Photoredox‐Catalyzed Functionalization of Alkenes with Thiourea Dioxide: Construction of Alkyl Sulfones or Sulfonamides

Photoinduced Alkylsulfonylation and Cyanoalkylsulfonylation of Morita-Baylis-Hillman Adducts via Multicomponent Insertion of Sulfur Dioxide

General and convenient visible-light-promoted alkylsulfonylation and cyanoalkylsulfonylation of MBH adducts have been developed through the multicomponent insertion of sulfur dioxide, enabling the assembly of two C-S bonds to generate structurally diverse allylic alkylsulfones (43 examples in total). The reaction of MBH adducts with potassium alkyltrifluoroborates and 1,4-diazabicyclo[2.2.2]octane bis(sulfur dioxide) adduct afforded sulfones with generally good yields. Notably, the addition of N,N,N',N'-tetramethylethylenediamine as a base into the photocatalytic system led to yielding an alkyl sulfonyl unit and cyano group-anchored trisubstituted alkenes by utilizing cycloketone oxime esters as C-radical precursors. Both of these reactions have constructed two C-S bonds, and all desired products were obtained in moderate to excellent yields with complete stereospecificity.

Molecular mechanism of the metal-independent production of hydroxyl radicals by thiourea dioxide and H2O2

It is well-known that highly reactive hydroxyl radicals (HO•) can be produced by the classic Fenton system and our recently discovered haloquinone/H2O2 system, but rarely from thiol-derivatives. Here, we found, unexpectedly, that HO• can be generated from H2O2 and thiourea dioxide (TUO2), a widely used and environmentally friendly bleaching agent. A carbon-centered radical and sulfite were detected and identified as the transient intermediates, and urea and sulfate as the final products, with the complementary application of electron spin-trapping, oxygen-18 isotope labeling coupled with HPLC/MS analysis. Density functional theory calculations were conducted to further elucidate the detailed pathways for HO• production. Taken together, we proposed that the molecular mechanism for HO• generation by TUO2/H2O2: TUO2 tautomerizes from sulfinic acid into ketone isomer (TUO2-K) through proton transfer, then a nucleophilic addition of H2O2 on the S atom of TUO2-K, forming a S-hydroperoxide intermediate TUO2-OOH, which dissociates homolytically to produce HO•. Our findings represent the first experimental and computational study on an unprecedented new molecular mechanism of HO• production from simple thiol-derived sulfinic acids, which may have broad chemical, environmental, and biomedical significance for future research on the application of the well-known bleaching agent and its analogs.

A visible-light-catalyzed sulfonylation reaction of an aryl selenonium salt via an electron donor-acceptor complex

An efficient synthesis of sulfone structures through selenonium salts and sodium sulfinates was developed. Under the irradiation of a blue LED lamp, the two substrates generate aryl and sulfonyl radicals through the activation of the intermediate electron donor acceptor (EDA) complex, thereby synthesizing aromatic, heteroaromatic and aliphatic sulfones in medium to good yields. The advantages of this strategy are metal-free, mild conditions and the leaving group is recycled to construct new selenonium salts.

Iron-catalyzed fluoroalkylative alkylsulfonylation of alkenes via radical-anion relay

Transition metal-catalyzed reductive difunctionalization of alkenes with alkyl halides is a powerful method for upgrading commodity chemicals into densely functionalized molecules. However, super stoichiometric amounts of metal reductant and the requirement of installing a directing group into alkenes to suppress the inherent β-H elimination bring great limitations to this type of reaction. We demonstrate herein that the difunctionalization of alkenes with two different alkyl halides is accessible via a radical-anion relay with Na2S2O4 as both reductant and sulfone-source. The Na2S2O4 together with the electron-shuttle catalyst is crucial to divert the mechanistic pathway toward the formation of alkyl sulfone anion instead of the previously reported alkylmetal intermediates. Mechanistic studies allow the identification of carbon-centered alkyl radical and sulfur-centered alkyl sulfone radical, which are in equilibrium via capture or extrusion of SO2 and could be converted to alkyl sulfone anion accelerated by iron electron-shuttle catalysis, leading to the observed high chemoselectivity.

