Radical C−C Bond Formation using Sulfonium Salts and Light

Radical Cascade Cyclization of N-(o-Cyanobiaryl)acrylamides with Sulfonium Salts via Synergetic Photoredox and Copper Catalysis

As the magic methyl effect is well acknowledged in pharmaceutical molecules, the development of simple and efficient methods for the installment of methyl groups on complex molecules is highly coveted. Hence, we provide a general strategy for radical cascade cyclization of N-(o-cyanobiaryl)acrylamides by utilizing sulfonium salts as the sources of methyl radical and merging photoredox and copper catalysis. This novel protocol can access a wide variety of methylation or remote thioether-substituted benzo-fused N-heterocycle derivatives, which can be easily transformed into diverse highly valuable sulfone and sulfoximine compounds via late-stage diversification. Moreover, to further demonstrate the synthetic utility of this conversion, the methyl(phenyl)sulfide, which serves as both raw material and byproduct, can be recovered and reused in this transformation. The scale-up experiment for the one-pot two-step process directly offers the target product in good yield under the standard conditions.

Cu-Promoted Divergent Phosphination of Alkynylsulfonium Salts with Diarylphosphines

An efficient copper-promoted divergent phosphination of alkynylsulfonium salts 1 with secondary diarylphosphines 2 that tolerates a wide range of functional groups under mild conditions is reported. The use of excess alkynyl dibenzothiophenium salts (1/2 > 1, mole ratio) enables the phosphination to deliver alkynyl monophosphine products via a C(sp)-P cross-coupling in good to high yields, while the use of excess secondary diarylphosphines (1/2 < 0.5, mole ratio) leads to a type of cis-ethenyl bisphosphine products via sequential stereoselective double phosphination.

Ring-Opening Sulfonylation of Cyclic Sulfonium Salts with Sodium Sulfinates under Transition-Metal- and Additive-Free Conditions

Incorporating a sulfonyl group into parent molecules has been shown to effectively improve their synthetic applications and bioactivities. In this study, we present a straightforward and practical approach for the ring-opening reaction of alkenyl-aryl sulfonium salts with sodium sulfinates to produce a range of sulfur-containing alkyl sulfones. This method offers the benefits of mild reaction conditions, easily accessible raw materials, wide substrate applicability, good functional group compatibility, and operational simplicity. Importantly, the resulting products can be readily converted into sulfoxides, sulfones, sulfoximines, and some heterocyclic compounds.

Hydrodeuteroalkylation of Unactivated Olefins Using Thianthrenium Salts

Isotopically labeled alkanes play a crucial role in organic and pharmaceutical chemistry. While some deuterated methylating agents are readily available, the limited accessibility of other deuteroalkyl reagents has hindered the synthesis of corresponding products. In this study, we introduce a nickel-catalyzed system that facilitates the synthesis of various deuterium-labeled alkanes through the hydrodeuteroalkylation of d2-labeled alkyl TT salts with unactivated alkenes. Diverging from traditional deuterated alkyl reagents, alkyl thianthrenium (TT) salts can effectively and selectively introduce deuterium at α position of alkyl chains using D2O as the deuterium source via a single-step pH-dependent hydrogen isotope exchange (HIE). Our method allows for high deuterium incorporation, and offers precise control over the site of deuterium insertion within an alkyl chain. This technique proves to be invaluable for the synthesis of various deuterium-labeled compounds, especially those of pharmaceutical relevance.

Unusual Regio- and Chemoselectivity in Oxidation of Pyrroles and Indoles Enabled by a Thianthrenium Salt Intermediate

A dearomative oxidation of pyrroles to Δ3-pyrrol-2-ones is described, which employs a sulfoxide as oxidant, in conjunction with a carboxylic acid anhydride and a Brønsted acid additive. 3-substituted pyrroles undergo regioselective oxidation to give the product isomer in which oxygen has been introduced at the more hindered position. Regioselectivity is rationalized by a proposed mechanism that proceeds by initial thianthrenium introduction at the less-hindered pyrrole α-position, followed by distal attack of an oxygen nucleophile and subsequent elimination of thianthrene. The same reaction conditions are also able to effect a chemoselective oxidation of indoles to indolin-3-ones and additionally of indolin-3-ones to 2-hydroxyindolin-3-ones. Here again, the regio- and chemoselectivities are rationalized through the intermediacy of a thianthrenium salt.

