Bond-Forming and -Breaking Reactions at Sulfur(IV): Sulfoxides, Sulfonium Salts, Sulfur Ylides, and Sulfinate Salts

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.

From perfluoroalkyl aryl sulfoxides to ortho thioethers

Access to original ortho thioether derivatives was achieved through a [3,3]-rearrangement in a one-pot two-step protocol. Several aryl-SCF3 compounds are reported by variation of the nitrile or of the trifluoroalkyl sulfoxide starting material. The variation of the perfluoroalkyl chain was also possible.

Catalyst-Free Domino Cross-Olefinations and Intramolecular Cyclizations of Morita-Baylis-Hillman Carbonates with Sulfur Ylides

A novel strategy for the catalyst-free domino cross-olefination and intramolecular cyclization of Morita-Baylis-Hillman carbonates with sulfur ylides has been established, resulting in the synthesis of 1,2-dihydroquinolines with a broad scope, excellent chemoselectivity, and high efficiency. Mechanistic experiments revealed the process of 1,1-dipole cross-olefination and highlighted the assistance of the amino group in achieving successful transformations.

Catalytic Enantioselective Sulfoxidation of Functionalized Thioethers Mediated by Aspartic Acid-Containing Peptides

A peptide-catalyzed enantioselective oxidation of sulfides to yield pharmaceutically relevant chiral sulfoxides is reported. Experimental evidence suggesting that a hydrogen bond-donating moiety must be present in the substrate to achieve high levels of enantioinduction is supported by computational modeling of transition states. These models also indicate that dual points of contact between the peptidic catalyst and substrate are likely responsible for the formation of one desired sulfoxide in 94:6 er.

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.

Electron Donor-Acceptor Complex Enabled Cyclization/Sulfonylation Cascade of N-Heterocycles with Thianthrenium Salts

We report a visible-light-promoted cyclization/sulfonylation cascade of N-heterocycles with thianthrenium salts using DABSO as the SO2 surrogate. This method features excellent functional group tolerance, wide substrate scope, and late-stage elaboration of bioactive relevant molecules. Mechanistic investigations reveal that the photoactive electron donor-acceptor (EDA) complexes between thianthrenium salts and DABCO are capable of the generation of aryl radicals, which induce the following SO2 insertion by attacking DABSO, thus triggering the key radical cyclization step.

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.

Synthesis of indolines via palladium-catalyzed [4 + 1] annulation of (2-aminophenyl)methanols with sulfoxonium ylides

A facile strategy for the synthesis of valuable indolines has been developed, involving a palladium(II)/Brønsted acid co-catalyzed annulation of readily available (2-aminophenyl)methanols and sulfoxonium ylides. This protocol allows for the direct utilization of the OH group as a leaving group, tolerates alkyl and aryl groups on the N atom of the aniline moiety, operates under mild reaction conditions, and exhibits good efficiency.

Photocatalytic Synthesis and Functionalization of Sulfones, Sulfonamides and Sulfoximines

Sulfur(VI)-based functional groups are popular scaffolds in a wide variety of research fields including synthetic and medicinal chemistry, as well as chemical biology. The growing interest in sulfur(VI)-containing molecules has motivated the scientific community to explore new methods to synthesize and modify them. Here, photocatalysis plays a key role granting access to new types of reactivity under mild reaction conditions. In this Perspective, we present a selection of works reported in the last six years focused on the photocatalytic assembly and reactivity of sulfones, sulfonamides, and sulfoximines. We addressed the key synthetic intermediates for each transformation, while discussing limitations and strength points of the protocols. Future directions of the field are finally presented.

