Covalent Organic Frameworks: A Sustainable Photocatalyst toward Visible-Light-Accelerated C3 Arylation and Alkylation of Quinoxalin-2(1H)-ones

Photocatalytic applications of covalent organic frameworks: synthesis, characterization, and utility

Photocatalysis has emerged as an energy efficient and safe method to perform organic transformations, and many semiconductors have been studied for use as photocatalysts. Covalent organic frameworks (COFs) are an established class of crystalline, porous materials constructed from organic units that are easily tunable. COFs importantly display semiconductor properties and respectable photoelectric behaviour, making them a strong prospect as photocatalysts. In this review, we summarize the design, synthetic methods, and characterization techniques for COFs. Strategies to boost photocatalytic performance are also discussed. Then the applications of COFs as photocatalysts in a variety of reactions are detailed. Finally, a summary, challenges, and future opportunities for the development of COFs as efficient photocatalysts are entailed.

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.

Unveiling the catalytic potency of a novel hydrazone-linked covalent organic framework for the highly efficient one-pot synthesis of 1,2,4-triazolidine-3-thiones

A novel hydrazone-linked covalent organic framework (TRIPOD-DHTH COF) was synthesized through the ultrasonic treatment of 2,5-dihydroxyterephthalohydrazide (DHTH) and 4,4',4''-[1,3,5-triazine-2,4,6-triyltris(oxy)]tris-benzaldehyde (TRIPOD). The COF was extensively analyzed using FT-IR, PXRD, SEM, TEM, BET, XPS, TGA, and DTA techniques. The characterization studies revealed the presence of mesoporous properties and high thermal stability, with a surface area measuring 2.78 m2 g-1 and an average pore size of 8.88 nm. The developed COF demonstrated exceptional catalytic activity in synthesizing 1,2,4-triazolidine-3-thiones from thiosemicarbazide and various ketones and aldehydes using a water : ethanol (1 : 2) medium at room temperature. A significant yield (80-98%) of 1,2,4-triazolidine-3-thiones was obtained in a low reaction time (4-20 min). The role of TRIPOD as a precursor in the synthesis of the COF and as a reactant in the synthesis of 1,2,4-triazolidine-3-thione (3l) was found to be fascinating. The synthesized COF maintained its catalytic activity over eight runs, underscoring its efficiency and reusability, highlighting its potential for sustainable chemical syntheses.

Hydrazone-Linked Covalent Organic Frameworks

Hydrazone-linked covalent organic frameworks (COFs) with structural flexibility, heteroatomic sites, post-modification ability and high hydrolytic stability have attracted great attention from scientific community. Hydrazone-linked COFs, as a subclass of Schiff-base COFs, was firstly reported in 2011 by Yaghi's group and later witnessed prosperous development in various aspects. Their adjustable structures, precise pore channels and plentiful heteroatomic sites of hydrazone-linked structures possess much potential in diverse applications, for example, adsorption/separation, chemical sensing, catalysis and energy storage, etc. Up to date, the systematic reviews about the reported hydrazone-linked COFs are still rare. Therefore, in this review, we will summarize their preparation methods, characteristics and related applications, and discuss the opportunity or challenge of hydrazone-linked COFs. We hope this review could provide new insights about hydrazone-linked COFs for exploring more appealing functions or applications.

The Application of Porous Organic Polymers as Metal Free Photocatalysts in Organic Synthesis

Concerns about increasing greenhouse gas emissions and their effect on our environment highlight the urgent need for new sustainable technologies. Visible light photocatalysis allows the clean and selective generation of reactive intermediates under mild conditions. The more widespread adoption of the current generation of photocatalysts, particularly those using precious metals, is hampered by drawbacks such as their cost, toxicity, difficult separation, and limited recyclability. This is driving the search for alternatives, such as porous organic polymers (POPs). This new class of materials is made entirely from organic building blocks, can possess high surface area and stability, and has a controllable composition and functionality. This review focuses on the application of POPs as photocatalysts in organic synthesis. For each reaction type, a representative material is discussed, with special attention to the mechanism of the reaction. Additionally, an overview is given, comparing POPs with other classes of photocatalysts, and critical conclusions and future perspectives are provided on this important field.

Post-synthetic Rhodium (III) Complexes in Covalent Organic Frameworks for Photothermal Heterogeneous C-H Activation

Although photocatalytic C-H activation has been realized by using heterogeneous catalysts, most of them require high-temperature conditions to provide the energy required for C-H bond breakage. The catalysts with photothermal conversion properties can catalyze this reaction efficiently at room temperature, but so far, these catalysts have been rarely developed. Here, we construct bifunctional catalysts Rh-COF-316 and -318 to combine photosensitive covalent organic frameworks (COFs) and transition-metal catalytic moiety using a post-synthetic approach. The Rh-COF enable the heterogeneous C-H activation reaction by photothermal conversion for the first time, and exhibit excellent yields (up to 98 %) and broad scope of substrates in [4+2] annulation at room temperature, while maintaining the high stability and recyclability. Significantly, this work is the highest yield reported so far in porous materials catalyzing C(sp2)-C(sp2) formation at room temperature. The excellent performances can be attributed to the COF-316, which enhances the photothermal effect (ΔT=50.9 °C), thus accelerating C-H bond activation and the exchange of catalyst with substrates.

