Visible-Light-Induced Singlet Oxygen-Promoted Arylation and Alkylation of Quinoxalin-2(1H)-ones and Quinolines

We report a green and efficient visible-light-driven method for the arylation and alkylation of quinoxalin-2(1H)-ones and quinolines. This catalyst-free process utilizes air as the oxidant, offering mild reaction conditions, environmental sustainability, and broad functional group compatibility. The approach enables the synthesis of aryl and alkyl derivatives of quinoxalin-2(1H)-ones and quinolines with high to excellent yields.

Pyrazinoquinoxaline derivatives for flexible electronic devices: effect of the mechanical properties of the crystals on device durability

Understanding the interplay between the molecular structure and material properties of emerging p-type organic semiconductors marks a significant stride in the advancement of molecular electronics. Among the array of promising materials, mechanically flexible single crystals of π-conjugated molecules stand out due to their potential for cutting-edge applications in organic electronics. Notably, derivatives of pyrazinoquinoxaline (PQ) are recognized as versatile building blocks for constructing π-conjugated systems, showcasing good semiconductor performance in organic field-effect transistors (OFETs). In this study, we present an exploration into the p-type charge transport and mechanical characteristics of two newly synthesized PQ derivatives: 5,10-diphenyl-2,3,7,8-tetra(thiophen-2-yl)pyrazino[2,3-g]quinoxaline (DPTTQ) and 2,3,5,7,8,10-hexa(thiophen-2-yl)pyrazino[2,3-g]quinoxaline (HTPQ). HTPQ crystals exhibit flexural behaviour under applied stress, effortlessly returning to their initial configuration upon relaxation. Conversely, two polymorphic forms of DPTTQ crystals display brittle fracture when subjected to a similar stress. Specifically, DPTTQ molecules adopt a β-sheet packing, while HTPQ presents a γ-packing with a corrugated arrangement. Field-effect charge transport measurements reveal p-type charge transport in both DPTTQ and HTPQ, with HTPQ showcasing hole mobility up to 0.01 cm2 V-1 s-1, while DPTTQ exhibits mobility that is at least one order of magnitude lower. This variance in the field effect mobility can be directly correlated to the difference in crystal packing, highlighting a clear structure-property correlation. Moreover, taking advantage of the flexural nature of the HTPQ crystals, we fabricated durable electronic devices, which retain their conductivity for over 60 cycles of strain, indicating the efficacy of our chemical design in demonstrating high-performance flexible devices. These findings underscore the promise of semiconducting organics with γ-packing for achieving both better mobility and elasticity for integration into organic electronic devices.

Base-Mediated Visible-Light-Driven C-H Arylation of Quinoxalin-2(1H)-Ones in Ethanol

Prior methods for visible-light-driven C-H arylation of quinoxalin-2(1H)-ones relied on external photocatalysts. Herein, we report a photocatalyst-free approach for this arylation. In this approach, β-dicarbonyl iodonium ylides, combined with t-BuOK in ethanol, act as aryl precursors, forming electron donor-acceptor (EDA) complexes. These complexes enhance light absorption, facilitating efficient single electron transfer and aryl radical formation. Consequently, various quinoxalin-2(1H)-ones undergo precise and efficient arylation without external photocatalysts. This protocol exhibits excellent tolerance toward diverse functional groups, with mild reaction conditions and eco-friendly solvents, revealing a high Ecoscale value.

Photoexcited nitroarenes for alkylation of quinoxalin-2(1H)-ones

A straightforward method for the dehydrogenative alkylation of quinoxalin-2(1H)-ones with alkylbenzenes has been developed, facilitated by a photoexcited nitroarene. The reaction's success hinges on the dual role of the photoexcited nitroarene molecule, acting as both a hydrogen atom transfer (HAT) reagent and an oxidant. This technique is both atom-economical and cost-effective, due to the readily available nitroarene, which serves as the sole intermediary in the reaction process.

Mild Iron-Catalyzed Oxidative Cross-Coupling of Quinoxalinones with Indoles

Utilizing iron chloride as a Lewis acid catalyst, we developed a straightforward and mild oxidative cross-coupling reaction between quinoxalinones and indoles, yielding a series of versatile 3-(indol-3-yl)quinoxalin-2-one derivatives. This approach allows for the incorporation of a wide array of functional groups into the final products, demonstrating its synthetic versatility. Notably, the method was successfully scaled up to gram-scale reactions while maintaining high yields. Our mechanistic investigation indicates that iron chloride serves as a catalyst to facilitate the formation of key intermediates which subsequently undergo oxidation to afford the desired products. The merits of this protocol include its cost effectiveness, operational simplicity, and the ease of product isolation via filtration.