Single-electron transfer between sulfonium and tryptophan enables site-selective photo crosslinking of methyllysine reader proteins

The identification of readers, an important class of proteins that recognize modified residues at specific sites, is essential to uncover the biological roles of post-translational modifications. Photoreactive crosslinkers are powerful tools for investigating readers. However, existing methods usually employ synthetically challenging photoreactive warheads, and their high-energy intermediates generated upon irradiation, such as nitrene and carbene, may cause substantial non-specific crosslinking. Here we report dimethylsulfonium as a methyllysine mimic that binds to specific readers and subsequently crosslinks to a conserved tryptophan inside the binding pocket through single-electron transfer under ultraviolet irradiation. The crosslinking relies on a protein-templated σ-π electron donor-acceptor interaction between sulfonium and indole, ensuring excellent site selectivity for tryptophan in the active site and orthogonality to other methyllysine readers. This method could escalate the discovery of methyllysine readers from complex cell samples. Furthermore, this photo crosslinking strategy could be extended to develop other types of microenvironment-dependent conjugations to site-specific tryptophan.

Glycosylation with sulfoxide-based glycosyl donors

Sulfoxides have emerged as pivotal constituents in modern carbohydrate chemistry. As anomeric leaving groups, sulfinyl moieties may occupy positions directly at the anomeric position or at a more remote site. This feature article is focused on the evolution and notable advancements of glycosyl sulfoxide donors in glycosylation reactions. Its objective is to elucidate the obstacles and prospects within this evolving research domain, with the aim of enhancing comprehension and progress in the field of carbohydrate chemistry.

Domino Conjugate Addition-1,4-Aryl Migration for the Synthesis of α,β-Difunctionalized Amides

A domino difunctionalization of sulfonyl(acryl)imides to form β-substituted α-aryl amides is reported. This transformation involves a 1,4-addition followed by a polar Truce-Smiles rearrangement process, entropically driven by release of SO2. A wide range of carbon- and heteroatom-based nucleophiles and sulfonyl imides were employed, allowing rapid access to highly functionalized amides. In contrast to related reactions with a radical pathway, unbiased substrates could be employed. Despite the usual requirement of an electron-poor migrating moiety for the SNAr event, we herein report unique and unprecedented vinylogous migrations of electron-neutral arenes. Additionally, a one-pot process toward β-amido amides starting from acrylic acids has been developed.

HCl/DMSO/HFIP-Mediated Chlorination of Pyrrolo[2,1-a]isoquinolines and Other Electron-Rich Heteroarenes

An efficient oxidative chlorination of pyrrolo[2,1-a]isoquinolines has been established using HCl (aq) as the chlorine source and DMSO as the terminal oxidant in HFIP at ambient temperature. A variety of chlorinated pyrrolo[2,1-a]isoquinoline derivatives have been prepared readily in 23 to 99% yields. This chlorination strategy can be expanded to the functionalization of other electron-rich heteroarenes including substituted pyrroles, indoles, and naphthols.

Synthesis of 6/5/3-Fused Tricyclic Scaffolds via Multistep Cascade Cyclization of α-Aryl Vinylsulfoniums with para-Quinamines and para-Quinols

Herein, we present a straightforward approach to access hydroindoline-5-one-based 6/5/3-fused polycyclic ring structures through multistep cascade reactions involving α-aryl vinylsulfoniums and para-quinamines. The reactions proceed smoothly under mild conditions to deliver the desired products in generally good isolated yields. This protocol is also applicable to the cascade cycloaddition reactions of α-aryl vinylsulfoniums and para-quinols, effectively generating complex tricyclic scaffolds. In addition, the scale-up synthesis and further derivatizations demonstrate the potential synthetic application of the protocol.

