Sulfur(IV)-mediated transformations: from ylide transfer to metal-free arylation of carbonyl compounds

Hypervalent Sulfur Derivatives as Sulfenylating Reagents: Visible-Light-Mediated Direct Thiolation of Activated C(sp2)-H Bonds with Dihalosulfuranes

In contrast to hypervalent iodine compounds, the chemistry of their sulfur analogues has been considerably less explored. Herein, we report the direct C-H bond thiolation of electron-rich heterocycles, arenes, and 1,3-dicarbonyls by dichlorosulfuranes under mild conditions. Mechanistic studies and density functional theory calculations suggest the radical chain mechanism of the disclosed transformation. The key to success is attributed to a strikingly low S-Cl bond dissociation energy, which enables the generation of radical species upon exposure to daylight.

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.

Transfer freier Aminogruppen via α-Aminierung von Carbonylen

Eine Strategie zur direkten α‐Aminierung unfunktionalisierter Carbonylverbindungen wird berichtet. Unter Verwendung einer kommerziell verfügbaren Stickstoffquelle zur Übertragung der freien Aminogruppe (NH2) werden primäre α‐Aminocarbonylverbindungen unter besonders milden Bedingungen hergestellt. Die direkte Einführung einer ungeschützten, primären Aminogruppe ermöglicht in der Folge zahlreiche in situ Funktionalisierungen der erhaltenen Reaktionsprodukte, einschließlich Peptidkupplungen und Pictet–Spengler Cyclisierungen.

Free Amino Group Transfer via α-Amination of Native Carbonyls

We report herein a straightforward transfer of a free amino group (NH2 ) from a commercially available nitrogen source to unfunctionalized, native carbonyls (amides and ketones) resulting in direct α-amination. Primary α-amino carbonyls are readily produced under mild conditions, further enabling diverse in situ functionalization reactions-including peptide coupling and Pictet-Spengler cyclization-that capitalize on the presence of the unprotected primary amine.

Halide-promoted pyridinylation of α-acylmethylides with 2-halo-1-methylpyridinium iodides as reagents

Halide-promoted pyridinylation between α-acyl sulfonylmethylides and 2-halo-1-methylpyridinium iodides in a transition-metal-free protocol is described. A broad range of α-acyl sulfonylmethylides were transformed to bifunctionalized vinylsulfones in moderate to good yields, thereby providing a facile and practical approach for constructing methylthio- and pyridinoxyl-substituted vinylsulfones. The substrates can be extended to other acyl methylides. The reaction was shown to entail the formation of a C-O bond and consecutive breaking of C-S, C-Cl and C-N bonds.

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.

Deployment of Sulfinimines in Charge-Accelerated Sulfonium Rearrangement Enables a Surrogate Asymmetric Mannich Reaction

β-Amino acid derivatives are key structural elements in synthetic and biological chemistry. Despite being a hallmark method for their preparation, the direct Mannich reaction encounters significant challenges when carboxylic acid derivatives are employed. Indeed, not only is chemoselective enolate formation a pitfall (particularly with carboxamides), but most importantly the inability to reliably access α-tertiary amines through an enolate/ketimine coupling is an unsolved problem of this century-old reaction. Herein, we report a strategy enabling the first direct coupling of carboxamides with ketimines for the diastereo- and enantioselective synthesis of β-amino amides. This conceptually novel approach hinges on the innovative deployment of enantiopure sulfinimines in sulfonium rearrangements, and at once solves the problems of chemoselectivity, reactivity, and (relative and absolute) stereoselectivity of the Mannich process. In-depth computational studies explain the observed, unexpected (dia)stereoselectivity and showcase the key role of intramolecular interactions, including London dispersion, for the accurate description of the reaction mechanism.

Direct Stereodivergent Olefination of Carbonyl Compounds with Sulfur Ylides

The reactivity of phosphorus and sulfur ylides toward carbonyl compounds constitutes a well-known dichotomy that is a common educational device in organic chemistry─the former gives olefins, while the latter gives epoxides. Herein, we report a stereodivergent carbonyl olefination that challenges this dichotomy, showcasing thiouronium ylides as valuable olefination reagents. With this method, aldehydes are converted to Z-alkenes with high stereoselectivity and broad substrate scope, while N-tosylimines provide a similarly proficient entry to E-alkenes. In-depth computational and experimental studies clarified the mechanistic details of this unusual reactivity.

