Transition Metal-Free Iodosobenzene-Promoted Direct Oxidative 3-Arylation of Quinoxalin-2(H)-ones with Arylhydrazines

Metal- and photocatalyst-free approach to visible-light-induced acylation of quinoxalinones

A transition-metal- and photocatalyst-free photochemical reaction was successfully developed for the direct acylation of quinoxalin-2(1H)-ones, which was enabled by the formation of electron donor-acceptor (EDA) complexes. The use of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as the electron donor allows efficient and operationally simple access to a series of C3-aroylated and acylated quinoxalin-2(1H)-ones with moderate to good yields.

Visible Light-Induced Electron-Donor-Acceptor-Mediated C-3 Coupling of Quinoxalin-2(1H)-ones with Unactivated Aryl Iodides

Visible light-induced C-3 arylation of quinoxalin-2(1H)-ones with abundantly available aryl iodides with good yields via an electron-donor-acceptor (EDA)-complex formation have been accomplished. The radical scavenging, Electron paramagnetic resonance (EPR), UV-visible experiments, density functional theory (DFT), and quantum yield studies revealed that the reaction went through a radical pathway via a single electron transfer (SET) process. Furthermore, the protocol could also be applied to the synthesis of biologically active molecules, illustrating the practicality of the present protocol.

Paired Electrolysis-Enabled Arylation of Quinoxalin-2(1H)-ones

The first paired electrolysis-enabled arylation of quinoxalin-2(1H)-ones was achieved using cyanoarenes as the arylation reagents. A variety of 3-arylquinoxalin-2(1H)-ones with various important functional groups were obtained in moderate to good yields under metal- and chemical oxidant-free conditions. With a pair of reductive and oxidative processes occurring among the substrates and reaction intermediates, the power consumption can be dramatically reduced.

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.

Recent Developments in Direct C-H Functionalization of Quinoxalin-2(1H)-Ones via Heterogeneous Catalysis Reactions

In recent years, Web of Science has published nearly one hundred reports per year on quinoxalin-2(1H)-ones, which have attracted great interest due to their wide applications in pharmaceutical and materials fields, especially in recyclable heterogeneous catalytic reactions for direct C-H functionalisation. This review summarises for the first time the methods and reaction mechanisms of heterogeneous catalytic reactions of quinoxalin-2(1H)-ones, including six major types of heterogeneous catalysts involved. The heterogeneous reactions of quinoxalin-2(1H)-ones are summarised by classifying different types of catalytic materials (graphitic phase carbon nitride, MOF, COF, ion exchange resin, piezoelectric materials, and microsphere catalysis). In addition, this review discusses the future development of heterogeneous catalytic reactions of quinoxalin-2(1H)-ones, including the construction of C-B/Si/P/R_F/X/Se bonds by heterogeneous catalytic reactions, the enrichment of heterogeneous catalysts such as metal oxides, graphene-based composites, doped metal nanoparticles, and molecular sieve-based porous materials, asymmetric synthesis, and other areas. The aim of this review is to contribute to the development of green and sustainable heterogeneous reaction methods for quinoxalin-2(1H)-ones with applications in materials chemistry and pharmacology.

Metal-free visible-induced C(sp2)-C(sp2) coupling of quinoxalin-2(H)-ones via oxidative cleavage of the C-N bond

A C(sp²)-C(sp²) reaction between aromatic hydrazines and quinoxalines has been developed through a photocatalytic system. The protocol is established for C(sp²)-N bond cleavage and direct C(sp²)-H functionalization for the coupling of C(sp²)-C(sp²) via photocatalysis under mild and ideal air conditions without the presence of a strong base and metal. The mechanistic studies reveal that the generation of a benzene radical via the oxidative cleavage of aromatic hydrazines for the cross-coupling of C(sp²)-C(sp²) with the assistance of a photocatalyst is essential. The process exhibits excellent compatibility with functional groups and provides convenient access to various 3-arylquinoxalin-2(1H)-ones in good to excellent yields.

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.

