A general electron donor-acceptor complex for photoactivation of arenes via thianthrenation

Visible-Light-Mediated Borylation of Arenes via an Electron Donor-Acceptor Complex of Thianthrenium Salts

Aryl borates are one of the most important synthetic materials in organic synthesis. Herein, we report a visible-light-induced electron donor-acceptor (EDA)-based borylation protocol between thianthrenium salts and 1,4-diazabicyclo[2.2.2]octane (DABCO) for the synthesis of various aryl boronates without a photocatalyst or stepwise processing. Remarkably, this photochemical strategy is compatible with a wide range of sensitive functional groups and affords excellent yields. Moreover, the quantitative recovery of thianthrene for feedstock synthesis lowers the reaction cost.

Photoactivation of Thianthrenium Salts: An Electron-Donor-Acceptor (EDA)-Complex Approach

Thianthrenium salts have emerged as one of the most versatile reagents, gaining significant popularity within the synthetic community for their utility in the construction of C-C and C-X (X = N, O, S, P, halogens) bonds. The use of photoredox and transition metal catalysis with thianthrenium salts for C-C and C-heteroatom bond formation is well established. However, most of these methods require elevated temperatures, expensive catalysts, and ligands under stringent conditions for effective execution. In contrast, the photocatalysis- and transition-metal-free approaches for constructing C-C and C-X bonds using thianthrenium salt derivatives have become increasingly sought after. In this regard, electron-donor-acceptor (EDA)-complex reactions have emerged as a powerful strategy in organic synthesis, eliminating the need for photocatalysts under visible light irradiation. EDA-complex photochemistry exploits the electron-acceptor properties of thianthrenium salts, facilitating the rapid generation of radical intermediates via the C-S bond cleavage. These radical intermediates play a pivotal role in enabling a variety of valuable C-C and C-X formations. In this Perspective, we highlight significant advances in the EDA-complex-mediated reactions involving thianthrenium salts with mechanisms, substrate scope, and limitations for constructing C-C and C-heteroatom bonds. For the sake of brevity, the article is organized into five main sections: (1) Nitrogen-based donor reactions, (2) Oxygen-based donor reactions, (3) Sulfur-based donor reactions, (4) Phosphorus-based donor reactions, and (5) π-based donor reactions, with a focus on C-C, C-S, C-B and C-P bond formations.

Palladium-Catalyzed α-Arylation of Sulfoxonium Ylides with Aryl Thianthrenium Salts via C-S and C-H Bond Activation

Diverse α-aryl α-carbonyl sulfoxonium ylides were efficiently synthesized in yields ranging from moderate to high via a palladium-catalyzed α-arylation of sulfoxonium ylides with aryl thianthrenium salts. The reactions proceeded smoothly via C-S and C-H bond functionalization, exhibiting broad substrate scope and good compatibility to various functionalities. In addition, the scale-up synthesis could be achieved, and the one-pot protocol commencing from the use of simple arene as the precursor of aryl thianthrenium salt could also be accomplished.

Aryl-Radical-Initiated Z-Olefin Synthesis via Synergistic Palladium and Photocatalysis

We report a novel photocatalyzed selective functionalization of terminal olefins for the synthesis of Z-olefins and cyclobutanes under mild conditions in excellent yields. A series of Z-cinnamic acid derivatives, enamines, and amide-containing cyclobutanes were efficiently synthesized using this method. Mechanistic studies indicated that the success of this reaction is based on the triple radical relay strategy, anion-π interaction for aryl halide activation and aryl radical generation, palladium for controlling chemoselectivity of the aryl radical, and photocatalyst for controlling stereoselectivity via diradical species, respectively.

Photoredox cobalt dual catalysis toward C3-functionalization of quinoxalinones with alkenes and alkynes

A site-selective coupling involving quinoxalin-2-ones with alkenes and alkynes has been developed through synergistic visible-light photoredox cobalt catalysis. This method enables C3-selective alkylation and alkenylation of both N-substituted and N-unsubstituted quinoxalin-2-ones, achieving high yields under mild conditions. Of note, the protocol facilitates the incorporation of two alkene units, leading to a formal three-component coupling, whereas a two-component coupling is preferred for alkynes.