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.

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.

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.

A Density Functional Theory Study on Et-BAC-Catalyzed 1,6-Conjugate Addition of p-Chlorobenzaldehyde to p-Quinone Methide for the Synthesis of α,α'-Diarylated Ketones

Umpolung-based organocatalysis has made a remarkable breakthrough in the field of synthetic organic chemistry. Among a plethora of umpolung catalysts, bis(amino)cyclopropenylidenes (BACs) have emerged as efficient organocatalysts with potential applications in synthesizing numerous essential organic moieties. In this study, a plausible mechanism for bis(diethylamino)cyclopropenylidene (Et-BAC)-catalyzed synthesis of α,α'-diarylated ketones has been established using the density functional theory (DFT) method. The proposed catalytic cycle of the studied reaction initiates with the nucleophilic interaction of Et-BAC with p-chlorobenzaldehyde to form a zwitterionic intermediate, which is then transformed to a reactive Breslow intermediate. The Breslow intermediate further undergoes a chemoselective and stereoselective 1,6-conjugate addition reaction with p-quinone methide to form a new C-C bond connection. Finally, the generated adduct undergoes a proton shift reaction with the assistance of both 8-diazabicyclo(5.4.0)undec-7-ene (DBU) and protonated DBU to yield the desired product. Conceptual DFT-derived reactivity indices and frontier molecular orbital theory analysis have been successfully utilized to unravel the role of Et-BAC in this studied reaction. In addition to Et-BAC, DBU and protonated DBU also play a very important role in lowering the activation energy barrier of proton transfer steps. This investigation will help in the rational designing of simple nonheterocyclic carbene-mediated novel organic transformations.

Formal Enone α-Arylation via I(III)-Mediated Aryl Migration/Elimination

A formal enone α-arylation is described. This metal-free transformation relies on the I(III)-mediated skeletal reorganization of silyl enol ethers and features mild conditions, good yields, and high stereoselectivities for β-substituted enones.

Z-Selective α-Arylation of α,β-Unsaturated Nitriles via [3,3]-Sigmatropic Rearrangement

The Morita-Baylis-Hillman (MBH) reaction and [3, 3]-sigmatropic rearrangement are two paradigms in organic synthesis. We have merged the two types of reactions to achieve [3,3]-rearrangement of aryl sulfoxides with α,β-unsaturated nitriles. The reaction was achieved by sequentially treating both coupling partners with electrophilic activator (Tf₂ O) and base, offering an effective approach to prepare synthetically versatile α-aryl α,β-unsaturated nitriles with Z-selectivity through direct α-C-H arylation of unmodified α,β-unsaturated nitriles. The control experiments and DFT calculations support a four-stage reaction sequence, including the assembly of Tf₂ O activated aryl sulfoxide with α,β-unsaturated nitrile, MBH-like Lewis base addition, [3,3]-rearrangement, and E1cB-elimination. Among these stages, the Lewis base addition is diastereoselective and E1cB-elimination is cis-selective, which could account for the remarkable Z-selectivity of the reaction.

Chemoselective Alpha-Deuteration of Amides via Retro-ene Reaction

A synthetically convenient approach for the direct α‐deuteration of amides is reported. This mechanistically unusual process relies on a retro‐ene‐type process, triggered by the addition of deuterated dimethyl sulfoxide to a keteniminium intermediate, generated through electrophilic amide activation. The transformation displays broad functional‐group tolerance and high deuterium incorporation.

Iodane-Guided ortho C-H Allylation

A metal-free C-H allylation strategy is described to access diverse functionalized ortho-allyl-iodoarenes. The method employs hypervalent (diacetoxy)iodoarenes and proceeds through the iodane-guided "iodonio-Claisen" allyl transfer. The use of allylsilanes bearing electron-withdrawing functional groups unlocks the functionalization of a broad range of substrates, including electron-neutral and electron-poor rings. The resulting ortho-allylated iodoarenes are versatile building blocks, with examples of downstream transformation including a concise synthesis of the experimental antimitotic core of Dosabulin. DFT calculations shed additional light on the reaction mechanism, with notable aspects including the aromatic character of the transition-state structure for the [3,3] sigmatropic rearrangement, as well as the highly stereoconvergent nature of the trans-product formation.

