C-H Coupling Reactions Directed by Sulfoxides: Teaching an Old Functional Group New Tricks

Brønsted Acid Catalyzed Asymmetric Synthesis of Cyclopentenones with C4-Quaternary Centers Starting from Vinyl Sulfoxides and Allenyl Ketones or Allenoates

Starting with chiral vinyl sulfoxides and allenyl ketones or allenoates, a triflic acid-catalyzed asymmetric [3,3]-sigmatropic rearrangement of sulfoniums is reported to have a direct access to highly functionalized C4-chiral cyclopentenones (19 examples, up to 85% yield and >95% enantiomeric excesses). In addition to the use of these chiral compounds as key building blocks in organic synthesis, the antiproliferative activities of sulfoxide substrates and the corresponding cyclopentenones were evaluated, and promising cytotoxicity against the HL-60 human tumor cell line was found.

Sulfoxide-containing polymers conjugated prodrug micelles with enhanced anticancer activity and reduced intestinal toxicity

Poly(ethylene glycol) (PEG) is widely utilized as a hydrophilic coating to extend the circulation time and improve the tumor accumulation of polymeric micelles. Nonetheless, PEGylated micelles often activate complement proteins, leading to accelerated blood clearance and negatively impacting drug efficacy and safety. Here, we have crafted amphiphilic block copolymers that merge hydrophilic sulfoxide-containing polymers (psulfoxides) with the hydrophobic drug 7-ethyl-10-hydroxylcamptothecin (SN38) into drug-conjugate micelles. Our findings show that the specific variant, PMSEA-PSN38 micelles, remarkably reduce protein fouling, prolong blood circulation, and improve intratumoral accumulation, culminating in significantly increased anti-cancer efficacy compared with PEG-PSN38 counterpart. Additionally, PMSEA-PSN38 micelles effectively inhibit complement activation, mitigate leukocyte uptake, and attenuate hyperactivation of inflammatory cells, diminishing their ability to stimulate tumor metastasis and cause inflammation. As a result, PMSEA-PSN38 micelles show exceptional promise in the realm of anti-metastasis and significantly abate SN38-induced intestinal toxicity. This study underscores the promising role of psulfoxides as viable PEG substitutes in the design of polymeric micelles for efficacious anti-cancer drug delivery.

Aryl Sulfoxides: A Traceless Directing Group for Catalytic C-H Activation of Arenes

The transition metal-catalyzed C-H activation of arenes directed by sulfoxides represents a compelling strategy in organic synthesis, owing to its exceptional regioselectivity and high efficiency. This innovative approach stands out for its traceless character, enabling the direct functionalization of arenes, before the easy removal or conversion of the key sulfinyl moiety. Beyond their utility as a directing group, sulfoxides have proved particularly valuable to mediate as chiral auxiliaries, presenting exciting prospects for the synthesis of stereo-enriched compounds upon C-H functionalization. The versatility demonstrated by the method paves the way to different structures with potential applications ranging from medicinal chemistry to organic electronics.

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.

Arylation of benzazoles at the 4 positions by activation of their 2-methylsulfinyl groups

Treatment of 2-methylsulfinylbenzazoles with triflic anhydride in the presence of phenols yields the corresponding 4-(p-hydroxyphenyl)-2-methylsulfanylbenzazoles. This regioselective dehydrative C-H/C-H coupling arylation represents a rare example of functionalizations on the benzene rings of benzo-fused azoles.

ortho-Cyanomethylation of aryl fluoroalkyl sulfoxides via a sulfonium-Claisen rearrangement

We hereby report the ortho-cyanomethylation of aryl fluoroalkyl sulfoxides with acetonitrile through a sulfonium-Claisen-type rearrangement. This reaction enables the incorporation of two valuable functional groups, such as the cyanomethyl group and the fluoroalkylthio group, into arenes. Remarkably, fluoroalkylthio groups, such as SCFH2 and SCF2H, bearing active hydrogen, are well tolerated by the reaction. The success of the reaction relies on the use of an excess amount of acetonitrile and the electronegative effect of fluoroalkyl substituents, both of which promote the electrophilic assembly of sulfoxides with acetonitrile. Consequently, the sulfonium-Claisen rearrangement reaction tolerates a wide variety of fluoroalkyl sulfoxides bearing functional groups including halides, nitriles, ketones, sulfones, and amides, which are appealing for subsequent elaboration and exploration.

