The Emerging Applications of Sulfur(VI) Fluorides in Catalysis

Radical Fluorosulfonamidation: A Facile Access to Sulfamoyl Fluorides

Recently, the introduction of fluorosulfonyl (-SO2F) groups have attracted considerable research interests, as this moiety could often afford enhanced activities and new functions in the context of chemical biology and drug discovery. Herein, we report the design and synthesis of 1-fluorosulfamoyl-pyridinium (FSAP) salts, which could serve as an effective photoredox-active precursor to fluorosulfamoyl radicals and enable the direct radical C-H fluorosulfonamidation of a variety of (hetero)arenes. This method features mild conditions, visible light, broad substrate scope, good group tolerance, etc., and a metal-free protocol is also viable by using organic photocatalysts. Further, FSAP can also be applied to the radical functionalization of alkenes via 1,2-difunctionalization, radical distal migration, tandem radical-polar crossover reactions, etc. In addition, a formal C-H methylamination of (hetero)arenes by combining this radical C-H fluorosulfonamidation with subsequent hydrolysis as well as product derivatization are also demonstrated.

Intramolecular chalcogen bonding activated SuFEx click chemistry for efficient organic-inorganic linking

SuFEx click chemistry demonstrates remarkable molecular assembly capabilities. However, the effective utilization of alkyl sulfonyl fluoride hubs in SuFEx chemistry, particularly in reactions with alcohols and primary amines, presents considerable challenges. This study pioneers an intramolecular chalcogen bonding activated SuFEx (S-SuFEx) click chemistry employing alkyl sulfonyl fluorides with γ-S as the activating group. The ChB-activated alkyl sulfonyl fluorides can react smoothly with phenols, alcohols, and amines, exhibiting enhanced reactivity compared to SO2F2. Excellent yields have been achieved with all 75 tested substrates. Pioneering the application of S-SuFEx chemistry, we highlight its immense potential in organic-inorganic linking, considering the critical role of interfacial covalent bonding in material fabrication. The S-SuFEx hub 1c, incorporating a trialkoxy silane group has been specifically designed and synthesized for organic-inorganic linking. In a simple step, 1c efficiently anchors various organic compounds onto surfaces of inorganic materials, forming functionalized surfaces with properties such as antibacterial activity, hydrophobicity, and fluorescence.

Selective Ring-Opening Polymerization of Silyl Glycidyl Ether through Organocatalysis

Silyl ether constitutes a multipurpose (macro)molecular functionality for being, e.g., SuFEx-clickable and easily cleavable as a hydroxyl precursor. Its direct incorporation by anionic polymerization is challenged by its base susceptibility. In this study, a two-component organocatalyst shows strict epoxy-selectivity in the anionic ring-opening polymerization (ROP) of commercially available tert-butyldimethylsilyl (R)-(-)-glycidyl ether (TBSGE). The silyl ether pendant groups are fully preserved in the resultant polyether and readily undergo acidic hydrolysis to yield well-defined linear polyglycerol (PGC). Combination of the ROP with mechanistically distinct polymerization chemistries delivers PGC-based polyurethane and a hybrid amphiphilic block copolymer. The SuFEx reaction with sulfonyl fluoride shows effective tuning of polyTBSGE into a sulfonate-functionalized polyether. We have thus exploited the chemoselectivity of organocatalysis to facilitate access to polymers carrying reactive pendant functionalities.

Radical Ring-Opening Fluorosulfonylation of Methylenecyclobutanols via Electron Donor-Acceptor Photoactivation

A visible-light-mediated catalyst- and additive-free method for radical ring-opening fluorosulfonylation of methylenecyclobutanols is reported. Sulfuryl chlorofluoride acts as a FSO2 radical precursor as well as an electron acceptor to form electron donor-acceptor complexes with various methylenecyclobutanol substrates. This method shows fully regioselective and (E)-stereoselective ring-opening processes, providing a variety of FSO2-functionalized γ,δ-unsaturated carbonyls in 38-77% yields. A selection of product diversifications has been studied to demonstrate the versatility of these sulfonyl fluoride products.

Perfluorooxosulfate Salts as SOF4-Gas-Free Precursors to Multidimensional SuFEx Electrophiles

Sulfur(VI) Fluoride Exchange (SuFEx) chemistry stands as a well-established method for swiftly constructing complex molecules in a modular fashion. An especially promising segment of this toolbox is reserved for multidimensional SuFEx hubs: three or more substituents pluggable into a singular SVI centre to make 'beyond-linear' clicked constructions. Sulfurimidoyl difluorides (RNSOF2) stand out as the prime example of this, however their preparation from the scarcely available thionyl tetrafluoride (SOF4) limits this chemistry to only a few laboratories with access to this gas. In this work, we identify silver pentafluorooxosulfate (AgOSF5) as a viable SuFEx hub with reactivity equal to SOF4. The AgF2-mediated oxidation of SOCl2 gives rise to the hexacoordinate AgOSF5 adduct, which in contact with primary amines produces the sulfurimidoyl fluorides in high yields. In addition, we have found this workflow to be fully extendable to the trifluoromethyl homologue, AgOSF4CF3, and we propose the use of AgOSF4X salts as a general route to azasulfur SuFEx electrophiles from commercial starting materials.

