Sulfonium Salts as Acceptors in Electron Donor-Acceptor Complexes

Efficient late-stage synthesis of quaternary phosphonium salts from organothianthrenium salts via photocatalysis

Quaternary phosphonium salts (QPS) are significant structural motifs in drugs, materials, and catalysts. Here, a photoactivated approach for the selective late-stage synthesis of QPS utilizing organothianthrenium salts and tertiary phosphines is presented with high yields and broad functional group compatibility. Additionally, the synthetic utility of this protocol is demonstrated by in situ generation of QPS via C-H functionalization and its fluorescence confocal imaging of mitochondrial localization in cells.

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.

EDA Complex-Promoted Cascade Cyclization of Alkynes Enabling the Rapid Assembly of 3-Sulfonylindoles and Vinyl Sulfone Oxindoles

Herein, we disclose a photoinduced radical cascade cyclization of alkynes with sulfinates via a novel EDA complex for the synthesis of various 3-sulfonylindoles and vinyl sulfone oxindoles, which are crucial motifs in medicinal and biological chemistry. The reaction proceeds under mild, photocatalyst- and transition-metal-free conditions, featuring operational simplicity, broad substrate scope, and easy scalability. Mechanistic studies reveal that the reaction is initiated with a photoinduced intermolecular charge transfer from sulfinates to N-sulfonamides, generating a sulfonyl radical followed by an N-centered radical, thus enabling the cascade cyclization process.

Base-Mediated Divergent Synthesis of Spiro-heterocycles Using Pronucleophiles and Ethylene via Thianthrenation

Spirocyclic compounds are abundant in biologically active products. However, the divergent synthesis of spirocyclic compounds using low-cost and abundant available starting materials remains a challenge. Herein, we report an effective method for producing spirocyclic motifs using a cyclic β-carbonyl ester or amide and ethylene via thianthrenation. This strategy highlights the exciting possibility of utilizing abundant ethylene as a C-2 synthon and allows regulating the core structure of the spirocyclic compound by simply altering the base type.

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.

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.

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.

Mechanistic studies on single-electron transfer in frustrated Lewis pairs and its application to main-group chemistry

Advances in the field of frustrated Lewis pair (FLP) chemistry have led to the discovery of radical pairs, obtained by a single-electron transfer (SET) from the Lewis base to the Lewis acid. Radical pairs are intriguing for their potential to enable cooperative activation of challenging substrates (e.g., CH4, N2) in a homolytic fashion, as well as the exploration of novel radical reactions. In this review, we will cover the two known mechanisms of SET in FLPs-thermal and photoinduced-along with methods (i.e., CV, DFT, UV-vis) to predict the mechanism and to characterise the involved electron donors and acceptors. Furthermore, the available techniques (i.e., EPR, UV-vis, transient absorption spectroscopy) for studying the corresponding radical pairs will be discussed. Initially, two model systems (PMes3/CPh3+ and PMes3/B(C6F5)3) will be reviewed to highlight the difference between a thermal and a photoinduced SET mechanism. Additionally, three cases are analysed to provide further tools and insights into characterizing electron donors and acceptors, and the associated radical pairs. Firstly, a thermal SET process between LiHMDS and [TEMPO][BF4] is discussed. Next, the influence of Lewis acid complexation on the electron acceptor will be highlighted to facilitate a SET between (pBrPh)3N and TCNQ. Finally, an analysis of sulfonium salts as electron acceptors will demonstrate how to manage systems with rapidly decomposing radical species. This framework equips the reader with an expanded array of tools for both predicting and characterizing SET events within FLP chemistry, thereby enabling its extension and application to the broader domain of main-group (photo)redox chemistry.

Palladium-catalyzed regio- and stereo-selective phosphination of cyclic biarylsulfonium salts to access atropoisomeric phosphines

A palladium-catalyzed regio- and stereo-selective phosphination of cyclic biarylsulfonium salts (racemic) with HPAr3Ar4 for straightforward synthesis of atropoisomeric phosphines (P,S-ligands) bearing a stereogenic axis or both a stereogenic axis and a P-stereogenic center is reported. The high reactivity and regio- and stereo-selectivity originate from the torsional strain release and palladium catalysis, and the construction of a P-stereogenic center is enabled by an efficient dynamic kinetic resolution. The high performance of the nascent P,S-ligands has been demonstrated in palladium-catalyzed asymmetric allylic substitutions, indicating the great potential of the present methodology.