Electro-oxidative coupling of Bunte salts with aryldiazonium tetrafluoroborates: a benign access to unsymmetrical sulfoxides

An electrochemical strategy for the synthesis of unsymmetrical sulfoxides has been explored using Bunte salts and aryldiazonium tetrafluoroborates under constant current electrolysis at room temperature. In addition to being eco-safe and using mild conditions, the present protocol is free from the use of metal/oxidant, and is endowed with a broad substrate scope and good functional group tolerance.

Defluorinative Multi-Functionalization of Fluoroaryl Sulfoxides Enabled by Fluorine-Assisted Temporary Dearomatization

Owing to its unique physical properties, fluorine is often used to open up new reaction channels. In this report, we establish a cooperation of [5,5]-rearrangement and fluorine-assisted temporary dearomatization for arene multi-functionalization. Specifically, the [5,5]-rearrangement of fluoroaryl sulfoxides with β,γ-unsaturated nitriles generates an intriguing dearomatized sulfonium species which is short-lived but exhibits unusually high electrophilicity and thus can be instantly trapped by nucleophiles and dienes at a remarkably low temperature (-95 °C) to produce four types of valuable multi-functionalized benzenes, respectively, involving appealing processes of defluorination, desulfurization, and sulfur shift. Mechanistic studies indicate that the use of fluorine on arenes not only circumvents the generally inevitable [3,3]-rearrangement but also impedes the undesired rearomatization process, thus provides a precious space for constructing and elaborating the temporarily dearomatized fluorinated sulfonium species.

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.

Metal-Free Arylation of Benzothiophenes at C4 by Activation as their Benzothiophene S-Oxides

Benzothiophenes, activated by oxidation to the corresponding S-oxides, undergo C-H/C-H-type coupling with phenols to give C4 arylation products. While an electron-withdrawing group at C3 of the benzothiophene is important, the process operates without a directing group and a metal catalyst, thus rendering it compatible with sensitive functionalities-e.g. halides and formyl groups. Quantum chemical calculations suggest a formal stepwise mechanism involving heterolytic cleavage of an aryloxysulfur species to give a π-complex of the corresponding benzothiophene and a phenoxonium cation. Subsequent addition of the phenoxonium cation to the C4 position of the benzothiophene is favored over the addition to C3; Fukui functions predict that the major regioisomer is formed at the more electron-rich position between C3 and C4. Varied selective manipulation of the benzothiophene products showcase the synthetic utility of the metal-free arylation process.

Catalytic Decarboxylative Radical Sulfinylation

Sulfoxides are ubiquitous in both naturally and synthetically bioactive molecules. We report herein a redox-neutral and mild approach for radical sulfinylation of redox-active esters via dual photoredox and copper catalysis, furnishing a series of functionalized sulfoxides. The reaction could accommodate a range of tertiary, secondary, and primary carboxylic acids, as well as exhibit wide functional group compatibility. The chemistry features a high degree of practicality, is scalable, and allows late-stage modification of bioactive pharmaceuticals.

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.

