Heterogeneous Carbon Nitrides Photocatalysis Multicomponent Hydrosulfonylation of Alkynes To Access β-Keto Sulfones with the Insertion of Sulfur Dioxide in Aerobic Aqueous Medium

Photocatalytic Synthesis and Functionalization of Sulfones, Sulfonamides and Sulfoximines

Sulfur(VI)-based functional groups are popular scaffolds in a wide variety of research fields including synthetic and medicinal chemistry, as well as chemical biology. The growing interest in sulfur(VI)-containing molecules has motivated the scientific community to explore new methods to synthesize and modify them. Here, photocatalysis plays a key role granting access to new types of reactivity under mild reaction conditions. In this Perspective, we present a selection of works reported in the last six years focused on the photocatalytic assembly and reactivity of sulfones, sulfonamides, and sulfoximines. We addressed the key synthetic intermediates for each transformation, while discussing limitations and strength points of the protocols. Future directions of the field are finally presented.

Oxysulfonylation of Alkynes with Sodium Sulfinates to Access β-Keto Sulfones Catalyzed by BF3·OEt2

An efficient and operationally simple method for the synthesis of β-keto sulfones through the BF3·OEt2-promoted reaction of alkynes and sodium sulfinates is developed. With its facile and selective access to the targets, it features good functional group compatibility, mild conditions, easily available starting materials, and good yields. Notably, the reaction does not require metal catalysts or chemical reagents with pungent odors.

Regulation of Photogenerated Redox Species through High Crystallinity Carbon Nitride for Improved C-S Coupling Reactions

A novel and efficient approach for the synthesis of α, β-unsaturated sulfones through heterogeneous photocatalyzed C-S coupling reactions have been developed. The use of molten-salt method derived carbon nitride (MCN), a transition metal-free polymeric photocatalyst, combined with enhanced crystallinity and potassium iodide as an additive, effectively modulates photogenerated reactive redox species, markedly increasing the overall reaction selectivity. This method achieves the shortest reaction time (2 h) with high yield (up to 95 %) among the reported heterogeneous catalytic C-S bond formation reactions, matching the efficiency of the homogeneous photocatalysts. Furthermore, the application to challenging alkyne substrates has been demonstrated, underscoring the potential for a broad range of applications in pharmaceutical research and synthetic chemistry.

1,1-Oxycarbonation of Terminal Alkynes via Sequential Borylation, 1,2-Migration, and Oxidation with Oxone

Alkynes are readily available and multifunctional synthetic intermediates, but their 1,1-oxofunctionalization remains challenging. Herein, we report a 1,1-oxycarbonation of terminal alkynes to construct ketones through sequential borylation, 1,2-carbon migration, and oxidation with Oxone as the proton source and oxidant. The synthetic potential of this transformation is showcased by the broad functional groups, scale-up synthesis, and diverse product transformations.

Visible-Light-Mediated Divergent and Regioselective Vicinal Difunctionalization of Styrenes with Arylazo Sulfones

Activated by visible light, arylazo sulfones can serve as multifaceted reactants and are employed in diazenylation, sulfonylation, and arylation reactions under (photo)catalyst-free conditions. Such versatile reactivity enabled us to develop an operationally simple, regioselective, and tunable difunctionalization of styrenes with arylazo sulfones to produce α-sulfonyl arylhydrazones and 1,2-alkoxyarylated products in moderate to excellent yields. Furthermore, such difunctionalized products have been exploited as key building blocks for the synthesis of various heterocycles.