Photoinduced arylation or alkylation of 1,2,4-triazine-3,5(2H,4H)-diones with hydrazines

A light-induced method is developed for synthesizing azauracils. This method is independent from traditional methodology. Remarkably, this reaction can also be powered by sunlight. The applicability of this method is further demonstrated through its successful implementation in large-scale reactions and its use in synthesizing derivatives.

Visible Light-Driven Flexible Synthesis of α-Alkylated Glycine Derivatives Catalyzed by Reusable Covalent Organic Frameworks

Herein, we report a heterogeneous visible light-driven preparation of α-alkylated glycine derivatives. This approach employed a β-ketoenamine-linked covalent organic framework (2D-COF-4) as the heterogeneous photocatalyst and N-hydroxy phthalimide (NHPI) esters as the alkyl radical sources. Numerous glycine derivatives, including dipeptides, were precisely and efficiently alkylated under visible light-driven reaction conditions. Based on the excellent photoactivity and organic reaction compatibility of 2D-COF-4, this alkylation could proceed flexibly in a green solvent (ethanol) without any other additives. The photocatalyst and phthalimide were fruitfully recycled with a simple workup procedure, revealing a high ecoscale value and low environmental factor (E-factor).

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.

Recent Advances in the Use of Covalent Organic Frameworks as Heterogenous Photocatalysts in Organic Synthesis

Organic photochemistry is intensely developed in the 1980s, in which the nature of excited electronic states and the energy and electron transfer processes are thoroughly studied and finally well-understood. This knowledge from molecular organic photochemistry can be transferred to the design of covalent organic frameworks (COFs) as active visible-light photocatalysts. COFs constitute a new class of crystalline porous materials with substantial application potentials. Featured with outstanding structural tunability, large porosity, high surface area, excellent stability, and unique photoelectronic properties, COFs are studied as potential candidates in various research areas (e.g., photocatalysis). This review aims to provide the state-of-the-art insights into the design of COF photocatalysts (pristine, functionalized, and hybrid COFs) for organic transformations. The catalytic reaction mechanism of COF-based photocatalysts and the influence of dimensionality and crystallinity on heterogenous photocatalysis performance are also discussed, followed by perspectives and prospects on the main challenges and opportunities in future research of COFs and COF-based photocatalysts.

Photoinduced Decarboxylative C3-H Alkylation of Quinoxalin-2(1H)-ones

An efficient, catalyst- and additive-free, visible-light-driven radical C3-H alkylation of quinoxalin-2(1H)-one derivatives has been developed. This reaction utilizes alkyl-NHP-esters as an alkyl radical donor and quinoxalin-2(1H)-one derivatives as an alkyl radical acceptor. The operationally simple protocol works under mild reaction conditions and tolerates a variety of functional groups. Furthermore, the synthetic utility of the methodology was successfully implemented for synthesizing biologically relevant C3-alkyl substituted quinoxalin-2(1H)-one derivatives.

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.

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.

Phenanthrenequinone (PQ) catalyzed cross-dehydrogenative coupling of alkanes with quinoxalin-2(1H)-ones and simple N-heteroarenes under visible light irradiation

A direct and convenient strategy to 3-alkylquinoxalin-2(1H)-ones and other alkyl N-heteroarenes via a photocatalyzed alkylation of quinoxalin-2(1H)-ones and other N-heterocycles with commercially available, low-cost alkanes under ambient conditions using phenanthrenequinone (PQ) as a photocatalyst was developed. This transformation has advantages of environment-friendly protocol, mild conditions, good functional-group tolerance, and high yields of products.

Photoinduced ligand to metal charge transfer enabling cerium mediated decarboxylative alkylation of quinoxalin-2(1H)-ones

Here, we report the cerium-mediated decarboxylative alkylation of quinoxalin-2(1H)-ones utilizing feedstock carboxylic acids as a radical precursor via photoinduced-LMCT. This operationally simple protocol overcomes the limitation of the direct use of carboxylic acids to access alkyl radicals. Spectroscopic investigations reveal the photoinduced LMCT and CO₂ evolving events. We have utilized a broad range of alkyl carboxylic acids (1° to 3° acids), amino acids and pharmaceutically-important acids as a coupling partner to synthesise the desired alkylated heterocyclic product in good to excellent yields.

Photochemical and electrochemical C-N borylation of arylhydrazines

The C-N borylation of arylhydrazine hydrochlorides with bis(pinacolato)diboron was achieved under photochemical and electrochemical conditions, respectively. This novel and scalable transformation provides two efficient and mild transition-metal-free synthetic routes towards aryl boronate esters from easily available arylhydrazines.