Transition metal-catalyzed C-H/C-C activation and coupling with 1,3-diyne

This review provides a broad overview of the recent developments in the field of transition metal-catalyzed C-H/C-C bond activation and coupling with 1,3-diyne for assembling alkynylated heterocycles, bis-heterocycles, and 1,3-enynes. Transition metal-catalyzed inert bond (C-H/C-C) activation has been the focus of attention among synthetic chemists in recent times. Enormous developments have taken place in C-H/C-C bond activation chemistry in the last two decades. In recent years the use of 2π-unsaturated units as coupling partners for the synthesis of heterocycles through C-H/C-C bond activation and annulation sequence has received immense attention. Among the unsaturated units employed for assembling heterocycles, the use of 1,3-diynes has garnered significant attention due to its ability to render bis-heterocycles in a straightforward manner. The C-H bond activation and coupling with 1,3-diyne has been very much explored in recent years. However, the development of strategies for the use of 1,3-diynes in the analogous C-C bond activation chemistry is less explored. Earlier methods employed to assemble bis-heterocycle used heterocycles that were preformed and pre-functionalized via transition metal-catalyzed coupling reactions. The expensive pre-functionalized halo-heterocycles and sensitive and expensive heterocyclic metal reagents limit its broad application. However, the transition metal-catalyzed C-H activation obviates the need for expensive heterocyclic metal reagents and pre-functionalized halo-heterocycles. The C-H bond activation strategy makes use of C-H bonds as functional groups for effecting the transformation. This renders the overall synthetic sequence both step and cost economic. Hence, this strategy of C-H activation and subsequent reaction with 1,3-diyne could be used for the larger-scale synthesis of chemicals in the pharmaceutical industry. Despite these advances, there is still the possibility of exploration of earth-abundant and cost-effective first-row transition metals (Ni, Cu, Mn. Fe, etc.) for the synthesis of bis-heterocycles. Moreover, the Cp*-ligand-free, simple metal-salt-mediated synthesis of bis-heterocycles is also less explored. Thus, more exploration of reaction conditions for the Cp*-free synthesis of bis-heterocycles is called for. We hope this review will inspire scientists to investigate these unexplored domains.

Visible-Light-Induced Phenoxyl Radical-based Metal-Organic Framework for Selective Photooxidation of Sulfides

Phenoxyl radicals originating from phenols through oxidation or photoinduction are relatively stable and exhibit mild oxidative activity, which endows them with the potential for photocatalysis. Herein, a stable and recyclable metal-organic framework Zr-MOF-OH constructed of a binaphthol derivative ligand has been synthesized and functions as an efficient heterogeneous photocatalyst. Zr-MOF-OH shows fairly good catalytic activity and substrate compatibility toward the selective oxidation of sulfides to sulfoxides under visible light irradiation. Such irradiation of Zr-MOF-OH converts the phenolic hydroxyl groups of the binaphthol derivative ligand to phenoxyl radicals through excited state intramolecular proton transfer, and the excited state photocatalyst triggers the single-electron oxidation of the sulfide. No reactive oxygen species are produced in the photocatalytic process, and triplet O₂ directly participates in the reaction, endowing Zr-MOF-OH with wide substrate compatibility and high selectivity, which also proposes a promising pathway for the direct activation of substrates via phenoxyl radicals.

Oxidative Dehydrogenative Coupling of Thiols with Alkanes for the Synthesis of Sulfoxides

An oxidative dehydrogenative coupling of thiols with alkanes via direct C(sp³)-H bond functionalization to form a new C-S bond and S═O double bond was developed. The present reaction features the use of readily available reagents and high step- and atom-efficiency, thus providing an efficient access to sulfoxides. A possible mechanism is proposed.

Photocatalytic Late-Stage C-H Functionalization

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

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.

Zr-Based Metal-Organic Frameworks with Phosphoric Acids for the Photo-Oxidation of Sulfides

Heterogeneous Brønsted acidic catalysts such as phosphoric acids are the conventional activators for organic transformations. However, the photocatalytic performance of these catalysts is still rarely explored. Herein, a novel Zr-based metal-organic framework Zr-MOF-P with phosphoric acids as a heterogeneous photocatalyst has been fabricated, which shows high selectivity and reactivity towards the photo-oxidation of sulfides under white light illumination. A mechanism study indicates that the selective oxygenation of sulfides occurs with triplet oxygen rather than common reactive oxygen species (ROS). When Zr-MOF-P is irradiated, the hydroxyl group of phosphoric acid is converted into oxygen radical, which takes an electron from the sulfides, and then the activated substrates react with the triplet oxygen to form sulfoxides, avoiding the destruction of the catalysts and endowing the reaction with high substrate compatibility and fine recyclability.

