Recent advances in photochemical and electrochemical strategies for the synthesis of sulfonyl fluorides

Abstract

Sulfonyl fluorides display unique reactivity in organic synthesis and have wide applications in drug discovery, chemical biology, and materials science. Against the backdrop of increasing demands for environmental friendliness and sustainability, photochemical and electrochemical strategies have recently emerged as alternative methods for synthesizing diverse sulfonyl fluorides. This review highlights the recent advancements in these strategies, emphasizing their novel features and proposed mechanisms. We expect this review to inspire additional novel approaches to access diverse sulfonyl fluorides.

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1. Introduction

It has been more than a century since the first sulfonyl fluoride was synthesized.¹ Since then, this class of compounds has continued to garner considerable attention from organic chemists and biochemists, particularly after sulfur(VI) fluoride exchange (SuFEx) came to be regarded as the second-generation click reaction introduced by Sharpless and colleagues in 2014.² Sulfonyl fluorides are used to synthesize³ sulfonamides,⁴ sulfones,⁵ and sulfonate esters,⁶ and in ¹⁸F radiolabelling,⁷ and have applications⁸ in chemical biology,⁹ drug discovery,¹⁰ and materials science.¹¹ Consequently, numerous methods have been developed for the synthesis of sulfonyl fluorides. The classical synthetic route involves chloride–fluoride exchange with a parent sulfonyl chloride,¹² but the low accessibility and stability of sulfonyl chlorides limit their utility. Afterward, various alternative methods were developed using more readily available starting materials such as sulfonic acid, sulfinates, sulfonamides, and aryl halides.¹³ However, these approaches often require stoichiometric oxidants, harsh reaction conditions, or transition metals, leading to high costs, poor chemo-selectivity, and toxicity. Therefore, environmentally benign and sustainable methodologies are highly desirable for the efficient synthesis of sulfonyl fluorides.

Photocatalysis, which harnesses light to form new chemical bonds under mild conditions, typically enables reactions with exceptional functional group compatibility and avoids the use of toxic and hazardous reagents.¹⁴ Electrosynthesis has also emerged as an efficient and sustainable platform for selective molecular transformations.¹⁵ The direct use of traceless and clean electrons as redox reagents obviates the need for stoichiometric chemical redox reagents, making such reactions more efficient and environmentally friendly. Although several reviews have been published on synthetic methodologies to access sulfonyl fluorides,¹⁶ none have focused solely on photochemical and electrochemical strategies. Thus, this review exclusively summarizes the achievements in the construction of sulfonyl fluorides via photocatalysis and electrocatalysis in the past several years, with emphasis on their novel features and proposed mechanisms (Scheme 1). A comprehensive overview of the current challenges and future prospects for advancing the practical applications of photo- and electro-reactions for sulfonyl fluoride synthesis is also provided.

Scheme 1 Photochemical and electrochemical strategies towards diverse sulfonyl fluorides.

2. Photochemical synthesis of sulfonyl fluorides

As photocatalysis has matured over the past several decades, many photoreactions have been accomplished, but this practical and operationally simple technology is seldom implemented for sulfonyl fluorides. In this section, we will present the progress in the synthesis of sulfonyl fluorides by photocatalysis using different sulfonyl fluoride sources.

2.1 Direct use of sulfonyl fluoride hubs

Ethylene sulfonyl fluoride (ESF) is a readily available and versatile compound that has been widely utilized as a sulfonyl fluoride hub to access various alkyl sulfonyl fluorides.¹⁷ On the one hand, owing to their highly electrophilic C=C double bond, ESF and other alkenylsulfonyl fluorides are strong Michel acceptors and readily react with various nucleophiles, such as amines, alcohols, and 1,3-dicarbonyl compounds.^2,17a–c On the other hand, the strong electron-withdrawing property of the SO₂F group also makes ESF act as a dienophile in Diels–Alder reactions.^17h In 2018, Mykhailiuk et al. reported one example of a photocatalytic [2 + 2] cycloaddition reaction of ESF with N-benzylmaleimide to access the corresponding adduct in 29% yield (Scheme 2, top).¹⁸ In 2020, the Qin group reported another efficient photocatalytic [2 + 2] cycloaddition reaction between ESF and pyridine or isoquinolone (Scheme 2, bottom).¹⁹ In this photo-induced cycloaddition reaction, various pyridones and isoquinolones were transformed into unique cyclobutane-fused pyridinyl sulfonyl fluorides in good to excellent yields. In addition, 1-bromoethene-1-sulfonyl fluoride, as another sulfonyl fluoride hub prepared from ESF, was also applied to this cycloaddition reaction under the same conditions and afforded the desired products in moderate to good yields. Besides, upon the reliable SuFEx reaction, adduct products were smoothly converted into sulfonates and sulphonamides, which have potential applications in drug discovery and development.

Scheme 2 Photocatalytic cycloaddition reactions of ethylene sulfonyl fluoride (ESF).