Metal-Organic Framework Based on Pyrazinoquinoxaline Tetracarboxylic Acid for Fluorescence Sensing for Nitro Explosives

The rapid and selective detection of nitro explosives has become one of the current urgent environmental and safety issues. Fluorescent metal-organic frameworks (MOFs) provide strong support for the development of photoactive materials with excellent sensing performances. In this work, Zn2+ and pyrazinoquinoxaline tetracarboxylic acid with high nitrogen content were selected to construct a MOF structure termed Zn-MOF, which had excellent optical properties. The fluorescence sensing performance of Zn-MOF for nitro explosives was also investigated. The structural advantages of Zn-MOF, such as its porous structure, abundant host-guest interaction sites, and stable framework, ensure the prerequisites for various applications. Zn-MOF is not only capable of responding to a wide range of substrates, such as Fe3+, Cr2O72-, and MnO4-, to achieve fluorescence quenching detection but also able to achieve sensitive fluorescence sensing behavior for nitro explosives. In particular, for trinitrotoluene, the Ksv value can reach 8.72 × 103 M-1. The results show that the introduction of pyrazinoquinoxaline groups into MOFs can be an effective strategy for the preparation of highly efficient fluorescent sensing materials for nitro explosives.

Divergent Synthesis of 3-(Indol-2-yl)quinoxalin-2-ones and 4-(Benzimidazol-2-yl)-3-methyl(aryl)cinnolines via Polyphosphoric Acid (PPA)-Mediated Intramolecular Rearrangements of 3-(Methyl/aryl(2-phenylhydrazono)methyl)quinoxalin-2-ones

Herein, we report a polyphosphoric acid (PPA)-mediated divergent metal-free operation to access a diverse collection of 3-(indol-2-yl)quinoxalin-2-ones and 4-(benzimidazol-2-yl)-3-methylcinnolines in moderate to excellent overall yields. The described process involves two distinct, and competing rearrangements of 3-(methyl(2-phenylhydrazono)methyl)quinoxalin-2-ones, namely [3,3]-sigmatropic Fischer rearrangement with the formation of an indole ring to produce 3-(indol-2-yl)-quinoxalin-2-ones, and Mamedov rearrangement with simultaneous construction of benzimidazole and cinnoline rings to form the new biheterocyclic system─4-(benzimidazol-2-yl)-3-methylcinnolines. The reaction mechanism of both rearrangement channels is explored by extensive dispersion-corrected DFT calculations. It is partcularly remarkable that when 3-(aryl(2-phenylhydrazono)methyl)quinoxalin-2-ones is used, instead of 3-(methyl(2-phenylhydrazono)methyl)quinoxalin-2-ones, reactions proceed regioselectively with the formation of only rearrangement products─4-(benzimidazol-2-yl)-3-arylcinnolines with high yields. This operationally simple protocol enables a rapid access to these scaffolds and is compatible with a wide scope of starting materials. In addition, the new rearrangement found features a promising approach for the design of unique compound libraries for drug design and discovery programs.

DABCO-Catalyzed DMSO-Promoted Sulfurative 1,2-Diamination of Phenylacetylenes with Elemental Sulfur and o-Phenylenediamines: Access to Quinoxaline-2-thiones

The oxidative amination of alkynes typically requires transition metal catalysts and strong oxidants. Herein, we alternatively utilize DABCO as a sulfur-activating catalyst to achieve the sulfurative 1,2-diamination of phenylacetylenes with elemental sulfur and o-phenylenediamines. DMSO was found to be particularly suitable for use as a terminal oxidant for this three-component process. A mechanistic study has shown that this cascade reaction is triggered by the addition of active sulfur species to the triple bond of phenylacetylenes.

Oxidative cross-coupling of quinoxalinones with indoles enabled by acidochromism

An oxidative cross-coupling of quinoxalinones with indole derivatives via B(C₆F₅)₃·H₂O induced acidochromism of quinoxalinone derivatives was developed under mild and external photocatalyst-free conditions. The reaction shows excellent substrate scope, accommodating a wide range of functional groups. The usefulness of this strategy was demonstrated by the synthesis of the natural products Azacephalandole A and Cephalandole A in high yields. Moreover, the products are fluorophores showing prevalent fluorescence properties with a wide emission range and good relative quantum yields.

Post-synthesis from Lewis acid–base interaction: an alternative way to generate light and harvest triplet excitons

The changes in absorption and emission of fluorescent materials with the introduction of Lewis acids have been frequently observed due to either physical or chemical interactions. In this mini-review, we elaborate how Lewis acids adjust the optical properties and the bandgap of luminescent materials by simple coordination reactions. It is common that fluorescent materials containing Lewis basic nitrogen heterocycles are more likely to provide the feasible band gap modulation. The essence of such phenomenon originates from Lewis acid–base coordination and adducts, which highly depends on the electron-accepting property of the Lewis acids. This intermolecular mechanism, considered as post-synthesis of new luminescent compounds offers promising applications in sensing and electroluminescence by manipulating the frontier molecular orbital energy levels of organic conjugated materials, simply based on Lewis acid–base chemistry.

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.

