A direct ylide transfer to carbonyl derivatives and heteroaromatic compounds

Expanding Natural Diversity: Tailored Enrichment of the 8,12-Sesquiterpenoid Lactone Chemical Space through Divergent Synthesis

The divergent synthesis of a non-natural 8,12-sesquiterpenoid lactone collection is described. The synthesis relies on a rationally designed guaianolide scaffold bearing a tertiary hydroxyl as the pinpoint for inducing its selective diversification. Key reactions include an unprecedented Suarez-type CH lactonization and a highly diastereoselective oxy-Cope/ene cascade that allows the introduction of three stereocenters in a single operation. Selective oxidative/reductive and redox neutral transformations follow to highlight the synthesis of naturally unpresented highly substituted 8,12-guaianolides.

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.

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.

Synthesis of I(III)/S(VI) reagents and their reactivity in photochemical cycloaddition reactions with unsaturated bonds

The development of novel methodologies for the introduction of the sulfoxonium group under mild conditions is appealing but remains underexplored. Herein we report the synthesis of a class of hypervalent iodine reagents with a transferrable sulfoxonium group. These compounds enable mixed iodonium-sulfoxonium ylide reactivity. These well-defined reagents are examined in visible-light-promoted cyclization reactions with a wide range of unsaturated bonds including alkenes, alkynes, nitriles, and allenes. Two distinct cyclization pathways are identified, which are controlled by the substituent of the unsaturated bond. The cycloaddition protocol features simple operation, mild reaction conditions, and excellent functional group tolerance, affording a broad range of sulfoxonium-containing cyclic structures in moderate to excellent yields. Furthermore, the sufoxonium group in the product can be transformed into diverse functional groups and structural motifs via single electron transfer and transition-metal catalysis.

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.

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

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

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.

Sulfur-Based Ylides in Transition-Metal-Catalysed Processes

Traditionally employed in the synthesis of small ring systems and rearrangement chemistry, sulfur-based ylides occupy a unique position in the toolbox of the synthetic organic chemist. In recent years a number of pioneering researchers have looked to expand the application of these unorthodox reagents through the use of transition metal catalysis. The strength and flexibility of such a combination have been shown to be of key importance in developing powerful novel methodologies. This chapter summarises recent developments in transition metal-catalysed sulfonium/sulfoxonium ylide reactions, as well as providing a historical perspective. In overviewing the successes in this area, the authors hope to encourage others into this growing field.

One-pot sequential synthesis of tetrasubstituted thiophenes via sulfur ylide-like intermediates

Herein, we describe a novel approach for the practical synthesis of tetrasubstituted thiophenes 8. The developed method was particularly used for the facile preparation of thienyl heterocycles 8. The mechanism for this reaction is based on the formation of a sulfur ylide-like intermediate. It was clearly suggested by (i) the intramolecular cyclization of ketene N,S-acetals 7 to the corresponding thiophenes 8, (ii) 1H NMR studies of Meldrum's acid-substituted aminothioacetals 9, and (iii) substitution studies of the methoxy group on Meldrum's acid containing N,S-acetals 9b. Notably, in terms of structural effects on the reactivity and stability of sulfur ylide-like intermediates, 2-pyridyl substituted compound 7a exhibited superior properties over those of others.

Rhodium-Catalyzed Synthesis of Sulfur Ylides via in Situ Generated Iodonium Ylides

A convenient strategy for the synthesis of sulfur ylides via rhodium-catalyzed coupling of in situ generated iodonium ylides with sulfides or sulfoxides has been developed. A wide range of sulfur ylides were obtained in moderate to good yields from inexpensive sulfur compounds and active methylene compounds with a short reaction time (MW, 5-10 min) or 12-16 h at rt. Furthermore, these sulfoxonium ylides were used as novel acceptor/acceptor carbenes for N-H insertion reactions.