Inexpensive and bench stable diarylmethylium tetrafluoroborates as organocatalysts in the light mediated hydrosulfonylation of unactivated alkenes

In this paper, we present the synthetic potential of diarylmethylium tetrafluoroborates as catalysts for the visible light promoted hydrosulfonylation of unactivated alkenes. For the first time, these salts, which are bench stable and easily preparable on a multi-gram scale, were employed as organocatalysts. Interestingly, a catalyst loading of only 1 mol% allowed sulfone products to be efficiently obtained from good-to-excellent yields with high functional-group tolerance and scalability up to 15 mmol of alkene. The mechanistic study, both experimental and computational, presented here, revealed an alternative mechanism for the formation of the key sulfonyl radical. Indeed, the photoactive species was proved not to be the diarylcarbenium salt itself, but two intermediates, a stable S-C adduct and an ion couple, that were formed after its interaction with sodium benzenesulfinate. Upon absorbing light, the ion couple could reach an excited state with a charge-transfer character which gave the fundamental sulfonyl radical. A PCET (proton-coupled electron transfer) closes the catalytic cycle reforming the diarylcarbenium salt.

Lighting Up the Organochalcogen Synthesis: A Concise Update of Recent Photocatalyzed Approaches

This review describes the recent advances in photocatalyzed reactions to form new carbon–sulfur and carbon–selenium bonds. With a total of 136 references, of which 81 articles are presented, the authors introduce in five sections an updated picture of the state of the art in the light-promoted synthesis of organochalcogen compounds (from 2019 to present). The light-promoted synthesis of sulfides by direct sulfenylation of C–C π-bonds; synthesis of sulfones; the activation of Csp2–N bond in the formation of Csp2–S bonds; synthesis of thiol ester, thioether and thioacetal; and the synthesis of organoselenium compounds are discussed, with detailed reaction conditions and selected examples for each protocol.

Visible light driven multicomponent synthesis of difluoroamidosulfonyl quinoline derivatives

A visible-light-induced photocatalyst-free three-component radical tandem cyclization of N-propargylamine and N-allylbromodifluoroacetamides with the insertion of sulfur dioxide has been developed. Diverse difluoroamidosulfonylated quinolines are obtained in moderate to good yields. This protocol features broad functional group tolerance and high regioselectivity. Moreover, mechanistic studies reveal the involvement of the radical pathway and the formation of an electron donor-acceptor (EDA) complex in this reaction.

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.

Recent advances in visible-light graphitic carbon nitride (g-C3N4) photocatalysts for chemical transformations

Graphitic carbon nitride (g-C3N4) has emerged as a new research hotspot, attracting broad interdisciplinary attention in the form of metal-free and visible-light-responsive photocatalysts in the field of solar energy conversion and environmental remediation. These photocatalysts have evolved as attractive candidates due to their non-toxicity, chemical stability, efficient light absorption capacity in the visible and near-infrared regions, and adaptability as a platform for the fabrication of hybrid materials. This review mainly describes the latest advances in g-C3N4 photocatalysts for chemical transformations. In addition, the typical applications of g-C3N4-based photocatalysts involving organic transformation reactions are discussed (synthesis of heterocycles, hydrosulfonylation, hydration, oxygenation, arylation, coupling reactions, etc.).

Multicomponent reactions and photo/electrochemistry join forces: atom economy meets energy efficiency

Visible-light photoredox catalysis has been regarded as an extremely powerful tool in organic chemistry, bringing the spotlight back to radical processes. The versatility of photocatalyzed reactions has already been demonstrated to be effective in providing alternative routes for cross-coupling as well as multicomponent reactions. The photocatalyst allows the generation of high-energy intermediates through light irradiation rather than using highly reactive reagents or harsh reaction conditions. In a similar vein, organic electrochemistry has experienced a fruitful renaissance as a tool for generating reactive intermediates without the need for any catalyst. Such milder approaches pose the basis toward higher selectivity and broader applicability. In photocatalyzed and electrochemical multicomponent reactions, the generation of the radical species acts as a starter of the cascade of events. This allows for diverse reactivity and the use of reagents is usually not covered by classical methods. Owing to the availability of cheaper and more standardized photo- and electrochemical reactors, as well as easily scalable flow-setups, it is not surprising that these two fields have become areas of increased research interest. Keeping these in view, this review is aimed at providing an overview of the synthetic approaches in the design of MCRs involving photoredox catalysis and/or electrochemical activation as a crucial step with particular focus on the choice of the difunctionalized reagent.