Recent developments in thiochromene chemistry

Thiochromenes are versatile sulfur-containing heterocyclic compounds that have received considerable interest in drug discovery because of their ability to act as crucial building blocks for synthesizing bioactive compounds. In particular, these scaffolds have found utility in the design of anticancer, anti-HIV, antioxidant, and antimicrobial agents, among others. Despite their pharmacological potential, the synthesis of these scaffolds is less explored in contrast to their oxygen-containing counterparts. This review classifies the synthetic processes into Michael addition, cycloaddition, ring-opening, coupling, cyclization and Diels-Alder reactions, and others. Reaction mechanisms, circumstances, and important instances are thoroughly discussed in each area. For instance, chiral catalysts and substrates like mercaptobenzaldehyde and cinnamaldehyde are used in Michael addition processes to achieve excellent enantioselectivity. In cycloaddition reactions, readily available substrates such as thioisatins and alkynes achieve regioselectivity and product production. Thiochromenes are also synthesized by ring-opening reactions with epoxides or aziridines. These reactions demonstrate the importance of catalysts and solvents in reaction control, particularly palladium catalysts for aryl halides and thiol coupling processes. Another major class discussed is cyclization reactions with alkynyl thiols and alkynes under regulated temperature and pressure conditions to efficiently synthesize thiochromenes. With the use of chiral substrates and catalysts, Diels-Alder processes increase yields and selectivity and enhance the variety of thiochromene compounds. This review emphasizes the versatility of thiochromenes in drug discovery and consolidates the existing literature on thiochromenes, scrutinizing the gaps and opportunities for synthesizing novel thiochromene-containing lead molecules.

[3 + 2] Cycloaddition Reaction of Vinylsulfonium Salts with Hydrazonoyl Halides: Synthesis of Pyrazoles

An efficient [3 + 2] cycloaddition reaction between in situ generated nitrile imines from hydrazonoyl halides and vinylsulfonium salts is developed. The nitrile imines are demonstrated to be a new class of reaction partner for vinylsulfonium salts to conduct the [3 + 2] cycloaddition reaction. The process provides a concise and efficient method for the construction of pyrazole derivatives under mild reaction conditions with broad substrate scope, good product yields, and high regioselectivity.

Thianthrenation-promoted photoinduced alkene difunctionalization and aryl allylation with Morita-Baylis-Hillman adducts

An organophotoredox-catalyzed alkoxyallylation of feed-stock olefins, through thianthrenation using a Morita-Baylis-Hillman adduct as an allylating agent, is described. Site-selective addition of MeOH to an alkene-thianthrenium salt and its subsequent conversion into a nucleophilic radical species forms the basis of this unique difunctionalization strategy. The scope is also expanded into radical aryl allylation.

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.

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.

Visible Light-Induced Reactions of Diazo Compounds and Their Precursors

In recent years, visible light-induced reactions of diazo compounds have attracted increasing attention in organic synthesis, leading to improvement of existing reactions, as well as to the discovery of unprecedented transformations. Thus, photochemical or photocatalytic generation of both carbenes and radicals provide milder tools toward these key intermediates for many valuable transformations. However, the vast majority of the transformations represent new reactivity modes of diazo compounds, which are achieved by the photochemical decomposition of diazo compounds and photoredox catalysis. In particular, the use of a redox-active photocatalysts opens the avenue to a plethora of radical reactions. The application of these methods to diazo compounds led to discovery of transformations inaccessible by the classical reactivity associated with carbenes and metal carbenes. In most cases, diazo compounds act as radical sources but can also serve as radical acceptors. Importantly, the described processes operate under mild, practical conditions. This Review describes this subfield of diazo compound chemistry, particularly focusing on recent advancements.

Synthesis of thioesters using an electrochemical three-component reaction involving elemental sulfur

An electrochemical three-component reaction involving elemental sulfur is disclosed for achieving a metal-free, oxidant-free synthesis of thioesters in a high atom-economical, step-economical and chemoselective manner. A mechanistic investigation indicates that the use of elemental sulfur to trap acyl radical derived from radical umpolung of α-keto acid with an electrochemical design can efficiently generate a carbonyl thiyl radical, which can further be captured by diazoalkane to afford various thioesters.

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.