Electrocatalytic Formal C(sp2)-H Alkylations via Nickel-Catalyzed Cross-Electrophile Coupling with Versatile Arylsulfonium Salts

Producing sp3-hybridized carbon-enriched molecules is of particular interest due to their high success rate in clinical trials. The installation of aliphatic chains onto aromatic scaffolds was accomplished by nickel-catalyzed C(sp2)-C(sp3) cross-electrophile coupling with arylsulfonium salts. Thus, simple non-prefunctionalized arenes could be alkylated through the formation of aryldibenzothiophenium salts. The reaction employs an electrochemical approach to avoid potentially hazardous chemical redox agents, and importantly, the one-pot alkylation proved also viable, highlighting the robustness of our approach.

Base-Mediated Divergent Synthesis of Spiro-heterocycles Using Pronucleophiles and Ethylene via Thianthrenation

Spirocyclic compounds are abundant in biologically active products. However, the divergent synthesis of spirocyclic compounds using low-cost and abundant available starting materials remains a challenge. Herein, we report an effective method for producing spirocyclic motifs using a cyclic β-carbonyl ester or amide and ethylene via thianthrenation. This strategy highlights the exciting possibility of utilizing abundant ethylene as a C-2 synthon and allows regulating the core structure of the spirocyclic compound by simply altering the base type.

Modular assembly of arenes, ethylene and heteroarenes for the synthesis of 1,2-arylheteroaryl ethanes

The 1,2-arylheteroaryl ethane motif stands as a privileged scaffold with promising implications in drug discovery. Conventional de novo syntheses of these molecules have relied heavily on pre-functionalized synthons, entailing harsh conditions and multi-step processes. Here, to address these limitations, we present a modular approach for the direct synthesis of 1,2-arylheteroaryl ethanes using feedstock chemicals, including ethylene, arenes and heteroarenes. We disclosed a photo triplet-energy-transfer-initiated radical cascade process, leveraging homolytic cleavage of C-S bonds in aryl sulfonium salts as the key step to access aryl radicals with excellent regioselectivity. This method allows for rapid structural diversification of bioactive molecules, showcasing excellent functional group tolerance and streamlining the synthesis of bioactive compounds and their derivatives. Furthermore, our approach can be extended to propylene, non-gaseous terminal alkenes and various other electrophilic radical precursors, including heteroaryl radicals, hydroxyl radicals, trifluoromethyl radicals and α-carbonyl alkyl radicals. This study highlights the significance of radical polarity matching in designing selective multi-component couplings.

Photoredox Ring Opening 1,2-Alkylarylation of Alkenes with Sulfonium Salts Toward Thioether-Substituted Oxindoles

A novel strategy for the difunctionalization of electron-deficient alkenes with aryl sulfonium salts to access remote sulfur-containing oxindole derivatives by using in situ-formed copper(I)-based complexes as a photoredox catalyst is presented. This method enables the generation of the C(sp3)-centered radicals through site selective cleavage of the C-S bond of aryl sulfonium salts under mild conditions. Moreover, the oxidation reactions of desired products provide a new strategy for the preparation of sulfoxide or sulfone-containing compounds. Importantly, this approach can be easily applied to late-stage modification of pharmaceuticals molecules.

N-(Sulfonio)Sulfilimine Reagents: Non-Oxidizing Sources of Electrophilic Nitrogen Atom for Skeletal Editing

The one-pot synthesis of λ4-dibenzothiophen-5-imino-N-dibenzothiophenium triflate (1) in multigram scale is reported. This compound reacts with Rh2(esp)2 (esp=α,α,α',α'-tetramethyl-1,3-benzenedipropionic acid) generating a Rh-coordinated sulfonitrene species, which is able to transfer the electrophilic nitrene moiety to olefins. When indenes are used as substrates, isoquinolines are obtained in good yields. We assumed that after formation of the corresponding N-sulfonio aziridine, a ring expansion occurs via selective C-C bond cleavage and concomitant elimination of dibenzothiophene. Unexpectedly, a similar protocol transforms 1-arylcyclobutenes into 1-cyano-1-arylcyclopropanes. Our calculations indicate that aziridination is not favored in this case; instead, sulfilimine-substituted cyclobutyl carbocations are initially formed, and these evolve to the isolated cyclopropanes via ring contraction. Both procedures are operationally simple, tolerate a range of functional groups, including oxidation-sensitive alcohols and aldehydes, and enable the convenient preparation of valuable 15N-labelled products. These results demonstrate the potential of 1 to provide alternative pathways for the selective transfer of N-atoms in organic molecules.

