Azulenesulfonium Salts: Accessible, Stable, and Versatile Reagents for Cross-Coupling

Palladium/norbornene-catalyzed diversified trifunctionalization of aryl-thianthreniums

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

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.

Cobalt-Catalyzed Reductive Cross-Coupling To Construct Csp3-Csp3 Bonds via Csp3-S and Csp3-X Bonds Activation

A cross-electrophilic coupling of benzyl sulfonium salts with alkyl halides forming Csp3-Csp3 bonds is described by using a Co-based catalytic system. The activation of the stable Csp3-S bond of benzyl sulfonium salts under mild reaction conditions leads to various functionalized alkyl derivatives. Preliminary mechanistic studies suggest the involvement of alkyl radicals formed from both alkyl halides and benzyl sulfoniums through a single electron transfer.

Construction of a 2,2'-biazulene framework via Brønsted acid-promoted annulation of 2,3-di(1-azulenyl)benzofurans

Dibenzofurans featuring a 2,2'-biazulene framework were prepared in good yields by Brønsted acid-promoted annulation of 2,3-di(1-azulenyl)benzofurans in 100% H₃PO₄. NMR, UV-Vis, and fluorescence spectroscopies were used to investigate the structural and optical properties of the products prepared. Remarkably, the annulated products exhibited fluorescence, with the longest wavelength of azulene derivatives reported to date, which extended into the near-infrared region under acidic conditions.

Sulfur(iv)-mediated umpolung α-heterofunctionalization of 2-oxazolines

The α-umpolung of carbonyl compounds significantly expands the boundaries of traditional carbonyl chemistry. Despite various umpolung methods available today, reversing the inherent reactivity of carbonyls still remains a substantial challenge. In this article, we report the first use of sulfonium salts, in lieu of well-established hypervalent iodines, for the carbonyl umpolung event. The protocol enables the incorporation of a wide variety of heteroatom nucleophiles into the α-carbon of 2-oxazolines. The success of this investigation hinges on the following factors: (1) the use of sulfoxides, which are abundant, structurally diverse and tunable, and easily accessible, ensures the identification of a superior oxidant namely phenoxathiin sulfoxide for the umpolung reaction; (2) the "assembly/deprotonation" protocol previously developed for rearrangement reactions in our laboratory was successfully applied here for the construction of α-umpoled 2-oxazolines.

Sulfonium cations as versatile strongly π-acidic ligands

More than a century old, sulfonium cations are still intriguing species in the landscape of organic chemistry. On one hand they have found broad applications in organic synthesis and materials science, but on the other hand, while isoelectronic to the ubiquitous tertiary phosphine ligands, their own coordination chemistry has been neglected for the last three decades. Here we report the synthesis and full characterization of the first Rh(i) and Pt(ii) complexes of sulfonium. Moreover, for the first time, coordination of an aromatic sulfonium has been established. A thorough computational analysis of the exceptionally short S–Rh bonds obtained attests to the strongly π-accepting nature of sulfonium cations and places them among the best π-acceptor ligands available today. Our calculations also show that embedding within a pincer framework enhances their π-acidity even further. Therefore, in addition to the stability and modularity that these frameworks offer, our pincer complexes might open the way for sulfonium cations to become powerful tools in π-acid catalysis.

Back to the scene: while isolobal to the ubiquitous tertiary phosphines, sulfonium cations as ligands were neglected for decades. This work revives the coordination chemistry of these species showing their potential as ligands for π-acid catalysis.

Nickel-Catalyzed Direct Cross-Coupling of Aryl Sulfonium Salt with Aryl Bromide

The direct cross-couplings of aryl sulfonium salts with aryl halides could be achieved by using nickel as a reaction catalyst. The reactions proceeded efficiently via C-S bond activation in the presence of magnesium turnings and lithium chloride in THF at ambient temperature to afford the corresponding biaryls in moderate to good yields, potentially serving as an attractive alternative to conventional cross-coupling reactions employing preprepared organometallic reagents.

Modular synthesis of unsymmetrical [1]benzothieno[3,2-b][1]benzothiophene molecular semiconductors for organic transistors

A modular approach to underexplored, unsymmetrical [1]benzothieno[3,2-b][1]benzothiophene (BTBT) scaffolds delivers a library of BTBT materials from readily available coupling partners by combining a transition-metal free Pummerer CH-CH-type cross-coupling and a Newman-Kwart reaction. This effective approach to unsymmetrical BTBT materials has allowed their properties to be studied. In particular, tuning the functional groups on the BTBT scaffold allows the solid-state assembly and molecular orbital energy levels to be modulated. Investigation of the charge transport properties of BTBT-containing small-molecule:polymer blends revealed the importance of molecular ordering during phase segregation and matching the highest occupied molecular orbital energy level with that of the semiconducting polymer binder, polyindacenodithiophene-benzothiadiazole (PIDTBT). The hole mobilities extracted from transistors fabricated using blends of PIDTBT with phenyl or methoxy functionalized unsymmetrical BTBTs were double those measured for devices fabricated using pristine PIDTBT. This study underscores the value of the synthetic methodology in providing a platform from which to study structure-property relationships in an underrepresented family of unsymmetrical BTBT molecular semiconductors.

