Synthesis, Structure, and Reactivity of 5-(Aryl)dibenzothiophenium Triflates

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.

Palladium-Catalyzed Carbonylative Sonogashira Coupling of Aryl Thianthrenium Salts with Arylalkynes

Alkynones are valuable compounds with applications in various areas. In this work, we developed an efficient carbonylation procedure for the carbonylative cross-coupling of aryl thianthrenium salts with aromatic alkynes. Various useful alkynones were produced in moderate to excellent yields under mild conditions. Notably, among the various tolerated functional groups, the bromide group can be maintained, which is ready for further coupling reactions.

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.

Highlights from the 57th Bürgenstock Conference on Stereochemistry 2024

Herein, we share an overview of the scientific highlights from speakers at the latest edition of the longstanding Bürgenstock Conference.

Ruthenium(II)-Catalyzed Late-Stage Incorporation of N-Aryl Triazoles and Tetrazoles with Sulfonium Salts via C-H Activation

The late-stage functionalization of active pharmaceutical ingredients is a key challenge in medicinal chemistry. Furthermore, N-aryl triazoles and tetrazoles are important structural motifs with the potential to boost the activity of diverse drug molecules. Using easily accessible dibenzothiophenium salts for the ruthenium-catalyzed C-H arylation, these scaffolds were introduced into a variety of bioactive compounds. Our methodology uses cost-efficient ruthenium, KOAc as a mild base and gives access to a plethora of highly decorated triazole and tetrazole containing drug derivatives.

Palladium/norbornene-catalyzed diversified trifunctionalization of aryl-thianthreniums

A novel Catellani-type conversion is reported using aryl-thianthreniums (aryl-TTs) instead of aryl halides. Three classes of ortho-dual C-H functionalization involving alkylation, amination, and deuterated methylation and five types of ipso-operation including alkenylation, cyanation, methylation, hydrogenation, and alkynylation all proceed well in this procedure. In this conversion, aryl-TTs exhibit satisfactory reactivity and feature the advantage that the leaving TT unit can be recovered. More strikingly, this finding represents a new chemistry conversion of aryl-TTs, wherein contiguous tri-functionalization in a single chemical manipulation is realized.

Photocatalytic Functionalization of Dehydroalanine-Derived Peptides in Batch and Flow

Unnatural amino acids, and their synthesis by the late-stage functionalization (LSF) of peptides, play a crucial role in areas such as drug design and discovery. Historically, the LSF of biomolecules has predominantly utilized traditional synthetic methodologies that exploit nucleophilic residues, such as cysteine, lysine or tyrosine. Herein, we present a photocatalytic hydroarylation process targeting the electrophilic residue dehydroalanine (Dha). This residue possesses an α,β-unsaturated moiety and can be combined with various arylthianthrenium salts, both in batch and flow reactors. Notably, the flow setup proved instrumental for efficient scale-up, paving the way for the synthesis of unnatural amino acids and peptides in substantial quantities. Our photocatalytic approach, being inherently mild, permits the diversification of peptides even when they contain sensitive functional groups. The readily available arylthianthrenium salts facilitate the seamless integration of Dha-containing peptides with a wide range of arenes, drug blueprints, and natural products, culminating in the creation of unconventional phenylalanine derivatives. The synergistic effect of the high functional group tolerance and the modular characteristic of the aryl electrophile enables efficient peptide conjugation and ligation in both batch and flow conditions.

Synthesis of Sulfur-Containing Trisubstituted Imidazoles by One-Pot, Multicomponent Reaction via Electron Donor-Acceptor Complex Photoactivation

Rapid and efficient construction of multifunctionalized skeletons through a one-pot multicompound domino reaction has been recognized as a simple and practical strategy. Herein, a visible-light-enabled three-component reaction of isothiocyanates, isocyanides, and thianthrenium salt-functionalized arenes is presented, which affords a facile approach to sulfur-containing trisubstituted imidazoles in good yields with a broad substrate scope and excellent functional group tolerance. The byproduct thianthrene is recovered in quantity, thereby ultimately reducing the production of chemical waste. The developed methodology has potential value for the discovery and development of thioimidazole-based drugs.