Synthesis of Sulfur-Containing Trisubstituted Imidazoles by One-Pot, Multicomponent Reaction via Electron Donor-Acceptor Complex Photoactivation

Rapid and efficient construction of multifunctionalized skeletons through a one-pot multicompound domino reaction has been recognized as a simple and practical strategy. Herein, a visible-light-enabled three-component reaction of isothiocyanates, isocyanides, and thianthrenium salt-functionalized arenes is presented, which affords a facile approach to sulfur-containing trisubstituted imidazoles in good yields with a broad substrate scope and excellent functional group tolerance. The byproduct thianthrene is recovered in quantity, thereby ultimately reducing the production of chemical waste. The developed methodology has potential value for the discovery and development of thioimidazole-based drugs.

Photo- and Cu-Mediated 11C Cyanation of (Hetero)Aryl Thianthrenium Salts

We present a photo- and Cu-mediated 11C cyanation of bench-stable (hetero)aryl thianthrenium salts via an aryl radical addition pathway. The thianthrenium substrates can be readily accessed via C-H functionalization, and the radiocyanation protocol proceeds under mild conditions (<50 °C, 5 min) and can be automated using open-source, readily accessible augmentations to existing radiochemistry equipment.

Reactivity of α-diazo sulfonium salts: rhodium-catalysed ring expansion of indenes to naphthalenes

In the presence of catalytic amounts of the paddlewheel dirhodium complex Rh2(esp)2, α-diazo dibenzothiophenium salts generate highly electrophilic Rh-coordinated carbenes, which evolve differently depending on their substitution pattern. Keto-moieties directly attached to the azomethinic carbon promote carbene insertion into one of the adjacent C-S bonds, giving rise to highly electrophilic dibenzothiopyrilium salts. This intramolecular pathway is not operative when the carbene carbon bears ester or trifluoromethyl substituents; in fact, these species react with olefins delivering easy to handle cyclopropyl-substituted sulfonium salts. When indenes are the olefins of choice, the initially formed cyclopropyl rings smoothly open with concomitant departure of dibenzothiophene, enabling access to a series of 2-functionalized naphthalenes.

Synthesis of alkynyl sulfides via base-promoted nucleophilic ring-opening of α-bromostyrene sulfonium salt

An efficient method for the synthesis of alkynyl sulfides via a C(sp3)-S bond cleavage of α-bromostyrene sulfonium salts has been developed. This base-promoted nucleophilic ring-opening pathway allows the preparation of diverse alkynyl sulfide compounds using tetramethylene sulfoxide as the sulfur source. The reaction proceeds with good functional group tolerance and could be applied to the late-stage functionalization of bioactive molecules and drugs. Furthermore, the synthetic utility of this method was demonstrated by a one-pot synthesis, scale-up reaction and further modification of various alkynyl sulfide products.

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.

Exploring Tunable Properties, Solvent-Modulated Dynamics, and Novel C(sp3)-H Activation Mechanisms in Electron Donor-Acceptor Complexes

Electron donor-acceptor (EDA) complex photochemistry has emerged as a vibrant area in visible-light-mediated synthetic radical chemistry. However, theoretical insights into the reaction mechanisms remain limited. Our study investigates the influence of solvent polarity and halogen atom types on radical reaction pathways in EDA complexes. We demonstrate that solvent polarity modulates the charge transfer and spatial arrangement within EDA complexes, thereby influencing their stability and reaction kinetics. Iodide ions play a crucial role in facilitating free radical generation and stabilizing reaction intermediates. Different halogen atom types exhibit distinct effects on radical reactions. Variations in radical concentration and solvent environment further affect the pathway selectivity. Additionally, light conditions influence the free radical generation and pathway selectivity. Our findings enhance the understanding of EDA complex photochemistry and radical reactions, offering insights for organic synthesis and photochemistry applications.

Base-Promoted Ring-Opening Hydroxylation of Cyclic Sulfonium Salts

Herein, we reported a general strategy for the synthesis of sulfur-containing primary alcohol derivatives by base-promoted ring-opening hydroxylation of cyclic sulfonium salts. A variety of sulfonium salts were successfully transformed into the desired hydroxylated products in moderate to excellent yields with good functional group tolerance. Moreover, the one-pot synthesis, scale-up reaction, and late-stage functionalization of complex molecules demonstrated the practicability of this synthetic protocol in the field of synthetic chemistry.