Rapid and Convenient Potentiometric Method for Determining Fluorosulfate, a Byproduct of the Fumigant and Greenhouse Gas Sulfuryl Fluoride

A fluorosulfate ion (FSO3-) is a hydrolysis product of sulfuryl fluoride (SO2F2), which is widely used to fumigate buildings, soil, construction materials, and postharvest commodities, and is a potent greenhouse gas. It is a potential marker for biological exposure to SO2F2 and for monitoring the progress of reactions used to scrub SO2F2 from fumigation vent gases. Here, we report a simple and inexpensive potentiometric method for determining FSO3- using a commercial nitrate-selective electrode and discuss its application. The method is suitable for solutions between 0.0025 mM and 660 mM FSO3- at initial pH between 5 and 9. Halide interference depends on its molar ratio to FSO3- and follows the sequence, F- < Cl- < Br- ≪ I-. Halide interference can be eliminated by adding silver sulfate. Interference by bicarbonate can be eliminated by H2SO4 pretreatment, and interference by phosphate or pyrophosphate by MgSO4 addition. Sulfate does not interfere, as it does in ion chromatography. Satisfactory method detection limits for FSO3- in spiked aqueous extracts of 11 fruits were obtained. The method accurately quantified the yield of FSO3- relative to that of F- in base hydrolysis of SO2F2. This study demonstrates that the developed method is highly selective, convenient, and sensitive and thus can be of great value in practice.

Ultrafast Flow Synthesis of o-Functionalized Benzenesulfonyl Fluorides and Subsequent SuFEx Connections via Lithiated Chemistry

Herein we present a flow-based, rapid, and straightforward approach to synthesize diverse functionalized sulfonyl fluorides by harnessing an aryllithium intermediate. The aryllithium intermediate was fully utilized under optimized conditions (0.016 s, -18 °C) to afford various functionalized sulfonyl fluorides and also intramolecular SuFEx cyclization products in high yields (27-94%). Furthermore, the integrated synthesis incorporating subsequent SuFEx connections with even unstable organolithium nucleophiles facilitated one-flow molecular assembly in high yields (42-72%).

Palladium-Catalyzed Remote Hydrosulfonamidation of Alkenes: Access to Primary N-Alkyl Sulfamides by the SuFEx Reaction

Herein, we establish a remote hydrosulfonamidation (HSA) of alkenes using palladium catalysis, where N-fluoro-N-(fluoro-sulfonyl)-carbamate with a sulfur(VI) fluoride moiety is demonstrated as a good amidation reagent. The anti-Markovnikov HSA reaction of terminal alkenes and the remote HSA of internal alkenes are achieved to efficiently yield primary N-alkyl-N-(fluorosulfonyl)-carbamates. In addition, this protocol enables the high-value utilization of alkane by combining the dehydrogenation process. The generated N-alkyl products exhibit a unique reactivity of sulfur(VI) fluorides, which can be directly transferred to N-alkyl sulfamides or amines via the sulfur(VI) fluoride exchange reaction, thereby streamlining their synthesis. Moreover, a (pyridyl) benzazole-type ligand proved to be vital for the excellent chemo- and regioselectivities.

SuFEx-Enabled Direct Deoxy-Diversification of Alcohols

We introduce a new use of sulfonyl fluoride as a bifunctional reagent that facilitates the one-step deoxy-diversification of complex alcohol libraries. Our reaction design features a Sulfur(VI) Fluoride Exchange (SuFEx) mediated activation of alcohols and fluoride-induced activation of silicon-bound nucleophiles. This method enables the direct conversion of alcoholic C-O bonds in complex molecules into diverse analogues via C-C, C-N, C-Cl, and C-Br bond formation while suppressing any elimination side-products.

Iterative SuFEx approach for sequence-regulated oligosulfates and its extension to periodic copolymers

The synthesis of sequence-regulated oligosulfates has not yet been established due to the difficulties in precise reactivity control. In this work, we report an example of a multi-directional divergent iterative method to furnish oligosulfates based on a chain homologation approach, in which the fluorosulfate unit is regenerated. The oligosulfate sequences are determined by high resolution mass spectrometry of the hydrolyzed fragments, and polysulfate periodic copolymers are synthesized by using oligomeric bisfluorosulfates in a bi-directional fashion. The synthetic utility of this iterative ligation is demonstrated by preparing crosslinked network polymers as synthetic adhesive materials.