Metallaphotoredox deuteroalkylation utilizing thianthrenium salts

Deuterium labeling compounds play a crucial role in organic and pharmaceutical chemistry. The synthesis of such compounds typically involves deuterated building blocks, allowing for the incorporation of deuterium atoms and functional groups into a target molecule in a single step. Unfortunately, the limited availability of synthetic approaches to deuterated synthons has impeded progress in this field. Here, we present an approach utilizing alkyl-substituted thianthrenium salts that efficiently and selectively introduce deuterium at the α position of alkyl chains through a pH-dependent HIE process, using D2O as the deuterium source. The resulting α-deuterated alkyl thianthrenium salts, which bear two deuterium atoms, exhibit excellent selectivity and deuterium incorporation in electrophilic substitution reactions. Through in situ formation of isotopically labelled alkyl halides, these thianthrenium salts demonstrate excellent compatibility in a series of metallaphotoredox cross-electrophile coupling with (hetero)aryl, alkenyl, alkyl bromides, and other alkyl thianthrenium salts. Our technique allows for a wide range of substrates, high deuterium incorporation, and precise control over the site of deuterium insertion within a molecule such as the benzyl position, allylic position, or any alkyl chain in between, as well as neighboring heteroatoms. This makes it invaluable for synthesizing various deuterium-labeled compounds, especially those with pharmaceutical significance.

Tuning Regioselectivity in the [3 + 2] Cycloaddition of Alkynyl Sulfonium Salts with Binucleophilic N-Aryl Amidines

A tunable reaction manifold of alkynyl sulfonium salts with binucleophilic N-aryl amidines in the absence of any transition metal catalyst is first reported. This methodology involves sequential addition/cyclization that is perfectly tuned by stepwise addition of K2CO3, affording a plethora of valuable 1,2,4- and 1,2,5-trisubstituted imidazoles in good yields with high regioselectivity. Importantly, trapping and isolation of the reactive intermediate unveiled the reaction mechanism of β-attack on the triple bond in this [3 + 2] cycloaddition reaction.

Palladium-catalyzed regio- and stereo-selective phosphination of cyclic biarylsulfonium salts to access atropoisomeric phosphines

A palladium-catalyzed regio- and stereo-selective phosphination of cyclic biarylsulfonium salts (racemic) with HPAr3Ar4 for straightforward synthesis of atropoisomeric phosphines (P,S-ligands) bearing a stereogenic axis or both a stereogenic axis and a P-stereogenic center is reported. The high reactivity and regio- and stereo-selectivity originate from the torsional strain release and palladium catalysis, and the construction of a P-stereogenic center is enabled by an efficient dynamic kinetic resolution. The high performance of the nascent P,S-ligands has been demonstrated in palladium-catalyzed asymmetric allylic substitutions, indicating the great potential of the present methodology.

Reactivity of α-diazo sulfonium salts: rhodium-catalysed ring expansion of indenes to naphthalenes

In the presence of catalytic amounts of the paddlewheel dirhodium complex Rh2(esp)2, α-diazo dibenzothiophenium salts generate highly electrophilic Rh-coordinated carbenes, which evolve differently depending on their substitution pattern. Keto-moieties directly attached to the azomethinic carbon promote carbene insertion into one of the adjacent C-S bonds, giving rise to highly electrophilic dibenzothiopyrilium salts. This intramolecular pathway is not operative when the carbene carbon bears ester or trifluoromethyl substituents; in fact, these species react with olefins delivering easy to handle cyclopropyl-substituted sulfonium salts. When indenes are the olefins of choice, the initially formed cyclopropyl rings smoothly open with concomitant departure of dibenzothiophene, enabling access to a series of 2-functionalized naphthalenes.