Dioxygen compatible electron donor-acceptor catalytic system and its enabled aerobic oxygenation

The photochemical properties of Electron Donor-Acceptor (EDA) complexes present exciting opportunities for synthetic chemistry. However, these strategies often require an inert atmosphere to maintain high efficiency. Herein, we develop an EDA complex photocatalytic system through rational design, which overcomes the oxygen-sensitive limitation of traditional EDA photocatalytic systems and enables aerobic oxygenation reactions through dioxygen activation. The mild oxidation system transfers electrons from the donor to the effective catalytic acceptor upon visible light irradiation, which are subsequently captured by molecular oxygen to form the superoxide radical ion, as demonstrated by the specific fluorescent probe, dihydroethidine (DHE). Furthermore, this visible-light mediated oxidative EDA protocol is successfully applied in the aerobic oxygenation of boronic acids. We believe that this photochemical dioxygen activation strategy enabled by EDA complex not only provides a practical approach to aerobic oxygenation but also promotes the design and application of EDA photocatalysis under ambient conditions.

Multicomponent reactions to access S-aryl dithiocarbamates via an electron donor-acceptor complex under open-to-air conditions

A simple and efficient transition-metal/photocatalyst-free visible-light-driven one-pot three-component reaction between thianthrenium salts, carbon disulfide and amines under an air atmosphere for the preparation of biologically relevant S-aryl dithiocarbamates is developed. This methodology is robust and scalable, and exhibits a broad substrate scope and excellent functional group tolerance. Of note, a wide range of primary aliphatic amines bearing different groups are suitable for this strategy. The synthetic utility was further demonstrated by a two-step one-pot multi-component reaction and photo-flow decagram-scale synthesis. Preliminary mechanistic studies suggest that the association of the dithiocarbamate anion with thianthrenium salts formed an electron donor-acceptor complex, which upon excitation with visible light produced an aryl radical via single-electron transfer.

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.

Reductive Radical-Polar Crossover Enabled Carboxylative Alkylation of Aryl Thianthrenium Salts with CO2 and Styrenes

Carboxylic functionalities are among the pivotal groups in bioactive molecules and in the synthesis of new lead compounds because of their unique character in the formation of hydrogen bonds and the possibility of constructing molecular complexes via amide couplings. We adopt the reductive radical-polar crossover strategy to introduce carboxyalkyl groups into arenes with styrenes and CO2 via thianthrenium salts. This protocol exhibits excellent potential as a straightforward and modular platform for site-selective carboxylative derivation of bioactive molecules.

An Expanded EDA Complex Profile: Construction of Aza-arenes and Their Synthetic Application as Fluorescence Probes

We developed a visible-light-driven expanded EDA complex profile for the synthesis of aza-arenes via aza-6π electrocyclization of 2-styrylanilines with aromatic aldehydes. This protocol relies on the EDA complexes of AlCl3 with imine to induce the absorption red-shift to visible light from ultraviolet light. An array of 2,3-disubstituted quinolines were constructed smoothly after excitation with blue-light-emitting diodes at room temperature. In addition, the resultant product, used as a cell permeable lipid droplet-specific probe, shows a low working concentration, a short staining time, and functionality in living and fixed cells.

Electron Donor-Acceptor Complex Enabled Radical Cyclization of α-Diazodifluoroethyl Sulfonium Salt with Unactivated Alkynes

An α-diazodifluoroethane sulfonium reagent was developed in this study to undergo [3 + 2] radical cyclization with unactivated alkynes to give the corresponding 3-difluoromethyl pyrazoles under blue light irradiation conditions. The key to the success of this transformation lies in the formation of an electron donor-acceptor (EDA) complex between an electron-deficient α-diazo sulfonium salt and an electron-rich triaryl amine. This study circumvents a major substrate scope limitation in polar cycloaddition reactions of existent diazodifluoroethane reagents.

A General Electron Donor-Acceptor Photoactivation Platform of Diaryliodonium Reagents: Arylation of Heterocycles

We report a photoredox system comprising sodium iodide, triphenyl phosphine, and N,N,N',N'-tetramethylethylenediamine (TMEDA) that can form a self-assembled tetrameric electron donor-acceptor (EDA) complex with diaryliodonium reagents (DAIRs) and furnish aryl radicals upon visible light irradiation. This practical mode of activation of DAIRs enables arylation of an array of heterocycles under mild conditions to provide the respective heteroaryl-(hetero)aryl assembly in moderate to excellent yields. Detailed mechanistic investigations comprising photophysical and DFT studies provided insight into the reaction mechanism.

Dithiolation of Alkenyl Sulfonium Salts with Arylthiols to Access 1,2-Dithioalkanes

A dithiolation of alkenyl sulfonium salts with arylthiols is described, affording a series of 1,2-dithioalkanes in high yields. This protocol features mild and catalyst-free conditions and involves the formation of two C-S bonds sequentially via the regioselective addition of an arylthiol to the unsaturated C═C bonds, followed by the attack of another arylthiol to form 1,2-dithioalkanes exclusively.