Apart from being involved in cycloaddition reactions, ESF can also serve as a radical acceptor for the photochemical synthesis of aliphatic sulfonyl fluorides. In 2019, Liao and co-workers demonstrated a photocatalytic strategy for the preparation of a series of sulfonyl fluorides from redox-active N-hydroxyphthalimide (NHPI) esters derived from naturally abundant carboxylic acids and ESF (Scheme 3).²⁰ This method employed Eosin Y-Na₂ as a metal-free photocatalyst and a Hantzsch ester as the reductant to produce various aliphatic sulfonyl fluorides in moderate to high yields. Notably, amino acids, peptides, and several drugs were also applicable to this transformation. Derivatization to pharmaceutically relevant motifs such as sultams, sulfonate, and sulfonyl azide demonstrated the potential application of SO₂F-containing products. A plausible mechanism for this reaction is shown in Scheme 3, in which Eosin Y-Na₂ is initially excited to Eosin Y-Na₂*, which is then reduced by the Hantzsch ester. The resultant Eosin Y-Na₂ radical anion is oxidized by the NHPI ester to regenerate the catalyst and afford the NHPI ester radical anion, which undergoes decarboxylation to release CO₂ and give an alkyl radical. The latter is added to ESF and subsequently abstracts a hydrogen atom from the Hantzsch ester radical to generate the desired product.

Scheme 3 Photocatalytic radical addition reaction of N-hydroxyphthalimide (NHPI) esters and ESF.

In 2020, Qin and co-workers developed a pre-activation-free method for synthesizing sulfonyl fluorides from readily-available alkyl iodides and ESF. In this approach, Mn₂(CO)₁₀ was used as the photocatalyst under visible-light irradiation (Scheme 4).²¹ This methodology converted a wide range of 1°, 2°, and 3° alkyl iodides, many of which were derived from natural products, into aliphatic sulfonyl fluorides in the presence of a Hantzsch ester as the hydrogen source. Several large-scale experiments also proceeded smoothly. The proposed mechanism of this reaction is shown in Scheme 4, in which Mn₂(CO)₁₀ first undergoes homolytic cleavage of its Mn–Mn bond to generate a Mn(CO)₅ radical. The Mn(CO)₅ radical reacts with an alkyl iodide to produce alkyl radical A and MnI(CO)₅. The radical addition of alkyl radical A to ESF affords a new alkyl radical B, which then traps hydrogen from the Hantzsch ester to generate the final product and dienyl radical C. Finally, the resulting dienyl radical C undergoes aromatization in the presence of MnI(CO)₅ to produce the pyridine-HI salt D. The Mn(CO)₅ radical is regenerated, thus completing the catalytic cycle.

Scheme 4 Photocatalytic radical addition reaction of alkyl iodides with ESF.

Su, Li, and collaborators recently disclosed the conversion of vinyl fluorosulfates, as FSO₂-containing compounds, into β-keto sulfonyl fluorides through visible light-induced catalytic radical rearrangement (Scheme 5).²² Under the optimal conditions, a variety of aromatic vinyl fluorosulfates with different electronic and steric properties were tolerated, generating the corresponding products in 33–90% yields. However, acyclic or aliphatic ketones were unsuitable for this transformation. The authors proposed a mechanism based on the control experiments, crossover experiments, and DFT calculations, as outlined in Scheme 5. In this mechanism, an excited-state species [Ir^III]* is generated from [Ir^III] under visible light irradiation. There are two pathways by which [Ir^III]* can interact with the vinyl fluorosulfate substrate. In pathway A, vinyl fluorosulfate (E_T = 134 kJ mol⁻¹) undergoes homolytic cleavage via energy transfer from the excited state [Ir^III]* (E_T = 251 kJ mol⁻¹) to produce an enol radical and FSO₂ radical. Subsequent recombination of these two radicals generates the final β-keto sulfonyl fluoride. In pathway B, the excited-state [Ir^III]* is oxidized by vinyl fluorosulfate (E^red_1/2 = +1.30 V vs. SCE) to produce the [Ir^IV] species and a radical anion. Upon subsequent radical fragmentation/reconstruction and oxidation by [Ir^IV], the final product is formed, and the catalyst is regenerated.

Scheme 5 Photocatalytic rearrangement of vinyl fluorosulfates.

Glorius and colleagues very recently developed a bench-stable bifunctional reagent, (diphenylmethylene)sulfamoyl fluoride. This compound undergoes σ-bond homolysis via the inherent weak N–S bond (≈70 kcal mol⁻¹) through energy transfer catalysis to construct protected β-amino sulfonyl fluorides from alkenes (Scheme 6).²³ This novel approach featured a wide substrate scope with good functional group tolerance. Gaseous ethylene and propylene afforded the target sulfonate esters from SuFEx and sulfonyl fluoride, respectively. The reaction could also be controllably switched to yield aliphatic sulfonyl fluorides in the presence of a thiol-based hydrogen atom donor. Under the optimal conditions, terminal alkenes, unactivated 1,2-substituted alkenes, and 1,1-disubstituted alkenes were all tolerated in this hydro-fluorosulfonylation reaction. Additionally, the synthetic scope of β-imino sulfonyl fluorides was showcased by transforming them into biologically-active β-lactams. The proposed mechanism of this divergent reaction is shown in Scheme 6, in which the photocatalyst is excited by a blue LED and undergoes energy transfer with (diphenylmethylene)sulfamoyl fluoride to form the corresponding triplet state. After homolysis of the N–S σ-bond, a sulfonyl fluoride radical and an iminyl radical are formed. The former is then added to the alkene to form a carbon-centered radical that recombines with the iminyl radical to give the imino-fluorosulfonylated product. It may also abstract a hydrogen atom from a hydrogen donor to yield an aliphatic sulfonyl fluoride.