A desulphurization strategy for Sonogashira couplings by visible light/copper catalysis

We have developed a new copper-based photocatalyst, [(binap)(tpy)Cu]Cl, and applied it in the visible-light promoted Sonogashira coupling reactions.

Alkylsulfonium salts for the photochemical desulphurizative functionalization of heteroarenes

A metal-free organic photoredox-catalyzed alkylation of heteroarenes using alkylsulfonium salts as alkylation reagents has been developed.

Palladium-catalyzed phosphorylation of arylsulfonium salts with P(O)H compounds via C–S bond cleavage

Herein we report a novel palladium-catalyzed phosphorylation of arylsulfonium salts with P(O)H compounds via C–S bond cleavage under mild conditions.

Synthesis of C3-functionalized indole derivatives via Brønsted acid-catalyzed regioselective arylation of 2-indolylmethanols with guaiazulene

The first Brønsted acid catalyzed method for the construction of guaiazulenyl C3-functionalized indole derivatives was established. The reactions proceeded smoothly at ambient temperature by used (±)-10-camphorsulfonic acid (CSA) as catalyst,...

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

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

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

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

Enabling the Use of Alkyl Thianthrenium Salts in Cross-Coupling Reactions by Copper Catalysis

Alkyl groups are one of the most widely used groups in organic synthesis. Here, a a series of thianthrenium salts have been synthesized that act as reliable alkylation reagents and readily engage in copper-catalyzed Sonogashira reactions to build C(sp³ )-C(sp) bonds under mild photochemical conditions. Diverse alkyl thianthrenium salts, including methyl and disubstituted thianthrenium salts, are employed with great functional breadth, since sensitive Cl, Br, and I atoms, which are poorly tolerated in conventional approaches, are compatible. The generality of the developed alkyl reagents has also been demonstrated in copper-catalyzed Kumada reactions.

Palladium-Catalyzed Fluoroalkylation via C(sp3)-S Bond Cleavage of Vinylsulfonium Salts

An interrupted Pummerer/palladium-catalyzed fluoro-alkylation strategy was developed for alkenyl C-H fluoroalkylthiolation. Palladium-catalyzed ring-opening fluoroalkylation via aliphatic C-S bond cleavage of the vinylsulfonium salts efficiently afforded fluoroalkylthiolated alkene derivatives from readily available alkene substrates and CsF. The protocol features broad substrate scopes and good functional group tolerance under an air atmosphere. The practicability of the synthetic method was demonstrated by transforming the multisubstituted alkene products to diverse fluoroalkylthiolated N-heterocycles.

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

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

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

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

C4-aldehyde of guaiazulene: synthesis and derivatisation

Guaiazulene is an alkyl-substituted azulene available from natural sources and is a much lower cost starting material for the synthesis of azulene derivatives than azulene itself. Here we report an approach for the selective functionalisation of guaiazulene which takes advantage of the acidity of the protons on the guaiazulene C4 methyl group. The aldehyde produced by this approach constitutes a building block for the construction of azulenes substituted on the seven-membered ring. Derivatives of this aldehyde synthesised by alkenylation, reduction and condensation are reported, and the halochromic properties of a subset of these derivatives have been studied.

Azulene-A Bright Core for Sensing and Imaging

Azulene is a hydrocarbon isomer of naphthalene known for its unusual colour and fluorescence properties. Through the harnessing of these properties, the literature has been enriched with a series of chemical sensors and dosimeters with distinct colorimetric and fluorescence responses. This review focuses specifically on the latter of these phenomena. The review is subdivided into two sections. Section one discusses turn-on fluorescent sensors employing azulene, for which the literature is dominated by examples of the unusual phenomenon of azulene protonation-dependent fluorescence. Section two focuses on fluorescent azulenes that have been used in the context of biological sensing and imaging. To aid the reader, the azulene skeleton is highlighted in blue in each compound.