Radical Hydro-Fluorosulfonylation of Propargylic Alcohols via Electron Donor-Acceptor Photoactivation

A radical hydro-fluorosulfonylation of propargyl alcohols with FSO2Cl is presented based on the photoactivation of the electron donor-acceptor (EDA) complex. The reaction avoids the requirement for photocatalysts, bases, hydrogen donor reagents, any other additives, and harsh conditions, enabling the facile synthesis of various functionalized γ-hydroxy (E)-alkenylsulfonyl fluorides. These multifunctional sulfonyl fluorides can be further diversified, providing access to various privileged molecules of biological relevance.

Arylazo sulfones: multifaceted photochemical reagents and beyond

The photochemical action of arylazo sulfones under visible light irradiation has recently gained considerable attention for the construction of carbon-carbon and carbon-heteroatom bonds in organic synthesis. The inherent dyedauxiliary group (-N2SO2R) embedded in the reagent is responsible for the absorption of visible light even in the absence of a photocatalyst, additive or oxidant, leading to the generation of three different radicals, viz. aryl (carbon-centred), sulfonyl (sulphur-centred) and diazenyl (nitrogen-centred) radicals, under different reaction conditions. Encountering a reagent with such a versatile behaviour is quite rare, which makes arylazo sulfones a highly interesting class of compounds. The mild reaction conditions under which these reagents can operate are an added advantage. Recently, they are also being used as non-ionic photoacid generators (PAGs), electron acceptors, and hydrogen atom transfer (HAT) and imination reagents in a number of synthetic transformations. They have displayed substantial damaging effect on the structure of DNA in the presence of light which can lead to their use as phototoxic pharmaceuticals for cancer treatment. Moreover, their photochemistry is also being exploited in polymerization reactions (as photoinitiators) and in materials chemistry (surface modification).

Electroreductive hydroxy fluorosulfonylation of alkenes

An electroreductive strategy for radical hydroxyl fluorosulfonylation of alkenes with sulfuryl chlorofluoride and molecular oxygen from air is described. This mild protocol displays excellent functional group compatibility, broad scope, and good scalability, providing convenient access to diverse β-hydroxy sulfonyl fluorides. These β-hydroxy sulfonyl fluoride products can be further converted to valuable aliphatic sulfonyl fluorides, β-keto sulfonyl fluorides, and β-alkenyl sulfonyl fluorides. Further, some of these products showed excellent inhibitory activity against Botrytis cinerea or Bursaphelenchus xylophilus, which could be useful for potent agrochemical discovery. Preliminary mechanistic studies indicate that this transformation is achieved through rapid O2 interception by the alkyl radical and subsequent reduction of the peroxy radical, which outcompete other side reactions such as chlorine atom transfer, hydrogen atom transfer, and Russell fragmentation.

Hydrosulfonylation of Unactivated Alkenes and Alkynes by Halogen-Atom Transfer (XAT) Cleavage of SVI-F Bond

A photochemical halogen-atom transfer (XAT) method for generating sulfonyl radicals from aryl sulfonyl fluorides has been developed. It allows the hydrosulfonylation of unactivated alkenes, which was challenging to achieve through our previous single-electron transfer route. This reaction has excellent functional group tolerance and substrate scope under mild conditions.

Photocatalytic C-C Bond Cleavage and Fluorosulfonylation of Strained Cycloalkanols for Carbonyl-Containing Aliphatic Sulfonyl Fluorides

In this report, we present a photocatalytic ring-opening fluorosulfonylation of strained cycloalkanols with sulfur dioxide and NFSI under mild conditions for the synthesis of carbonyl-containing aliphatic sulfonyl fluorides. The synthetic potential of the carbonyl-containing aliphatic sulfonyl fluoride products has been examined by diverse transformations, including SuFEx reactions and Baeyer-Villiger oxidation reactions. Mechanistic studies demonstrate that the reaction operates through a radical C-C bond cleavage/SO2 insertion/fluorination cascade process.

Aryl sulfonyl fluoride synthesis via organophotocatalytic fluorosulfonylation of diaryliodonium salts

A mild and efficient synthesis of various aryl sulfonyl fluorides from diaryliodonium salts under organophotocatalysis via a radical sulfur dioxide insertion and fluorination strategy is presented. Diaryliodonium salts are used as aryl radical precursors, the 1,4-diazabicyclo[2.2.2]octane bis(sulfur dioxide) adduct (DABSO) as a sulfonyl source and cheap KHF2 as a desirable fluorine source, respectively. Notably, the electronic properties of substituents on the aromatic rings in diaryliodonium salts have a significant influence on the reaction yields.