Three-Component Tandem Amidosulfenylation of Alkenes for the Synthesis of β-Succinimide Sulfides

An environmentally benign multicomponent strategy for the amidosulfenylation of alkenes for the synthesis of β-succinimide sulfides is disclosed. In this process, common disulfides smoothly act as a sulfur-based source, and N-iodosuccinimide (NIS) is used not only as a free radical initiator but also as an N-nucleophile. A broad range of functional groups are tolerated in this reaction system.

Visible-light-driven reactions for the synthesis of sulfur dioxide-inserted compounds: generation of S-F, S-O, and S-N bonds

Sulfur dioxide-containing compounds such as sulfonyl fluorides, sulfonyl esters, and sulfonyl amides are important structural frameworks in many natural products, pharmaceuticals, and organic compounds. Thus, synthesis of these molecules is a very valuable research topic in organic chemistry. Various synthetic methods to introduce SO₂ groups into the structure of organic compounds have been developed for the synthesis of biologically and pharmaceutically useful compounds. Recently, visible-light-driven reactions were carried out to create SO₂-X (X = F, O, N) bonds, and their effective synthetic approaches were demonstrated. In this review, we summarized recent advances in visible-light-mediated synthetic strategies for generation of SO₂-X (X = F, O, N) bonds for various synthetic applications along with proposed reaction mechanisms.

Copper-Catalyzed Synthesis of Functionalized Aryl Sulfonamides from Sodium Sulfinates in Green Solvents

Functionalized aryl sulfonamides are important building blocks in the pharmaceutical industry. A one-step synthesis catalyzed by a copper salt was developed using stable solid commodity chemicals in sulfolane or, alternatively, in green solvents such as γ-valerolactone, iPrOAc, or nBuOAc with acetic acid. The method tolerated diverse functional groups commonly presented in current medicines and drug intermediates. The mechanistic study showed a radical coupling pathway between the sulfonyl and anilinium radicals through the use of K₂S₂O₈ and copper catalyst, respectively.

Sulfoxylate Anion Radical-Induced Aryl Radical Generation and Intramolecular Arylation for the Synthesis of Biarylsultams

Aryl radical generation from the corresponding aryl halides using an electron donor and subsequent intramolecular cyclization with arenes could be an important advancement in contemporary biaryl synthesis. A green and practically useful synthetic protocol to access diverse six- and seven-membered biarylsultams especially with a free NH group including demonstration of a gram-scale synthesis is reported herein. The sulfoxylate anion radical (SO₂^-•), generated in situ from the reagents rongalite or sodium dithionite (Na₂S₂O₄), was found to be the key single electron transfer agent forming aryl radicals from aryl halides, which upon intramolecular arylation gives biarylsultams with good to excellent yields. The approach features generation of aryl radicals that remained underexplored, use of a cheap and readily available industrial reagents, and transition metal-free, mild, and green reaction conditions.

An iron-catalyzed multicomponent reaction of cycloketone oxime esters, alkenes, DABCO·(SO2)2 and trimethylsilyl azide

The synthesis of β-azidosulfones starting from alkenes, cycloketone oxime esters, trimethylsilyl azide and a sulfur dioxide surrogate of DABCO·(SO2)2 under iron catalysis is developed.

Construction of sulfonated spiro[5,5]trienones from sulfur dioxide via iron-catalyzed dearomative spirocyclization of biaryls

An iron-catalyzed dearomative spirocyclization of biaryl ynones with sodium metabisulfite and cycloketone oxime esters is developed. By using sodium metabisulfite as the source of sulfur dioxide, this approach enables the...

Formation of the Tertiary Sulfonamide C(sp3)-N Bond Using Alkyl Boronic Ester via Intramolecular and Intermolecular Copper-Catalyzed Oxidative Cross-Coupling

A synthetic strategy for the formation of C(sp³)-N bonds, particularly through a copper-catalyzed oxidative cross-coupling, is rare. Herein, we report a novel synthetic approach for the preparation of tertiary sulfonamides via copper-catalyzed intra- and intermolecular oxidative C(sp³)-N cross-coupling reactions. This method allows the utilization of the readily available C(sp³)-based pinacol boronate as a substrate and the tolerance of a wide range of functional groups under mild reaction conditions. The success of this strategy relies on the unprecedented additive effects of silanol and NaIO₄.