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.

Palladium-Catalyzed Domino Conversion of Aryl-Thianthreniums with Anhydrides: Rapidly Building Highly Functionalized Fluorenones

As a class of rising electrophilic coupling reagents, aryl-thianthreniums (aryl-TTs) have been gaining immense attention. Herein, a novel palladium-catalyzed domino annulation of aryl-TTs with anhydrides is proposed to rapidly assemble a collection of highly functionalized fluorenones. This finding presents an innovative reaction pattern of aryl-TTs wherein the domino annulation version is first involved. Heavily compared with the existing conversions with aryl-TTs, this identified process successively functions as four aryl C-H bonds.

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.

Preparation of Alkyl Di(p-tolyl)sulfonium Salts and Their Application in Metal-Free C(sp3)-C(sp3) and C(sp3)-C(sp2) Bond Formations

Alkyldiarylsulfonium salts were synthesized by a combination of active sulfonium species, prepared through the activation of diarylsulfoxide, and alkyl nucleophiles. The isolated sulfonium salts were subjected to the allylation and cyclopropanation of the active methylene compounds and metal-free C(sp³)-C(sp²) couplings via oxyallyl cation intermediates under mild conditions. The series of reactions included an umpolung strategy for the coupling of alkyl nucleophiles and metal-free C-C bond formation using sulfonium salts.

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.

Palladium-Catalyzed Carbonylative Synthesis of Diaryl Ketones from Arenes and Arylboronic Acids through C(sp2)-H Thianthrenation

The development of mild methodology for converting inert C-H bonds to value-added molecules has been an attractive research topic during the last few decades as it offers efficient preparation. Meanwhile, diaryl ketones hold potent applications in antitumor drugs, the agrochemical industry, and synthetic chemistry. Herein, we report versatile palladium-catalyzed carbonylative cross-coupling reactions of aryl thianthrenium salts with arylboronic acids. Arenes were transformed site selectively via C(sp²)-H thianthrenation, and various desired diaryl ketones were produced in good to excellent yields.

Late-stage C-H Functionalization of Tryptophan-Containing Peptides with Thianthrenium Salts: Conjugation and Ligation

Bioorthogonal late-stage diversification of structurally complex peptides bears enormous potential for drug discovery and molecular imaging, among other applications. Herein, we report on a palladium-catalyzed C-H arylation of tryptophan-containing peptides with readily accessible and modular arylthianthrenium salts. Under exceedingly mild reaction conditions, the late-stage diversification of structurally complex peptides was accomplished. The tunability and ease of preparation of arylthianthrenium salts allowed the expedient stitching of tryptophan-containing peptides with drug, natural product, and peptidic scaffolds by forging sterically congested biaryl linkages. The robustness of the palladium catalysis regime was reflected by the full tolerance of a plethora of sensitive and coordinating functional groups. Hence, our manifold enabled efficient access to highly decorated, labelled, conjugated, and ligated linear and cyclic peptides.

Synthesis of Arynes via Formal Dehydrogenation of Arenes

Arynes offer immense potential for diversification of benzenoid rings, which occur in pharmaceuticals, agrochemicals, and liquid crystals. However, accessing these high-energy intermediates requires synthetic precursors, which involve either harsh conditions or multistep syntheses. The development of alternative methods to access arynes using simpler substrates and milder conditions is necessary for a more streamlined approach. Here, we describe a two-step formal dehydrogenation of simple arenes to generate arynes at a remote position relative to traditionally reactive groups, e.g., halides. This approach is enabled by regioselective installation and ejection of an "onium" leaving group, and we demonstrate the compatibility of simple arenes (20 examples) and arynophiles (8 examples). Moreover, through direct comparison, we show that our formal dehydrogenation method is both more functional group tolerant and efficient in generating arynes than the current state-of-the-art aryne precursors. Finally, we show that aryne intermediates offer opportunities for regioselective C-H amination that are distinct from other methods.

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.