Pseudocyclic Arylbenziodoxaboroles as Water-Triggered Aryne Precursors in Reactions with Organic Sulfides

Pseudocyclic arylbenziodoxaboroles are unique aryne precursors under neutral aqueous conditions that selectively react with organic sulfides, forming the corresponding sulfonium salts. This reaction is compatible with various substituents (alkyl, halogen, CN, NO2, CHO, and cyclopropyl) in the aromatic ring or alkyl group of the sulfide. Similar reactions of sulfoxides afford o-hydroxy-substituted sulfonium salts. The structures of key products were confirmed by X-ray analysis.

Nucleophilic functionalization of thianthrenium salts under basic conditions

In recent years, S-(alkyl)thianthrenium salts have become an important means of functionalizing alcohol compounds. However, additional transition metal catalysts and/or visible light are required. Herein, a direct thioetherification/amination reaction of thianthrenium salts is realized under metal-free conditions. This strategy exhibits good functional-group tolerance, operational simplicity, and an extensive range of compatible substrates.

Catalytic Enantioselective Nucleophilic α-Chlorination of Ketones with NaCl

Catalytic enantioselective α-chlorination of ketones is a highly desirable process. Different from the conventional approaches that employ corrosive electrophilic chlorination reagents, the process disclosed here employs nucleophilic chloride, aqueous NaCl solution, and even seawater, as green inexpensive chlorine sources. This mechanistically distinct and electronically opposite approach provides facile access to diverse highly enantioenriched acyclic α-chloro ketones that are less straightforward by conventional approaches. With a chiral thiourea catalyst, a range of racemic α-keto sulfonium salts underwent enantioconvergent carbon-chlorine bond formation with high efficiency and excellent enantioselectivity under mild conditions. The sulfonium motif plays a crucial triple role by permitting smooth dynamic kinetic resolution to take place via a chiral anion binding mechanism in a well-designed phase-transfer system. This protocol represents a new general platform for the asymmetric nucleophilic α-functionalization of carbonyl compounds.

Dithiolation of Alkenyl Sulfonium Salts with Arylthiols to Access 1,2-Dithioalkanes

A dithiolation of alkenyl sulfonium salts with arylthiols is described, affording a series of 1,2-dithioalkanes in high yields. This protocol features mild and catalyst-free conditions and involves the formation of two C-S bonds sequentially via the regioselective addition of an arylthiol to the unsaturated C═C bonds, followed by the attack of another arylthiol to form 1,2-dithioalkanes exclusively.

Cobalt-Catalyzed Reductive Cross-Coupling To Construct Csp3-Csp3 Bonds via Csp3-S and Csp3-X Bonds Activation

A cross-electrophilic coupling of benzyl sulfonium salts with alkyl halides forming Csp3-Csp3 bonds is described by using a Co-based catalytic system. The activation of the stable Csp3-S bond of benzyl sulfonium salts under mild reaction conditions leads to various functionalized alkyl derivatives. Preliminary mechanistic studies suggest the involvement of alkyl radicals formed from both alkyl halides and benzyl sulfoniums through a single electron transfer.

Copper and Silver Catalysis in the (3 + 2) Cycloaddition of Neutral Three-Atom Components with Terminal Alkynes

The introduction of the copper-catalyzed azide-alkyne coupling (CuAAC) to 1,3-dipolar cycloadditions was pivotal to their popularization in synthetic chemistry and to their application to multiple other domains of science. The reaction rate enhancement observed when coinage metal acetylide intermediates are involved in the cyclization process greatly expanded the structural and conditional range in which (3 + 2) cycloadditions may take place with terminal alkynes. Herein, we report that comparable rate enhancements, in nature and level, are induced by copper and silver catalysts in the intramolecular (3 + 2) cycloaddition of terminal alkynes with "neutral" three-atom components (TACs), specifically alkynyl sulfides. Through careful observations amidst reaction optimization, experimental, and DFT mechanistic studies, a pathway involving a proton-coupled cyclometallation key step is proposed. The sets of catalytic conditions that have been developed allow us to overcome several scope limitations previously presented by the thermally promoted (3 + 2) cycloaddition of "neutral" TACs, thus expanding their synthetic and applicative potential.

