The Same Oxidation-State Introduction of Hypervalent Sulfur via Transition-Metal Catalysis

Redox Relay-Induced C-S Radical Cross-Coupling Strategy: Application in Nontraditional Site-Selective Thiocyanation of Quinoxalinones

Oxidative cross-coupling is a powerful strategy to form C-heteroatom bonds. However, oxidative cross-coupling for constructing C-S bond is still a challenge due to sulfur overoxidation and poisoning transition-metal catalysts. Now, electrochemical redox relay using sulfur radicals formed in situ from inorganic sulfur source offers a solution to this problem. Herein, electrochemical redox relay-induced C-S radical cross-coupling of quinoxalinones and ammonium thiocyanate with bromine anion as mediator is presented. The electrochemical redox relay comprised initially the formation of sulfur radical via indirect electrochemical oxidation, simultaneous electrochemical reduction of the imine bond, electro-oxidation-triggered radical coupling involving dearomatization-rearomatization, and the reformation of the imine bond through anodic oxidation. Applying this strategy, various quinoxalinones bearing multifarious electron-deficient/-rich substituents at different positions were well compatible with moderate to excellent yields and good steric hindrance compatibility under constant current conditions in an undivided cell without transition-metal catalysts and additional redox reagents. Synthetic applications of this methodology were demonstrated through gram-scale preparation and follow-up transformation. Notably, such a unique strategy may offer new opportunities for the development of new quinoxalinone-core leads.

Electro-oxidative three-component cascade coupling of isocyanides with elemental sulfur and amines for the synthesis of 2-aminobenzothiazoles

2-Aminobenzothiazoles are commonly encountered in various functional compounds. Herein, we disclose an electro-oxidative three-component reaction for the effective synthesis of 2-aminobenzothiazoles under mild conditions, utilizing non-toxic and abundant elemental sulfur as the sulfur source. Both aliphatic amines and aryl amines demonstrate good compatibility at room temperature, highlighting the broad functional group tolerance of this approach. Additionally, elemental selenium demonstrated reactivities comparable to those of elemental sulfur.

The Chan-Lam-type synthesis of thioimidazolium salts for thiol-(hetero)arene conjugation

The design of stable and variable aryl linkers for conjugating drug moieties to the metabolism-related thiols is of importance in drug discovery. We disclosed that thioimidazolium groups are unique scaffolds for the thiol-(hetero)arene conjugation under mild conditions. The drug bound thioimidazolium salts, which are easily accessible via a copper-mediated Chan-Lam process in gram-scale, could be successfully applied to the late-stage coupling of bioactive thiols to construct a broad array of drug-like molecules.

Electrochemical selective divergent C-H chalcogenocyanation of N-heterocyclic scaffolds

An electrochemical direct selective C-H chalcogenocyanation approach for indolizine derivatives under mild conditions has been described. Cyclic enone-fused, chromone-fused and 2-substituted indolizines possessing EDGs (electron donating groups) and EWGs (electron withdrawing groups) were successfully reacted with NH4SCN and KSeCN under electrochemical conditions to provide a wide array of mono and bis-chalcogenocyanate-indolizines in 75-94% yields. In addition, 1-substituted imidazo[1,5-a]quinolines were also successfully chalcogenocyanated under the optimized reaction conditions providing a platform for the synthesis of pharmaceutically privileged molecules. By switching the reaction conditions, the developed protocol offers selective synthesis of C-3 thiocyanate and 1,3 bis-thiocyanate indolizines in good to excellent yields under catalyst-free conditions. On the basis of control experiments and cyclic voltammetry data, a plausible reaction pathway is also presented.

Neutral nickel-catalyzed dehydrosulfonylation of unactivated allylic alcohols under mild conditions

Allyl sulfones are important sulfur-containing compounds that have widespread applications in organic synthesis, medicinal chemistry and materials science. Herein, nickel-catalysed dehydrosulfonylation of unactivated allyl alcohols with aryl sulfonyl hydrazides without additional active agents under mild conditions was developed. A variety of functional allyl sulfones could be efficiently synthesized in the presence of air-stable Ni(acac)2 as the catalyst and 1,1'-bis(diphenylphosphino)ferrocene (DPPF) as the ligand.