Enroute sustainability: metal free C-H bond functionalisation

The term "C-H functionalisation" incorporates C-H activation followed by its transformation. In a single line, this can be defined as the conversion of carbon-hydrogen bonds into carbon-carbon or carbon-heteroatom bonds. The catalytic functionalisation of C-H bonds using transition metals has emerged as an atom-economical technique to engender new bonds without activated precursors which can be considered as a major drawback while attempting large-scale synthesis. Replacing the transition-metal-catalysed approach with a metal-free strategy significantly offers an alternative route that is not only inexpensive but also environmentally benign to functionalize C-H bonds. Recently metal free synthetic approaches have been flourishing to functionalize C-H bonds, motivated by the search for greener, cost-effective, and non-toxic catalysts. In this review, we will highlight the comprehensive and up-to-date discussion on recent examples of ground-breaking research on green and sustainable metal-free C-H bond functionalisation.

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.

Visible Light Photoredox-Catalyzed Direct C-H Arylation of Quinoxalin-2(1H)-ones with Diaryliodonium Salts

A photoredox-catalyzed direct arylation of quinoxalin-2-(1H)-ones using diaryliodonium triflates as the convenient, stable, and cheap aryl source is described. A broad variety of quinoxalin-2-(1H)-ones are shown to react with structurally and electronically diverse diaryliodonium triflates, allowing efficient access to a wide variety of pharmaceutically important 3-arylquinoxalin-2-(1H)-ones. The presented method is attractive with regard to operational simplicity, mild conditions, broad scope, scalability, and high functional group tolerance.

A HCl-Mediated, Metal- and Oxidant-Free Photocatalytic Strategy for C3 Arylation of Quinoxalin(on)es with Arylhydrazine

A novel and simple HCl-mediated, photocatalytic method for quinoxaline(on)es C3-H arylation with arylhydrazine under transition metal catalyst- and oxidant-free conditions is presented. Various quinoxaline(on)es underwent this transformation smoothly, demonstrating a broad substrate tolerance and providing the corresponding aryl products in moderate to excellent yields. Mechanistic studies indicated that a radical pathway may be involved in this transformation.

Copper-catalyzed C-3 benzylation of quinoxalin-2(1H)-ones with benzylsulfonyl hydrazides

An unprecedented use of benzylsulfonyl hydrazides as benzylating agents has been demonstrated in the direct C-3 benzylation of quinoxalin-2(1H)-ones. A range of benzylsulfonyl hydrazides participated in the C-3 benzylation of quinoxalin-2(1H)-ones with CuCN as the catalyst and DTBP as the oxidant, delivering structurally diverse 3-benzylquinoxalin-2(1H)-ones in moderate to good yields.

Regioselective C-3-alkylation of quinoxalin-2(1H)-ones via C-N bond cleavage of amine derived Katritzky salts enabled by continuous-flow photoredox catalysis

An efficient, transition metal-free visible-light-driven continuous-flow C-3-alkylation of quinoxalin-2(1H)-ones has been demonstrated by employing Katritzky salts as alkylating agents in the presence of eosin-y as a photoredox catalyst and DIPEA as a base at room temperature. The present protocol was accomplished by utilizing abundant and inexpensive alkyl amine (both primary and secondary alkyl) and as well as this a few amino acid feedstocks were converted into their corresponding redox-active pyridinium salts and subsequently into alkyl radicals. A wide variety of C-3-alkylated quinoxalin-2(1H)-ones were synthesized in moderate to high yields. Further this environmentally benign protocol is carried out in a PFA (Perfluoroalkoxy alkane) capillary based micro reactor under blue LED irradiation, enabling excellent yields (72% to 91%) and shorter reaction times (0.81 min) as compared to a batch system (16 h).

An efficient, transition metal-free visible-light-driven continuous-flow C-3-alkylation of quinoxalin-2(1H)-ones has been demonstrated enabling excellent yields (72% to 91%) and shorter reaction time (0.81 min) as compared to batch system (16 h).

PhIO-Mediated Oxidative C═C Bond Cleavage and Reassembly toward Highly Functionalized Oxazolones

An efficient PhIO-mediated oxidative C═C bond cleavage and reassembly of enaminone toward oxazolone with high regioselectivity has been reported. DFT calculations revealed that the reaction proceeded through an oxygen atom transfer, C═C bond cleavage, alkylthio migration, and reassembly cascade. This strategy is highlighted by high atom and step economy with formation of five bonds in one pot and generation of a high-valued oxazolone skeleton under mild conditions.