Synthesis of β-cyanoalkylsulfonylated vinyl selenides through a four-component reaction

A mild copper-catalyzed four-component selenosulfonylation of alkynes, cycloketone oxime esters, DABCO (SO₂)₂ and diselenides has been developed. This method enables the rapid assembly of β-cyanoalkylsulfonylated vinyl selenides in moderate to good yields. Advantages of this protocol include a broad substrate scope, good functional group tolerance and the late-stage functionalization of complex molecules. Moreover, the potential utility of this methodology is demonstrated through simple oxidation of the products to access synthetically important alkynyl sulfones. Mechanistic studies suggest that a cyanoalkylsulfonyl radical intermediate is involved in this process.

Recent Progress and Emerging Technologies towards a Sustainable Synthesis of Sulfones

Sulfones play a pivotal role in modern organic chemistry. They are highly versatile building blocks and find various applications as drugs, agrochemicals, or functional materials. Therefore, sustainable access to this class of molecules is of great interest. Herein, the goal was to provide a summary on recent developments in the field of sustainable sulfone synthesis. Advances and existing limitations in traditional approaches towards sulfones were reviewed on selected examples. Furthermore, novel emerging technologies for a more sustainable sulfone synthesis and future directions were discussed.

Photoinduced efficient synthesis of cyanoalkylsulfonylated oxindoles via sulfur dioxide insertion

A visible-light-promoted radical cascade reaction of N-arylacrylamide and cyclobutanone oxime esters with sulfur dioxide insertion is established. Mainly through the exploration of the visible light wavelength, it is found that the light source has a certain influence on the formation of cyanoalkylsulfonylated oxindoles, furnishing a range of sulfones in good to excellent yields. This protocol presents good functional group compatibility and does not require transition metals, photosensitizers, external bases, or oxidants.

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

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.

Generation of (E)-β-sulfonyl enamines from sulfur dioxide via a radical process

An iron(ii)-catalyzed three-component reaction of O-acyl oximes, sulfur dioxide, and N-vinylacetamides is accomplished. Diverse (E)-β-sulfonyl enamines are obtained in moderate to good yields by using this protocol with excellent stereoselectivity and regioselectivity.

Iminyl radical initiated sulfonylation of alkenes with rongalite under photoredox conditions

A photoredox-catalyzed reaction of oximes, rongalite and electrophiles is accomplished, affording pyrrole-substituted aliphatic sulfones or sulfonamides in moderate to good yields.

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.

Generation of Sulfonylated Tetrazoles through an Iron-Catalyzed Multicomponent Reaction Involving Sulfur Dioxide

As a privileged motif, tetrazoles can be widely found in pharmaceuticals and materials science. Herein, a five-component reaction of cycloketone oxime esters, alkynes, DABCO·(SO₂)₂, and two molecules of trimethylsilyl azide under iron catalysis is developed, giving rise to a range of cyano-containing sulfonylated tetrazoles in moderate to good yields. This multicomponent reaction exhibits excellent selectivity and enables the formation of multiple new chemical bonds in one pot. A possible mechanism involving azidosulfonylation of alkynes, C-C bond cleavage of both cycloketone oxime esters and alkynes, and [3 + 2] cycloaddition of trimethylsilyl azide and the nitrilium cation intermediate is proposed. Additionally, the potential of terminal alkynes acting as powerful synthons for the synthesis of tetrazoles in a radical initiated process is demonstrated for the first time.

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High-value tetrazole motifs were synthesized via a five-component reaction

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Fixing sulfur dioxide into tetrazole molecules under mild conditions

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Low-cost iron catalyst initiated the transformation

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Excellent selectivity with the formation of multiple new chemical bonds

Visible-Light-Driven Radical Multicomponent Reaction of 2-Vinylanilines, Sulfonyl Chlorides, and Sulfur Ylides for Synthesis of Indolines

A visible-light-driven photoredox-catalyzed multicomponent reaction of 2-vinylanilines, sulfonyl chlorides, and sulfur ylides is described. This protocol features redox-neutral mild conditions, a broad substrate scope, and good functional group tolerance, providing access to various sulfonated 2,3-disubstituted indolines. The product can be transformed to a diverse range of functionalized indoles by a selective aromatization/nucleophilic substitution process. Mechanistic investigations suggest that both sulfonyl chlorides and sulfur ylides serve as radical sources, and the reaction proceeds through a sequential radical addition/addition/thermal S_N2-substitution process.