Photo-induced versatile aliphatic C-H functionalization via electron donor-acceptor complex

The ability to selectively introduce diverse functionality onto hydrocarbons is of substantial value in the synthesis of both small molecules and pharmaceuticals. In this endeavour, as a photocatalyst- and metal-free process, the electron donor-acceptor (EDA) strategy has not been well explored. Here we report an approach to aliphatic carbon-hydrogen bond diversification through an EDA complex constituted by HCl and SIV=O groups. As an efficient hydrogen atom transfer (HAT) reagent, chlorine radical can be produced via a proton-coupled electron transfer process in this system. Based on this unusual path, a photo-promoted versatile aliphatic C-H functionalization is developed without photo- and metal-catalysts, including thiolation, arylation, alkynylation, and allylation. This conversion has concise and ambient reaction conditions, good functional group tolerance, and substrate diversity, and provides an alternative solution for the high value-added utilization of bulk light alkanes.

A Redox-neutral Nickel-catalysed Sulfonylation of (Hetero)aryl Boronic Acids with 2-Chlorothiazoles

A redox-neutral nickel-catalysed sulfonylation for arylsulfone synthesis was developed. (Hetero)aryl boronic acids reacted with potassium metabisulfite (K2 S2 O5 ) and readily available 2-chlorothiazoles in the presence of air-stable Ni(OTf)2 and 4,4-di-tert-butyl bipyridine (dtbpy) as a commercially available ligand to produce the corresponding 2-sulfonylthiazoles in moderate to excellent yields. This practical protocol tolerates a wide range of substrates including boronic acids and 2-chloro(benzo)thiazoles without additional bases, allowing the direct synthesis of functional arylsulfones.

An efficient and mild oxidative approach from thiols to sulfonyl derivatives with DMSO/HBr

A mild and practical method for synthesizing sulfonyl derivatives, which have a wide range of applications in pharmaceuticals, materials, and organic synthesis, was described through the oxidative functionalization of thiols with DMSO/HBr. The simple conditions, low cost and ready availability of DMSO/HBr, as well as the versatility of the transformations, make this strategy very powerful in synthesizing a variety of sulfonyl derivatives including sulfonamides, sulfonyl fluorides, sulfonyl azides, and sulfonates. Mechanistic studies revealed that DMSO served as the terminal oxidant, and HBr acted as both a nucleophile and a redox mediator to transfer the oxygen atom.

Pyrosulfite-Involved Synthesis of Sulfides by Palladium-Catalyzed Decarboxylative Couplings

The decarboxylative coupling using carboxylic acid and potassium metabisulfite, promoted by a palladium catalyst, is reported for the generation of sulfides. The coupling is performed using the easily available carboxylic acid and environmentally friendly inorganic sulfides as a divalent inorganic sulfur source. Not only aromatic acids but also aliphatic carboxylic acids are workable during the couplings. The method is applicable and practical to a scope of 20 examples and drug molecules.

Hydrazinosulfonylation of aryl electrophiles: a straightforward approach for the synthesis of aryl N-aminosulfonamides

In recent years, the direct hydrazinosulfonylation of aryl electrophiles with SO₂ and hydrazines has emerged as an efficient and versatile method for the synthesis of aryl N-aminosulfonamides. This method has the advantages of being operationally simple and requiring only readily available starting materials. This review article is an attempt to survey literature describing the preparation of aryl N-aminosulfonamides through the direct hydrazinosulfonylation of aryl electrophiles with SO₂ and hydrazines, with special attention paid to the mechanistic features of the reactions. It can be used as a guide for chemists to apply the best hydrazinosulfonylation conditions in their work or serve as inspiration for future research related to the topic.

KI-Catalyzed Allylic Sulfonation of α-Methylstyrene Derivatives with Sulfonylhydrazides via Electrochemistry

A direct allylic C-H bond activation of α-methylstyrene derivatives with sulfonylhydrazines for the synthesis of allylic sulfones has been developed under exogenous oxidant- and metal-catalyst-free electrochemical conditions. Using the transfer of electrons in the current instead of a stoichiometric chemical oxidant, a series of valuable allylic sulfones were accessed with a wide substrate scope and excellent regioselectivity via radical coupling.

Recent advances in the synthesis of cyclic sulfinic acid derivatives (sultines and cyclic sulfinamides)

The chemistry of cyclic sulfinic acid derivatives (sultines and cyclic sulfinamides) was underdeveloped for a long time due to their inaccessibility. Considering the importance of cyclic sulfinate esters and amides in the fields of chemistry, pharmaceutical science, and material science, synthesis strategies involving cyclic sulfinic acid derivatives have been paid more attention in recent years, and have been widely used in the synthesis of sulfur-containing compounds such as sulfoxides, sulfones, sulfinates and thioethers. Despite the impressive improvements that have been made in last twenty years with the new strategies, to date, no reviews have been published, to the best of our knowledge, dealing with the preparation of cyclic sulfinic acid derivatives. This review summarizes the latest advances in the development of new synthesis methods to access cyclic sulfinic acid derivatives in the last two decades. The synthetic strategies are reviewed by highlighting their product diversity, selectivity and applicability, and the mechanistic rationale is presented where possible. We wish to bring readers a comprehensive understanding of the state-of-play of cyclic sulfinic acid derivative formation and make a contribution to future research.

