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.

Photoinduced Vicinal Sulfamoyloximation of Alkenes: Harnessing Bifunctional Nitrosamines via a Rapid Radical Trapping Strategy

We developed a photoinduced method for vicinal sulfamoyloximation of alkenes using N-nitrosamines as bifunctional reagents, with DABSO serving as both a sulfonyl source and a rapid aminyl radical trap. This strategy prevents radical recombination, enabling bifunctional activation under neutral conditions to generate key sulfamoyl radicals. It accommodates broad substrate scope and functional group compatibility, enabling late-stage modifications of bioactive molecules and expanding sulfonamide diversity in organic synthesis.

Recent advances in palladium-catalyzed sulfonylation via SO2 insertion

The importance of sulfonyl-group-containing compounds, such as sulfonamides, sulfones, sulfinate esters, and sulfonyl fluorides, in pharmaceuticals, bioactive molecules, and natural products cannot be overstated. The new development of palladium-catalyzed sulfonylation via SO2 insertion represents a crucial advancement in organic synthesis, enabling the direct α,α-difunctionalization of SO2 and providing efficient access to an array of structure-diverse sulfonyl-containing compounds. Although there have been numerous reviews about SO2 insertion, many of them only cover specific aspects of palladium-catalyzed reactions, leading to an oversight of some important works. Besides, these reviews often lack detailed discussions and systematic conclusion on reaction mechanisms, and fail to comprehensively summarize the significant research achievements in palladium-catalyzed reactions over the past few years. Herein, we aim to systematically consolidate the recent advances in palladium-catalyzed sulfonylation via SO2 insertion, elucidate the underlying reaction mechanism, and highlight some unsolved challenges in this segment. This review seeks to serve as a valuable resource for researchers, assisting in the continued development of palladium-catalyzed sulfonylation methodologies.

Sandmeyer Chlorosulfonylation of (Hetero)Aromatic Amines Using DABSO as an SO2 Surrogate

Sulfonyl chlorides not only play a crucial role in protecting group chemistry but also are important starting materials in the synthesis of sulfonamides, which are in-demand motifs in drug discovery chemistry. Despite their importance, the number of different synthetic approaches to sulfonyl chlorides is limited, and most of them rely on traditional oxidative chlorination chemistry from thiol precursors. In this report, we disclose a novel Sandmeyer-type sulfonyl chloride synthesis from feedstock anilines and DABSO, used as a stable SO2 surrogate, in the presence of HCl and a Cu catalyst. The method works on a wide range of anilines and allows for the isolation of the sulfonyl chloride after aqueous workup or its direct conversion into the sulfonamide by simple addition of an amine after the completion of the Sandmeyer reaction. The scalability of this method was demonstrated on a 20 g scale, and the corresponding heterocyclic sulfonyl chloride was isolated in 80% yield and excellent purity.

C(sp3)-H sulfinylation of light hydrocarbons with sulfur dioxide via hydrogen atom transfer photocatalysis in flow

Sulfur-containing scaffolds originating from small alkyl fragments play a crucial role in various pharmaceuticals, agrochemicals, and materials. Nonetheless, their synthesis using conventional methods presents significant challenges. In this study, we introduce a practical and efficient approach that harnesses hydrogen atom transfer photocatalysis to activate volatile alkanes, such as isobutane, butane, propane, ethane, and methane. Subsequently, these nucleophilic radicals react with SO2 to yield the corresponding sulfinates. These sulfinates then serve as versatile building blocks for the synthesis of diverse sulfur-containing organic compounds, including sulfones, sulfonamides, and sulfonate esters. Our use of flow technology offers a robust, safe and scalable platform for effectively activating these challenging gaseous alkanes, facilitating their transformation into valuable sulfinates.

Photoredox-Catalyzed Preparation of Sulfones Using Bis-Piperidine Sulfur Dioxide - An Underutilized Reagent for SO2 Transfer

Sulfonyl groups are widely observed in biologically relevant molecules and consequently, SO2 capture is an increasingly attractive method to prepare these sulfonyl-containing compounds given the range of SO2 -surrogates now available as alternatives to using the neat gas. This, along with the advent of photoredox catalysis, has enabled mild radical capture of SO2 to emerge as an effective route to sulfonyl compounds. Here we report a photoredox-catalyzed cross-electrophile sulfonylation of aryl and alkyl bromides making use of a previously under-used amine-SO2 surrogate; bis(piperidine) sulfur dioxide (PIPSO). A broad selection of alkyl and aryl bromides were photocatalytically converted to their corresponding sulfinates and then trapped with various electrophiles in a one-pot multistep procedure to prepare sulfones and sulfonamides.

Chemodivergent Sulfonylation of Enynones via Ionic and Radical Addition Modes of Sulfinic Acids

Different addition modes of sulfinic acids were developed for the chemodivergent sulfonylation of enynones, where the ionic sulfonylation to an alkyne moiety of enynones was effected through a salt-controlled syn-addition pathway. The radical sulfonylation of an alkene moiety also provided the stereodefined sulfonylated alkenes. A one-pot tandem sequence of the Ti(Oi-Pr)₄-catalyzed α-vinyl aldol condensation of (E)-β-chlorovinyl ketones followed by the chemodivergent sulfonylations was also explored, allowing for ready access to highly substituted dienes and enynes.

Photocatalytic conversion of aryl diazonium salts to sulfonyl fluorides

Sulfonyl fluorides have emerged as powerful tools in chemical biology for the selective labelling of proteins. A photocatalytic method is described for the conversion of aryl diazonium salts to aryl sulfonyl fluorides. The diazonium substrates are easily obtained in one step from functionalized anilines. We present the optimization of this mild method for the synthesis of sulfonyl fluorides, the scope of the transformation with a series of functionalized diazonium salts, and we discuss photophysical measurements that provide detailed information about the mechanism of the photochemical process.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

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.

Ligand-Enabled Gold-Catalyzed C(sp2)-S Cross-Coupling Reactions

Herein we report C(sp²)-S cross-coupling reactions of aryl iodides and arylsulfonyl hydrazides under ligand-enabled, Au(I)/Au(III) redox catalysis. This strategy operates under mild reaction conditions, requires no prefunctionalized aryl coupling partner, and works across several aryl iodides. The utility of this protocol is highlighted through the synthesis of various medicinally relevant biaryl sulfones. The reaction mechanism is supported with control experiments, mass spectrometry, and NMR studies.

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.

Three-component synthesis of arylsulfonyl-substituted indolo[2,1-a]isoquinolinones and benzimidazo-[2,1-a]isoquinolin-6(5H)-ones by SO2 insertion and radical cascade cyclization

An efficient arylsulfonylation/cyclization of 2-aryl-N-methacryloyl indoles with potassium metabisulfite and aryldiazonium tetrafluoroborates was developed. A series of variously substituted arylsulfonyl indolo[2,1-a]isoquinolin-6(5H)-ones were formed in moderate to good yields via utilization of the nature abundant inorganic salt potassium metabisulfite as a SO₂ surrogate. Additionally, this three-component protocol can also be employed for the synthesis of arylsulfonyl-substituted benzimidazo-[2,1-a]isoquinolin-6(5H)-ones.

Photocatalytic fluorosulfonylation of aliphatic carboxylic acid NHPI esters

SO2 radical insertion/fluorination via a photocatalytic redox strategy is developed, providing an efficient and reliable approach for the synthesis of alkylsulfonyl fluorides.

Divergent cyanoalkylation/cyanoalkylsulfonylation of enamides under organophotoredox catalytic conditions

An organophotoredox catalyzed divergent cyanoalkylation/cyanoalkylsulfonylation of enamides is described.