Fluorosulfate as a Latent Sulfate in Peptides and Proteins

Sulfation widely exists in the eukaryotic proteome. However, understanding the biological functions of sulfation in peptides and proteins has been hampered by the lack of methods to control its spatial or temporal distribution in the proteome. Herein, we report that fluorosulfate can serve as a latent precursor of sulfate in peptides and proteins, which can be efficiently converted to sulfate by hydroxamic acid reagents under physiologically relevant conditions. Photocaging the hydroxamic acid reagents further allowed for the light-controlled activation of functional sulfopeptides. This work provides a valuable tool for probing the functional roles of sulfation in peptides and proteins.

Aliphatic Sulfonyl Fluoride Synthesis via Decarboxylative Fluorosulfonylation of Hypervalent Iodine(III) Carboxylates

We disclose herein a photocatalytic decarboxylative fluorosulfonylation reaction of various hypervalent iodine(III) carboxylates in combination with 1,4-diazabicyclo[2.2.2]octane-bis(sulfur dioxide) adduct as a sulfonyl source and KHF2 as a desirable fluorine source via a radical sulfur dioxide insertion and fluorination strategy. A one-pot photocatalytic decarboxylative fluorosulfonylation reaction of various carboxylic acids mediated by PhI(OAc)2 was realized, as well. Notably, this transformation can be performed under heating conditions without the need for catalysts.

New Chemical Transformations Involving SO2 F2 -Mediated Alcohol Activation

Sulfuryl fluoride is a gas produced on a multi-ton scale for its use as a fumigant. In the last decades, it has gained interest in organic synthesis as a reagent with unique properties in terms of stability and reactivity when compared to other sulfur-based reagents. Sulfuryl fluoride has not only been used for sulfur-fluoride exchange (SuFEx) chemistry but also encountered applications in classic organic synthesis as an efficient activator of both alcohols and phenols, forming a triflate surrogate, namely a fluorosulfonate. A long-standing industrial collaboration in our research group drove our work on the sulfuryl fluoride-mediated transformations that will be highlighted below. We will first describe recent works on metal-catalyzed transformations from aryl fluorosulfonates while emphasizing the one-pot processes from phenol derivatives. In a second section, nucleophilic substitution reactions on polyfluoroalkyl alcohols will be discussed and the value of polyfluoroalkyl fluorosulfonates in comparison to alternative triflate and halide reagents will be brought to light.

Turning sulfonyl and sulfonimidoyl fluoride electrophiles into sulfur(VI) radicals for alkene ligation

Sulfonyl and sulfonimidoyl fluorides are versatile substrates in organic synthesis and medicinal chemistry. However, they have been exclusively used as S(VI)+ electrophiles for defluorinative ligations. Converting sulfonyl and sulfonimidoyl fluorides to S(VI) radicals is challenging and underexplored due to the strong bond dissociation energy of SVI-F and high reduction potentials, but once achieved would enable dramatically expanded synthetic utility and downstream applications. In this report, we disclose a general platform to address this issue through cooperative organosuperbase activation and photoredox catalysis. Vinyl sulfones and sulfoximines are obtained with excellent E selectivity under mild conditions by coupling reactions with alkenes. The synthetic utility of this method in the preparation of functional polymers and dyes is also demonstrated.

Phosphorus fluoride exchange: Multidimensional catalytic click chemistry from phosphorus connective hubs

Phosphorus Fluoride Exchange (PFEx) represents a cutting-edge advancement in catalytic click-reaction technology. Drawing inspiration from Nature's phosphate connectors, PFEx facilitates the reliable coupling of P(V)-F loaded hubs with aryl alcohols, alkyl alcohols, and amines to produce stable, multidimensional P(V)-O and P(V)-N linked products. The rate of P-F exchange is significantly enhanced by Lewis amine base catalysis, such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). PFEx substrates containing multiple P-F bonds are capable of selective, serial exchange reactions via judicious catalyst selection. In fewer than four synthetic steps, controlled projections can be deliberately incorporated along three of the four tetrahedral axes departing from the P(V) central hub, thus taking full advantage of the potential for generating three-dimensional diversity. Furthermore, late-stage functionalization of drugs and drug fragments can be achieved with the polyvalent PFEx hub, hexafluorocyclotriphosphazene (HFP), as has been demonstrated in prior research.

Sulfur fluoride exchange

Sulfur Fluoride Exchange (SuFEx) is a click reaction par excellence that has revolutionized multiple research fields. In this Primer, we delve into the essential elements of SuFEx operation, catalysis, and SuFExable connective hubs. We also explore the cutting-edge applications of SuFEx in drug development, polymer science, and biochemistry. Additionally, we examine the potential limitations and promising prospects for this versatile click reaction.