Electrochemical synthesis of sulfinic and sulfonic esters from sulfonyl hydrazides

An electrochemical synthetic method for the synthesis of sulfinic esters and sulfonic esters from sulfonyl hydrazides was developed. Alkyl sulfinic esters were synthesized by treating sulfonyl hydrazides with trialkyl orthoformate in a DMF solvent at a constant current of 5 mA and then optimizing the reaction conditions. Conversely, alkyl sulfonic esters were exclusively obtained when the reaction was conducted in alkyl alcohol solvents at a constant current of 15 mA. The various substituted arylsulfonyl hydrazides afforded moderate to good yields of the desired sulfinic esters and sulfonic esters. Mechanistic investigations revealed that sulfonyl radicals were formed through electrochemical oxidation and that they react with alkyl radicals or alkoxy radicals to generate the respective ester products.

Synthesis of β,β-Dithioketones by Merging C-C and C-S Bond Cleavage in [1 + 1 + 1 + 1 + 1 + 1] Annulation

An unconventional [1 + 1 + 1 + 1 + 1 + 1] annulation process was developed for the construction of β,β-dithioketones by merging C-C and C-S bond cleavage. In this reaction, rongalite concurrently served as triple C1 units, dual sulfur(II) synthons, and a reductant for the first time. Mechanism investigation indicated that the reaction involved the self-mediated valence state change of rongalite. By performing this step-economical method, the challenging construction of C5-substituted 1,3-dithiane can be achieved under mild and simple conditions.

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.

Stereodivergent Synthesis of 1,4-Dicarbonyl Compounds through Sulfonium Rearrangement: Mechanistic Investigation, Stereocontrolled Access to γ-Lactones and γ-Lactams, and Total Synthesis of Paraconic Acids

Although simple γ-lactones and γ-lactams have received considerable attention from the synthetic community, particularly due to their relevance in biological and medicinal contexts, stereoselective synthetic approaches to more densely substituted derivatives remain scarce. The in-depth study presented herein, showcasing a straightforward method for the stereocontrolled synthesis of γ-lactones and γ-lactams, builds on and considerably expands the stereodivergent synthesis of 1,4-dicarbonyl compounds by a ynamide/vinyl sulfoxide coupling. A full mechanistic and computational study of the rearrangement was conducted, uncovering the role of all of the reaction components and providing a rationale for stereoselection. The broad applicability of the developed tools to streamlining synthesis is demonstrated by concise enantioselective total syntheses of (+)-nephrosteranic acid, (+)-rocellaric acid, and (+)-nephromopsinic acid.

Density Functional Theory Study of the Mechanism of Ni-Catalyzed Carboxylation of Aryl C(sp2)-S Bonds with CO2: Computational Evidence for the Multifaceted Role of Additive Zn

The mechanism of Ni-catalyzed carboxylation of aryl C(sp2)-S bonds with CO2 was studied for the first time by density functional theory calculations. We first proposed another possible reaction pathway in which CO2 insertion occurs prior to reduction. Then, we performed calculations on all proposed reaction pathways, and our calculation results show that the pathway in which reduction occurs prior to CO2 insertion is the favored pathway for this reaction. Additionally, our calculations disclose that additive Zn0 acts in multifaceted roles. (1) Zn0 acts as a reductant to reduce the NiI and NiII intermediates. (2) The simultaneously formed ZnIIBr2 can undergo transmetalation with NiI or NiII intermediates to produce an aryl reservoir by forming arylzinc species. (3) ZnIIBr2 can also coordinate to the CO2 to lower the energy barrier of the CO2 insertion step. Moreover, the calculation results suggest that CO2 insertion is the rate-determining step of the reaction, and CO2 is easier to insert into the NiI-Ph bond rather than into the NiII-Ph bond. These calculation results can improve our understanding of the mechanism of the carboxylation process and the multifaceted roles of metal additive Zn0 and provide theoretical guidance for improving the carboxylation reaction.