Scheme 6 Chemodivergent fluorosulfonylation of alkenes with a bifunctional reagent via energy transfer.

2.2 Radical SO₂ insertion and fluorination strategy

Another important strategy for directly installing the sulfonyl fluoride motif involves sulfonylation with SO₂ surrogates (DABSO, Na₂S₂O₅, etc.), followed by oxidation and fluorination using F surrogates (NFSI, Selectfluor, etc.). This strategy typically involves transition-metal catalyzed cross-coupling and produces only aryl and alkenyl sulfonyl fluorides.²⁴ A similar radical SO₂ insertion and fluorination strategy has also emerged for the same purpose, which typically involves the generation of alkyl radicals.²⁵ Conventional approaches require the use of excess oxidants or transition metals such as Ag and Cu.

Photocatalysis is one of the most effective strategies for generating reactive radicals under mild conditions. Accordingly, the radical SO₂ insertion and fluorination strategy has been used in several examples of decarboxylative fluorosulfonylation through visible-light photocatalysis. In 2022, Weng and co-workers developed the first three-component decarboxylative fluorosulfonylation of aldoxime esters to prepare sulfonyl fluorides via photocatalysis. In this approach, DABSO and N-fluorobenzenesulfonimide (NFSI) were used as the sulfonyl source and fluorination reagent, respectively (Scheme 7).²⁶ The reaction was driven by energy transfer-mediated photocatalysis, which produced diverse aliphatic sulfonyl fluorides under mild conditions with high functional group compatibility. The proposed mechanism is shown in Scheme 7, in which the triplet state [Ir^III]* is generated upon irradiation with visible light. Energy transfer between [Ir^III]* and the aldoxime ester produces the catalyst and the excited-state aldoxime ester, which then undergoes N–O bond homolysis to produce an iminyl radical and an alkyl radical after expelling CO₂. The alkyl radical is trapped by SO₂ from DABSO to afford the alkylsulfonyl radical, which then reacts with NFSI to form the final fluorosulfonylated product.

Scheme 7 Catalytic decarboxylative fluorosulfonylation of aldoxime esters through energy-transfer-mediated photocatalysis.

Beyond the energy-transfer-mediated reaction pathway, the radical SO₂ insertion/fluorination strategy was also realized via a photo-redox process. Nie and co-workers reported the photoredox decarboxylative fluorosulfonylation of various NHPI esters derived from aliphatic carboxylic acids using DABSO and NFSI (Scheme 8).²⁷ This method enabled the construction of primary, secondary, and tertiary aliphatic sulfonyl fluorides in good to excellent yields under mild conditions. The proposed mechanism is shown in Scheme 8, in which the excited photocatalyst Ir[dF(CF₃)ppy]₂(dtbbpy)PF₆ and DIPEA first undergo single-electron transfer (SET), which forms DIPEA˙⁺ and the reducing [Ir^II] species. Then, the reduction of NHPI esters by [Ir^II] produces an alkyl radical after releasing CO₂, and then the catalyst is regenerated. Upon capturing SO₂ from DABSO, the alkyl radical is transformed into the sulfonyl radical. Finally, the fluorosulfonylation product is generated from the reaction of an alkyl radical and NFSI via fluorine atom transfer.

Scheme 8 Catalytic photo-redox decarboxylative fluorosulfonylation of NHPI esters.

The Liao,²⁸ Larionov,²⁹ Weng,³⁰ and Liu³¹ groups independently reported the construction of sulfonyl fluorides under blue-light irradiation using the same decarboxylative fluorosulfonylation approach but different radical precursors, fluoride sources, and reaction conditions. Their approaches are briefly summarized in Scheme 9.

Scheme 9 Catalytic photo-redox decarboxylative fluorosulfonylation to form sulfonyl fluorides.

In addition to using widely available carboxylic acids and their redox-active derivatives as radical precursors to synthesize sulfonyl fluorides, commonly used organotrifluoroborate salts and boronic acid pinacol esters have also been converted into alkyl sulfonyl fluorides using photocatalysis. In 2023, the Tambar group reported a method (Scheme 10)³² that enabled the synthesis of primary, secondary, and tertiary sulfonyl fluorides under mild conditions with broad functional group tolerance. Mechanistic studies, including UV-visible absorption, steady-state and time-resolved spectroscopy, and control experiments, were used to obtain the mechanism outlined in Scheme 10. An alkyl radical was generated from the oxidation of the trifluoroborate salt by the photo-excited catalyst PC3*. After capturing SO₂ from DABSO, an alkyl sulfonyl radical was formed, which then oxidized the reduced acridinium species PC3⁻¹ to regenerate the catalyst and afford the alkyl sulfonyl anion. Finally, the reaction of the alkyl sulfonyl anion with Selectfluor produced the desired sulfonyl fluoride.

Scheme 10 Catalytic photo-redox fluorosulfonylation of alkyl organoboron substrates.

