenation cross-coupling reaction proceeds smoothly in the absence of traditional redox reagents.

Electrocatalytic C-H/S-H Coupling of Amino Pyrazoles and Thiophenols: Synthesis of Amino Pyrazole Thioether Derivatives

A mild method for the C-H/S-H coupling of pyrazol-5-amines and thiophenols was developed via electrochemistry, giving diverse amino pyrazole thioether derivatives in 37-98% yields. This electrochemical reaction is sustainable and an atom-efficient approach with good functional group tolerance and scalability by avoiding metal and external chemical oxidants.

Pushing Boundaries: What's Next in Metal-Free C-H Functionalization for Sulfenylation?

The synthesis of thioether derivatives has been explored widely due to the potential application of these derivatives in medicinal chemistry, pharmaceutical industry and material chemistry. Within this context, there has been an increasing demand for the environmentally benign construction of C-S bonds via C-H functionalization under metal-free conditions. In the present article, we highlight recent developments in metal-free sulfenylation that have occurred in the past three years. The synthesis of organosulfur compounds via a metal-free approach using a variety of sulfur sources, including thiophenols, disulfides, sulfonyl hydrazides, sulfonyl chlorides, elemental sulfur and sulfinates, is discussed. Non-conventional strategies, which refer to the development of thioether derivatives under visible light and electrochemically mediated conditions, are also discussed. The key advantages of the reviewed methodologies include broad substrate scope and high reaction yields under environmentally benign conditions. This comprehensive review will provide chemists with a synthetic tool that will facilitate further development in this field.

Design, synthesis and bioevaluation of PI3Kα-selective inhibitors as potential colorectal cancer drugs

The dysregulation of the phosphoinositide 3-kinase (PI3K)/mammalian target of rapamycin signaling pathway has been implicated in various human cancers, and isoform-selective inhibitors targeting PI3Kα have received significant interest in recent years. In this study, we have designed and synthesized three series of substituted benzoxazole derivatives based on the clinical candidate TAK-117 (8a). A detailed structure-activity relationship (SAR) study has identified the optimal compound 18a bearing a quinoxaline scaffold. Compared to the control 8a, 18a exhibited 4.4-fold more potent inhibitory activity against PI3Kα (IC50: 2.5 vs 11 nM) and better isoform-selective profiles over other PI3Ks. In addition, 18a showed a 1.5-fold more potent antiproliferative effect against HCT-116 cell lines (IC50: 3.79 vs 5.80 μM) and a better selectivity over the normal tissue cells. The potential antitumor mechanism and in vitro metabolic stability of 18a were also investigated. Notably, pharmacokinetic assays indicated that 18a had a higher plasma exposure, a higher maximum concentration and shorter elimination time compared to 8a.

Direct Access to Strained Fused Dihalo-Aziridino Quinoxalinones via C3-Alkylation Followed by Tandem Cyclization

Quinoxalinones are a privileged class of compounds, and their structural framework is found in many bioactive compounds, natural compounds, and pharmaceuticals. Quinoxalinone is a promising scaffold for different types of functionalization, and the slight modification of the quinoxalinone skeleton is known to offer a wide range of compounds for drug discovery. Owing to the importance of the quinoxalinone scaffold, we have developed a base-mediated protocol for the C3-alkylation of quinoxalinone followed by tandem cyclization to access novel types of strenuous and fused dihalo-aziridino-quinoxalinone heterocycles via the construction of C-C and C-N bonds. The protocol proved to be simple and practical to access desired fused quinoxalinone heterocycles in excellent yields (up to 98% yield). As an application, the highly functionalized fused dihalo-aziridino-quinoxalinone molecule has been further utilized for mono-dehalogenation under visible light irradiation and selective amide reduction. Moreover, the protocol has also been demonstrated on a gram scale.

Visible-Light-Induced Regioselective C-H Sulfenylation of Pyrazolo[1,5-a]pyrimidines via Cross-Dehydrogenative Coupling

A visible-light-induced cross-dehydrogenative methodology has been developed for the regioselective sulfenylation of pyrazolo[1,5-a]pyrimidine derivatives. Rose bengal, blue LEDs, KI, K₂S₂O₈, and DMSO are all essential for this photocatalytic transformation. The protocol is applicable for the synthesis of a library of 3-(aryl/heteroaryl thio)pyrazolo[1,5-a]pyrimidine derivatives with broad functionalities. The selectivity and scalability of the methodology have been also demonstrated. Moreover, the efficiency of this strategy for sulfenylation of pyrazoles, indole, imidazoheterocycles, and 4-hydroxy coumarin has been proven. The mechanistic investigation revealed the radical-based mechanism and formation of diaryl disulfide as a key intermediate for this cross-dehydrogenative coupling reaction.