Trifluoromethyl Sulfoxides: Reagents for Metal-Free C-H Trifluoromethylthiolation

Trifluoromethyl sulfoxides are a new class of trifluoromethylthiolating reagent. The sulfoxides engage in metal-free C-H trifluoromethylthiolation with a range of (hetero)arenes. The method is also applicable to the functionalization of important compound classes, such as ligand derivatives and polyaromatics, and in the late-stage trifluoromethylthiolation of medicines and agrochemicals. The isolation and characterization of a sulfonium salt intermediate supports an interrupted Pummerer reaction mechanism.

Dearomatization of aryl sulfoxides: a switch between mono- and dual-difluoroalkylation

Herein we describe the dearomatization of aryl sulfoxides with difluoroenol silyl ether (DFESE) using a rearrangement/addition protocol. The selection of the sulfoxide activator determines whether one or two difluoroalkyl groups are incorporated into dearomatized products. Using TFAA can deliberately halt the reaction at the mono-difluoroalkylated dearomatized intermediate formed via a [3,3]-rearrangement, which can be further trapped by external nucleophiles to give mono-difluoroalkylated alicycles. In contrast, switching to Tf₂O enhances the electrophilicity of dearomatized intermediates, thus allowing for the adoption of a second DFESE to produce dual-difluoroalkylated alicycles.

Herein we describe the dearomatization of aryl sulfoxides with difluoroenol silyl ether (DFESE) using a rearrangement/addition protocol.

Sulfoxide-mediated oxidative cross-coupling of phenols

A metal-free, oxidative coupling of phenols with various nucleophiles, including arenes, 1,3-diketones and other phenols, is reported. Cross-coupling is mediated by a sulfoxide which inverts the reactivity of the phenol partner. Crucially, the process shows high selectivity for cross-versus homo-coupling and allows efficient access to a variety of aromatic scaffolds including biaryls, benzofurans and, through an iterative procedure, aromatic oligomers.

A metal-free, oxidative coupling of phenols with various nucleophiles, including arenes, 1,3-diketones and other phenols, is reported.

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

Organosulfur compounds have long played a vital role in organic chemistry and in the development of novel chemical structures and architectures. Prominent among these organosulfur compounds are those involving a sulfur(IV) center, which have been the subject of countless investigations over more than a hundred years. In addition to a long list of textbook sulfur-based reactions, there has been a sustained interest in the chemistry of organosulfur(IV) compounds in recent years. Of particular interest within organosulfur chemistry is the ease with which the synthetic chemist can effect a wide range of transformations through either bond formation or bond cleavage at sulfur. This review aims to cover the developments of the past decade in the chemistry of organic sulfur(IV) molecules and provide insight into both the wide range of reactions which critically rely on this versatile element and the diverse scaffolds that can thereby be synthesized.

Transition-Metal-Free Cross-Coupling of Benzothiophenes and Styrenes in a Stereoselective Synthesis of Substituted (E,Z)-1,3-Dienes

A transition metal-free one-pot stereoselective approach to substituted (E,Z)-1,3-dienes was developed by using an interrupted Pummerer reaction/ligand-coupling strategy. Readily available benzothiophene S-oxides, which can be conveniently prepared by oxidation of the parent benzothiophenes, undergo Pummerer coupling with styrenes. Reaction of the resultant sulfonium salts with alkyllithium/magnesium reagents generates underexploited hypervalent sulfurane intermediates that undergo selective ligand coupling, resulting in dismantling of the benzothiophene motif and the formation of decorated (E,Z)-1,3-dienes.

Synthesis of Polysubstituted Iodoarenes Enabled by Iterative Iodine-Directed para and ortho C-H Functionalization

Among halogenated aromatics, iodoarenes are unique in their ability to produce the bench-stable halogen(III) form. Earlier, such iodine(III) centers were shown to enable C-H functionalization ortho to iodine via halogen-centered rearrangement. The broader implications of this phenomenon are explored by testing the extent of an unusual iodane-directed para C-H benzylation, as well as by developing an efficient C-H coupling with sulfonyl-substituted allylic silanes. Through the combination of the one-shot nature of the coupling event and the iodine retention, multisubstituted arenes can be prepared by sequentially engaging up to three aromatic C-H sites. This type of iodine-based iterative synthesis will serve as a tool for the formation of value-added aromatic cores.