Light-Induced Transformations of Donor-Donor Diazo Compounds Derived from N-Sulfonylhydrazones

The donor-donor carbene chemistry field is underdeveloped and often relies on harsh reaction conditions, utilizing either thermal or oxidative process with or without transition-metal catalysts. In this review, we discussed the synthesis and transformation of donor-donor diazo compounds from N-sulfonylhydrazones in the presence of light and base. The N-sulfonylhydrazones are easily accessible from the corresponding carbonyl compounds and sulfonyl hydrazides through condensation. The in situ generated N-sulfonyl anion in the presence of base would undergo the N-S bond cleavage with the aid of light to generate the donor-donor diazo compounds. The donor-donor diazo compounds showed various reactivity in the presence of light for the C-C and C-X bond formation, cyclopropanation reactions, and synthesis of nitrogen, oxygen-containing heterocyclic compounds, which all are discussed under metal-free conditions.

Cu(I)-Catalyzed Silylation and Germylation of Azauracils

The current study demonstrates a Cu(I) catalyzed direct C(sp2)-H silylation and germylation of azauracil using triphenylsilane and triphenylgermane. A broad scope, excellent functional group tolerance, and suitability for large-scale reactions are exhibited in this protocol. Moreover, this method can be readily applied to structurally complex bioactive molecules. Experimental results suggest a radical mechanistic pathway for the reaction.

Light-assisted functionalization of aryl radicals towards metal-free cross-coupling

The many synthetic possibilities that arise when using radical intermediates, in place of their polar counterparts, make contemporary radical chemistry research an exhilarating field. The introduction of photocatalysis has helped tame aryl radicals, leading to a resurgence of interest in their chemistry, and an expansion of viable coupling partners and attainable transformations. These methods are more selective and safer than classical approaches, and they utilize new radical precursors. Given the importance of sustainability in current organic synthesis and our interest in light-assisted metal-free transformations, this Review focuses on recent advances in the use of aryl radicals in photoinduced cross-couplings that do not rely on metals for the crucial bond-forming event, and it is structured according to the key step that the aryl radicals engage in.

Stabilized Carbon Radical-Mediated Assembly of Arylthianthrenium Salts, Alkenes and Amino Acid/Peptide Derivatives

Efficiently assembling amino acids and peptides with bioactive molecules facilitates the modular and streamlined synthesis of a diverse library of peptide-related compounds. Particularly notable is their application in pharmaceutical development, leveraging site-selective late-stage functionalization. Here, a visible light-induced three-component reaction involving arylthianthrenium salts, amino acid/peptide derivatives, and alkenes are introduced. This approach utilizes captodatively-stabilized carbon radicals to enable radical-radical C─C coupling, effectively constructing complex bioactive molecules. This method offers a promising alternative route for modular synthesis of peptide-derived bio-relevant compounds.

Alkyl Sulfonium Salts Enabled Radical Thiocyanohydroxylation of Alkenes

Herein, we report a novel and practical strategy for alkyl sulfonium salts mediating radical thiocyanohydroxylation of alkenes. This reaction features metal-free and mild conditions, generation of noncarbon radicals from the readily available alkyl sulfonium salts, easy handling, and excellent functional group compatibility. A diverse range of activated and unactivated alkenes worked well to deliver various β-thiocyanato alcohols.

C-H Functionalization of Heteroarenes via Electron Donor-Acceptor Complex Photoactivation

C-H Functionalization of heteroarenes stands as a potent instrument in organic synthesis, and with the incorporation of visible light, it emerged as a transformative game-changer. In this domain, electron donor-acceptor (EDA) complex, formed through the pairing of an electron-rich substrate with an electron-accepting molecule, has garnered substantial consideration in recent years due to the related avoidance of the requirement of photocatalyst as well as oxidant. EDA complexes can undergo photoactivation under mild conditions and exhibit high functional group tolerance, making them potentially suitable for the functionalization of biologically relevant heteroarenes. This review article provides an overview of recent advancements in the field of C-H functionalization of heteroarenes via EDA complex photoactivation with literature coverage up to April, 2024.