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.

Radical coupling reactions of hydrazines via photochemical and electrochemical strategies

Hydrazines are versatile building blocks in organic synthesis.

A covalent organic framework-catalyzed visible-light-induced three-component cascade synthesis of trifluoroalkyl and trifluoroalkenyl quinoxalin-2(1H)-one derivatives

A COF-catalyzed visible-light-induced three-component synthesis of trifluoroalkyl and trifluoroalkenyl quinoxalin-2(1H)-one derivatives features robust substrate adaptability, good sustainability, and Z-selectivity.

Metal–organic framework (MOF)-, covalent-organic framework (COF)-, and porous-organic polymers (POP)-catalyzed selective C–H bond activation and functionalization reactions

The review summarizes the state-of-the-art of C–H active transformations over crystalline and amorphous porous materials as new emerging heterogeneous (photo)catalysts.

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.

Direct functionalization of quinoxalin-2(1H)-one with alkanes: C(sp2)-H/C(sp3)-H cross coupling in transition metal-free mode

Considering the significance of pharmaceutically important heterocycles, efficient and highly versatile protocols for the functionalization of diverse heterocycles with easily accessible feedstock are crucial. Here, we have reported selective alkylation of quinoxalin-2(1H)-one with a broad class of hydrocarbons having different C(sp³)-H bonds with varying bond strengths using di-tert-butyl peroxide (DTBP) as an alkoxyl radical mediator for hydrogen atom transfer (HAT). This dehydrogenative coupling approach utilizes feedstock chemicals such as cycloalkanes, cyclic ethers and alkyl arenes as coupling partners. This protocol exhibits good functional group compatibility and selectivity regarding both heterocycles and unactivated alkanes. Moreover, this methodology allows functionalization of relatively strong C-H bonds of adamantane and exclusive selectivity towards 3° C(sp³)-H bonds is observed. We also illustrate the applicability of this C(sp²)-H/C(sp³)-H cross-coupling for practical access to bioactive pharmaceuticals.

Covalent Organic Frameworks toward Diverse Photocatalytic Aerobic Oxidations

Photoactive two-dimensional covalent organic frameworks (2D-COFs) have become promising heterogenous photocatalysts in visible-light-driven organic transformations. Herein, a visible-light-driven selective aerobic oxidation of various small organic molecules by using 2D-COFs as the photocatalyst was developed. In this protocol, due to the remarkable photocatalytic capability of hydrazone-based 2D-COF-1 on molecular oxygen activation, a wide range of amides, quinolones, heterocyclic compounds, and sulfoxides were obtained with high efficiency and excellent functional group tolerance under very mild reaction conditions. Furthermore, benefiting from the inherent advantage of heterogenous photocatalysis, prominent sustainability and easy photocatalyst recyclability, a drug molecule (modafinil) and an oxidized mustard gas simulant (2-chloroethyl ethyl sulfoxide) were selectively and easily obtained in scale-up reactions. Mechanistic investigations were conducted using radical quenching experiments and in situ ESR spectroscopy, all corroborating the proposed role of 2D-COF-1 in photocatalytic cycle.

Metal-Free Photocatalysis: Two-Dimensional Nanomaterial Connection toward Advanced Organic Synthesis

Two-dimensional (2D) nanostructures are a frontier in materials chemistry as a result of their extraordinary properties. Metal-free 2D nanomaterials possess extra appeal due to their improved cost-effectiveness and lower toxicity with respect to many inorganic structures. The outstanding electronic characteristics of some metal-free 2D semiconductors have projected them into the world of organic synthesis, where they can function as high-performance photocatalysts to drive the sustainable synthesis of high-value organic molecules. Recent reports on this topic have inspired a stream of research and opened up a theme that we believe will become one of the most dominant trends in the forthcoming years.

Visible-Light-Mediated Oxidative Amidation of Aldehydes by Using Magnetic CdS Quantum Dots as a Photocatalyst

A magnetic CdS quantum dot (Fe₃ O₄ /polydopamine (PDA)/CdS) was synthesized through a facile and convenient method from inexpensive starting materials. Characterization of the prepared catalyst was performed by means of FTIR spectroscopy, XRD, SEM, TEM, energy-dispersive X-ray spectroscopy, and vibrating-sample magnetometer techniques. Fe₃ O₄ /PDA/CdS was found to be a highly active photocatalyst for the amidation of aromatic aldehydes by using air as a clean oxidant under mild conditions. The photocatalyst can be recovered by magnetic separation and successfully reused for five cycles without considerable loss of its catalytic activity.

An ultrastable olefin-linked covalent organic framework for photocatalytic decarboxylative alkylations under highly acidic conditions

An ultrastable olefin-linked covalent organic framework 2D-COF-2 offers an alternative heterogeneous photocatalyst for photocatalytic decarboxylative alkylations, exhibiting impressive effciency, sustainabilty and promising industrial potential.

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.