Multicomponent Sulfonylation of Alkenes to Access β-Substituted Arylsulfones

A novel multicomponent sulfonylation of alkenes is described for the assembly of various β-substituted arylsulfones using cheap and easily available K₂S₂O₅ as a sulfur dioxide source. Of note, the procedure does not need any extra oxidants and metal catalysts and exhibits a relatively wide substrate scope and good functional group compatibility. Mechanistically, an initial arylsulfonyl radical is formed involving the insertion of sulfur dioxide with aryl diazonium salt, followed by alkoxyarylsulfonylation or hydroxysulfonylation of alkenes.

Photosensitized Vicinal Sulfonylamination of Alkenes with Oxime Ester and DABCO·(SO2)2

A metal-free photosensitized three-component reaction of oxime esters, alkenes, and DABCO·(SO₂)₂ was developed. This protocol could accommodate a wide substrate scope, including activated and unactivated alkenes and aryl and aliphatic carboxylic acid oxime esters, delivering a broad range of β-amino sulfones in moderate to high yields. The insertion of SO₂ as a linker moiety allows the manipulation of the functionality in the reaction process, expanding the utility of oxime esters as bifunctional reagents.

Recent Advances on Direct Functionalization of Indoles in Aqueous Media

Indoles and their derivatives have dominated a significant proportion of nitrogen-containing heterocyclic compounds and play an essential role in synthetic and medicinal chemistry, pesticides, and advanced materials. Compared with conventional synthetic strategies, direct functionalization of indoles provides straightforward access to construct diverse indole scaffolds. As we enter an era emphasizing green and sustainable chemistry, utilizing environment-friendly solvents represented by water demonstrates great potential in synthesizing valuable indole derivatives. This review aims to depict the critical aspects of aqueous-mediated indoles functionalization over the past decade and discusses the future challenges and prospects in this fast-growing field. For the convenience of readers, this review is classified into three parts according to the bonding modes (C-C, C-N, and C-S bonds), which focus on the diversity of indole derivatives, the prominent role of water in the chemical process, and the types of catalyst systems and mechanisms. We hope this review can promote the sustainable development of the direct functionalization of indoles and their derivatives and the discovery of novel and practical organic methods in aqueous phase.

Copper-Catalyzed Sulfonylation Reaction of NH-Sulfoximines with Aryldiazonium Tetrafluoroborates and Sulfur Dioxide: Formation of N-Sulfonyl Sulfoximines

An efficient and practical SO₂ insertion protocol of NH-sulfoximines with aryldiazonium tetrafluoroborates and DABSO toward N-sulfonyl sulfoximines has been developed under mildly basic conditions. This transformation features easy operation, readily available substrates, and mild conditions. A tentative mechanism is proposed, which indicates that the aryldiazonium tetrafluoroborates would be radical donors under standard reaction conditions. The aryl radical produced in situ from diazonium salts would be trapped by SO₂ to generate an arylsulfonyl radical and then undergo further transformation to generate the final N-sulfonyl sulfoximines.

Nickel-catalyzed sulfonylative coupling of 2-chlorobenzothiazoles with sulfinates at room temperature

An efficient nickel-catalyzed cross-coupling for the synthesis of 2-sulfonylthiazoles from readily available 2-chlorobenzothiazoles and sodium sulfinates has been developed. A variety of 2-chlorobenzothiazoles and sulfinates having a diverse range of substitution patterns can undergo the coupling process successfully at room temperature. Avoiding the use of precious catalysts and sensitive ligands, moderate to excellent yields of various 2-sulfonylthiazoles were observed.

Photoredox-catalyzed intermolecular azidosulfonylation of alkenes with DABCO·(SO2)2, trimethylsilyl azide and thianthrenium salts

Synthesis of β-azido alkylsulfones through a photoredox-catalyzed azido sulfonylation of alkenes with DABCO·(SO2)2, trimethylsilyl azide and alkyl thianthrenium salts is developed.