Recent Applications of Sulfonium Salts in Synthesis and Catalysis

The use of sulfonium salts in organic synthesis has experienced a dramatic increase during the last years that can arguably be attributed to three main factors; the development of more direct and efficient synthetic methods that make easily available sulfonium reagents of a wide structural variety, their intrinsic thermal stability, which facilitates their structural modification, handling and purification even on large scale, and the recognition that their reactivity resembles that of hypervalent iodine compounds and therefore, they can be used as replacement of such reagents for most of their uses. This renewed interest has led to the improvement of already existing reactions, as well as to the discovery of unprecedented transformations; in particular, by the implementation of photocatalytic protocols. This review aims to summarize the most recent advancements on the area focusing on the work published during and after 2020. The scope of the methods developed will be highlighted and their limitations critically evaluated.

Self-Catalyzed Sono-Photoinduced Arylation of Quinoxalin-2(1H)-ones with Arylhydrazines

Herein, the first example of self-catalyzed sono-photoinduced carbon-carbon bond formation was described. Combining the advantages of phototriggered self-catalysis and ultrasonic catalysis, a wide range of 3-arylquinoxalin-2(1H)-ones can be efficiently synthesized in good yields with high functional-group compatibility under mild and eco-friendly conditions.

Late-stage-functionalization of anti-depressant molecule buspirone

Buspirone, a well-established anxiolytic agent, has gained attention for its potential role as an antidepressant, primarily due to its unique pharmacological profile and the ability to modulate serotonin receptors effectively. Late-stage functionalization is considered as a pivotal strategy in drug synthesis that enhances the therapeutic efficacy of existing molecules. This review summarizes various late-stage functionalization techniques applicable to Buspirone, including photocatalyzed, metal-catalyzed, and enzyme-catalyzed reactions.

Energy-transfer-enabled photocatalytic transformations of aryl thianthrenium salts

Aryl thianthrenium salts are valuable in photocatalysis but traditionally require external electron donors for activation. This study introduces an energy transfer (EnT) strategy for the activation of aryl thianthrenium salts using 2,3,4,5,6-penta(carbazol-9-yl)benzonitrile (5CzBN) as a metal-free photocatalyst, eliminating the need for external donors. Utilizing this EnT approach, we achieve C-H deuteration of arenes under visible light with CDCl3 as a deuterium source to synthesize various deuterated aromatic compounds, including important natural products and pharmaceuticals. Additionally, this strategy enables diverse functionalizations including borylation, arylation, cyanation, and selenylation, enhancing the applicability of aryl sulfonium salts in environmentally friendly photocatalysis.

Overriding Cage Effect in Electron Donor-Acceptor Photoactivation of Diaryliodonium Reagents: Synthesis of Chalcogenides

In recent times, diaryliodonium reagents (DAIRs) have witnessed a resurgence as arylating reagents, especially under photoinduced conditions. However, reactions proceeding through electron donor-acceptor (EDA) complex formation with DAIRs are restricted to electron-rich reacting partners serving as donors due to the well-known cage effect. We discovered a practical and high-yielding visible-light-induced EDA platform to generate aryl radicals from the corresponding DAIRs and use them to synthesize key chalcogenides. In this process, an array of DAIRs and dichalcogenides react in the presence of 1,4 diazabicyclo[2.2.2]octane (DABCO) as a cheap and readily available donor, furnishing a variety of di(hetero)aryl and aryl/alkyl chalcogenides in good yields. The method is scalable, features a broad scope with good yields, and operates under open-to-air conditions. The photoinduced chalcogenation technology is suitable for late-stage functionalizations and disulfide bioconjugations and facilitates access to biologically relevant thioesters, dithiocarbamates, sulfoximines, and sulfones. Moreover, the method applies to synthesizing diverse pharmaceuticals, such as vortioxetine, promazine, mequitazine, and dapsone, under amenable conditions.