Synthesis of Polycyclic Hetero-Fused 7-Deazapurine Heterocycles and Nucleosides through C-H Dibenzothiophenation and Negishi Coupling

A new approach for synthesizing polycyclic heterofused 7-deazapurine heterocycles and the corresponding nucleosides was developed based on C–H functionalization of diverse (hetero)aromatics with dibenzothiophene-S-oxide followed by the Negishi cross-cooupling with bis(4,6-dichloropyrimidin-5-yl)zinc. This cross-coupling afforded a series of (het)aryl-pyrimidines that were converted to fused deazapurine heterocycles through azidation and thermal cyclization. The fused heterocycles were glycosylated to the corresponding 2′-deoxy- and ribonucleosides, and a series of derivatives were prepared by nucleophilic substitutions at position 4. Four series of new polycyclic thieno-fused 7-deazapurine nucleosides were synthesized using this strategy. Most of the deoxyribonucleosides showed good cytotoxic activity, especially for the CCRF-CEM cell line. Phenyl- and thienyl-substituted thieno-fused 7-deazapurine nucleosides were fluorescent, and the former one was converted to 2′-deoxyribonucleoside triphosphate for enzymatic synthesis of labeled oligonucleotides.

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-Catalyzed Synthesis of Esters from Arenes through C-H Thianthrenation

The efficient palladium-catalyzed synthesis of esters from readily available arenes has been developed. These C-H bond esterifications were achieved relying on the regioselective thianthrenation to generate the aryl-TT salts, which were treated as reactive electrophilic substrates to couple with phenol formate and N-hydroxysuccinimide (NHS) formate giving access to phenol esters and NHS esters, respectively, in the absence of carbon monoxide. A wide range of functional esters could be prepared with high efficiency under this redox-neutral palladium-catalytic condition. Late-stage functionalization and investigations of synthetic applications demonstrated the potential application of the established platform and these products.

Palladium-Catalyzed Synthesis of C-1 Deuterated Aldehydes from (Hetero) Arenes Mediated by C (sp2)-H Thianthrenation

A palladium-catalyzed deuterated formylation of aryl sulfonium salts is prepared conveniently from readily available arenes, which enables the expedient synthesis of a series of structurally diverse C-1 deuterated aldehydes with 96%-99% deuterium incorporation. The easy to handle and cost-effective DCOONa provides a deuterium source, which can be introduced onto the formyl units with excellent selectivity under the palladium-catalytic redox neutral conditions. This catalytic route can accomplish the direct late-stage C-H functionalization of bioactive molecules and natural product derivatives assisted by C (sp²)-H thianthrenation. Moreover, on the basis of this practical approach, several deuterated drugs and analogues could be prepared with excellent levels of deuterium incorporation.

Sulfonium and Selenonium Salts as Noncovalent Organocatalysts for the Multicomponent Groebke-Blackburn-Bienaymé Reaction

Sulfonium and selenonium salts, represented by S-aryl dibenzothiophenium and Se-aryl dibenzoselenophenium triflates, were found to exhibit remarkable catalytic activity in the model Groebke-Blackburn-Bienaymé reaction. Kinetic analysis and density functional theory (DFT) calculations indicated that their catalytic effect is induced by the ligation of the reaction substrates to the σ-holes on the S or Se atom of the cations. The experimental data indicated that although 10-fold excess of the chloride totally inhibits the catalytic activity of the sulfonium salts, the selenonium salt remains catalytically active, which can be explained by the experimentally found lower binding constant of the selenonium derivative to chloride in comparison with the sulfonium analogue. Both types of salts exhibit lower catalytic activity in the model reaction than dibenziodolium species.

Transition-Metal-Free Cascade Approach for the Synthesis of Functionalized Biaryls by SNAr of Arylhydroxylamines with Arylsulfonium Salts

We report a transition-metal-free protocol for the synthesis of functionalized biaryls through nucleophilic aromatic substitution (S_NAr) of arylhydroxylamines to arylsulfonium salts. With this protocol, structurally diverse functionalized biaryls were obtained smoothly in moderate to good yields. Merits of this transformation include mild reaction conditions, broad substrate scope, great functional group tolerance, feasibility of a one-pot procedure, and ease of handing and scale-up.