Lithium Bromide-Promoted Formal C(sp3)-H Bond Insertion Reactions of β-Carbonyl Esters with Sulfoxonium Ylides to Synthesize 1,4-Dicarbonyl Compounds

A LiBr-promoted formal C(sp3)-H bond insertion reaction between β-carbonyl esters and sulfoxonium ylides is established. This practical reaction has a wide range of substrate scope for both β-carbonyl esters and sulfoxonium ylides to give a variety of 1,4-dicarbonyl compounds with 43-94% yields. The reaction features transition-metal-free reaction conditions and exclusive C-alkylation chemselectivity. The use of bench-stable sulfoxonium ylides overcomes previous methods that require transition metal as catalysts and unstable diazo compounds or toxic haloketones as alkylation reagents.

Recent trends in incorporation of CO2 into organosulfur compounds via C-S bond cleavage

Desulfurative functionalization of organosulfur compounds to form various carbon-carbon and carbon-heteroatom bonds has become established as a powerful tool in organic chemistry. In this context, desulfurative carboxylation of this class of compounds using carbon dioxide (CO2) as a sustainable and renewable source of carboxyl has recently been developed as an efficient option for the synthesis of carboxylic acid derivatives. The aim of this Focus Review is to summarize the major progress in this appealing research field with particular emphasis on the mechanistic features of the reactions. Literature has been surveyed until the end of February 2024, according to the data collected using SciFinder and Google Scholar engines.

Neighboring Effects of Sulfur Oxidation State on Dynamic Covalent Bonds and Assemblies

The impact of a varied sulfur oxidation state (sulfide, sulfoxide, and sulfone) on imine dynamic covalent chemistry is presented. The role of noncovalent interactions, including chalcogen bonds and CH hydrogen bonds, on aldehyde/imine structures and imine exchange reactions was elucidated through experimental and computational evidence. The change in the sulfur oxidation state and diamine linkage further allowed the regulation of imine macrocycles, providing a platform for controlling molecular assemblies.

Generation of sulfones utilizing β-sulfinyl esters as masked aryl sulfinates under redox-neutral conditions

A method for generation of SVI sulfones from β-sulfinyl esters (SIV) under transition-metal-free non-oxidative mild conditions is presented. Various sulfones have been achieved with moderate to excellent yields. The advantage of using β-sulfinyl esters as masked aryl sulfinates has also been exemplified using brominated substrates. Oxygen isotope-labeling experiments indicated that the oxygen atoms incorporated into the sulfone product come from the sulfoxide of the β-sulfinyl ester. Successive β-elimination/O-addition/sulfinate esterification/β-elimination processes are proposed for the mechanism of generating SVI from SIV.

Sulfur in Waste-Free Sustainable Synthesis: Advancing Carbon-Carbon Coupling Techniques

This review explores the pivotal role of sulfur in advancing sustainable carbon-carbon (C-C) coupling reactions. The unique electronic properties of sulfur, as a soft Lewis base with significant mesomeric effect make it an excellent candidate for initiating radical transformations, directing C-H-activation, and facilitating cycloaddition and C-S bond dissociation reactions. These attributes are crucial for developing waste-free methodologies in green chemistry. Our mini-review is focused on existing sulfur-directed C-C coupling techniques, emphasizing their sustainability and comparing state-of-the-art methods with traditional approaches. The review highlights the importance of this research in addressing current challenges in organic synthesis and catalysis. The innovative use of sulfur in photocatalytic, electrochemical and metal-catalyzed processes not only exemplifies significant advancements in the field but also opens new avenues for environmentally friendly chemical processes. By focusing on atom economy and waste minimization, the analysis provides broad appeal and potential for future developments in sustainable organic chemistry.

Ring expansion of spirocyclopropanes with stabilized sulfonium ylides: highly diastereoselective synthesis of cyclobutanes

A novel method was devised for regioselective ring expansion of Meldrum's acid-derived spirocyclopropanes to spirocyclobutanes with stabilized sulfonium ylides, affording 1,2-trans-disubstituted 6,8-dioxaspiro[3.5]nonane-5,9-diones in up to 87% yields without the formation of any isomers. The aforementioned reaction was also applied to the barbituric acid-derived spirocyclopropane, resulting in the formation of the corresponding cyclobutanes.