Fluorosulfonylvinylation of Unactivated C(sp3)-H via Electron Donor-Acceptor Photoactivation

An electron donor-acceptor (EDA) complex photoactivation strategy for radical fluorosulfonylation is disclosed for the first time. Simply upon blue light irradiation, the FSO₂ radical can be generated efficiently under catalyst-free, base-free, and additive-free conditions, which enables facile access to 6-keto alkenylsulfonyl fluorides from readily available propargyl alcohols and FSO₂Cl. The 6-keto alkenylsulfonyl fluoride motif has been showcased as a versatile SuFEx hub with diverse follow-up derivatizations.

Photocatalytic Sulfonyl Fluorination of Alkyl Organoboron Substrates

Sulfonyl fluorides are highly versatile molecules for click chemistry that have found applications in many areas of chemistry and biology. Recent chemical approaches have focused on the synthesis of alkyl sulfonyl fluorides from readily available starting materials. Here, we report a photocatalytic synthesis of alkyl sulfonyl fluorides from organotrifluoroborates and boronic acid pinacol esters, which are building blocks commonly employed by medicinal chemists in the synthesis of bioactive molecules. Steady-state and time-resolved spectroscopy have confirmed that the absorption of photons by the acridinium catalysts leads to the oxidation of the organotrifluoroborate substrates. The reaction exhibits broad functional group tolerance, which can be attributed to the mild activation with visible light. Importantly, this general approach provides easy access to primary, secondary, and tertiary alkyl sulfonyl fluorides.

Synthesis of Sulfonyl Fluorides Using Direct Chloride/Fluoride Exchange in Potassium Fluoride and Water/Acetone

A multitude of synthetic routes are available for the preparation of sulfonyl fluorides (R-SO₂F) whose discovery is now almost a century old. These syntheses are often limited by the scope, hazardous materials, and elaborate procedures. The simple and mild procedure based on direct chloride/fluoride exchange from a sulfonyl chloride (R-SO₂Cl) starting material in a KF and water/acetone biphasic mixture is investigated. Seventeen examples in high yield (84-100%) with wide scope are described.

Radical Fluorosulfonyl Heteroarylation of Unactivated Alkenes with Quinoxalin-2(1H)-ones and Related N-Heterocycles

The incorporation of sulfonyl fluoride groups into molecules has been proved effective to enhance their biological activities or introduce new functions. Herein, we report a transition-metal-free and visible-light-mediated radical 1-fluorosulfonyl-2-heteroarylation of alkenes, which could allow access to a series of SO₂F-containing quinoxalin-2(1H)-ones, which are a critical structural motif widely present in a number of biologically active molecules. Further application of the method to the modification of other heterocycles and drug molecules as well as ligation chemistry via SuFEx click reactions is also demonstrated.

EnT-Mediated N-S Bond Homolysis of a Bifunctional Reagent Leading to Aliphatic Sulfonyl Fluorides

Sulfur(VI) fluoride exchange (SuFEx) gives rise to a plethora of high-valent sulfur linkages; however, the availability of (aliphatic) sulfonyl fluoride manifolds lag behind, owing to the limited sources of introducing the SO₂F moiety via a classical two-electron approach. Recently, radical-based methodologies have emerged as a complementary strategy to increase the diversity of accessible click partners. In this work, synthesis of a bench-stable sulfamoyl fluoride reagent is presented, which may undergo sigma-bond homolysis upon visible-light-induced sensitization to form protected β-amino sulfonyl fluorides from alkene feedstocks. Notably, this offers an appealing strategy to access various building blocks for peptido sulfonyl fluorides, relevant in a medicinal chemistry context, as well as an intriguing entry to β-ammonium sulfonates and β-sultams, from alkenes. Densely functionalized 1,3-sultones were obtained by employing allyl alcohols as substrates. Surprisingly, allyl chloride-derived β-imino sulfonyl fluoride underwent S-O bond formation and ring closure to yield rigid cyclopropyl β-imino sulfonate ester under SuFEx conditions. Furthermore, by engaging a thiol-based hydrogen atom donor in the reaction, the reactivity of the same reagent can be tuned toward the direct synthesis of aliphatic sulfonyl fluorides. Mechanistic experiments indicate an energy transfer (EnT)-mediated process. The transient sulfonyl fluoride radical adds to the alkene and product formation occurs upon either radical-radical coupling or hydrogen atom transfer (HAT), respectively.

Facile synthesis of sulfonyl fluorides from sulfonic acids

Herein, we demonstrate two complementary strategies for the syntheses of sulfonyl fluorides using sulfonic acids and their salts. One strategy involves the conversion of sulfonic acid sodium salts to sulfonyl fluorides using thionyl fluoride in 90-99% yields in one hour. Lessons learned from the mechanism of this reaction also have enabled a complementary deoxyfluorination of sulfonic acids using Xtalfluor-E® - a bench stable solid - allowing for the conversion of both aryl and alkyl sulfonic acids and salts to sulfonyl fluorides in 41-94% yields. Notably, using Xtalfluor-E® enabled milder conditions and the use of both sulfonic acids and their sodium salts.