Visible Light-Induced Radical Cascade Functionalization of Quinoxalin-2(1H)-ones: Three-Component 1,2-Di(hetero)arylation Approach with Styrenes and Thianthrenium Salts

The additive-free visible light-induced three-component 1,2-di(hetero)arylation of styrenes was developed using quinoxalin-2(1H)-ones and thianthrenium salts. The purple visible light excitable quinoxalin-2(1H)-ones were utilized for the single-electron transfer to aryl thianthrenium salts, where the generated aryl radical species underwent the addition cascade to styrenes and quinoxalin-2(1H)-ones. The direct aryl radical addition to quinoxalin-2(1H)-ones also led to the formation of a side product, C3-aryl quinoxalin-2(1H)-ones, capable of a photoredox process to help the formation of 1,2-di(hetero)arylation products.

Base-Mediated Visible-Light-Driven C-H Arylation of Quinoxalin-2(1H)-Ones in Ethanol

Prior methods for visible-light-driven C-H arylation of quinoxalin-2(1H)-ones relied on external photocatalysts. Herein, we report a photocatalyst-free approach for this arylation. In this approach, β-dicarbonyl iodonium ylides, combined with t-BuOK in ethanol, act as aryl precursors, forming electron donor-acceptor (EDA) complexes. These complexes enhance light absorption, facilitating efficient single electron transfer and aryl radical formation. Consequently, various quinoxalin-2(1H)-ones undergo precise and efficient arylation without external photocatalysts. This protocol exhibits excellent tolerance toward diverse functional groups, with mild reaction conditions and eco-friendly solvents, revealing a high Ecoscale value.

Visible Light/Copper Catalysis-Enabled Arylation and Alkenylation of Phosphorothioates via Site-Selective C-H Thianthrenation

An efficient visible light/copper-enabled arylation and alkenylation of phosphorothioates with thianthrenium salts via a C(sp2)-S cross-coupling reaction have been demonstrated. This strategy uses aryl/alkenyl thianthrenium salts as new electrophilic reagents, which can be easily prepared by the site-selective C-H thianthrenation of arenes/alkenes with high regioselectivity. Mechanistic studies revealed a crucial role of the in situ formed copper-sulfur complex, which undergoes a facile SET process with the thianthrenium salts under visible light conditions, thereby successfully achieving the desired cross-coupling reactivity.

Photoinduced Deconstructive Alkylation Approach Enabled by Oxy-Radicals from Alcohols

Alcohols are the most commercially abundant, synthetically versatile and operationally convenient functional groups in organic chemistry. Therefore, a strategy that utilizes hydroxy-containing compounds to develop novel bond disconnection and formation process would achieve molecular diversity. Herein, a deconstructive strategy for the generation of quinoxalin-2(1H)-one derivatives has been developed from alcohol precursors via oxy-radical-induced β-fragmentation. Additionally, 1,5-HAT and deoxygenation by P(III) along with oxy-radical were demonstrated as alternative pathways for this transformation. Furthermore, with the deep-seated reorganization of a few terpenes carbon framework, a unique activity with inhibition against the growth of pathogenic fungi was observed.

Arylcyanation of Styrenes by Photoactive Electron Donor-Acceptor Complexes/Copper Catalysis

A novel electron donor-acceptor (EDA) complex/copper catalysis model has been proposed for the construction of 2,3-diarylpropionitriles under visible light conditions. The developed protocol proceeds via intermolecular charge transfer between the photoactive EDA complex of dibutamine (DBA), aryl thianthrenium salts, and trimethylsilyl cyanide (TMSCN), followed by a copper catalytic cycle. UV-vis absorption measurements confirm the participation of EDA complexes as reactive intermediates. This three-component process proceeds smoothly in the presence of pharmaceutically relevant core structures and sensitive functional groups, which offers the possibility of the precise editing of drug molecules with important scaffolds.

Semiheterogeneous Photoredox Catalysis: Synthesis of 3-Sulfonylquinolines via Radical Multicomponent Cascade Annulation

An efficient photocatalytic protocol combining heterogeneous semiconductors as photocatalysts and NHPI as a redox reagent was disclosed. Under this protocol, 3-sulfonylquinolines were formed in up to 92% yield via a multicomponent radical cascade annulation. Good substituent tolerance and gram-scale reaction showed the potential in fine chemicals modification and pharmaceutical synthesis.