Sulfonium Salts as Acceptors in Electron Donor-Acceptor Complexes

The photoactivation of electron donor-acceptor complexes has emerged as a sustainable, selective and versatile strategy for the generation of radical species. Electron donor-acceptor (EDA) complexation, however, imposes electronic constraints on the donor and acceptor components and this can limit the range of radicals that can be generated using the approach. New EDA complexation strategies exploiting sulfonium salts allow radicals to be generated from native functionality. For example, aryl sulfonium salts, formed by the activation of arenes, can serve as the acceptor components in EDA complexes due to their electron-deficient nature. This "sulfonium tag" approach relaxes the electronic constraints on the parent substrate and dramatically expands the range of radicals that can be generated using EDA complexation. In this review, these new applications of sulfonium salts will be introduced and the areas of chemical space rendered accessible through this innovation will be highlighted.

Highly Regioselective Dichalcogenation of Alkenyl Sulfonium Salts to Access 1,1-Dichalcogenalkenes

An unprecedented geminal olefinic dichalcogenation of alkenyl sulfonium salts with dichalcogenides ArYYAr (Y = S, Se, Te) is reported, providing various trisubstituted 1,1-dichalcogenalkenes [Ar¹CH = C(YAr²)₂] in a highly selective manner under mild and catalyst-free conditions. The key process involves the formation of two geminal olefinic C-Y bonds via sequential C-Y cross-coupling and C-H chalcogenation. A mechanistic rationale is further supported by control experiments and density functional theory calculations.

Thianthrenium-Enabled Phosphorylation of Aryl C-H Bonds via Electron Donor-Acceptor Complex Photoactivation

An efficient strategy for the preparation of aryl phosphonates via blue-light-promoted single electron transfer process of an EDA complex between phosphites and thianthrenium salts has been demonstrated. The corresponding substituted aryl phosphonates were obtained in good to excellent yields, and the byproduct thianthrene can be recovered and reused in quantity. This developed method realizes the construction of aryl phosphonates through the indirect C-H functionalization of arenes, which has potential application value in drug discovery and development.

Visible light/copper catalysis enabled alkylation of silyl enol ethers with arylsulfonium salts

An efficient protocol has been developed herein for the site-selective alkylation of silyl enol ethers with arylsulfonium salts giving access to valuable aryl alkyl thioethers under visible light conditions. Enabled by copper (I) photocatalysis, the C-S bond of arylsulfonium salts can be selectively cleaved to deliver C-centered radicals under mild conditions. This developed method provides a straightforward approach to utilize arylsulfonium salts as sulfur sources for the synthesis of aryl alkyl thioethers.

Visible-Light Copper Catalysis for the Synthesis of α-Alkyl-Acetophenones by the Radical-Type Ring Opening of Sulfonium Salts and Oxidative Alkylation of Alkenes

Direct difunctionalization of simple alkenes has been treated as a powerful synthetic strategy for the construction of highly functionalized skeletons. In this study, direct oxidative coupling of sulfonium salts with alkenes was achieved under mild conditions by a blue-light-driven photoredox process using a copper complex as a photosensitizer. This protocol allows regioselective synthesis of aryl/alkyl ketones from simple sulfonium salts and aromatic alkenes via selective C-S bond cleavage of sulfonium salts and oxidative alkylation of aromatic alkenes using dimethyl sulfoxide (DMSO) as a mild oxidant.

Nickel Meets Aryl Thianthrenium Salts: Ni(I)-Catalyzed Halogenation of Arenes

Herein, a regioselective, late-stage two-step arene halogenation method is reported. We propose how unusual Ni(I)/(III) catalysis is enabled by a combination of aryl thianthrenium and Ni redox properties that is hitherto unachieved with other (pseudo)halides. The catalyst is accessed in situ from inexpensive NiCl₂·6(H₂O) and zinc without the need of supporting ligands.

Polarity Transduction Enables the Formal Electronically Mismatched Radical Addition to Alkenes

The formation of carbon-carbon bonds via the intermolecular addition of alkyl radicals to alkenes is a cornerstone of organic chemistry and plays a central role in synthesis. However, unless specific electrophilic radicals are involved, polarity matching requirements restrict the alkene component to be electron deficient. This limits the scope of a fundamentally important carbon-carbon bond forming process that could otherwise be more universally applied. Herein, we introduce a polarity transduction strategy that formally overcomes this electronic limitation. Vinyl sulfonium ions are demonstrated to react with carbon-centered radicals, giving adducts that undergo in situ or sequential nucleophilic displacement to provide products that would be inaccessible via traditional methods. The broad generality of this strategy is demonstrated through the derivatization of unmodified complex bioactive molecules.