Contrasting the Noncovalent Interactions of Aromatic Sulfonyl Fluoride and Sulfonyl Chloride Motifs via Crystallography and Hirshfeld Surfaces

A heteroaryl sulfonyl(VI) fluoride, 4-chloro-7-fluorosulfonyl-2,1,3-benzoxadiazole, was synthesized from its chloride counterpart (4-chloro-7-chlorosulfonyl-2,1,3-benzoxadiazole) and the X-ray structure analysis of these compounds and the interactions in the solid-state were thoroughly examined. Hirshfeld surface analysis is used to provide a thorough and complete picture of the changes arising from the different halides in the functional groups. Surface analysis reveals that the fluoride does not participate in any hydrogen interactions as opposed to the chloride. However, the fluorine atom is observed to form close interactions with several π bonds. For both moieties, however, the sulfonyl oxygens show comparable interactions with respect to both magnitude and interatomic distances. The Hirshfeld surface analysis is coupled with computational studies to help elucidate the observed interactions that are found from the distinct nitrogen, chlorine, and oxygen atoms present in the molecules, providing new physical insights to the correlation between their structures and properties.

Accelerated and Concerted Aza-Michael Addition and SuFEx Reaction in Microdroplets in Unitary and High-Throughput Formats

The sulfur fluoride exchange (SuFEx) reaction is significant in drug discovery, materials science, and chemical biology. Conventionally, it involves installation of SO₂ F followed by fluoride exchange by a catalyst. We report catalyst-free Aza-Michael addition to install SO₂ F and then SuFEx reaction with amines, both occurring in concert, in microdroplets under ambient conditions. The microdroplet reaction is accelerated by a factor of ∼10⁴ relative to the corresponding bulk reaction. We suggest that the superacidic microdroplet surface assists SuFEx reaction by protonating fluorine to create a good leaving group. The reaction scope was established by performing individual reactions in microdroplets of 18 amines in four solvents and confirmed using high-throughput desorption electrospray ionization experiments. The study demonstrates the value of microdroplet-assisted accelerated reactions in combination with high-throughput experimentation for characterization of reaction scope.

A reductive dehalogenative process for chemo- and stereoselective synthesis of 1,3-dienylsulfonyl fluorides

A method for the mild and efficient synthesis of 1,3-dienylsulfonyl fluorides was developed via dehalogenation of α-halo-1,3-dienylsulfonyl fluorides in the presence of zinc powder and acetic acid, achieving exclusive chemo- and stereoselectivities. This protocol was successfully applied to the synthesis of heterocyclic dienylsulfonyl fluorides and polyene sulfonyl fluoride.

Diversity oriented clicking delivers β-substituted alkenyl sulfonyl fluorides as covalent human neutrophil elastase inhibitors

Diversity Oriented Clicking (DOC) is a discovery method geared toward the rapid synthesis of functional libraries. It combines the best attributes of both classical and modern click chemistries. DOC strategies center upon the chemical diversification of core "SuFExable" hubs-exemplified by 2-Substituted-Alkynyl-1-Sulfonyl Fluorides (SASFs)-enabling the modular assembly of compounds through multiple reaction pathways. We report here a range of stereoselective Michael-type addition pathways from SASF hubs including reactions with secondary amines, carboxylates, 1H-1,2,3-triazole, and halides. These high yielding conjugate addition pathways deliver unprecedented β-substituted alkenyl sulfonyl fluorides as single isomers with minimal purification, greatly enriching the repertoire of DOC and holding true to the fundamentals of modular click chemistry. Further, we demonstrate the potential for biological function - a key objective of click chemistry - of this family of SASF-derived molecules as covalent inhibitors of human neutrophil elastase.

Photo-organocatalytic Synthesis of β-Keto Sulfonyl Fluorides via Radical Fluorosulfonylation of Vinyl Acetates

A metal-free synthesis of useful β-keto sulfonyl fluorides has been established via radical fluorosulfonylation of ketone-derived vinyl acetates under photoredox organocatalysis by using 1-fluorosulfonyl benzoimidazolium (FABI) as the fluorosulfonyl radical source and oxygen-doped anthanthrene (ODA) as the photocatalyst. A series of aryl and alkyl β-keto sulfonyl fluorides as well as cyclic analogues can be readily obtained in moderate to high yields from widely available ketone starting materials.