Synthesis of 3-SCF2H-/3-SCF3-chromones via Interrupted Pummerer Reaction/Intramolecular Cyclization Mediated by Difluoromethyl or Trifluoromethyl Sulfoxide and Tf2O

The reaction of alkynyl aryl ketones bearing an o-methoxy group with difluoromethyl sulfoxide in the presence of Tf₂O was found to conveniently afford the corresponding 3-SCF₂H-substituted chromones. The combining use of difluoromethyl sulfoxide/Tf₂O could represent the first reagents system that can introduce the biologically important SCF₂H moiety under base-free conditions via an interrupted Pummerer reaction. The same protocol could also be applied to the synthesis of 3-SCF₃-substituted chromones by replacing difluoromethyl sulfoxide with trifluoromethyl sulfoxide and CH₃CN with toluene.

Regio- and Stereoselective Synthesis of β-Methylthio Vinyl Triflates

Vinyl triflates are commonly employed as electrophilic vinyl sources in complex synthesis. The triflation of enolates is commonly required for the preparation of vinyl triflates, generally under strongly basic conditions. Herein, the reaction between alkynes and dimethyl(methylthio)sulfonium trifluoromethanesulfonate is presented, which leads to the development of a facile synthesis of β-methylthio vinyl triflates in a chemo-, regio-, and stereoselective manner under neutral and extremely simple conditions.

Dearomative di- and trifunctionalization of aryl sulfoxides via [5,5]-rearrangement

Aromatic [5,5]-rearrangement can in principle be an ideal protocol to access dearomative compounds. However, the lack of competent [5,5]-rearrangement impedes the advance of the protocol. In this Article, we showcase the power of [5,5]-rearrangement recently developed in our laboratory for constructing an intriguing dearomative sulfonium specie which features versatile and unique reactivities to perform nucleophilic 1,2- and 1,4-addition and cyclization, thus achieving dearomative di- and trifunctionalization of easily accessible aryl sulfoxides. Impressively, the dearomatization products can be readily converted to sulfur-removed cyclohexenones, naphthalenones, bicyclic cyclohexadienones, and multi-substituted benzenes. Mechanistic studies shed light on the key intermediates and the remarkable chemo-, regio- and stereoselectivities of the reactions.

Alkyl/Glycosyl Sulfoxides as Radical Precursors and Their Use in the Synthesis of Pyridine Derivatives

We report here the use of simple and readily available alkyl sulfoxides as precursors to radicals and their application in the preparation of pyridine derivatives. We show that alkyl sulfoxides, N-methoxy pyridinium salts and fluoride anions form electron donor-acceptor (EDA) complexes in solution, which, upon visible light irradiation, undergo a radical chain process to afford various pyridine derivatives smoothly. This reaction displays broad scope with respect to both sulfoxides and N-methoxy pyridiniums. The synthetic versatility of sulfoxides as a handle in chemistry adds to their power as radical precursors. Glycosyl sulfoxides are converted to the corresponding pyridyl C-glycosides with high stereoselectivities. Computational and experimental studies provide insights into the reaction mechanism.

Revisiting Aromatic Claisen Rearrangement Using Unstable Aryl Sulfonium/Iodonium Species: The Strategy of Breaking Up the Whole into Parts