β-Amino sulfonyl fluorides are key intermediates in the synthesis of peptide sulfonyl fluorides, which are promising covalent inhibitors.³³ However, these compounds require tedious procedures that typically need 6–8 steps. Recently, Weng and co-workers reported the photoredox catalytic aminofluorosulfonylation of unactivated olefins to rapidly construct β-amino sulfonyl fluorides (Scheme 11).³⁴ The catalytic system involved the generation of an amidyl radical, 5-exo-trig cyclization, SO₂ insertion, and subsequent fluorine atom transfer. This strategy was used to obtain various aminofluorosulfonylated products in moderate to good yields under mild conditions. The authors proposed a mechanism for this three-component cascade reaction, as shown in Scheme 11. First, an amidyl radical and [Ir^II] are produced via the excited state [Ir^III]* with the assistance of a base via proton-coupled electron transfer (PCET) under visible-light irradiation. Subsequently, 5-exo-trig cyclization of the amidyl radical generates an alkyl radical, which is then captured by SO₂ from DABSO to produce an alkylsulfonyl radical. Then, the sulfonyl fluoride product and N(SO₂Ph)₂ radical are produced by subsequent fluorine atom transfer between the alkylsulfonyl radical and NFSI. Finally, the N(SO₂Ph)₂ radical is reduced by [Ir^II] to complete the catalytic cycle.

Scheme 11 Catalytic photo-redox fluorosulfonylation of unactivated alkenes.

Four types of activated substrates, including Katritzky pyridinium salts,³⁵ aryl diazonium salts,^36,37 arylazo sulfones,³⁸ and thianthrenium salts,³⁹ and unactivated strained cycloalkanols,⁴⁰ have also been used in this radical SO₂ insertion and fluorination strategy under visible-light irradiation to synthesize aromatic and aliphatic sulfonyl fluorides (Scheme 12). For their mechanisms, radicals are typically generated through single electron transfer to the activated substrates, whereas the strained cycloalkanols are oxidized by the excited catalyst to initiate the reaction in these examples.

Scheme 12 Catalytic photo-redox fluorosulfonylation of different substrates for the construction of sulfonyl fluorides.

2.3 FSO₂Cl as a fluorosulfonyl radical precursor

In comparison with the classical synthetic route through chloride–fluoride exchange from sulfonyl chlorides and the above two strategies, direct one-step fluorosulfonylation to introduce the FSO₂ group is an attractive alternative to access sulfonyl fluorides. Gaseous sulfuryl fluoride (SO₂F₂) is inexpensive and abundant but inert towards hydrolysis and is a relatively inert electrophile, which is attributed to the homolytic bond dissociation energy of its S–F bonds (81 ± 2 kcal mol⁻¹).⁴¹ In addition, this is also exemplified by sulfuryl chloride fluoride (FSO₂Cl), in which the homolytic bond dissociation energy of the S–F bond (84 ± 10 kcal mol⁻¹) is considerably higher than that of the S–Cl bond (43 ± 10 kcal mol⁻¹).⁴² In 2014, the Sharpless group disclosed that a proton can activate the exchange of S–F bonds for S–O bonds by using SO₂F₂ as electrophilic “FSO₂⁺” synthons.² Besides, other fluorosulfonylating reagents including the fluorosulfuryl imidazolium triflate salt (FDIT)⁴³ and [4-(acetylamino)phenyl]imidodisulfuryl difluoride (AISF)⁴⁴ have been widely used as electrophilic “FSO₂⁺” synthons to synthesize sulfonyl fluorides.⁴⁵ In contrast, the use of the fluorosulfonyl radical (˙SO₂F) from an appropriate reagent for the radical fluorosulfonylation reaction was unknown until the first seminal example was disclosed by Liao and colleagues in 2020 (Scheme 13).⁴⁶ This approach utilizes FSO₂Cl as the fluorosulfonyl radical precursor and [Ir(dFCF₃ppy)₂dtbbpy]PF₆ as the photocatalyst to obtain diverse alkenyl sulfonyl fluorides with excellent functional group compatibility. Several natural products could also be fluorosulfonylated for late-stage modification. The proposed mechanism of this photocatalytic fluorosulfonylation reaction is shown in Scheme 14. First, the excited state [Ir^III]* reduces FSO₂Cl via single-electron transfer to produce a fluorosulfonyl radical, which then attacks the alkene to form a carbon-centered radical. This radical reacts with another molecule of FSO₂Cl to create a chlorinated product via a radical chain process. Subsequently, the fluorosulfonylation product is obtained after losing HCl. Alternatively, the redox pathway may also involve the single-electron oxidation of the carbon radical by [Ir^IV] and deprotonation.

Scheme 13 Fluorosulfonylation of alkenes with FSO₂Cl as a radical precursor.

Scheme 14 Chloro-fluorosulfonylation of alkynes with FSO₂Cl as the radical precursor.