Bromine-radical-induced Csp2–H difluoroalkylation of quinoxalinones and hydrazones through visible-light-promoted Csp3–Br bond homolysis

Bromine radicals derived from photo-induced Csp3–Br bond homolysis can mediate H abstraction/imine radical formation from quinoxalinones and hydrazones, which in turn quench the in situ-generated difluoroalkyl radicals to furnish the products.

C–H benzylation of quinoxalin-2(1H)-ones via visible-light riboflavin photocatalysis

An efficient visible light promoted riboflavin-catalyzed direct benzylation of substituted quinoxalin-2(1H)-ones for the synthesis of various C3-benzylated quinoxalin-2(1H)-one derivatives has been developed under mild conditions.

Sustainable C–H activation approach for palladium-catalyzed, regioselective functionalization of 1-methyl-3-phenyl quinoxaline-2(1H)-ones in water

An environment-friendly approach for regioselective acylation of 1-methyl-3-phenyl quinoxaline-2(1H)-ones was developed using water as a solvent. The protocol exhibits a wide substrate scope and employs commercially available, non-toxic acyl surrogates.

H2O2-mediated room temperature synthesis of 2-arylacetophenones from arylhydrazines and vinyl azides in water

An environmentally benign, cost-efficient and practical methodology for the room temperature synthesis of 2-arylacetophenones in water has been discovered. The facile and efficient transformation involves the oxidative radical addition of arylhydrazines with α-aryl vinyl azides in the presence of H₂O₂ (as a radical initiator) and PEG-800 (as a phase-transfer catalyst). From the viewpoint of green chemistry and organic synthesis, the present protocol is of great significance because of using cheap, non-toxic and readily available starting materials and reagents as well as amenability to gram-scale synthesis, which provides an attractive strategy to access 2-arylacetophenones.

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.

Recent Advances in the Transition-Metal-Free Arylation of Hetero­arenes

Transition-metal-free arylation reactions have attracted considerable attention for economic and environmental reasons over the past 40 years. In recent years, much effort has been made to develop efficient transition-metal-free approaches for the arylation of heteroarenes. Covering the literature from 2015 to early 2021, this review aims to provide a thorough overview of the synthetic and mechanistic aspects of these atom-economical and environmentally benign reactions.

1 Introduction

2 Arylation of Pre-functionalized Heteroarenes

2.1 Arylation of Heteroaryl Halides

2.2 Decarboxylative Arylation of Heteroarenes

3 Direct C–H Arylation of Heteroarenes

3.1 C(sp2)–H Arylation

3.2 C(sp3)–H Arylation

4 N-Arylation of Heteroarenes

5 Summary and Outlook

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.

Visible-Light-Induced C(sp2)-C(sp3) Cross-Dehydrogenative-Coupling Reaction of N-Heterocycles with N-Alkyl-N-methylanilines under Mild Conditions

Disclosed herein is a cross-dehydrogenative-coupling reaction of N-heterocycles including 1,2,4-triazine-3,5(2H, 4H)-diones and quinoxaline-2(1H)-ones with N-methylanilines to form C(sp²)-C(sp³) under visible-light illumination and ambient air at room temperature. In this process, easily available Ru(bpy)₃Cl₂·6H₂O serves as the catalyst, and air acts as the green oxidant. This method features high atom economy, environmental friendliness, and convenient operation and provides an efficient and practical access to aminomethyl-substituted N-heterocycles with extensive functional group compatibility in 40-86% yields.

On-DNA Derivatization of Quinoxalin-2-ones by Visible-Light-Triggered Alkylation with Carboxylic Acids

An efficient visible-light-induced alkylation of DNA-tagged quinoxaline-2-ones was described. The methodology demonstrated moderate-to-excellent conversions under mild conditions. The reaction was found to be tolerant with both N-protected α-amino acids and aliphatic carboxylic acids and could be applied to the synthesis of focused DNA-encoded quinoxalin-2-one libraries.

Ni-Catalyzed radical cyclization of vinyl azides with cyclobutanone oxime esters to access cyanoalkyl containing quinoxalin-2(1H)-ones

A nickel-catalyzed cascade addition/cyclization of 2-azido-N-arylacrylamides and cyclobutanone oxime esters for the construction of 3-cyanoalkylated quinoxalin-2(1H)-ones is developed. This reaction proceeds under mild conditions with good functional group tolerance and broad substrate scope. A preliminary mechanistic experiment indicated that the cyanoalkyl radical might be involved in this transformation.

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.