ConspectusSince Ludwig Claisen's discovery of the sigmatropic rearrangement of allyl aryl ethers in 1912, aromatic Claisen rearrangement has continuously attracted the attention of both experimental and theoretical chemists. Over more than a century of growth, this protocol has proven to be a practical and powerful synthetic tool in many aspects. However, the reaction scope has long been limited to aryl ethers and their S or N analogs until the serendipitous discovery of aromatic iodonium-Claisen rearrangement by Oh et al. in 1988 and the development of aromatic sulfonium-Claisen rearrangement by Kita et al. in 2004. Unlike traditional Claisen rearrangements, these hypervalent-bonding-based Claisen-type rearrangements can be performed by simply mixing electrophilically activated aryl sulfoxides/iodanes with certain nucleophiles to directly deliver rearrangement products. In addition to the simple operation, remarkable features, such as readily available substrates, valuable products and intriguing rearrangement patterns, have led to a dramatic resurgence of this rearrangement chemistry.In this Account, we summarize our recent works on developing new aromatic rearrangement modes using sulfonium/iodonium reagents. Interestingly, the program started with an accidental discovery that aryl sulfoxides could be coupled with alkyl nitriles in the presence of Tf₂O and base. Mechanistic studies reveal that the reaction proceeds in three major steps, including the Tf₂O-triggered assembly of both coupling partners, base-promoted deprotonation of in situ-generated aryl sulfonium-imine species leading to a key rearrangement precursor called aryl sulfonium-ketenimine species, and subsequent facile and rapid [3,3]-rearrangement. On the basis of the mechanistic underpinning, we divided the one-step operation into two steps called the "assembly/deprotonation" protocol for constructing unstable rearrangement precursors. Most notably, the switch from the commonly used one-step to mechanism-based multiple-step manipulation, which can be termed "breaking up the whole into parts", not only enables the independent control of each step of the reaction, thus significantly expanding the accessible synthetic scope, but also raises opportunities for developing new rearrangement patterns. For example, the "assembly/deprotonation" protocol has also been applied to the development of [5,5]-rearrangement of aryl sulfoxides and the asymmetric rearrangement of aryl iodanes, thus enabling the unprecedented regio- and stereocontrol of the rearrangement process. Furthermore, the "breaking up the whole into parts" thinking triggered us to merge the Morita-Baylis-Hillman (MBH) reaction into the rearrangement process to accomplish Z-selective MBH-type [3,3]-rearrangement of α,β-unsaturated nitriles and E-selective MBH-type [3,3]-rearrangement of α,β-unsaturated 2-oxazolines, which expands the scope of rearrangement partners to include α,β-unsaturated carbonyls. In addition, the impressive rapidity of the rearrangement process found in our initial discovery has also been recognized as a congestion-acceleration effect, which was further utilized to forge the rapid ortho-cyanoalkylative rearrangement of aryl iodanes, and thus leading to the first dearomatization of aryl iodanes. We anticipate that our protocols and ideas behind the methods will be complementary to the traditional thinking of the aromatic Claisen rearrangement.

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.

Nucleophilic functionalizations of indole derivatives using the aromatic Pummerer reaction

Because of the electron-rich property of indoles, direct functionalization strategies towards indoles generally involve electrophilic substitutions. In this paper, an efficient protocol for nucleophilic hydroxylation, halogenation and esterification of indoles via the aromatic Pummerer process was developed. With the advantages of readily accessible starting materials, simple operation and mild conditions, this protocol should be of interest to synthetic scientists.

Alkynyl Sulfonium Salts Can Be Employed as Chalcogen-Bonding Catalysts and Generate Alkynyl Radicals under Blue-Light Irradiation

Chalcogen bonding (ChB) has emerged as a promising tool in organic synthesis. However, compared with the well-developed selenium- and tellurium-based salt catalysts, the ChB catalysis of sulfonium salts is still unknown. Here, we report a new type of alkynyl-sulfonium salt ChB catalysis for various ionic transformations, including transfer hydrogenation, bromination, bromolactonization, dimerization of 1,1-diphenylethylene, nitro-Michael addition reaction and Ritter reaction. More importantly, the photocapability of ChB was first demonstrated to generate alkynyl radicals for the synthesis of a variety of chalcogenoacetylenes. Mechanistic studies shed light on the mechanism of the photoinduced reactions and confirmed the involvement of alkynyl radicals which are difficult to generate otherwise.