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.

Transition Metal-Catalyzed Biaryl Atropisomer Synthesis via a Torsional Strain Promoted Ring-Opening Reaction

ConspectusArising from the restricted rotation of a single bond caused by steric or electronic effects, atropisomerism is one of the few fundamental categories for molecules to manifest their three-dimensional characters into which axially chiral biaryl compounds fall. Despite the widespread occurrence of axially chiral skeletons in natural products, bioactive molecules, and chiral ligands/organocatalysts, catalytic asymmetric methods for the synthesis of these structures still lag behind demand. Major challenges for the preparation of these chiral biaryls include accessing highly sterically hindered variants while controlling the stereoselectivity. A couple of useful strategies have emerged for the direct asymmetric synthesis of these molecules in the last two decades.Recently, we have engaged in catalytic asymmetric synthesis of biaryl atropisomers via transition metal catalysis, including asymmetric ring-openings of dibenzo cyclic compounds. During these studies, we serendipitously discovered that the two substituents adjacent to the axis cause these dibenzo cyclic molecules to be distorted to minimize steric repulsion. The distorted compounds display higher reactivity in the ring-opening reactions than the non-distorted molecules. In other words, torsional strain can promote a ring-opening reaction. On the basis of this concept, we have successfully realized the catalytic asymmetric ring-opening reaction of cyclic diaryliodoniums, dibenzo silanes, and 9H-fluoren-9-ols, which delivered several differently substituted ortho tetra-substituted biaryl atropisomers in high enantioselectivity. The torsional strain not only activates the substrates toward ring-opening under mild conditions but also changes the chemoselectivity of bond-breaking events. In the palladium-catalyzed carboxylation of S-aryl dibenzothiophenium, the torsional strain inversed the bond selectivity from exocyclic C-S bond cleavage to the ring-opening reaction.In this Account, we summarize our studies on copper-, rhodium-, or palladium-catalyzed asymmetric ring-opening reactions of dibenzo cyclic compounds as a useful collection of methods for the straightforward preparation of ortho tetra-substituted biaryl atropisomers with high enantiopurity on the basis of the above-mentioned torsional strain-promoted ring-opening coupling strategy. In the last part, the torsional strain energies are also discussed with the aid of density functional theory (DFT) calculations.

Transition-Metal-Free O-Arylation of Alcohols and Phenols with S-Arylphenothiaziniums

Herein, we report the transition-metal-free O-arylation of alcohols and phenols with S-arylphenothiaziniums, which can be easily synthesized from boronic acids. Aryl substituents derived from arylboronic acids were selectively introduced into the hydroxy groups in alcohols and phenols, and a variety of aryl ethers were synthesized. This selectivity is supported by theoretical calculations.

Torsional strain inversed chemoselectivity in a Pd-catalyzed atroposelective carbonylation reaction of dibenzothiophenium

A palladium-catalyzed enantioselective ring-opening/carbonylation of cyclic diarylsulfonium salts is reported. In comparison to thioethers, the sulfonium salts displayed high reactivity and enabled the reaction to be performed under mild conditions (room temperature). The steric repulsion of the two non-hydrogen substituents adjacent to the axis led cyclic diarylsulfonium salts to be distorted, which enabled the ring-opening reaction to proceed with significant preference for breaking the exocyclic C-S bond.

Visible-Light-Initiated Catalyst-Free Trifluoromethylselenolation of Arylsulfonium Salts with [Me4N][SeCF3]

The redox potential gap between arylsulfonium salt and [Me4N][SeCF3] has been clearly disclosed by CV measurements. Construction of carbon-selenium bond by overcoming this gap without using catalysts and additives is...