Radical Fluorosulfonyl Heteroarylation of Unactivated Alkenes with Quinoxalin-2(1H)-ones and Related N-Heterocycles

The incorporation of sulfonyl fluoride groups into molecules has been proved effective to enhance their biological activities or introduce new functions. Herein, we report a transition-metal-free and visible-light-mediated radical 1-fluorosulfonyl-2-heteroarylation of alkenes, which could allow access to a series of SO₂F-containing quinoxalin-2(1H)-ones, which are a critical structural motif widely present in a number of biologically active molecules. Further application of the method to the modification of other heterocycles and drug molecules as well as ligation chemistry via SuFEx click reactions is also demonstrated.

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.

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.

An Oxidant- and Catalyst-Free Electrooxidative Cross-Coupling Approach to Synthesize meso-Substituted Porphyrin Derivatives

The synthesis of porphyrin and chlorin derivatives has attracted significant attention due to their numerous applications. Herein, we report an environment friendly oxidant- and catalyst-free electrooxidative cross-coupling approach for multiple coupling reactions to synthesize meso C-N, C-O, and C-S substituted porphyrin and chlorin derivatives. For C-N cross-coupling reactions, diaminated porphyrins were obtained as the main products, while using 4-bromo-2,6-dimethyl aniline resulted in monoaminated product. Similarly, electrochemical catalysis of porphyrins with phenol and thiophene produced meso-disubstituted porphyrins in moderate yields under a smaller current. Chlorins were also applicable, and 20-substituted products were efficiently produced regioselectively. To the best of our knowledge, this work represents the first example of electrooxidative C-X cross-coupling of porphyrins and chlorins.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Metal-free regioselective nitration of quinoxalin-2(1H)-ones with tert-butyl nitrite

A metal-free coupling of quinoxalin-2(1H)-ones with tert-butyl nitrite has been developed. Distinctly from the previous functionalization of quinoxalin-2(1H)-ones, this nitration reaction took place selectively at the C7 or C5 position of the phenyl ring, affording a series of 7-nitro and 5-nitro quinoxalin-2(1H)-ones in moderate to good yields. Preliminary mechanistic studies revealed that the reaction may involve a radical process.

Visible Light-Promoted Radical Relay Cyclization/C-C Bond Formation of N-Allylbromodifluoroacetamides with Quinoxalin-2(1H)-ones

A visible light-promoted radical relay of N-allylbromodifluoroacetamide with quinoxalin-2(1H)-ones was developed in which 5-exo-trig cyclization and C-C bond formation were involved. This protocol was performed under mild conditions to facilely offer a variety of hybrid molecules bearing both quinoxalin-2(1H)-one and 3,3-difluoro-γ-lactam motifs. These prepared novel skeletons would expand the accessible chemical space for structurally complex heterocycles with potential biological activities.

Oxidative cross-coupling of thiols for S-X (X = S, N, O, P, and C) bond formation: mechanistic aspects

This review focuses on the reactive intermediates (disulfides, sulfenyl halides, thiyl radicals, sulfenium cations, and metal-organosulfur species) and the mechanisms of the recently reported oxidative couplings of thiols. These intermediates are generated by chemical oxidants, transition metal catalysts, electrochemistry, and photochemistry. Chemical oxidant-mediated reactions involve radical, halogenated, or cationic intermediates, or disulfides. Transition metal-catalyzed mechanisms proposed various metal-organosulfur intermediates to elucidate the reactivity and selectivity of metal catalysts. In electro- and photooxidation, direct oxidation/reduction mechanisms of reactants at the electrode or indirect oxidation/reduction of reactants in the presence of redox catalysts have been reported. The following sections are based on the products, thiosulfonates (S-S bond), sulfenamides, sulfinamides, and sulfonamides (S-N bond), sulfinates (S-O bond), thiophosphine oxides and thiophosphates (S-P bond), and sulfides, sulfoxides, and sulfones (S-C bond) and discuss the reaction mechanisms and the above-mentioned key intermediates for product formation. The contents of this review will provide helpful information, guiding the choice of oxidative coupling conditions for the synthesis of various organosulfur compounds with high yields and selectivity.