Organogermanes as Orthogonal Coupling Partners in Synthesis and Catalysis

Since the advent of metal-catalyzed cross-coupling technology more than 40 years ago, the field has grown to be ever-increasingly enabling, yet the employed coupling partners are largely still those that were originally employed in the context of Pd-catalyzed cross-coupling, namely, arylboronic esters/acids, aryl silanes, aryl stannanes, or organometallic reagents (RMgX, RZnX). Aryl germanes have little precedent in the literature; they were historically explored in the context of Pd⁰/Pd^II-catalyzed cross-coupling reactions but were found to be much less reactive than the already established reagents. Consequently, few efforts were made by the community on their further mechanistic or synthetic exploration.In 2019, our group described trialkyl aryl germanes as robust, convenient, and nontoxic reagents. Although structurally similar to trialkyl aryl stannanes or silanes, the GeEt₃ site does not engage in the traditional transmetalation mode of Pd^II complexes. Our studies instead provided strong support for an unprecedented and orthogonal reactivity of organogermanes that follows electrophilic aromatic substitution (S_EAr)-type reactivity. This mode of bond activation allowed us to devise a number of synthetic strategies in which the Ge functionality was for the first time more reactive and exclusively functionalized in preference over several of the established coupling partners (e.g., silanes, boronic acids/esters, halogens).Within the past year we have showcased the unique reactivity of organogermanes in C-C and C-X bond-forming transformations. Because of the exquisite mode of bond activation, the new strategies offer access to complementary chemical transformations, tolerating other cross-coupling enabling functionalities, and allow for their further downstream diversification. We have for instance demonstrated that organogermanes can be coupled efficiently with aryl halides under Pd nanoparticle conditions with tolerance of all other established cross-coupling partners, while under homogeneous Pd⁰/Pd^II catalysis all of the other established groups can be functionalized preferentially over the Ge functionality. We similarly were able to harness this orthogonal reactivity mode in oxidative gold catalysis, where organogermanes proved to be more reactive than the established silanes or boronic esters. We have also developed an orthogonal approach for metal-free halogenation of organogermanes with convenient halogenation agents, offering access to the chemo- and regioselective installation of valuable halide motifs in the presence of alternative groups that can also engage in electrophilic halogenations.In this Account, we wish to provide an overview of (i) the historic versus current reactivity findings and synthetic utility of organogermanes, (ii) the current state of mechanistic understanding of their reactivity, and (iii) the synthetic repertoire and ease of installing the germanium functionality in organic molecules.

Azulene Functionalization by Iron-Mediated Addition to a Cyclohexadiene Scaffold

The functionalization of azulenes via reaction with cationic η5-iron carbonyl diene complexes under mild reaction conditions is demonstrated. A range of azulenes, including derivatives of naturally occurring guaiazulene, were investigated in reactions with three electrophilic iron complexes of varying electronic properties, affording the desired coupling products in 43-98% yield. The products were examined with UV-vis/fluorescence spectroscopy and showed interesting halochromic properties. Decomplexation and further derivatization of the products provide access to several different classes of 1-substituted azulenes, including a conjugated ketone and a fused tetracycle.

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

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

Colorimetric detection of Hg2+ with an azulene-containing chemodosimeter via dithioacetal hydrolysis

Azulene is a bicyclic aromatic chromophore that absorbs in the visible region. Its absorption maximum undergoes a hypsochromic shift if a conjugated electron-withdrawing group is introduced at the C1 position. This fact can be exploited in the design of a colorimetric chemodosimeter that functions by the transformation of a dithioacetal to the corresponding aldehyde upon exposure to Hg2+ ions. This chemodosimeter exhibits good chemoselectivity over other metal cations, and responds with an unambiguous colour change clearly visible to the naked eye. Its synthesis is concise and its ease of use makes it appropriate in resource-constrained environments, for example in determing mercury content of drinking water sources in the developing world.

Development of Heterocycle-Substituted and Fused Azulenes in the Last Decade (2010-2020)

Azulene derivatives with heterocyclic moieties in the molecule have been synthesized for applications in materials science by taking advantage of their unique properties. These derivatives have been prepared by various methods, involving electrophilic substitution, condensation, cyclization, and transition metal-catalyzed cross-coupling reactions. Herein, we present the development of the synthetic methods, reactivities, and physical properties for the heterocycle-substituted and heterocycle-fused azulenes reported in the last decade.

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.

Synthetic Applications of Sulfonium Salts

This minireview aims to cover the developments over the past two decades in the chemistry of sulfonium salts. Specifically, insight is provided into the synthetic strategies available for the preparation of these compounds, the different reactivity patterns that are expected depending on their structural features or the reaction conditions applied, and the diversity of organic scaffolds that can thereby be synthesized. Additionally, the pros and cons derived from the use of sulfonium salts are presented and critically compared, when possible, in relation to reagents not based on sulfur but depicting similar reactivity.

Azulene in Polymers and Their Properties

Azulene, a unique isomer of naphthalene, has received much interest from researchers in different fields due to its unusual chemical structure with a negatively charged 5-membered ring fused with a positively charged 7-membered ring. In particular, incorporation of azulene into polymers has led to many interesting properties. This minireview covers functionalization methods of azulene at its various positions of 5- and 7-membered rings to form azulene derivatives including azulene monomers, and gives an overview of a wide range of azulene-containing polymers including poly(1,3-azulene), azulene-based copolymers with connectivity at 1,3-positions of the 5-membered ring, or 4,7-positions of the 7-membered ring, as well as copolymers with azulene units as side chains. Their chemical and physical properties together with applications of azulene-containing polymers have also been summarized.

Germylation of Arenes via Pd(I) Dimer Enabled Sulfonium Salt Functionalization

While aryl germanes have recently found usage as coupling partners in powerful catalytic applications, the synthetic access to this promising functionality is currently limited. This report details the straightforward synthesis of functionalized aryl triethylgermanes via formal C-H functionalization. Building on the concept of directing-group-free and site-selective C-H functionalization of arenes to thianthrenium salt intermediates, we showcase their efficient couplings with triethylgermane (Et₃Ge-H) at room temperature, which was enabled by the air- and moisture-stable Pd(I) dimer, [Pd(μ-I)(P^tBu₃)]₂. The method tolerates numerous functional groups, including valuable (pseudo)halides.