Dual vicinal functionalisation of heterocycles via an interrupted Pummerer coupling/[3,3]-sigmatropic rearrangement cascade

A dual vicinal functionalisation cascade involving the union of heterocycles and allyl sulfoxides is described. In particular, the approach provides efficient one-step access to biologically relevant and synthetically important C3 thio, C2 carbo substituted indoles. The reaction operates under mild, metal free conditions and without directing groups, via an interrupted Pummerer coupling of activated allyl sulfoxides, generating allyl heteroaryl sulfonium salts that are predisposed to a charge accelerated [3,3]-sigmatropic rearrangement.

Metal-free O-H/C-H difunctionalization of phenols by o-hydroxyarylsulfonium salts in water

An environmentally benign method for C-H/O-H difunctionalization of phenols with sulfoxides under mild conditions has been developed. The reaction process is mediated by an electrophilic aromatic substitution and subsequent selective aryl or alkyl migration, involving C-S and C-O bond formations with broad substrate scope.

N-Heterocyclic carbene-catalyzed stereoselective construction of olefinic carbon–sulfur bonds via cross-coupling reaction of gem-difluoroalkenes and thiols

A novel organocatalytic olefinic carbon–sulfur bond forming reaction was developed.

Synthesis and Photocatalytic Reactivity of Vinylsulfonium Ylides

Although sulfur ylides are textbook reagents in organic synthesis, surprisingly little variation of substituents on sulfur is usually observed. In particular, vinylsulfonium ylides have been neglected so far. Herein, we present a study on their synthesis and reactivity, including interesting behavior under photocatalytic conditions.

Stereoselective gold(I) domino catalysis of allylic isomerization and olefin cyclopropanation: mechanistic studies

Gold(I) catalysis of olefin activation is still a rare feature in the repertoire of that metal. Mechanistic studies on the gold(I)-catalyzed cyclopropanation of allyl-substituted sulfonium ylides, including kinetic analysis as well as detailed computational studies, reveal that the reaction proceeds through activation of the alkene moiety. Furthermore, a novel competitive allylic isomerization pathway that interconverts "linear" and "branched" allylic isomers is uncovered. The subtle interplay of cyclopropanation and olefin isomerization results in an intriguing domino process where two independent catalytic transformations combine with near-perfect regio- and stereoselectivities.

N-Heterocyclic carbene-catalyzed formal cross-coupling reaction of α-haloenals with thiols: organocatalytic construction of sp2 carbon-sulfur bonds

A novel N-heterocyclic carbene (NHC)-catalyzed formal cross-coupling reaction between α-haloenals and thiols was developed. In the presence of 5 mol% NHC precursors and 1.6 equiv. potassium carbonate, various thiols coupled with α-haloenals to produce α-thioenals in 53% to 91% yield and excellent Z-selectivity.

Gold-catalyzed intermolecular synthesis of alkylidenecyclopropanes through catalytic allene activation

A stereoselective gold(I)-catalyzed intermolecular cyclopropanation of allenamides with stabilized sulfonium ylides is reported. This transformation enables the direct synthesis of diacceptor alkylidenecyclopropanes and proceeds under very mild conditions through allene activation.

Double Smiles rearrangement of Passerini adducts towards benzoxazinones

A new straightforward access to benzoxazinones based on a three-component coupling is presented here.

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

The development of a direct ylide transfer to carbonyl derivatives and of a sulfoxide-mediated arylation is presented from a unified perspective. Mechanistic studies (including density functional calculations) support a common reaction pathway and showcase how subtle changes in reactant properties can lead to disparate and seemingly unrelated reaction outcomes.

Sulfoxide-mediated Umpolung of alkali halide salts

A new protocol for the direct two-electron oxidative Umpolung of alkali halide salts is reported. This procedure, relying on the use of a commercially available sulfoxide as the oxidant, allows the electrophilic halogenation of carbonyl compounds as well as halolactonisation reactions to proceed from the corresponding sodium salts, at room temperature and under mild conditions.

Sulfoxide-mediated α-arylation of carbonyl compounds

A novel sulfoxide-mediated α-arylation of carbonyl compounds is reported. This reaction proceeds under very mild conditions at room temperature and does not require any transition-metal promoter or catalyst.