An Interrupted Pummerer Reaction Mediated by a Hypervalent Iodine(III) Reagent: In Situ Formation of RSCl and Its Application for the Synthesis of 3-Sulfenylated Indoles

An interrupted Pummerer reaction of PhICl₂ and sulfoxides was found to in situ generate reactive organosulfenyl chloride, which enabled the intramolecular electrophilic cyclization of 2-alkynylanilines, generating 3-sulfenylated indole with a good to excellent yield under metal-free conditions. One striking feature of the approach is that sulfoxide regeneration can be realized via the oxidation of the formed sulfides by the generated hypervalent iodine species.

Construction of α-Amino Azines via Thianthrenation-Enabled Photocatalyzed Hydroarylation of Azine-Substituted Enamides with Arenes

α-Amino azines are widely found in pharmaceuticals and ligands. Herein, we report a practical method for accessing this class of compounds via photocatalyzed hydroarylation of azine-substituted enamides with the in situ-generated aryl thianthrenium salts as the radical precursor. This reaction features a broad substrate scope, good functional group tolerance, and mild conditions and is suitable for the late-stage installation of α-amino azines in complex structures.

Transition Metal-Free C-H Thiolation via Sulfonium Salts Using β-Sulfinylesters as the Sulfur Source

We disclose a direct C(sp)-, C(sp²)-, and C(sp³)-H thiolation reaction using β-sulfinylesters as the versatile sulfur source. The key step of this protocol is chemoselective C-S bond cleavage of the sulfonium salts that are formed in situ from the corresponding alkenes, alkynes, and 1,3-dicarboxyl compounds with β-sulfinylesters. The successful capture of the acrylate byproduct supports a retro-Michael reaction mechanism.

High Site Selectivity in Electrophilic Aromatic Substitutions: Mechanism of C-H Thianthrenation

The introduction of thianthrene as a linchpin has proven to be a versatile strategy for the C–H functionalization of aromatic compounds, featuring a broad scope and fast diversification. The synthesis of aryl thianthrenium salts has displayed an unusually high para regioselectivity, notably superior to those observed in halogenation or borylation reactions for various substrates. We report an experimental and computational study on the mechanism of aromatic C–H thianthrenation reactions, with an emphasis on the elucidation of the reactive species and the nature of the exquisite site selectivity. Mechanisms involving a direct attack of arene to the isolated O-trifluoracetylthianthrene S-oxide (TT⁺-TFA) or to the thianthrene dication (TT²⁺) via electron transfer under acidic conditions are identified. A reversible interconversion of the different Wheland-type intermediates before a subsequent, irreversible deprotonation is proposed to be responsible for the exceptional para selectivity of the reaction.

Late-stage C–H functionalization offers new opportunities in drug discovery

Over the past decade, the landscape of molecular synthesis has gained major impetus by the introduction of late-stage functionalization (LSF) methodologies. C-H functionalization approaches, particularly, set the stage for new retrosynthetic disconnections, while leading to improvements in resource economy. A variety of innovative techniques have been successfully applied to the C-H diversification of pharmaceuticals, and these key developments have enabled medicinal chemists to integrate LSF strategies in their drug discovery programmes. This Review highlights the significant advances achieved in the late-stage C-H functionalization of drugs and drug-like compounds, and showcases how the implementation of these modern strategies allows increased efficiency in the drug discovery process. Representative examples are examined and classified by mechanistic patterns involving directed or innate C-H functionalization, as well as emerging reaction manifolds, such as electrosynthesis and biocatalysis, among others. Structurally complex bioactive entities beyond small molecules are also covered, including diversification in the new modalities sphere. The challenges and limitations of current LSF methods are critically assessed, and avenues for future improvements of this rapidly expanding field are discussed. We, hereby, aim to provide a toolbox for chemists in academia as well as industrial practitioners, and introduce guiding principles for the application of LSF strategies to access new molecules of interest.

Pd/Cu-Catalyzed C-H/C-H Cross Coupling of (Hetero)Arenes with Azoles through Arylsulfonium Intermediates

A highly efficient method for the selective formal C-H/C-H cross-coupling of azoles and (hetero)arenes was established through arylsulfonium intermediates under transition-metal catalysis, which produced a variety of 2-(hetero)aryl azoles in good to excellent yields. Advantages of the reaction included mildness, a good functional group tolerance, a wide range of substrates, a high regio- and chemoselectivity, one-pot procedures, and the late-stage functionalization of complex molecules without the use of oxidants, offering a promising strategy for the transition-metal-catalyzed C-H arylation of azoles.