Photoinduced Phosphoniumation of Aryl Halides and Arylthianthrenium Salts via an Electron Donor-Acceptor Complex

Owing to their remarkable practicality and utility, phosphonium salts have attracted substantial interest and are widely applied in critical areas, such as medicine, materials science, and catalysis. Herein, we developed a facile and photocatalyst/metal-free synthetic strategy for the preparation of phosphonium salts utilizing aryl halides/arylthianthrenium salts as aryl radical precursors. This approach is disclosed to undergo an efficient light-induced electron donor-acceptor pathway, facilitating the synthesis of a structurally diverse range of phosphonium salts.

S-Arylation of Thioic S-Acid Using Thianthrenium Salts via Photoactivation of Electron Donor-Acceptor Complex

Thioesters make up an important class of bioactive compounds. Due to their chemoselectivity, they have been widely used in the synthesis of a wide range of complex bioactive molecules and natural products. At present, chemists have developed a variety of methods for the preparation of thioester compounds. However, these methods usually require the use of transition metal catalysis or harsh reaction conditions. The strategy of synthesizing thioester compounds via visible light-induced electron donor-acceptor (EDA) complex reactions avoids the problems associated with conventional methods through the development of photocatalysis. Here we report a sustainable method for thiocarbonylating aryl sulfonium salts via a visible light-induced EDA complex process without transition metals.

A Modular Three-Component Approach for Site-selective Tandem Arene Thiophosphorylation

Thiophosphates serve as pivotal reagents within the realms of both organic and inorganic synthesis, with their most notable applications observed in agricultural chemistry. This manuscript delineates a modular three-component synthetic strategy for site-selective arene C-H thiophosphorylation with thianthrenium salt, 1,4-diazabicyclo[2.2.2]octane-sulfur dioxide (DABSO), and diarylphosphine oxides as substrates. This approach facilitates the metal-free and green synthesis of a diverse spectrum of S-aryl phosphorothioates through C-H functionalization and late-stage modification showcasing practicality and broad applicability.

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.

Visible Light-Induced Electron-Donor-Acceptor-Mediated C-3 Coupling of Quinoxalin-2(1H)-ones with Unactivated Aryl Iodides

Visible light-induced C-3 arylation of quinoxalin-2(1H)-ones with abundantly available aryl iodides with good yields via an electron-donor-acceptor (EDA)-complex formation have been accomplished. The radical scavenging, Electron paramagnetic resonance (EPR), UV-visible experiments, density functional theory (DFT), and quantum yield studies revealed that the reaction went through a radical pathway via a single electron transfer (SET) process. Furthermore, the protocol could also be applied to the synthesis of biologically active molecules, illustrating the practicality of the present protocol.

Photocatalyzed Acylation of Azauracil Derivatives with Aldehydes

A novel approach for the acylation of azauracil derivatives with aldehydes has been developed utilizing sodium decatungstate (NaDT) as a photocatalyst. This method demonstrates broad substrate tolerance and yields moderate to excellent outcomes. Notably, it aligns with green chemistry principles by eliminating oxidants, utilizing eco-friendly energy sources, and offering high scalability and operational simplicity.

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.

Synthesis of Nonclassical Heteroaryl C-Glycosides via Decarboxylative C-H Glycosylation

A photoredox-promoted decarboxylative C-H glycosylation for the synthesis of nonclassical heteroaryl C-glycosides is reported. This methodology is characterized by an exceedingly simple reaction system, high diastereoselectivity, and good functional group tolerance. Moreover, the operational procedure is simple, and the gram-scale reaction highlights the practical applicability of this protocol.

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.

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.

Organohypervalent heterocycles

This review summarizes the structural and synthetic aspects of heterocyclic molecules incorporating an atom of a hypervalent main-group element. The term "hypervalent" has been suggested for derivatives of main-group elements with more than eight valence electrons, and the concept of hypervalency is commonly used despite some criticism from theoretical chemists. The significantly higher thermal stability of hypervalent heterocycles compared to their acyclic analogs adds special features to their chemistry, particularly for bromine and iodine. Heterocyclic compounds of elements with double bonds are not categorized as hypervalent molecules owing to the zwitterionic nature of these bonds, resulting in the conventional 8-electron species. This review is focused on hypervalent heterocyclic derivatives of nonmetal main-group elements, such as boron, silicon, nitrogen, carbon, phosphorus, sulfur, selenium, bromine, chlorine, iodine(III) and iodine(V).