Synthetic application of chalcogenonium salts: beyond sulfonium

The application of chalcogenonium salts in organic synthesis has grown enormously in the past decades since the discovery of the methyltransferase enzyme cofactor S-adenosyl-L-methionine (SAM), featuring a sulfonium center as the reactive functional group. Chalcogenonium salts can be employed as alkylating agents, sources of ylides and carbon-centered radicals, partners for metal-catalyzed cross-coupling reactions and organocatalysts. Herein, we will focus the discussion on heavier chalcogenonium salts (selenonium and telluronium), presenting their utility in synthetic organic transformations and, whenever possible, drawing comparisons in terms of reactivity and selectivity with the respective sulfonium analogues.

Radical SAM enzymes: Nature's choice for radical reactions

Enzymes that use a [4Fe-4S]1+ cluster plus S-adenosyl-l-methionine (SAM) to initiate radical reactions (radical SAM) form the largest enzyme superfamily, with over half a million members across the tree of life. This review summarizes recent work revealing the radical SAM reaction pathway, which ultimately liberates the 5'-deoxyadenosyl (5'-dAdo•) radical to perform extremely diverse, highly regio- and stereo-specific, transformations. Most surprising was the discovery of an organometallic intermediate Ω exhibiting an Fe-C5'-adenosyl bond. Ω liberates 5'-dAdo• through homolysis of the Fe-C5' bond, in analogy to Co-C5' bond homolysis in B12 , previously viewed as biology's paradigmatic radical generator. The 5'-dAdo• has been trapped and characterized in radical SAM enzymes via a recently discovered photoreactivity of the [4Fe-4S]+ /SAM complex, and has been confirmed as a catalytically active intermediate in enzyme catalysis. The regioselective SAM S-C bond cleavage to produce 5'-dAdo• originates in the Jahn-Teller effect. The simplicity of SAM as a radical precursor, and the exquisite control of 5'-dAdo• reactivity in radical SAM enzymes, may be why radical SAM enzymes pervade the tree of life, while B12 enzymes are only a few.

A general arene C-H functionalization strategy via electron donor-acceptor complex photoactivation

The photoactivation of electron donor-acceptor complexes has emerged as a sustainable, selective and versatile strategy for the generation of radical species. However, when it comes to aryl radical formation, this strategy remains hamstrung by the electronic properties of the aromatic radical precursors, and electron-deficient aryl halide acceptors are required. This has prevented the implementation of a general synthetic platform for aryl radical formation. Our study introduces triarylsulfonium salts as acceptors in photoactive electron donor-acceptor complexes, used in combination with catalytic amounts of newly designed amine donors. The sulfonium salt label renders inconsequential the electronic features of the aryl radical precursor and, more importantly, it is installed regioselectively in native aromatic compounds by C-H sulfenylation. Using this general, site-selective aromatic C-H functionalization approach, we developed metal-free protocols for the alkylation and cyanation of arenes, and showcased their application in both the synthesis and the late-stage modification of pharmaceuticals and agrochemicals.

5-(Trifluorovinyl)dibenzothiophenium Triflate: Introducing the 1,1,2-Trifluoroethylene Tether in Drug-Like Structures

This manuscript reports the synthesis and structure of an unprecedented sulfonium salt, 5-(trifluorovinyl)dibenzothiophenium triflate, and its use as a versatile reagent for the introduction of the bioisosteric 1,1,2-trifluoroethylene linker in drug-like structures. The protocol developed consists of the reaction of this compound with alcohols and phenols to deliver a complete set of 1,2,2-trifluoro-2-(alkoxy-/aryloxy)ethyl sulfonium salts, which have been purified by column chromatography and fully characterized. Subsequent single electron reduction under mild photochemical conditions efficiently affords the corresponding fluoroalkyl radicals that are trapped either intra- or intermolecularly through their reaction with (hetero)arenes. Theoretical calculations are used to evaluate the conformational consequences derived from the presence of the CF₂ -CHF tether.