Visible Light-Promoted Synthesis of β-Keto Sulfoximines from N-Tosyl-Protected Sulfoximidoyl Chlorides

Under visible light, N-tosyl-protected sulfoximidoyl chlorides react with aryl alkynes to give β-keto sulfoximines. The reaction is characterized by a high functional group tolerance and good yields. It can be improved by the presence of a ruthenium photocatalyst. Air is the source of the ketonic oxygen in the products.

Building a Pyrazole-Benzothiadiazole-Pyrazole Photosensitizer into Metal-Organic Frameworks for Photocatalytic Aerobic Oxidation

Charge separation plays a crucial role in regulating photochemical properties and therefore warrants consideration in designing photocatalysts. Metal-organic frameworks (MOFs) are emerging as promising candidates for heterogeneous photocatalysis due to their structural designability and tunability of photon absorption. Herein, we report the design of a pyrazole-benzothiadiazole-pyrazole organic molecule bearing a donor-acceptor-donor conjugated π-system for fast charge separation. Further attempts to integrate such a photosensitizer into MOFs afford a more effective heterogeneous photocatalyst (JNU-204). Under visible-light irradiation, three aerobic oxidation reactions involving different oxygenation pathways were achieved on JNU-204. Recycling experiments were conducted to demonstrate the stability and reusability of JNU-204 as a robust heterogeneous photocatalyst. Furthermore, we illustrate its applications in the facile synthesis of pyrrolo[2,1-a]isoquinoline-containing heterocycles, core skeletons of a family of marine natural products. JNU-204 is an exemplary MOF platform with good photon absorption, suitable band gap, fast charge separation, and extraordinary chemical stability for proceeding with aerobic oxidation reactions under visible-light irradiation.

Dioxygen-Triggered Oxosulfonylation/Sulfonylation of Terminal Olefins toward β-Keto Sulfones/Sulfones

A dioxygen-triggered oxosulfonylation/sulfonylation of unactivated olefins to achieve β-keto sulfones/sulfones has been developed. Interestingly, pluralistic mechanisms were found when different types of compounds were applied as substrates, and different products were achieved. The reaction is carried out with a high atomic efficiency in the absence of a metal and a catalyst at room temperature under an air atmosphere. Importantly, as a proof-of-concept, a bioactive molecule was synthesized on a gram-scale level using this method.

Photocatalytic functionalizations of alkynes

Visible light mediated functionalizations have significantly expanded the scope of alkynes by unraveling new mechanistic pathways and enabling their transformation to diverse structural entities. The photoredox reactions on alkynes rely on their innate capability to generate myriad carbon-centred radicals via single electron transfer (SET), thereby, allowing the introduction of new radical precursors. Moreover, an array of methods have been developed facilitating transformations such as vicinal or gem-difunctionalization, annulation, cycloaddition and oxidative reactions to construct numerous key building blocks of natural and pharmaceutically important molecules. In addition, the introduction of photoredox chemistry has successfully been used to deal with the challenges associated with alkyne functionalization such as stereoselective and regioselective control. This article accounts for several visible light mediated functionalization reactions of alkynes, wherein they have been transformed into α-oxo compounds, β-keto sulfoxides, substituted olefins, N-heterocycles, internal alkynes and sulfur containing compounds. The article has been primarily categorized into various sections based on the reaction type with particular attention being paid to mechanistic details, advancement and future applications.

Visible-Light Carbon Nitride-Catalyzed Aerobic Cyclization of Thiobenzanilides under Ambient Air Conditions

A metal-free heterogeneous photocatalysis has been developed for the synthesis of benzothiazoles via intramolecular C-H functionalization/C-S bond formation of thiobenzanilides by inexpensive graphitic carbon nitride (g-C₃N₄) under visible-light irradiation. This reaction provides access to a broad range of 2-substituted benzothiazoles in high yields under an air atmosphere at room temperature without addition of a strong base or organic oxidizing reagents. In addition, the catalyst was found to be stable and reusable after five reaction cycles.