Photoinduced Trifluoromethylselenolation of Aryl Halides with [Me4N][SeCF3]

A visible-light-induced metal-free trifluoromethylselenolation of aryl iodides and bromides with [Me4N][SeCF3] is described. The reaction was conducted at ambient temperature by successfully harnessing the light-sensitive SeCF3 reagent. Mechanistically, the EDA complexes between aryl halide and the -SeCF3 anion or the base might be formed and excited by light, which subsequently undergo intracomplex SET processes to generate aryl and •SeCF3 radicals as key intermediates, allowing a convenient and green access to various aryl trifluoromethyl selenoethers.

Copper and Silver Catalysis in the (3 + 2) Cycloaddition of Neutral Three-Atom Components with Terminal Alkynes

The introduction of the copper-catalyzed azide-alkyne coupling (CuAAC) to 1,3-dipolar cycloadditions was pivotal to their popularization in synthetic chemistry and to their application to multiple other domains of science. The reaction rate enhancement observed when coinage metal acetylide intermediates are involved in the cyclization process greatly expanded the structural and conditional range in which (3 + 2) cycloadditions may take place with terminal alkynes. Herein, we report that comparable rate enhancements, in nature and level, are induced by copper and silver catalysts in the intramolecular (3 + 2) cycloaddition of terminal alkynes with "neutral" three-atom components (TACs), specifically alkynyl sulfides. Through careful observations amidst reaction optimization, experimental, and DFT mechanistic studies, a pathway involving a proton-coupled cyclometallation key step is proposed. The sets of catalytic conditions that have been developed allow us to overcome several scope limitations previously presented by the thermally promoted (3 + 2) cycloaddition of "neutral" TACs, thus expanding their synthetic and applicative potential.

Pentafluorocyclopropanation of (Hetero)arenes Using Sulfonium Salts: Applications in Late-Stage Functionalization

The evaluation of the pentafluorocyclopropyl group as a chemotype in crop protection and medicinal chemistry has been hampered in the past by the lack of suitable methodologies that enable the practical incorporation of this moiety into advanced synthetic intermediates. Herein, we report the gram-scale synthesis of an unprecedented sulfonium salt, 5-(pentafluorocyclopropyl)dibenzothiophenium triflate, and its use as a versatile reagent for the photoinduced C-H pentafluorocyclopropylation of a broad series of non-previously functionalized (hetero)arenes through a radical mediated mechanism. The scope and potential benefits of the protocol developed are further demonstrated by the late-stage introduction of the pentafluorocyclopropyl unit into biologically relevant molecules and widely used pharmaceuticals.

Site-selective and metal-free C-H phosphonation of arenes via photoactivation of thianthrenium salts

Aryl phosphonates are prevalent moieties in medicinal chemistry and agrochemicals. Their chemical synthesis normally relies on the use of precious metals, harsh conditions or aryl halides as substrates. Herein, we describe a sustainable light-promoted and site-selective C-H phosphonation of arenes via thianthrenation and the formation of an electron donor-acceptor complex (EDA) as key steps. The method tolerates a wide range of functional groups including biomolecules. The use of sunlight also promotes this transformation and our mechanistic investigations support a radical chain mechanism.

Sulfonium Salts as Acceptors in Electron Donor-Acceptor Complexes

The photoactivation of electron donor-acceptor complexes has emerged as a sustainable, selective and versatile strategy for the generation of radical species. Electron donor-acceptor (EDA) complexation, however, imposes electronic constraints on the donor and acceptor components and this can limit the range of radicals that can be generated using the approach. New EDA complexation strategies exploiting sulfonium salts allow radicals to be generated from native functionality. For example, aryl sulfonium salts, formed by the activation of arenes, can serve as the acceptor components in EDA complexes due to their electron-deficient nature. This "sulfonium tag" approach relaxes the electronic constraints on the parent substrate and dramatically expands the range of radicals that can be generated using EDA complexation. In this review, these new applications of sulfonium salts will be introduced and the areas of chemical space rendered accessible through this innovation will be highlighted.

Highly Regioselective Dichalcogenation of Alkenyl Sulfonium Salts to Access 1,1-Dichalcogenalkenes

An unprecedented geminal olefinic dichalcogenation of alkenyl sulfonium salts with dichalcogenides ArYYAr (Y = S, Se, Te) is reported, providing various trisubstituted 1,1-dichalcogenalkenes [Ar¹CH = C(YAr²)₂] in a highly selective manner under mild and catalyst-free conditions. The key process involves the formation of two geminal olefinic C-Y bonds via sequential C-Y cross-coupling and C-H chalcogenation. A mechanistic rationale is further supported by control experiments and density functional theory calculations.