Synthesis of Highly Reactive Sulfone Iminium Fluorides and Their Use in Deoxyfluorination and Sulfur Fluoride Exchange Chemistry

We report the synthesis of sulfone iminium fluorides (SIFs), a reactive class of sulfur(VI) molecules. The synthesis is tolerant of a variety of substituents on the sulfur and nitrogen components. The SIF reagents were applied to the deoxyfluorination of alcohols and carboxylic acids, providing high yields of fluorinated products in 60 s at room temperature. The SIF reagents were then utilized in sulfur fluoride exchange (SuFEx), creating the first ionic SuFEx products to date.

Synthesis and 18F Labeling of Alkenyl Sulfonyl Fluorides via an Unconventional Elimination Pathway

A successful Cu-catalyzed addition of both Cl and SO₂OCF₂H groups into alkenes allows us to discover the unusual reactivity of the SO₂OCF₂H group. As opposed to common sulfonic esters (RSO₂-O-R'), in which the R' group is highly electrophilic, the SO₂ moiety demonstrates higher electrophilicity in RSO₂-OCF₂H. The unexpected reactivity is further developed not only as a synthetic tool for well-functionalized alkenyl sulfonyl fluorides but also for the first ¹⁸F labeling of alkenyl sulfonyl fluorides.

Calcium Bistriflimide-Mediated Sulfur(VI)-Fluoride Exchange (SuFEx): Mechanistic Insights toward Instigating Catalysis

We report a mechanistic investigation of calcium bistriflimide-mediated sulfur(VI)–fluoride exchange (SuFEx) between sulfonyl fluorides and amines. We determine the likely pre-activation resting state—a calcium bistriflimide complex with ligated amines—thus allowing for corroborated calculation of the SuFEx activation barrier at ∼21 kcal/mol, compared to 21.5 ± 0.14 kcal/mol derived via kinetics experiments. Transition state analysis revealed: (1) a two-point calcium-substrate contact that activates the sulfur(VI) center and stabilizes the leaving fluoride and (2) a 1,4-diazabicyclo[2.2.2]octane additive that provides Brønsted-base activation of the nucleophilic amine. Stable Ca–F complexes upon sulfonamide formation are likely contributors to inhibited catalytic turnover, and a proof-of-principle redesign provided evidence that sulfonamide formation is feasible with 10 mol % calcium bistriflimide.

We report a computational and experimental mechanistic study of sulfur(VI)-fluoride exchange mediated by a calcium salt. Ca²⁺ activates the substrate via two critical Lewis acid−base interactions. Stable fluoride-ligated Ca²⁺ complexes are formed upon sulfonamide formation, and computations indicate that heat and/or fluoride scavengers will facilitate regeneration of the active Ca²⁺ species. These calculations guided an experimental redesign, thus demonstrating improved catalytic efficiency using 10 mol % Ca²⁺.

Photoredox catalytic radical fluorosulfonylation of olefins enabled by a bench-stable redox-active fluorosulfonyl radical precursor

Sulfonyl fluorides have attracted considerable and growing research interests from various disciplines, which raises a high demand for novel and effective methods to access this class of compounds. Radical flurosulfonylation is recently emerging as a promising approach for the synthesis of sulfonyl fluorides. However, the scope of applicable substrate and reaction types are severely restricted by limited known radical reagents. Here, we introduce a solid state, redox-active type of fluorosulfonyl radical reagents, 1-fluorosulfonyl 2-aryl benzoimidazolium triflate (FABI) salts, which enable the radical fluorosulfonylation of olefins under photoredox conditions. In comparison with the known radical precursor, gaseous FSO₂Cl, FABI salts are bench-stable, easy to handle, affording high yields in the radical fluorosulfonylation of olefins with before challenging substrates. The advantage of FABIs is further demonstrated in the development of an alkoxyl-fluorosulfonyl difunctionalization reaction of olefins, which forges a facile access to useful β-alkoxyl sulfonyl fluorides and related compounds, and would thus benefit the related study in the context of chemical biology and drug discovery in the future.

Sulfonyl fluorides are compounds with potential application in chemical biology and drug discovery, but their preparation can be challenging. Here, the authors present a type of bench-stable fluorosulfonyl radical reagents that enable radical fluorosulfonylation reactions via photoredox catalysis.

(Hetero)aryl-SVI Fluorides: Synthetic Development and Opportunities

(Hetero)arylsulfur compounds where the S atom is in the oxidation state VI represent a large percentage of the molecular functionalities present in organic chemistry. More specifically, (hetero)aryl‐S^VI fluorides have recently received enormous attention because of their potential as chemical biology probes, as a result of their reactivity in a simple, modular, and efficient manner. Whereas the synthesis and application of the level 1 fluorination at S^VI atoms (sulfonyl and sulfonimidoyl fluorides) have been widely studied and reviewed, the synthetic strategies towards higher levels of fluorination (levels 2 to 5) are somewhat more limited. This Minireview evaluates and summarizes the progress in the synthesis of highly fluorinated aryl‐S^VI compounds at all levels, discussing synthetic strategies, reactivity, the advantages and disadvantages of the synthetic procedures, the proposed mechanisms, and the potential upcoming opportunities.