Visible Light as the Key for the Formation of Carbon–Sulfur Bonds in Sulfones, Thioethers, and Sulfonamides: An Update

This review summarizes the most relevant advancements made in the photocatalyzed synthesis of sulfones, thioethers, and sulfonamides from 2017 to the beginning of 2021. Synthetic strategies towards the construction of sulfur–carbon bonds are discussed together with the proposed reaction mechanisms. Interestingly, sulfur-based functional groups, which are of fundamental importance for the pharmaceutical field, can be assembled by photocatalysis in an easy and straightforward way under milder reaction conditions employing less toxic and expensive sulfur sources in comparison with common strategies.

1 Introduction

2 Sulfones

2.1 Sodium Sulfinates and Sulfinic Acids

2.2 Sulfonyl Halides

2.3 Sulfonyl Hydrazones

2.4 Sulfur Dioxide Surrogates

2.5 Miscellaneous

3 Thioethers

4 Sulfonamides

5 Conclusions

Recent advances in the synthesis and applications of β-keto sulfones: new prospects for the synthesis of β-keto thiosulfones

This review mainly focuses on recent developments in the preparation of β-keto sulfones and their extensive synthetic applications. New prospects for the synthesis of β-keto thiosulfones have also been highlighted. Over the last decade, there has been exponential growth in the direct construction of β-keto sulfones using a wide variety of keto and sulfonyl precursors. Of note, the most promising photoredox transformations and electrochemical synthesis methods of β-keto sulfones are also presented. Moreover, β-keto sulfones are versatile building blocks in organic synthesis due to their three essential functional groups: sulfonyl, carbonyl, and active methylene moieties. The convenient preparation of β-keto sulfones allows the synthesis of many valuable carbocyclic and heterocyclic compounds, and the effortless removal of the sulfonyl moiety via transformations is supported. The chemistry of β-keto sulfones (2013 to present) can be divided into several sections based on the sulfonyl surrogates, and ubiquitous synthetic strategies were systematically outlined.

Oxidative Sulfonylation of Hydrazones Enabled by Synergistic Copper/Silver Catalysis

A copper/silver-cocatalyzed protocol for oxidative sulfonylation of hydrazones is demonstrated. A wide range of β-ketosulfones and N-acylsulfonamides are directly synthesized in moderate to good yields. Our work provides a viable method for scalable preparation of β-ketosulfone derivatives that have found wide applications in the pharmaceutical industry.

Visible-light-induced remote C(sp3)–H sulfonylvinylation: assembly of cyanoalkylated vinyl sulfones

A photoinduced three-component sulfonylvinylation reaction of propargyl alcohols, potassium metabisulfite and cycloketone oxime esters is developed, affording cyanoalkylated vinyl sulfones in moderate to good yields.

Photoinduced intramolecular carbosulfonylation of alkynes: access to sulfone-containing dibenzazepines from sulfur dioxide

A visible-light-driven three-component carbosulfonylation of terminal alkynes, DABCO (SO₂)₂ and aryldiazonium tetrafluoroborates is developed, leading to sulfone-containing dibenzazepines in moderate to good yields under mild conditions.

A visible-light photoredox-catalyzed four-component reaction for the construction of sulfone-containing quinoxalin-2(1H)-ones

A visible-light photoredox-catalyzed four component reaction of quinoxalin-2(1H)-ones, alkenes, aryldiazonium, and sodium metabisulfite leading to sulfone-containing quinoxalin-2(1H)-ones has been developed.