Unconventional approaches for the introduction of sulfur-based functional groups

Organosulfur compounds have a pivotal role in the functionalities of many natural products, pharmaceuticals and organic materials. For these reasons, the search for new methodologies for the formation of carbon-sulfur bonds has been the object of intensive work for organic chemists. However, the proposed strategies suffer from various drawbacks, such as volatility, toxicity, and instability of the sulfur sources or the use of VOC solvents. In this review, we summarise the recent protocols which have the goal of obtaining sulfones, thioethers, thiazines, thiazepines and sulfonamides in an unconventional and/or sustainable way. The use of starting materials less invasive and toxic with respect to the traditional reagents, alternative solvents such as water, ionic liquids or deep eutectic solvents, the exploitation of ultrasound and electrochemistry, increasing the efficiency of the process, are reported. Moreover, representative reaction mechanisms are also discussed.

N,N,N',N'-Tetramethylethylenediamine-Enabled Photoredox-Catalyzed C-H Methylation of N-Heteroarenes

Aiming at the valuable methylation process, readily available and inexpensive N,N,N',N'-tetramethylethylenediamine (TMEDA) was first identified as a new methyl source in photoredox-catalyzed transformation in this work. By virtue of this simple methylating reagent, a facile and practical protocol for the direct C-H methylation of N-heteroarenes was developed, featuring mild reaction conditions, broad substrate scope, and scalability. Mechanistic studies disclosed that a sequential photoredox, base-assisted proton shift, fragmentation, and tautomerization process was essentially involved.

δ-Regioselective heteroarylation of free alcohols through 1,5-hydrogen-atom transfer

Silver-catalyzed δ-regioselective heteroarylation of C(sp³)–H bonds (1°, 2°, and 3°) in alcohols with divergent N-heteroarenes.

Transition metal-free C-3 functionalization of quinoxalin-2(1H)-ones: recent advances and sanguine future

A gradual shift from metal-catalyzed to metal-free methods is occurring, as the latter are more environmentally benign. This review discusses sustainable protocols for the construction of C–C, C–N, C–P, C–S, and C–O bonds via C–H functionalization of quinoxalin-2(1H)-ones.

Copper-catalyzed chemoselective C-H functionalization of quinoxalin-2(1H)-ones with hexafluoroisopropanol

An unexpected three-component reaction of quinoxalin-2(1H)-ones, tert-butyl peroxybenzoate (TBPB), and hexafluoroisopropanol (HFIP) is described. Under CuBr-catalyzed and TBPB-oxidized conditions, a variety of hydroxyhexafluoroisobutylated quinoxalin-2(1H)-ones were formed. Furthermore, the first hexafluoroisopropoxylation of the quinoxalin-2(1H)-ones with HFIP is also demonstrated with Cu2O as the catalyst and PhI(OAc)2 as the oxidant. These new transformations of HFIP furnish previously unknown and potentially useful fluorinated quinoxalin-2(1H)-one derivatives.

tert-Butylhydroperoxide (TBHP) mediated oxidative cross-dehydrogenative coupling of quinoxalin-2(1H)-ones with 4-hydroxycoumarins, 4-hydroxy-6-methyl-2-pyrone and 2-hydroxy-1,4-naphthoquinone under metal-free conditions

We report an efficient and atom-economical method of C-3 functionalization of quinoxalin-2(1H)-ones with 4-hydroxycoumarins, 4-hydroxy-6-methyl-2-pyrone, and 2-hydroxy-1,4-naphthoquinone via the free radical cross-coupling pathway under metal-free conditions. tert-Butylhydroperoxide (TBHP) smoothly promotes the reaction furnishing the cross-dehydrogenative coupling (CDC) products in very good to excellent yields. The protocol neither uses any toxic reagents nor metal catalysts to carry out the reaction, and all the products have been obtained without column chromatography purification. Different radical trapping experiments with 2,2,6,6-tetramethylpiperidine-1-oxyl, butylated hydroxytoluene, and diphenyl ethylene confirm the involvement of radicals.