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.

Palladium-Catalyzed Domino Conversion of Aryl-Thianthreniums with Anhydrides: Rapidly Building Highly Functionalized Fluorenones

As a class of rising electrophilic coupling reagents, aryl-thianthreniums (aryl-TTs) have been gaining immense attention. Herein, a novel palladium-catalyzed domino annulation of aryl-TTs with anhydrides is proposed to rapidly assemble a collection of highly functionalized fluorenones. This finding presents an innovative reaction pattern of aryl-TTs wherein the domino annulation version is first involved. Heavily compared with the existing conversions with aryl-TTs, this identified process successively functions as four aryl C-H bonds.

Tunable C-H functionalization and dearomatization enabled by an organic photocatalyst

C-H functionalization and dearomatization constitute fundamental transformations of aromatic compounds, which find wide applications in various research areas. However, achieving both transformations from the same substrates with a single catalyst by operating a distinct mechanism remains challenging. Here, we report a photocatalytic strategy to modulate the reaction pathways that can be directed toward either C-H functionalization or dearomatization under redox-neutral or net-reductive conditions, respectively. Two sets of indoles and indolines bearing tertiary alcohols are divergently furnished with good yields and high selectivity. The key to success is the introduction of isoazatruxene ITN-2 as a novel photocatalyst (PC), which outperforms the commonly used PCs. The ready synthesis and high modulability of isoazatruxene type PCs indicate their great application potential.

Arylthianthrenium Salts as the Aryl Sources: Visible Light/Copper Catalysis-Enabled Intermolecular Azidosulfonylation of Alkenes

The difunctionalization of alkenes using aryl thianthrenium salts as the aryl sources has been reported sporadically. However, the four-component difunctionalization of alkenes on the basis of aryl thianthrenium salts has not been reported thus far and still remains a challenge. Herein, a visible light/copper catalysis-enabled four-component reaction of aryl thianthrenium salts, DABCO·(SO2)2, alkenes, and TMSN3 is presented, which affords a facile approach to β-azidosulfones in good yields with broad substrate scope and excellent functional group tolerance. This strategy indirectly realizes the method for the synthesis of β-azidosulfones through site-selective aryl C-H bond functionalization and alkene difunctionalization. This developed method is an important complement to thianthrenium salts chemistry.

A Practical and Regioselective Strategy for Aromatic C-H Difunctionalization via Site-Selective C-H Thianthrenation

Herein, we present a novel Catellani-type reaction that employed aryl-thianthrenium salts as aryl substrates to trigger the subsequent palladium/norbornene cooperatively catalyzed progress. This strategy can achieve site-selective C-H difunctionalization of aryl compounds without directing groups or a known initiating reagent. A series of functionalized syntheses of bioactive molecules further demonstrated the potential of this strategy.

Photoinduced arylation or alkylation of 1,2,4-triazine-3,5(2H,4H)-diones with hydrazines

A light-induced method is developed for synthesizing azauracils. This method is independent from traditional methodology. Remarkably, this reaction can also be powered by sunlight. The applicability of this method is further demonstrated through its successful implementation in large-scale reactions and its use in synthesizing derivatives.

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.

Visible Light-Induced Imide Alkylation of Azauracils with Aryl Diazoesters

A visible light-induced green and sustainable N-H functionalization of (aza)uracils with α-diazo esters leading to imide alkylation is described. The reaction does not require any catalyst or additive and proceeds under mild conditions. Moreover, an intriguing three component coupling was observed when (aza)uracils were allowed to react with α-diazo esters in cyclic ethers (e. g. 1,4-dioxane, THF) as a solvent. Both the insertion and three-component coupling features broad scope with good to excellent yields and appreciable functional group tolerance. Notably, the divergent method enables modification of natural products and pharmaceuticals, thereby facilitates access to potentially biologically active compounds.