Gem-Difluoroallylation of Aryl Sulfonium Salts

The unprecedented photochemical late-stage defluorinative gem-difluoroallylation of aryl sulfonium salts, which are formed site-selectively by direct C(sp²)─H functionalization, is herein disclosed. This method is distinguished by its mild reaction conditions, wide scope, and excellent site-selectivity. As showcase examples, a Flurbiprofen and Pyriproxyfen derivatives could be late stage C(sp²)─H gem-difluoroallylated with high yields. Experimental and computational investigations were conducted.

Organothianthrenium salts: synthesis and utilization

Organothianthrenium salts are a class of compounds containing a positively charged sulfur atom and a neutral sulfur atom. Over the past years, organothianthrenium salts have been emerging as attractive precursors for a myriad of transformations to forge new C-C and C-X bonds due to their unique structural characteristics and chemical behaviors. The use of the thianthrenation strategy selectively transforms C-H, C-O, and other chemical bonds into organothianthrenium salts in a predictable manner, providing a straightforward alternative for regioselective functionalizations for arenes, alkenes, alkanes, alcohols, amines and so on through diverse reaction mechanisms under mild conditions. In this review, the preparation of different organothianthrenium salts is summarized, including aryl, alkenyl and alkyl thianthrenium salts. Moreover, the utilization of organothianthrenium salts in different catalytic processes and their synthetic potentials are also discussed.

Meerwein-type Bromoarylation with Arylthianthrenium Salts

Herein, we report a photocatalyzed Meerwein-type bromoarylation of alkenes with stable arylthianthrenium salts, formed by site-selective C-H thianthrenation. This protocol can be applied to late-stage functionalization of a variety of biomolecules that are difficult to access by other aryl coupling reagents. Halogen introduction allows for a variety of follow-up transformations, affording numerous biologically active skeletons.

Base-Promoted Synthesis of Vinyl Sulfides from Sulfonium Triflates

A new protocol has been developed for vinyl sulfide synthesis promoted by an alkoxy base under metal-free conditions. In this reaction, aryl and alkenyl sulfonium triflates with diverse functionalities are converted into vinyl sulfides with excellent reactivity. This transformation features mild and safe reaction conditions that avoid catalyst, transition metal, high-pressure gas, and high reaction temperature without compromising efficiency.

Zincation of Styrylsulfonium Salts

We report the formation of zinc reagents by the reaction of styrylsulfonium salts with zinc powder. Transition metals and other additives are not required for promoting zincation. Zincation tolerates a variety of sensitive functional groups, including esters, bromides, and boronic esters, and proceeds with complete retention of stereochemistry. This method presents a practical approach to the formation of zinc reagents that can be used in a variety of functionalizations, such as halogenation, carboxylation, and Negishi cross-couplings.

Catalyst-Free C(sp2)-H Borylation through Aryl Radical Generation from Thiophenium Salts via Electron Donor-Acceptor Complex Formation

Aryl borates lie at the heart of carbon-carbon bond couplings, and they are widely applied to the synthesis of functional materials, pharmaceutical compounds, and natural products. Currently, synthetic methods for aryl borates are mostly limited to metal-catalyzed routes, and nonmetallic strategies remain comparatively underdeveloped. Herein, we report a mild, scalable, visible-light-induced cross-coupling between aryl dibenzothiophenium triflate salts and bis(catecholato)-diboron for the construction of C-B bonds in the absence of base, transition metal-ligand complex, or photoredox catalyst. Mechanistic studies reveal that this transformation is achieved through an electron donor-acceptor (EDA) complex activation in the absence of a catalyst. The mild reaction conditions allow the preparation of aromatic borates in good yields with excellent functional group tolerance. This photochemical protocol was also successfully applied to the late-stage modification of natural products and the synthesis of a drug intermediate, greatly demonstrating broadened utility.

Palladium Metallaphotoredox-Catalyzed 2-Arylation of Indole Derivatives

Given that biaryl motifs are found in many useful molecules, including pesticides, pharmaceuticals, functional materials, and polymers, the development of methods for their construction is important. Herein, we report a two-step method for C(sp²)-H/C(sp²)-H cross-coupling reactions to synthesize 2-arylindole derivatives by combining palladium catalysis and photocatalysis. This mild, dual-catalysis method showed good functional group tolerance and a wide substrate scope and could be used for late-stage functionalization of oligopeptides, drugs, and natural products.