Highly substituted benzo[b]furan synthesis through substituent migration

An unusual benzofuran synthesis from 2,6-disubstituted phenols and alkynyl sulfoxides is disclosed. Various highly substituted benzofurans were synthesized via the charge-accelerated [3,3]-sigmatropic rearrangement and subsequent substituent migration. Multiaryl-substituted benzofurans and fully substituted benzofurans were prepared on the basis of the unique reaction mechanism.

Alkene Thianthrenation Unlocks Diverse Cation Synthons: Recent Progress and New Opportunities

Oxidative alkene functionalization reactions are a fundamental class of complexity-building organic transformations. However, the majority of established approaches rely on electrophilic reagents that limit the diversity of groups that can be installed. Recent advances have established a new approach that instead relies on the transformation of alkenes into thianthrene-derived cationic electrophiles. These linchpin intermediates can be generated selectively and undergo a diverse array of mechanistically distinct reactions with abundant nucleophiles. Taken together, this unlocks a suite of net oxidative alkene transformations that have been elusive using conventional strategies. This Minireview describes these advances and is organized around the three distinct synthons formally accessible from alkenes via thianthrenation: 1) alkenyl cations; 2) vicinal dications; 3) allyl cations. Throughout the Minireview, we illustrate how thianthrenium salts address key limitations endemic to classic alkene-derived electrophiles and highlight the mechanistic origins of these distinctions wherever possible.

Synthesis of gem-Difluorinated Keto-Sulfoxides from Sulfoxonium Ylides

Organic molecules containing fluorine and sulfur atoms represent a large percentage of approved pharmaceuticals. Those with combination of both S and F atoms in their structure such as Xtandi, approved in 2012 for prostate cancer, indicates the importance of synthetic methods that accommodates both atoms in an organic moiety. In this study, a novel aspect of sulfoxonium ylide reactivity was explored, unveiling a streamlined and mild synthesis method for gem-difluorinated keto-sulfoxides. Our protocol offers a direct and practical approach to prepare these compounds in 14-80 % chemical yields, that were represented by 21 examples. NMR studies and Hammett correlations gave strong evidence about the mechanism of this transformation.

Controllable Double Difluoromethylene Insertions into S-Cu Bonds: (Arylthio)tetrafluoroethylation of Aryl Iodides with TMSCF2Br

A new method of constructing "ArSCF2CF2Cu" from ArSCu and TMSCF2Br (TMS=trimethylsilyl) has been developed. The cross-coupling reactions of the obtained "ArSCF2CF2Cu" with diverse aryl iodides (Ar'I) provide an efficient access to Ar'CF2CF2SAr. Mechanistic studies demonstrate that the "ArSCF2CF2Cu" species were generated through controllable double difluoromethylene insertions into ArS-Cu bonds rather than the 1,2-addition of ArSCu to tetrafluoroethylene.

Substrate-directed divergent annulations of sulfur ylides: synthesis of functionalized bispirocyclopentane and bispirocyclopropane derivatives

Herein, an efficient, substrate-directed divergent [2 + 3]/[2 + 1] annulation of tetra-substituted oxa-dienes and allylic sulfur ylides has been successfully developed. Under precise annulation regulation, a series of functionalized bispirocyclopentane and bispirocyclopropane derivatives were synthesized in a highly stereoselective and economical manner (up to 95% yield, >20 : 1 dr or >20 : 1 E/Z). This protocol offers the advantages of mild conditions, high chemo-, regio- and diastereoselectivity and broad substrate compatibility. In addition, the synthetic practicality of this protocol was evaluated through a scale-up preparation and a series of three-component reactions utilizing in situ generated sulfur ylides.