Synthesis of I(III)/S(VI) reagents and their reactivity in photochemical cycloaddition reactions with unsaturated bonds

The development of novel methodologies for the introduction of the sulfoxonium group under mild conditions is appealing but remains underexplored. Herein we report the synthesis of a class of hypervalent iodine reagents with a transferrable sulfoxonium group. These compounds enable mixed iodonium-sulfoxonium ylide reactivity. These well-defined reagents are examined in visible-light-promoted cyclization reactions with a wide range of unsaturated bonds including alkenes, alkynes, nitriles, and allenes. Two distinct cyclization pathways are identified, which are controlled by the substituent of the unsaturated bond. The cycloaddition protocol features simple operation, mild reaction conditions, and excellent functional group tolerance, affording a broad range of sulfoxonium-containing cyclic structures in moderate to excellent yields. Furthermore, the sufoxonium group in the product can be transformed into diverse functional groups and structural motifs via single electron transfer and transition-metal catalysis.

Iron-Catalyzed Sulfonylthiocyanation of α,β-Unsaturated Amides/Esters via the Insertion of Sulfur Dioxide

An iron-catalyzed four-component sulfonylthiocyanation between α,β-unsaturated amides/esters, TMSNCS, aryldiazonium tetrafluoroborates, and sulfur dioxide (from SOgen) is demonstrated. This protocol is characterized by mild reaction conditions, good functional group compatibility, broad substrate scope, and good to excellent yields, providing a feasible method for the preparation of β-thiocyanated sulfone compounds. The preliminary mechanism investigation shows that a radical pathway may be involved in the process.

Ni-catalyzed C-S bond construction and cleavage

Sulfur-containing compounds have attracted considerable interest due to their wide-ranging applications in pharmaceuticals, agriculture, natural products, and organic materials. The development of efficient and rapid methods for the construction and transformation of sulfur-containing compounds is of great importance. Since nickel is inexpensive and has a variety of valence states, strong nucleophilicity and low energy barriers for oxidative addition, the construction and transformation of sulfur-containing compounds by nickel-catalyzed cross-coupling have become important strategies. In addition, sulfur-containing compounds have also been playing increasingly important roles in the field of cross-coupling due to their thermodynamically stable but dynamic activity. This review will focus on nickel-catalyzed construction and transformation of various sulfide-containing compounds, such as sulfides, disulfides, and hypervalent sulfur-containing compounds.

Switchable In Situ SO2 Capture and CF3 Migration of Enol Triflates with Peroxyl Compounds under Iron Catalysis

Switchable in situ SO₂ capture and CF₃ migration of enol triflates with peroxyl compounds under iron catalysis are presented. By regulating the structure of peroxides, a variety of keto-functionalized dialkyl sulfones and α-trifluoromethyl ketones were selectively synthesized in good yields under mild conditions.

Visible-Light-Mediated Three-Component Cascade Sulfonylative Annulation

Visible-light-promoted cascade radical cyclization for the synthesis of sulfonylated benzimidazo/indolo[2,1-a]iso-quinolin-6(5H)-ones has been reported. The reaction provides transition-metal-free and expeditious access to sulfonylated polyaromatics. The use of sodium metabisulfite as a SO₂ surrogate and the rapid generation of molecular complexity using a three-component photochemical protocol are the salient features of this reaction manifold.

Cyclization/hydrolysis of 1,5-enenitriles initiated by sulfonyl radicals in the aqueous phase in the presence of the I2/TBHP system

A novel cyclization/hydrolysis of 1,5-enenitriles for the synthesis of valuable pyrrolidine-2,4-diones in the aqueous phase using I₂ as the catalyst and tert-butyl hydroperoxide (TBHP) as the oxidant is reported. In the presence of the I₂/TBHP system, sulfonyl hydrazides produce sulfonyl radicals, which undergo radical addition, intramolecular cyclization, hydrogen abstraction, and hydrolysis to give the final products. The use of the inexpensive and environmentally friendly I₂/TBHP catalytic oxidation system in the aqueous phase makes it a benign and sustainable strategy.

Rongalite-Mediated Transition Metal- and Hydride-Free Chemoselective Reduction of α-Keto Esters and α-Keto Amides

A transition metal- and hydride-free protocol has been developed for the chemoselective reduction of α-keto esters and α-keto amides using rongalite as a reducing agent. Here, rongalite acts as a hydride-free reducing agent via a radical mechanism. This protocol offers the synthesis of a wide range of α-hydroxy esters and α-hydroxy amides with 85-98% yields. This chemoselective method is compatible with other reducible functionalities such as halides, alkenes, amides, and nitriles. The use of inexpensive rongalite (ca. $0.03/1 g), mild reaction conditions, and gram-scale synthesis are some of the key features of this methodology. Also, cyclandelate, a vasodilator drug, has been synthesized in gram scale with 79% yield.