Visible-light-catalyzed C-H arylation of (hetero)arenes via arylselenonium salts

A novel photo-induced C-H arylation of (hetero)arenes has been developed. Aryl selenonium salts as an aryl source led to the arylation of aromatic (hetero)cyclic compounds via C-Se bond activation under blue LED irradiation. The method simply utilizes the safe and clean energy source and yields a range of site-selective biphenyl or bi-heterocyclic products in medium to good yields. Furthermore, the borylation and Sonogashira coupling of aryl selenonium salts proceed in good yields as well. From the results, it is shown that selenonium salts are more reactive than sulfonium salts.

Sustainable Thioetherification via Electron Donor-Acceptor Photoactivation Using Thianthrenium Salts

The synthesis of sulfides has been widely studied because this functional subunit is prevalent in biomolecules and pharmaceuticals, as well as being a useful synthetic platform for further elaboration. Thus, various methods to build C-S bonds have been developed, but typically they require the use of precious metals or harsh conditions. Electron donor-acceptor (EDA) complex photoactivation strategies have emerged as versatile and sustainable ways to achieve C-S bond formation, avoiding challenges associated with previous methods. This work describes an open-to-air, photoinduced, site-selective C-H thioetherification from readily available reagents via EDA complex formation that tolerates a wide range of different functional groups. Moreover, C(sp2 )-halogen bonds remain intact using this protocol, allowing late-stage installation of the sulfide motif in various bioactive scaffolds, while allowing yet further modification through more traditional C-X bond cleavage protocols. Additionally, various mechanistic investigations support the envisioned EDA complex scenario.

Photocatalytic C(sp3) radical generation via C-H, C-C, and C-X bond cleavage

C(sp³) radicals (R˙) are of broad research interest and synthetic utility. This review collects some of the most recent advancements in photocatalytic R˙ generation and highlights representative examples in this field. Based on the key bond cleavages that generate R˙, these contributions are divided into C–H, C–C, and C–X bond cleavages. A general mechanistic scenario and key R˙-forming steps are presented and discussed in each section.

C(sp³) radicals (R˙) are of broad research interest and synthetic utility.

Sulfur(iv)-mediated umpolung α-heterofunctionalization of 2-oxazolines

The α-umpolung of carbonyl compounds significantly expands the boundaries of traditional carbonyl chemistry. Despite various umpolung methods available today, reversing the inherent reactivity of carbonyls still remains a substantial challenge. In this article, we report the first use of sulfonium salts, in lieu of well-established hypervalent iodines, for the carbonyl umpolung event. The protocol enables the incorporation of a wide variety of heteroatom nucleophiles into the α-carbon of 2-oxazolines. The success of this investigation hinges on the following factors: (1) the use of sulfoxides, which are abundant, structurally diverse and tunable, and easily accessible, ensures the identification of a superior oxidant namely phenoxathiin sulfoxide for the umpolung reaction; (2) the "assembly/deprotonation" protocol previously developed for rearrangement reactions in our laboratory was successfully applied here for the construction of α-umpoled 2-oxazolines.

Sulfonium cations as versatile strongly π-acidic ligands

More than a century old, sulfonium cations are still intriguing species in the landscape of organic chemistry. On one hand they have found broad applications in organic synthesis and materials science, but on the other hand, while isoelectronic to the ubiquitous tertiary phosphine ligands, their own coordination chemistry has been neglected for the last three decades. Here we report the synthesis and full characterization of the first Rh(i) and Pt(ii) complexes of sulfonium. Moreover, for the first time, coordination of an aromatic sulfonium has been established. A thorough computational analysis of the exceptionally short S-Rh bonds obtained attests to the strongly π-accepting nature of sulfonium cations and places them among the best π-acceptor ligands available today. Our calculations also show that embedding within a pincer framework enhances their π-acidity even further. Therefore, in addition to the stability and modularity that these frameworks offer, our pincer complexes might open the way for sulfonium cations to become powerful tools in π-acid catalysis.