Photosensitizer-free synthesis of β-keto sulfones via visible-light-induced oxysulfonylation of alkenes with sulfonic acids

A practical and environment-friendly methodology for the construction of β-keto sulfones through visible-light induced direct oxysulfonylation of alkenes with sulfonic acids at ambient temperature under open-air conditions was developed. Most importantly, the reaction proceeded smoothly without the addition of any photocatalyst or strong oxidant, ultimately minimizing the production of chemical waste.

Copper-catalyzed regio- and chemoselective selenosulfonylation of 1,6-enynes from sulfur dioxide

An efficient copper-catalyzed multicomponent reaction of 1,6-enynes, diselenides, DABCO·(SO2)2, and cycloketone oxime esters was achieved, providing cyanoalkylsulfonated pyrrolidines in moderate to good yields.

Difunctionalization of Alkenes and Alkynes via Intermolecular Radical and Nucleophilic Additions

Popular and readily available alkenes and alkynes are good substrates for the preparation of functionalized molecules through radical and/or ionic addition reactions. Difunctionalization is a topic of current interest due to its high efficiency, substrate versatility, and operational simplicity. Presented in this article are radical addition followed by oxidation and nucleophilic addition reactions for difunctionalization of alkenes or alkynes. The difunctionalization could be accomplished through 1,2-addition (vicinal) and 1,n-addition (distal or remote) if H-atom or group-transfer is involved in the reaction process. A wide range of moieties, such as alkyl (R), perfluoroalkyl (R_f), aryl (Ar), hydroxy (OH), alkoxy (OR), acetatic (O₂CR), halogenic (X), amino (NR₂), azido (N₃), cyano (CN), as well as sulfur- and phosphorous-containing groups can be incorporated through the difunctionalization reactions. Radicals generated from peroxides or single electron transfer (SET) agents, under photoredox or electrochemical reactions are employed for the reactions.

Emerging Concepts in Carbon Nitride Organic Photocatalysis

Carbon nitrides encompass a class of transition-metal-free materials possessing numerous advantages such as low cost (few Euros per gram), high chemical stability, broad tunability of redox potentials and optical bandgap, recyclability, and a high absorption coefficient (>10⁵  cm^-1 ), which make them highly attractive for application in photoredox catalysis. In this Review, we classify carbon nitrides based on their unique properties, structure, and redox potentials. We summarize recently emerging concepts in heterogeneous carbon nitride photocatalysis, with an emphasis on the synthesis of organic compounds: 1) Illumination-Driven Electron Accumulation in Semiconductors and Exploitation (IDEASE); 2) singlet-triplet intersystem crossing in carbon nitride excited states and related energy transfer; 3) architectures of flow photoreactors; and 4) dual metal/carbon nitride photocatalysis. The objective of this Review is to provide a detailed overview regarding innovative research in carbon nitride photocatalysis focusing on these topics.

Recent advances in sulfonylation reactions using potassium/sodium metabisulfite

Recently, sulfonylation reactions using potassium/sodium metabisulfite as the sulfur dioxide surrogate have been developed rapidly. In most cases, the transformations go through radical processes with the insertion of sulfur dioxide under mild conditions. Additionally, transition metal catalysis is applied in the reactions for the synthesis of sulfonyl-containing compounds. Among the approaches, photoinduced conversions under visible light or ultraviolet irradiation are also involved. In this updated report, the insertion of sulfur dioxide from potassium metabisulfite or sodium metabisulfite is summarized.

C3N4-Photocatalyzed aerobic oxidative cleavage of CC bonds in alkynes with diazonium salts leading to two different aldehydes or esters in one pot

C₃N₄-Photocatalyzed oxidative cleavage of CC bonds in alkynes with diazonium salts to obtain two different aldehydes or esters.

Generation of (Z)-β-alkenyl alkylsulfones via a copper-catalyzed decarboxylative alkylsulfonylation

A copper-catalyzed three-component reaction of acrylamides, sulfur dioxide and phenyliodine(iii) dicarboxylates is developed. The conversion using phenyliodine dicarboxylates as alkyl radical precursors provides diverse (Z)-β-alkenyl alkylsulfones.