Soon after, the same group developed a similar photo-redox catalytic system to synthesize β-chloro alkenylsulfonyl fluorides (BCASFs) from alkynes under mild conditions (Scheme 14).⁴⁷ Terminal and internal alkynes were transformed into the corresponding sulfonyl fluorides. As a new class of sulfonyl fluoride hubs, the BCASFs could be used in many reactions including Suzuki coupling, Sonogashira coupling, selective reduction, and nucleophilic substitution reactions with nitrogen, oxygen, and sulfur nucleophiles based on the β-chloro site. The SuFEx reactions were also applied for the late-stage modification of peptides and drugs, highlighting the potential applications of BCASFs for constructing more complex compounds. In the mechanism proposed by the authors, FSO₂Cl is first reduced by the excited state [Ir^III]* to produce the fluorosulfonyl radical, which undergoes radical addition to an alkyne to generate a vinyl radical. The vinyl radical further reacts with FSO₂Cl to give the final product and then initiates a radical chain process. The vinyl radical might also be converted into alkenylsulfonyl fluoride via hydrogen-atom transfer from Et₂O. The resulting ether-derived radical is then oxidized by [Ir^IV] to regenerate the catalyst and α-chlorinated diethyl ether upon addition of the chloride anion.

The above work showed that the FSO₂Cl radical precursor was reduced by an excited photocatalyst to form a fluorosulfonyl radical. Our recent study disclosed a facile visible light-mediated method to access 6-keto alkenylsulfonyl fluorides (Scheme 15).⁴⁸ This was based on the formation of an electron donor–acceptor (EDA) complex between FSO₂Cl and propargyl alcohol. This reaction is catalyst-free, base-free, and additive-free. It can also be performed in open-air and represents the first example of an EDA complex that uses FSO₂Cl as the acceptor. Radical trapping experiments indicated the participation of FSO₂ radicals, and on–off light experiments supported a chain radical pathway. Based on these findings and previous literature reports, the mechanism, shown in Scheme 15, was proposed. In this mechanism, FSO₂Cl and propargyl alcohol initially form an EDA complex in the group state. Upon irradiation of this EDA complex by blue LEDs, SET from propargyl alcohol to FSO₂Cl occurs, which forms a ˙SO₂F radical. The radical addition of ˙SO₂F to alkyne affords a vinyl radical that undergoes 1,5-HAT to generate an alkyl radical. After 5-exo-trig cyclization, a secondary alkyl radical is formed, and cleavage of the cyclic C–C bond produces a ketyl radical. Subsequent SET from the ketyl radical to FSO₂Cl yields the corresponding carbon cation intermediate, which finally loses a proton to give the desired product. Subsequently, this EDA complex photoactivation strategy was used to design a three-component reaction that furnished a wide range of FSO₂-functionalized tetrahydropyridines.⁴⁹

Scheme 15 Fluorosulfonylation of alkenes with FSO₂Cl as a radical precursor.

2.4 Imidazolium sulfonyl fluoride salt as a fluorosulfonyl radical precursor

Although FSO₂Cl has been used as a fluorosulfonyl radical precursor under photochemical conditions, it has inherent drawbacks. First, it is a low-boiling point (7 °C) compound, making it inconvenient to prepare and handle. Second, the highly electrophilic nature of the chloride causes it to easily react with electron-rich substrates to form chlorinated byproducts, resulting in a limited substrate scope.

To tackle these limitations, Liao and co-workers recently discovered that the solid-state imidazolium sulfonyl fluoride salt (FABI) could serve as a redox-active fluorosulfonyl radical precursor under photo-redox conditions (Scheme 16).⁵⁰ Compared with the previously-used FSO₂Cl, this newly-developed reagent is bench-stable and compatible with various olefins, especially electron-rich alkenes and triaryl ethylenes. This radical fluorosulfonylation was used to obtain several vinyl sulfonyl fluorides in good to excellent yields. The reaction could also switch to yield β-alkoxyl sulfonyl fluorides in the presence of alcohols or acids. As shown in Scheme 16, the authors proposed a mechanism for this fluorosulfonylation reaction based on their experimental findings. First, the excited state [Ir^III]* transfers an electron to the redox-active radical precursor FABI (E^red_1/2 = −1.07 V vs. SCE). Upon the homolytic cleavage of the N–S bond, the ˙SO₂F radical is generated, which undergoes addition to the alkene to produce the key alkyl radical. This radical is then oxidized by the highly-oxidative [Ir^IV] to provide the corresponding carbon cation, which is either deprotonated to yield the vinyl sulfonyl fluoride or captured by alcohols/acids to give the difunctionalization product.

Scheme 16 Photocatalytic fluorosulfonylation of alkenes with FABI as a radical precursor.

Continuing their previous work on the fluorocarboborylation of unsaturated hydrocarbons with the imidazolium sulfonate reagent,⁵¹ Wang and colleagues developed the same imidazolium sulfonyl fluoride salt for a radical fluorosulfonylation reaction (Scheme 17).⁵² Under their optimized conditions, this photocatalysis enabled the synthesis of a wide range of alkenyl sulfonyl fluorides and alkyl sulfonyl fluorides, as well as other functionalities in moderate to good yields with high regio- and stereoselectivity. Radical trapping and radical clock experiments supported the involvement of the fluorosulfonyl radical.

Scheme 17 Divergent fluorosulfonylation of alkenes with FABI as a radical precursor.