Electrochemical Synthesis of β-Keto Sulfonyl Fluorides via Radical Fluorosulfonylation of Vinyl Triflates

Electrochemical synthesis of versatile β-keto sulfonyl fluorides is accomplished by radical fluorosulfonylation of vinyl triflates with FSO₂Cl as the fluorosulfonyl radical source. This electroreductive protocol uses inexpensive graphite felt as electrodes, thus avoiding the use of a sacrificial anode. Moreover, this protocol, featuring metal-free, mild conditions and easy scalability, allows expedient access to valuable β-keto sulfonyl fluorides from readily available precursors, as well as the cyclic ones that are otherwise inaccessible using prior methods.

Silver-catalyzed Radical Cascade Arylthiodifluoromethylation/ Cyclization of Isonitriles for the Synthesis of 6-Phenanthridinyldifluoromethyl Aryl Thioethers

An efficient method for silver-catalyzed radical cascade arylthiodifluoromethylation/cyclization of isonitriles is disclosed. The transformation comprised addition of an arylthiodifluoromethyl radical generated in situ by the oxidative decarboxylation of arylthiodifluoroacetic salts to the isonitrile functionality to construct an ArSCF₂ -C bond, followed by intramolecular cyclization to eventually afford 6-phenanthridinyldifluoromethyl aryl thioethers. The protocol provided a variety of 6-phenanthridinyldifluoromethyl aryl thioethers in medium to excellent yields with a good functional group tolerance under mild reaction conditions.

SuFExable NH-Pyrazoles via 1,3-Dipolar Cycloadditions of Diazo Compounds with Bromoethenylsulfonyl Fluoride

"Click" reactions have transformed the molecular sciences. Augmenting cycloaddition reactions, sulfur(VI) fluoride exchange (SuFEx) chemistry has diversified the landscape of molecular assembly. Herein, we report a facile strategy to access SuFExable NH-pyrazoles via strain and catalyst-free 1,3-dipolar cycloadditions of stabilized diazo compounds under mild conditions. Subsequent SuFEx proceeds efficiently with various N- and O-nucleophiles. Access to SuFExable NH-pyrazoles─a class of compounds containing two common pharmacophores─enables future opportunities within drug discovery, chemical biology, materials chemistry, and related fields.

Amino-oxetanes as amide isosteres by an alternative defluorosulfonylative coupling of sulfonyl fluorides

Bioisosteres provide valuable design elements that medicinal chemists can use to adjust the structural and pharmacokinetic characteristics of bioactive compounds towards viable drug candidates. Aryl oxetane amines offer exciting potential as bioisosteres for benzamides-extremely common pharmacophores-but are rarely examined due to the lack of available synthetic methods. Here we describe a class of reactions for sulfonyl fluorides to form amino-oxetanes by an alternative pathway to the established SuFEx (sulfonyl-fluoride exchange) click reactivity. A defluorosulfonylation forms planar oxetane carbocations simply on warming. This disconnection, comparable to a typical amidation, will allow the application of vast existing amine libraries. The reaction is tolerant to a wide range of polar functionalities and is suitable for array formats. Ten oxetane analogues of bioactive benzamides and marketed drugs are prepared. Kinetic and computational studies support the formation of an oxetane carbocation as the rate-determining step, followed by a chemoselective nucleophile coupling step.

Accelerated SuFEx Click Chemistry For Modular Synthesis

SuFEx click chemistry is a powerful method designed for the selective, rapid, and modular synthesis of functional molecules. Classical SuFEx reactions form stable S-O linkages upon exchange of S-F bonds with aryl silyl-ether substrates, and while near-perfect in their outcome, are sometimes disadvantaged by relatively high catalyst loadings and prolonged reaction times. We herein report the development of accelerated SuFEx click chemistry (ASCC), an improved SuFEx method for the efficient and catalytic coupling of aryl and alkyl alcohols with a range of SuFExable hubs. We demonstrate Barton's hindered guanidine base (2-tert-butyl-1,1,3,3-tetramethylguanidine; BTMG) as a superb SuFEx catalyst that, when used in synergy with silicon additive hexamethyldisilazane (HMDS), yields stable S-O bond linkages in a single step; often within minutes. The powerful combination of BTMG and HMDS reagents allows for catalyst loadings as low as 1.0 mol % and, in congruence with click-principles, provides a scalable method that is safe, efficient, and practical for modular synthesis. ASSC expands the number of accessible SuFEx products and will find significant application in organic synthesis, medicinal chemistry, chemical biology, and materials science.