A metal-free four-component sulfonylation, Giese cyclization, selenylation cascade via insertion of sulfur dioxide

We hereby report a highly regio- and diastereoselective arylsulfonylation-radical cyclization-selenylation cascade of alkynyl cyclohexadienones for the facile synthesis of highly functionalized dihydrochromenones. The protocol utilizes aryldiazonium salts as aryl partners and DABSO as a benign SO₂ source and also as a redox mediator. Additionally, we also developed a visible light mediated protocol wherein diaryliodonium salts were used as the aryl partners at room temperature.

3-Arylsulfonylquinolines from N-Propargylamines via Cascaded Oxidative Sulfonylation Using DABSO

We report a cascaded oxidative sulfonylation of N-propargylamine via a three-component coupling reaction using DABCO·(SO₂)₂ (DABSO). 3-Arylsulfonylquinolines were obtained by mixing diazonium tetrafluoroborate, N-propargylamine, and DABSO under argon atmosphere in dichloroethane (DCE) for 1 h. In a radical pathway, DABSO was utilized as the sulfone source and an oxidant in this radical-mediated cascaded reaction.

Access to Sulfoxides under NHC/Photocatalysis via a Radical Pathway

A photocatalyzed transformation from sulfinic acids to sulfoxides under visible-light irradiation in the presence of N-heterocyclic carbene is established. Various alkyl groups from four-substituted Hantzsch esters or Meyer nitriles are smoothly converted to the corresponding sulfoxides through a radical coupling pathway in the presence of 1,1-carbonyldiimidazole. This method allows sulfoxide synthesis to refrain from relying on the oxidation of sulfides and provides an alternative route for the preparation of sulfoxides.

Copper-Catalyzed N-Directed Distal C(sp3)-H Sulfonylation and Thiolation with Sulfinate Salts

We herein report a selective and catalytic C(sp³)-H functionalization approach to access amines bearing organo-sulfonyl and organo-thiol groups. This reaction proceeds through a cascade process of N-radical formation, alkyl radical formation via 1,5-HAT, and C-S bond formation, thereby offering a series of functionalized amines. This method could enable primary, secondary, and tertiary C(sp³)-H sulfonylation and thiolation and also exhibits good functional group tolerance.

Sulfinimidate Esters as an Electrophilic Sulfinimidoyl Motif Source: Synthesis of N-Protected Sulfilimines from Grignard Reagents

In this work we investigated, for the first time, the reactivity of sulfinimidate esters as an electrophilic sulfinimidoyl motif source. The reaction of such sulfinimidate esters with Grignard reagents enables the preparation of protected sulfilimines in high yields and with a remarkable structural variability. Moreover, the transformation can be performed in CPME (cyclopentyl methyl ether) as a green solvent under environmentally responsible conditions.

Advances in photochemical and electrochemical incorporation of sulfur dioxide for the synthesis of value-added compounds

Organic photochemistry and electrochemistry currently receive tremendous attention in organic synthesis as both techniques enable the reagent-less activation of organic molecules without using expensive and hazardous redox reagents. The incorporation of SO₂ into organic molecules is a relatively modern research topic, which likewise gains immense popularity since the discovery of the SO₂ surrogate DABSO. Sulfur-containing organic molecules are omnipresent in pharmaceuticals and agrochemicals. This review covers the recent progress in electrochemical and photochemical methodologies for the incorporation and uses of SO₂ in the synthesis of value-added compounds. Additionally, different work techniques are demonstrated for the synthetic application of SO₂.

Dithionite-Involved Multicomponent Coupling for Alkenyl and Alkyl Tertiary Sulfones

A dithionite-involved multicomponent reaction of redox-active esters and alkenes/alkynes is comprehensively achieved for the construction of alkyl and alkenyl tertiary sulfones. The industrial feedstock sodium dithionite is employed as a sulfur dioxide surrogate and a single-electron reductant to initiate the decarboxylation of redox-active esters. Mechanistic studies further indicated that the transformation underwent a proton trapping process, which was different from the traditional radical trapping pathway.

Photocatalytic three-component annulation enabling stereoselective generation of (Z)-thiochromene 1,1-dioxides

A new photocatalytic three-component annulation of 2-alkynylaryldiazonium tetrafluoroborates with γ-hydroxyl terminal alkynes and sodium metabisulfite is reported and used to produce a series of (Z)-thiochromene 1,1-dioxides.