Using the same 1-fluorosulfonyl benzoimidazolium triflate salt as the fluorosulfonyl radical precursor, the Liao group developed a radical hydro-fluorosulfonylation procedure for alkenes using oxygen-doped anthanthrene (ODA) as the metal-free photocatalyst and 1,4-cyclohexadiene (1,4-CHD) as the hydrogen donor (Scheme 18).⁵³ This method constructed aliphatic sulfonyl fluorides under mild and metal-free conditions and enabled convenient later-stage modification of natural products and drugs in excellent yields in most cases. Interestingly, when (TMS)₃SiH instead of 1,4-CHD was used as the hydrogen donor, this photocatalytic system could be applied for the radical hydrofluorosulfonylation of alkynes, producing the desired alkenylsulfonyl fluorides with high Z-selectivity. Mechanistically, the excited photocatalyst ODA (E_1/2(ODA*/ODA˙⁺) = −1.76 V vs. SCE) is sufficient to reduce the FABI (E_1/2 = −1.07 V vs. SCE). Then, homolytic cleavage of the N–S bond produces the ˙SO₂F radical, which undergoes addition to an alkene to generate the key alkyl radical. HAT then occurs between the alkyl radical and 1,4-CHD, resulting in the formation of the desired product and a cyclohexadienyl radical. The latter can be oxidized by ODA˙⁺ (E_1/2(ODA˙⁺/ODA) = +0.82 V vs. SCE) to regenerate the photocatalyst and complete the catalytic cycle.

Scheme 18 Photocatalytic fluorosulfonylation of alkenes/alkynes with FABI as the radical precursor.

Liao and colleagues reported several radical fluorosulfonylation reactions to construct diverse sulfonyl fluorides (Scheme 19) using the same redox-active FABI as the sulfonyl fluoride radical precursor and a photocatalyst. These reactions included radical fluorosulfonylation of alkenes/intramolecular arylation via a cascade reaction to access FSO₂-functionalized chromanes,⁵⁴ radical fluorosulfonylation of vinyl acetates,⁵⁵ radical fluorosulfonyl heteroarylation of unactivated alkenes with quinoxalin-2(1H)-ones,⁵⁶ and a FSO₂ radical-initiated tandem addition reaction of two different olefins.⁵⁷

Scheme 19 Photocatalytic fluorosulfonylation of different alkenes with FABI as radical precursors.

3. Electrochemical synthesis of sulfonyl fluorides

In the second section, we showed that photochemical strategies have become alternatives to conventional methods to access sulfonyl fluorides. These reactions occur under milder conditions. In the last five years, electrochemistry has emerged as another choice for sulfonyl fluoride synthesis, and in this section, we will summarize relevant literature on using this strategy to synthesize sulfonyl fluorides.

3.1 Direct electrochemical oxidation of thiols

Thiols and disulfide substrates can be used to directly synthesize sulfonyl fluorides without involving a sulfonyl chloride intermediate. However, this method requires the excess use of the expensive Selectfluor as an oxidant and fluorine reagent.⁵⁸ In 2019, the Noël group described the first example of an electrochemical strategy to synthesize sulfonyl fluorides from thiols and disulfides. The approach employed widely-available thiols and disulfides as reagents and KF as an inexpensive and safe fluoride source under benign electrochemical conditions. This approach avoided the dependence on stoichiometric amounts of oxidants. The reaction featured a broad substrate scope, providing various aryl, heteroaryl, benzyl, and alkyl sulfonyl fluorides in moderate to high yields (Scheme 20).⁵⁹ Based on their kinetics experiments and radical-trapping experiments, the authors proposed the electrochemical transformation mechanism shown in Scheme 20. First, the anodic oxidation of a thiol affords the corresponding disulfide, which is further oxidized and then reacts with nucleophilic fluoride to produce a sulfenyl fluoride. Subsequently, two successive oxidation processes generate the target sulfonyl fluoride product via sulfinyl fluoride as an inseparable intermediate. Later, the same group developed a flow chemistry protocol for the electrochemical oxidative coupling of thiols and KF.⁶⁰ This approach requires only short reaction times (normally 5 min) and is scalable up to 10 mmol. This flow technology can be applied for the follow-up SuFEx reaction, which represents the first example of such a SuFEx flow reaction.

Scheme 20 Electrochemical oxidative coupling of thiols and potassium fluoride.

3.2 FSO₂Cl as a radical precursor

Sulfonyl fluoride radicals can be generated from FSO₂Cl via photocatalysis, as well as electrochemical methods. In 2021, our group disclosed the electrochemical oxo-fluorosulfonylation of alkynes with FSO₂Cl to synthesize β-keto sulfonyl fluorides (Scheme 21).⁶¹ This electrochemical fluorosulfonylation reaction displayed a broad substrate scope with good functional group tolerance and high chemoselectivity, under mild reaction conditions with air as the oxidant. It provided a wide range of β-keto sulfonyl fluorides in good to high yields. Interestingly, this protocol was applied to synthesize α-chloro β-keto sulfonyl fluorides using similar conditions as those for β-keto sulfonyl fluorides by simply changing the solvent from diethyl ether to tetrahydrofuran (THF). β-Keto sulfonyl fluorides are versatile building blocks that can participate in diverse reactions, including condensation, benzyne insertion, α-functionalization, and the SuFEx reaction. Preliminary studies showed that some β-keto sulfonyl fluoride derivatives display good antibacterial activities against Bursaphelenchus xylophilus and Colletotrichum gloeosporioides. Based on our control experiments, the mechanism for this diverse fluorosulfonylation was proposed, as depicted in Scheme 21. First, the reaction is initiated via single-electron transfer to FSO₂Cl to produce ˙SO₂F, which then adds to an alkyne to give a vinyl radical. This radical is then trapped by O₂ to generate a peroxy radical, which is converted to an oxygen-centered radical via recombination and the Russell mechanism. Subsequently, the oxygen-centered radical is reduced at the cathode, followed by protonation to deliver β-keto sulfonyl fluoride. In THF, α-chloro β-keto sulfonyl fluoride is obtained in the presence of in situ-generated MgCl₂ as a Lewis acid and FSO₂Cl as an electrophilic chlorination reagent.