Phosphorus-Alloying as a Powerful Method for Designing Highly Active and Durable Metal Nanoparticle Catalysts for the Deoxygenation of Sulfoxides: Ligand and Ensemble Effects of Phosphorus

The modification of metal nanoparticles (NPs) by incorporating additional metals is a key technique for developing novel catalysts. However, the effects of incorporating nonmetals into metal NPs have not been widely explored, particularly in the field of organic synthesis. In this study, we demonstrate that phosphorus (P)-alloying significantly increases the activity of precious metal NPs for the deoxygenation of sulfoxides into sulfides. In particular, ruthenium phosphide NPs exhibit an excellent catalytic activity and high durability against sulfur-poisoning, outperforming conventional catalysts. Various sulfoxides, including drug intermediates, were deoxygenated to sulfides with excellent yields. Detailed investigations into the structure-activity relationship revealed that P-alloying plays a dual role: it establishes a ligand effect on the electron transfer from Ru to P, facilitating the production of active hydrogen species, and has an ensemble effect on the formation of the Ru-P bond, preventing strong coordination with sulfide products. These effects combine to increase the catalytic performance of ruthenium phosphide NPs. These results demonstrate that P-alloying is an efficient method to improve the metal NP catalysis for diverse organic synthesis.

Direct Bis-Alkyl Thiolation for Indoles with Sulfinothioates under Pummerer-Type Conditions

A base-free bis-alkyl thiolation reaction of indoles with sulfinothioates under Pummerer-type conditions is described. Sulfinothioates, activated with 2,2,2-trifluoroacetic anhydride, are demonstrated to be an efficient thiolation reagent for wide applications. This approach enabled double C-H thiolation at the C2 and C3 of the indole in one pot. The mechanism studies suggested the thiolation was realized through the sulfoxonium salt rather than sulfenyl carboxylate.

Sulfonium-aided coupling of aromatic rings via sigmatropic rearrangement

Biaryl synthesis continues to occupy a central role in chemical synthesis. From blockbuster drug molecules to organic electronics, biaryls present numerous possibilities and new applications continue to emerge. Transition-metal-catalyzed coupling reactions represent the gold standard for biaryl synthesis and the mechanistic steps, such as reductive elimination, are well established. Developing routes that exploit alternative mechanistic scenarios could give unprecedented biaryl structures and expand the portfolio of biaryl applications. We have developed metal-free C-H/C-H couplings of aryl sulfoxides with phenols to afford 2-hydroxy-2'-sulfanylbiaryls. This cascade strategy consists of an interrupted Pummerer reaction and [3,3] sigmatropic rearrangement. Our method enables the synthesis of intriguing aromatic molecules, including oligoarenes, enantioenriched dihetero[8]helicenes, and polyfluorobiaryls. From our successes in aryl sulfoxide/phenol couplings and a deeper understanding of sigmatropic rearrangements for biaryl synthesis, we have established related methods, such as aryl sulfoxide/aniline and aryl iodane/phenol couplings. Overall, our fundamental interests in underexplored reaction mechanisms have led to various methods for accessing important biaryl architectures.

Ortho-selective C–H arylation of phenols with N-carboxyindoles under Brønsted acid- or Cu(i)-catalysis

Control over chemo- and regioselectivity is a critical issue in the heterobiaryl synthesis via C–H oxidative coupling. To address this challenge, a strategy to invert the normal polarity of indoles in the heterobiaryl coupling was developed. With N-carboxyindoles as umpoled indoles, an exclusively ortho-selective coupling with phenols has been realized, employing a Brønsted acid- or Cu(i)-catalyst (as low as 0.01 mol%). A range of phenols and N-carboxyindoles coupled with exceptional efficiency and selectivity at ambient temperature and the substrates bearing redox-active aryl halides (–Br and –I) smoothly coupled in an orthogonal manner. Notably, preliminary examples of atropselective heterobiaryl coupling have been demonstrated, based on a chiral disulfonimide or a Cu(i)/chiral bisphosphine catalytic system. The reaction was proposed to occur through S_N2′ substitution or a Cu(i)–Cu(iii) cycle, with Brønsted acid or Cu(i) catalysts, respectively.

Control over chemo- and regioselectivity is a critical issue in the heterobiaryl synthesis via C–H oxidative coupling. To address this challenge, a strategy to invert the normal polarity of indoles was developed.