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 C–H functionalization to synthesize 3-hydroxyalkylquinoxalin-2(1H)-ones via quinoxalin-2(1H)-ones and aldehydes

We reported an electrocatalytic direct C3-hydroxyalkylation of quinoxalin-2(1H)-ones to construct 3-hydroxyalkylquinoxalin-2(1H)-one derivatives, which uses unprotected quinoxalin-2(1H)-ones and aliphatic aldehydes as substrates.

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.

Diastereoselective [3 + 2] Cycloaddition of Quinoxalin-2(1H)-ones with Donor-Acceptor Cyclopropanes: Efficient Synthesis of Tetrahydro pyrrolo[1,2-a]quinoxalin-4(5H)-ones

A ytterbium triflate-catalyzed diastereoselective [3 + 2] cycloaddition of quinoxalinones with donor-acceptor cyclopropanes and cyclobutanes is described. A series of tetrahydropyrrolo-quinoxalinone derivatives were obtained in high yields (up to 96%) with excellent diastereoselectivities (up to 46:1). Other medicinally important heterocycles like benzoxazinone, isoquinoxalinone, and dibenzoxazepine derivatives were also suitable for the desired annulation reaction. The current method is applicable for the scale-up reaction. Further, the utility of this annulation reaction is demonstrated by the synthesis of densely functionalized proline derivatives.

Electroreductive C3 Pyridylation of Quinoxalin-2(1H)-ones: An Effective Way to Access Bidentate Nitrogen Ligands

The construction of functional N-containing active biomolecules and bidentate nitrogen ligands by electroreductive pyridylation of N-heteroaromatics is an eye-catching task and challenge. A simple and practical electroreductive-induced C3 pyridylation of quinoxalin-2(1H)-ones with readily available cyanopyridines is reported. More than 36 examples are supplied, and the reaction performed in >95% yield. The present protocol provides a convenient, efficient, and gram-scale synthesis strategy for a series of new types of potential bidentate nitrogen ligands.

Visible-light-promoted/PIFA-mediated direct C–H acylation of quinoxalin-2(1H)-ones with aldehydes

With aldehydes as the radical precursors under visible-light irradiation, a simple and mild PIFA-mediated C–H acylation reaction of quinoxalin-2(1H)-ones has been achieved.

Synthesis of (E)-Quinoxalinone Oximes through a Multicomponent Reaction under Mild Conditions

Herein, a novel method for the gram-scale synthesis of (E)-quinoxalinone oximes through a multicomponent reaction under mild conditions is described. Such a transformation was performed under transition-metal-free conditions, affording (E)-oximes in a moderate-to-good yield through recrystallization. Our methodology demonstrates a successful combination of a Mannich-type reaction and radical coupling, providing a green and practical approach for the synthesis of potentially bioactive quinoxalinone-containing molecules.

K2S2O8 mediated C-3 arylation of quinoxalin-2(1H)-ones under metal-, photocatalyst- and light-free conditions

Two facile and effective C-3 arylation protocols of quinoxalin-2(1H)-ones with arylhydrazines and aryl boronic acids respectively via free radical cross-coupling reactions under metal-, photocatalyst- and light-free conditions have been unveiled. K2S2O8 has been used as an efficient oxidant to generate aryl radicals from arylhydrazines and aryl boronic acids under two different reaction conditions. The generated aryl radicals undergo a free radical coupling reaction at the C-3 position of quinoxalin-2(1H)-ones producing 3-arylquinoxalin-2(1H)-ones in good to excellent yields. The involvement of radicals in the course of the reaction has been demonstrated by radical trapping experiments with 2,2,6,6-tetramethylpiperidine-1-oxyl.

Transition-metal-free direct C-3 cyanation of quinoxalin-2(1H)-ones with ammonium thiocyanate as the “CN” source

A practical protocol for TBHP-mediated oxidative C–H cyanation of quinoxalin-2(1H)-ones utilizing ammonium thiocyanate as the cyanide source has been developed under metal free conditions.

Metal-free direct oxidative C–C bond coupling of pyrazolones and quinoxalinones

An efficient oxidative dehydrogenative coupling of quinoxalinones and pyrazolones has been successfully developed using a readily available persulfate oxidant. This protocol provides facile access to a wide array of hydroxy-pyrazolyl quinoxalinones in good to excellent yields.

Persulfate-promoted oxidative C–N bond coupling of quinoxalinones andNH-sulfoximines

A simple approach for a direct sulfoximination of quinoxalinonesviaK₂S₂O₈-mediated oxidative coupling is reported.

Transition-Metal-Free Direct C-H Arylation of Quinoxalin-2(1H)-ones with Diaryliodonium Salts at Room Temperature

A method of synthesizing 3-arylquinoxalin-2(1H)-ones using diaryliodonium tetrafluoroborates under mild conditions is described. This protocol has a wide substrate scope and enables direct C-H functionalization. The synthetic potential of this coupling was explored using a range of readily accessible diaryliodonium salts and quinoxalin-2(1H)-ones.