Scheme 21 Electrochemical oxo-fluorosulfonylation of alkynes with FSO₂Cl.

Although we have developed the first one-step electrochemical method for synthesizing β-keto sulfonyl fluorides from alkynes by using FSO₂Cl as the radical precursor, the use of a sacrificial magnesium anode is unavoidable. Therefore, we developed an alternative electrochemical approach to obtain β-keto sulfonyl fluorides from readily-available vinyl triflates by using the inexpensive graphite felt as both the cathode and anode in 2021 (Scheme 22).⁶² This electrochemical fluorosulfonylation was tolerant to substrates bearing various functional groups, including halides, nitriles, methyl groups, and ketones. Notably, the substrate containing two vinyl triflates was fully fluorosulfonylated with FSO₂Cl to afford the corresponding product in moderate yields. Additionally, a gram-scale reaction could be easily accomplished with improved efficiency compared with the 0.2 mmol-scale reaction, and diverse derivatizations of the product were also accomplished. The proposed mechanism for this fluorosulfonylation approach is shown at the bottom of Scheme 22. First, the cathodic reduction of FSO₂Cl forms the sulfonyl fluoride radical, which upon radical addition to vinyl triflate gives an adduct radical species. β-Fragmentation of the resultant radical species forms the final product. The resulting ˙SO₂CF₃ radical releases SO₂ to produce a ˙CF₃ radical, which captures a hydrogen atom from diethyl ether to produce HCF₃, as confirmed by ¹⁹F NMR analysis. The chloride anion was oxidized to a chloride radical at the anode.

Scheme 22 Electrochemical approach towards β-keto sulfonyl fluorides from vinyl triflates.

3.3 Anodic oxidation and fluorination strategy

In 2022, Cheng, Zhou, and co-workers reported the anodic oxidation of sulfinic salts to construct sulfonyl fluorides, wherein commercially available Et₃N·HF served as the fluoride source (Scheme 23).⁶³ In this electrochemical transformation, a series of sulfinic salts was smoothly converted to the desired aryl, aliphatic, and alkenyl sulfonyl fluorides without requiring an external oxidant. Two possible pathways were proposed by the authors, as outlined in Scheme 23. Initially, sodium sulfinate is oxidized at the anode to give the sulfonyl radical. Meanwhile, the bromide anion undergoes single-electron oxidation to afford a bromo radical. Radical–radical coupling of these two radicals generates a sulfonyl bromide intermediate, as confirmed by isolation. The subsequent halogen exchange bromide by fluoride produces the final product (path A). Alternatively, sodium sulfinate can also successively lose two electrons to generate a sulfonyl cation, which then undergoes nucleophilic addition with the bromide anion and halogen exchange to produce the product (path B). The nucleophilic attack of the sulfonyl cation by the fluoride anion directly affords the product (path C). Based on this, the authors developed a one-pot three-component reaction via a Pd-catalyzed cross-coupling between an aryl iodide and sodium sulfite, followed by an electrochemical transformation to sulfonyl fluoride.

Scheme 23 Anodic oxidation of sulfinic salts with Et₃N·HF as the fluoride source.

Oh, Lee, and collaborators reported the synthesis of sulfonyl fluorides by reacting sulfonyl hydrazides and Et₃N·HF under electrochemical conditions (Scheme 24).⁶⁴ In contrast to the conventional method that uses the expensive Selectfluor as the fluoride reagent and elevated reaction temperatures (60 °C),⁶⁵ this protocol utilizes Et₃N·HF as a cheap fluoride source. The method was used to synthesize various aryl sulfonyl fluorides in good yields under mild conditions. However, some alkyl sulfonyl hydrazides were incompatible with this system, and no product was formed. Cyclic voltammetry (CV) studies indicated that ⁿBu₄NI acted both as the electrolyte and redox catalyst. Combined with the control experiments, a plausible mechanism was proposed by the authors, which is shown in Scheme 24. First, the iodide anion is oxidized to I₂via I₃⁻ at the anode. Iodine is then transformed into an iodine radical via homolysis and then oxidizes the sulfonyl hydrazide to the sulfonyl hydrazine radical (path A). The sulfonyl hydrazine radical can also be generated by direct oxidization at the anode (path B). The resultant sulfonyl hydrazine radical was then oxidized by the iodine radical followed by the release of N₂ to afford a sulfonyl radical, which can be oxidized either by the iodine radical or at the anode to produce the sulfonyl cation (path C or D). The cation is finally trapped by the fluoride anion to afford the target product.