Interrogating the Thionium Hydrogen Bond as a Noncovalent Stereocontrolling Interaction in Chiral Phosphate Catalysis

CH⋯O bonds are a privileged noncovalent interaction determining the energies and geometries of a large number of structures. In catalytic settings, these are invoked as a decisive feature controlling many asymmetric transformations involving aldehydes. However, little is known about their stereochemical role when the interaction involves other substrate types. We report the results of computations that show for the first time thionium hydrogen bonds to be an important noncovalent interaction in asymmetric catalysis. As a validating case study, we explored an asymmetric Pummerer rearrangement involving thionium intermediates to yield enantioenriched N,S-acetals under BINOL-derived chiral phosphate catalysis. DFT and QM/MM hybrid calculations showed that the lowest energy pathway corresponded to a transition state involving two hydrogen bonding interactions from the thionium intermediate to the catalyst. However, the enantiomer resulting from this process differed from the originally published absolute configuration. Experimental determination of the absolute configuration resolved this conflict in favor of our calculations. The reaction features required for enantioselectivity were further interrogated by statistical modeling analysis that utilized bespoke featurization techniques to enable the translation of enantioselectivity trends from intermolecular reactions to those proceeding intramolecularly. Through this suite of computational modeling techniques, a new model is revealed that provides a different explanation for the product outcome and enabled reassignment of the absolute product configuration.

Transferable selectivity profiles allow data from intermolecular reactions using iminium substrates to be applied to predict intramolecular reactions involving thioniums.

Advances of Sulfenate Anions in Catalytic Asymmetric Synthesis of Sulfoxides

In recent years, sulfenate anions as key intermediates in enantioselective synthesis have attracted considerable attention. Typically, development of novel synthetic methods to generate sulfenate anions allows for the preparation of various enantiopure sulfoxides, which are prevalently used as auxiliaries, ligands, organocatalysts, and biologically active compounds. This review presents the in situ preparation methods and the recent applications of sulfenate anions in catalytic asymmetric synthesis of chiral sulfoxides.

Synthesis of Cyclopentenones with C4-Quaternary Stereocenters via Stereospecific [3,3]-Sigmatropic Rearrangement and Applications in Total Synthesis of Sesquiterpenoids

A cationic gold(I)-catalyzed asymmetric [3,3]-sigmatropic rearrangement of sulfonium leads after cyclization to cyclopentenones with a C4-quaternary stereocenter. Starting with simple vinyl sulfoxides and propargyl silane, numerous compounds were isolated with moderate to good yields and excellent enantiomeric excesses (26 examples). The application of this simple methodology allowed the efficient total synthesis of five natural sesquiterpenoids, including enokipodin A and B, hitoyopodin A, lagopodin A, and isocuparene-3,4-diol.

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.

Mechanistic Investigation of a Synthetic Route to Biaryls by the Sigmatropic Rearrangement of Arylsulfonium Species

A comprehensive mechanistic investigation was conducted on the coupling reaction of aryl sulfoxides with phenols by using trifluoroacetic anhydride to yield biaryls. NMR experiments revealed that our previously proposed mechanism, which consists of a cascade of an interrupted Pummerer reaction and a rate-determining [3,3] sigmatropic rearrangement, is reasonable. The electronic effects of the substrates were also evaluated to elucidate the nature of the rearrangement step. Based on experimental observations and theoretical calculations, we conclude that the rearrangement is highly asynchronous and stepwise rather than concerted when electron-rich phenols are employed for the reaction.

Synthesis of Highly Substituted Biaryls by the Construction of a Benzene Ring via In Situ Formed Acetals

Herein, we present an interesting method for the construction of a benzene ring using propargylic alcohols and 1,3-dicarbonyls, which involves three new C-C bond formations via cascade alkylation, formylation, annulation, and aromatization to make substituted biaryls. This one-pot Brønsted acid-promoted protocol utilizes the unique reactivity of the acetal formed under the reaction conditions. Alkynyl methyl ketones could be employed instead of 1,3-dicarbonyls as they are converted to 1,3-dicarbonyls by hydration under the reaction conditions.