5-(Diarylimino)- and 5-(sulfoximido)dibenzothiophenium triflates: syntheses and applications as electrophilic aminating reagents

The one-pot synthesis of well-defined 5-(diarylimino) and 5-(sulfoximido)dibenzothiophenium triflates, respectively from diarylimines or sulfoximines, is reported and the structures of a series of these compounds are elucidated by X-ray crystallography. In analogy to their hypervalent I(iii) analogues, the iminoyl and sulfoximidoyl groups of these compounds can be selectively transferred to organic substrates. Specifically, the uncatalyzed imination of thiols or sulfinates proceeds with good yields, while under the mild reaction conditions offered by visible light photoredox catalysis, the radical amination of hydrazones or the sulfoximidation of benzylic, allylic and propargylic C-H bonds takes place satisfactorily.

α-Diazo Sulfonium Triflates: Synthesis, Structure, and Application to the Synthesis of 1-(Dialkylamino)-1,2,3-triazoles

The one-pot synthesis of a series of sulfonium salts containing transferable diazomethyl groups is described, and the structure of these compounds is elucidated by X-ray crystallography. Under photochemical conditions, reaction of these salts with N,N-dialkyl hydrazones affords 1-(dialkylamino)-1,2,3-triazoles via diazomethyl radical addition to the azomethine carbon followed by intramolecular ring closure. The straightforward transformation of the structures thus obtained into mesoionic carbene-metal complexes is also reported and the donor properties of these new ligands characterized.

Synthesis of 6H-Benzo[c]chromene Scaffolds from O-Benzylated Phenols through a C-H Sulfenylation/Radical Cyclization Sequence

S-Aryl dibenzothiophenium salts, obtained through a highly regioselective C–H sulfenylation of o-benzyl-protected phenols, are used as precursors of 6H-benzo[c]chromenes. The reaction starts with a photocatalytically triggered single-electron transfer to the sulfonium salt, which promotes the formation of an aryl radical via selective mesolitic cleavage of the S–Ar_exo bond. Mechanistic studies reveal that this initial radical species cyclizes following a kinetically favored 5-exo-trig pathway. Subsequent ring expansion, favored by rearomatization, delivers the desired tricyclic systems.

Diverse C-P Cross-Couplings of Arylsulfonium Salts with Diarylphosphines via Selective C-S Bond Cleavage

Diverse C-P cross-couplings of arylthianthrenium salts with diarylphosphines producing various triarylphosphines via highly selective C-S bond cleavage are reported. In the absence of catalyst, the reaction of arylthianthrenium salts with diarylphosphines undergoes phosphinative ring opening exclusively via the cleavage of an endocyclic C-S bond of a thianthrene skeleton. The use of a palladacycle catalyst under otherwise the same conditions enables the phosphination via the cleavage of an exocyclic C-S bond with significantly higher speed.

Cu-Catalyzed highly selective silylation and borylation of alkenylsulfonium salts

A Cu-catalyzed highly selective silylation and borylation of alkenylsulfonium salts under mild conditions is reported providing various alkenylsilanes and alkenylboranes.

Synthesis of Pyridylsulfonium Salts and Their Application in the Formation of Functionalized Bipyridines

An S-selective arylation of pyridylsulfides with good functional group tolerance was developed. To demonstrate synthetic utility, the resulting pyridylsulfonium salts were used in a scalable transition-metal-free coupling protocol, yielding functionalized bipyridines with extensive functional group tolerance. This modular methodology permits selective introduction of functional groups from commercially available pyridyl halides, furnishing symmetrical and unsymmetrical 2,2'- and 2,3'-bipyridines. Iterative application of the methodology enabled the synthesis of a functionalized terpyridine with three different pyridine components.