Scheme 24 Anodic oxidation of sulfonyl hydrazides with Et₃N·HF as the fluoride source.

3.4 SO₂ insertion and fluorination strategy

In 2022, the Baran group developed a novel strategy involving nickel electrocatalytic sulfinylation followed by fluorination to construct sulfonyl fluorides using a one-pot, two-step procedure (Scheme 25).⁶⁶ In most cases, this mild and economical approach provided higher yields of aryl sulfonyl fluorides than the established Pd-based catalysis approach.^6,13d A SO₂ stock solution was used as the sulfur source to avoid the poor solubility of other SO₂ surrogates. In addition, aryl bromides carrying an electron-withdrawing group were compatible with the reaction, wherein NFSI was used as the fluoride source. The reaction could also be scaled-up to 45 mmol using a recycling flow system to afford the desired sulfonyl fluoride in 50% isolated yield, which was the same as the 0.3 mmol-scale reaction. The CV study showed a reversible reduction of SO₂ at about −0.78 V in the presence of DIPEA, indicating that the amine coordination influenced its redox behavior. The addition of DIPEA was crucial to the reaction, as a significantly lower yield (<5%) was observed when it was absent. The proposed mechanism is presented in Scheme 25, in which SO₂ is preferentially reduced to give the long-lived SO₂˙⁻ at the cathode. Then, the reduction of an in situ-generated Ni^II complex affords Ni^I, which undergoes oxidation–addition with an aryl iodide to produce the Ni(III) intermediate. The substitution of iodide and the reduction of the metal center by SO₂˙⁻ formed a SO₂-coordinated aryl Ni^II complex. The subsequent SO₂ insertion provides Ni-sulfinate, which is finally substituted by a halide to yield the aryl sulfinate product, which regenerates the catalyst. A control experiment indicated that NMPI acts as a reductive mediator to reduce the Ni^II complex.

Scheme 25 Nickel electrocatalytic sulfinylation and fluorination.

Kong, Cao, and co-workers reported the electrochemical oxidation of thianthrenium salts in combination with DABSO as the SO₂ source and KHF₂ as the F source to assemble sulfonyl fluorides (Scheme 26).⁶⁷ Under their optimal conditions, aryl and alkyl thianthrenium salts were converted into the corresponding aryl and alkyl radicals, which was different from previous reports that mainly used thianthrenium salts as aryl radical precursors. This more practical approach does not require exogenous oxidants or catalysts. The authors proposed a mechanism for this fluorosulfonylation reaction, as outlined in Scheme 26. First, the single electron reduction of the thianthrenium salt affords a carbon-centered radical, which is captured by SO₂ to produce a sulfonyl radical. The sulfonyl radical may react via two possible pathways. In the first, it combines with DABCO˙⁺ to afford a tethered intermediate that undergoes nucleophilic substitution to provide the final product. In the other, it is oxidized at the anode to produce a sulfonyl cation, followed by nucleophilic attack to give the product.

Scheme 26 Electrochemical oxidation of thianthrenium salts.

4. Conclusions

As summarized above, several photocatalysis strategies have provided powerful alternatives for the synthesis of sulfonyl fluorides, but when using ESF as the radical acceptor, the products are limited to aliphatic ethyl-tethered ones. Radical fluorosulfonylation has been realized by using the solid-state imidazolium sulfonyl fluoride salt (FABI) as the radial precursor instead of gaseous FSO₂Cl. However, these radical fluorosulfonylation reactions are still restricted to alkenes and alkynes and produce aliphatic and alkenyl sulfonyl fluorides. The synthesis of aromatic sulfonyl fluorides via radical fluorosulfonylation has not yet been reported, and the formation of imidazole as a by-product reduces the reaction efficiency. In contrast to photocatalysis, the electrosynthesis of sulfonyl fluorides is still in its early stages, with only seven reported examples under these conditions. It is expected that more approaches will be developed using this green and efficient electrochemical technology. Furthermore, the utilization of photoelectrochemistry,⁶⁸ which integrates electrochemistry and molecular photocatalysis to enable transformations under mild electrode potentials without the need for exogenous chemical oxidants, remains unexplored in this field. While a flow system for the electrosynthesis of sulfonyl fluorides has been reported,^59,60 increasing demands are expected for both photo- and electrosynthesis examples in flow chemistry. Although there are many examples of alkyl and alkenyl sulfonyl fluorides, there are no photo- or electrochemical methods for producing alkynyl sulfonyl fluorides. It is currently not possible to synthesize sulfonyl fluorides via radical–radical coupling using any of these reactions. There is only one known example of the synthesis of sulfonyl fluorides via direct C–H functionalization under photocatalytic conditions.⁶⁹ More examples are desirable as this is more efficient than using pre-functionalized compounds as starting materials. It would also dramatically expand the reaction scope of sulfonyl fluorides. Until now, no chiral SO₂F compounds have been constructed, despite reported approaches towards enantiopure compounds with a SO₂F moiety.⁷⁰ We expect that all of the above-mentioned directions will continue to expand in the coming years.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the National Natural Science Foundation of China (22201133 and 32171724) and the Natural Science Foundation of Jiangsu Province (BK20210607) for their financial support of this work.

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