Acid-Catalyzed Highly Enantioselective Synthesis of α-Amino Acid Derivatives from Sulfinamides and Alkynes

An enantioselective difunctionalization of activated alkynes using chiral sulfinamide reagents is developed. It is an atom and chirality transfer process that allows for the modular synthesis of optically active α-amino acid derivatives under mild conditions. The reaction proceeds through an acid-catalyzed [2,3]-sigmatropic rearrangement mechanism with predictable stereochemistry and a broad scope.

Electrochemical desulfurative formation of C-N bonds through selective activation of inert C(sp3)-S bonds

In this study, we introduce an efficient, metal-free electrocatalytic desulfurative protocol for forming C-N bonds by selectively activating inert C(sp3)-S bonds of alkyl thioethers. This method offers a straightforward and environmentally friendly approach for modification of heterocyclic compounds from readily accessible thioethers. Preliminary mechanistic investigations suggest that the reaction proceeds via a carbocation intermediate. Furthermore, successful synthesis on a 10-gram scale was achieved in a continuous flow electrochemical reactor.

Sulfonium Rearrangements Enable the Direct Preparation of Sulfenyl Imidinium Salts

Sulfenyl imidinium salts are a virtually unexplored class of intermediates in organic chemistry. Herein, we demonstrate how sulfonium rearrangements can be deployed to access these versatile synthetic intermediates, bearing three contiguous (and congested) stereogenic centers, with high levels of selectivity. The synthetic value of the scaffold was unraveled by selective transformations into a range of building blocks, including 1,4-dicarbonyl derivatives and sulfonolactones.

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.

Hydroxylation of Aryl Sulfonium Salts for Phenol Synthesis under Mild Reaction Conditions

Hydroxylation of aryl sulfonium salts could be realized by utilizing acetohydroxamic acid and oxime as hydroxylative agents in the presence of cesium carbonate as a base, leading to a variety of structurally diverse hydroxylated arenes in 47-95% yields. In addition, the reaction exhibited broad functionality tolerance, and a range of important functional groups (e.g., cyano, nitro, sulfonyl, formyl, keto, and ester) could be well amenable to the mild reaction conditions.

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.

Indium-catalyzed inter- and intramolecular dithianyl-alkyne metathesis reactions

Herein, we reported an efficient indium catalyzed dithianyl-alkyne metathesis (DAM) reaction. This strategy allows for the formation of a new C-C double bond and valuable C-S bonds during the metathesis event, and was successfully applied to the synthesis of diverse vinyl dithianyl substituted organic molecules.

Recent Progress on the Synthesis of Bipyridine Derivatives

Bipyridine and related compounds are starting materials or precursors for a variety of valuable substances such as biologically active molecules, ligands for catalysts, photosensitizers, viologens, and supramolecular architectures. Thus, it is important to classify their synthesis methods and understand their characteristics. Representative examples include methods using homo and heterocoupling of pyridine derivatives in the presence of a catalyst. Because bipyridine compounds strongly coordinate with metal centers, a decrease in catalytic activity and yield is often observed in the reaction system. To address this issue, this review provides insights into advances over the last ~30 years in bipyridine synthesis using metal complexes under both homogeneous and heterogeneous conditions. Moreover, strategies for bipyridine synthesis involving sulfur and phosphorous compounds are examined. These alternative pathways offer promising avenues for overcoming the challenges associated with traditional catalysis methods, providing a more comprehensive understanding of the synthesis landscape.

Electro-oxidative quinylation of sulfides to sulfur ylides in batch and continuous flow

An unprecedented strategy for preparing a series of sulfur ylides through electro-oxidative quinylation of sulfides in batch and continuous flow has been developed. Good to excellent yields were obtained with excellent functional group compatibility and good concentration tolerance under exogenous oxidant- and transition metal-free conditions. Advantageously, this electrosynthesis methodology was scalable with higher daily production and steady production was achieved attributing to the use of micro-flow cells.