Synthetic exploration of sulfinyl radicals using sulfinyl sulfones

Sulfinyl radicals – one of the fundamental classes of S-centered radicals – have eluded synthetic application in organic chemistry for over 60 years, despite their potential to assemble valuable sulfoxide compounds. Here we report the successful generation and use of sulfinyl radicals in a dual radical addition/radical coupling with unsaturated hydrocarbons, where readily-accessed sulfinyl sulfones serve as the sulfinyl radical precursor. The strategy provides an entry to a variety of previously inaccessible linear and cyclic disulfurized adducts in a single step, and demonstrates tolerance to an extensive range of hydrocarbons and functional groups. Experimental and theoretical mechanistic investigations suggest that these reactions proceed through sequential sulfonyl and sulfinyl radical addition.

Sulfinyl radicals are an underexplored synthon in organic chemistry due to the fact that they reversibly add to pi systems and undergo homodimerization. Here the authors synthesize sulfonyl sulfones, previously thought to be unstable, and demonstrate their broad use as sulfinyl radical precursors in disulfurizations of alkenes and alkynes.

Sulfur stereogenic centers in transition-metal-catalyzed asymmetric C-H functionalization: generation and utilization

Transition-metal-catalyzed enantioselective C–H functionalization has emerged as a powerful tool for the synthesis of enantioenriched compounds in chemical and pharmaceutical industries. Sulfur-based functionalities are ubiquitous in many of the biologically active compounds, medicinal agents, functional materials, chiral auxiliaries and ligands. This perspective highlights recent advances in sulfur functional group enabled transition-metal-catalyzed enantioselective C–H functionalization for the construction of sulfur stereogenic centers, as well as the utilization of chiral sulfoxides to realize stereoselective C–H functionalization.

This perspective highlights sulfur functional groups enabled enantioselective C–H functionalization for the construction of sulfur stereogenic centers, and the utilization of chiral sulfoxide to realize stereoselective C–H functionalization.

Photochemical (Hetero-)Arylation of Aryl Sulfonium Salts

The construction of (hetero)biaryls, which are ubiquitous scaffolds among medical substances, functional materials, and agrochemicals, constitutes a key application of cross-coupling methods. However, these usually require multiple synthetic steps. Herein, we report a simple photoinduced and catalyst-free C–H/C–H (hetero)arylation cross-coupling through aryl thianthrenium salts, which are formed site-selectively by direct C–H functionalization. The key to this approach is the UV-light, which can disrupt the C–S bond to form thianthrene radical cations and aryl radicals.

Morita-Baylis-Hillman-Type [3,3]-Rearrangement: Switching from Z- to E-Selective α-Arylation by New Rearrangement Partners

α-aryl α,β-unsaturated carbonyls represent an important class of derivatizable synthetic intermediates, however, the synthesis of such compounds still remains a challenge. Recently, we showcased a novel Z-selective α-arylation of α,β-unsaturated nitriles with aryl sulfoxides via [3,3]-rearrangement involving an Morita-Baylis-Hillman (MBH) process. Herein, we demonstrate the feasibility of reversing the stereoselectivity of such MBH-type [3,3]-rearrangement by switching to a new pair of rearrangement partners consisting of aryl iodanes and α,β-unsaturated oxazolines. As a result, the two protocols complement each other in approaching E- or Z-α-aryl α,β-unsaturated carbonyl derivatives. Mechanistic studies reveal a possible reaction pathway and provide an explanation for the opposite stereoselectivities.

Palladium-Catalyzed Sulfinylation of Aryl- and Alkenylborons with Sulfinate Esters

An efficient, direct sulfinylation of organoborons catalyzed by palladium is disclosed. Treatment of organoborons and sulfinate esters in the presence of a palladium precatalyst provided a broad range of sulfoxides. Various organosulfur compounds having oxidizable functional groups were successfully prepared through the sulfoxide synthesis.