Photoinduced Phosphination of Arenes Enabled by an Electron Donor-Acceptor Complex Using Thianthrenium Salts

Herein, we describe a metal-free approach for the cross-coupling of arenes or styryl-thianthrenium salts (TTs) with diarylphosphines via an electron donor-acceptor (EDA) complex. Various tertiary phosphines were obtained with high site selectivity and good functional group tolerance. This method enables straightforward construction of C-P bonds via C-H activation of arenes and allows the late-stage functionalization of natural products or pharmaceutical molecules. Mechanistic studies support the approach involving a photoinduced EDA complex.

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.

Visible-light-induced [3+2] cycloadditions of donor/donor diazo intermediates with alkenes to achieve (spiro)-pyrazolines and pyrazoles

To date, [3 + 2] cycloadditions of diazo esters with alkynes or alkenes have been a robust tool to generate pyrazoles and pyrazolines. However, methods capable of generating donor/donor diazo species from readily available N-tosylhydrazones to furnish [3 + 2] cycloadditions, remain elusive. Herein, we describe the first visible-light-induced [3 + 2] cycloadditions of donor/donor diazo precursors with alkenes to afford pyrazoles and novel (spiro)pyrazolines bearing a quaternary center. This protocol shows a tolerable substrate scope covering versatile carbonyl compounds and alkenes. Late-stage functionalization of bioactive molecules, one-pot approach, and gram-scale synthesis have also been introduced successfully to prove the practicability. At last, mechanistic experiments and DFT studies suggested the formation of non-covalent interactions enabling the activation of N-tosylhydrazones and the formation of the donor/donor diazo intermediates.

Metal- and Photocatalyst-Free, Visible-Light-Initiated C3 α-Aminomethylation of Quinoxalin-2(1H)-ones via Electron Donor-Acceptor Complexes

We report a metal- and photocatalyst-free C3 α-aminomethylation of quinoxalin-2(1H)-ones with N-alkyl-N-methylanilines. The reaction proceeds through the formation of a photoactivated electron donor-acceptor complex between quinoxalin-2(1H)-ones and N-alkyl-N-methylanilines. The present method provides a mild and environmentally friendly protocol that exhibits good atom economy and excellent functional group tolerance to obtain a library of biologically significant C3 α-aminomethylated quinoxalin-2(1H)-ones in good yields.

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.

Metal-free visible-induced C(sp2)-C(sp2) coupling of quinoxalin-2(H)-ones via oxidative cleavage of the C-N bond

A C(sp²)-C(sp²) reaction between aromatic hydrazines and quinoxalines has been developed through a photocatalytic system. The protocol is established for C(sp²)-N bond cleavage and direct C(sp²)-H functionalization for the coupling of C(sp²)-C(sp²) via photocatalysis under mild and ideal air conditions without the presence of a strong base and metal. The mechanistic studies reveal that the generation of a benzene radical via the oxidative cleavage of aromatic hydrazines for the cross-coupling of C(sp²)-C(sp²) with the assistance of a photocatalyst is essential. The process exhibits excellent compatibility with functional groups and provides convenient access to various 3-arylquinoxalin-2(1H)-ones in good to excellent yields.

Thianthrenium-Enabled Phosphorylation of Aryl C-H Bonds via Electron Donor-Acceptor Complex Photoactivation

An efficient strategy for the preparation of aryl phosphonates via blue-light-promoted single electron transfer process of an EDA complex between phosphites and thianthrenium salts has been demonstrated. The corresponding substituted aryl phosphonates were obtained in good to excellent yields, and the byproduct thianthrene can be recovered and reused in quantity. This developed method realizes the construction of aryl phosphonates through the indirect C-H functionalization of arenes, which has potential application value in drug discovery and development.

Visible light/copper catalysis enabled alkylation of silyl enol ethers with arylsulfonium salts

An efficient protocol has been developed herein for the site-selective alkylation of silyl enol ethers with arylsulfonium salts giving access to valuable aryl alkyl thioethers under visible light conditions. Enabled by copper (I) photocatalysis, the C-S bond of arylsulfonium salts can be selectively cleaved to deliver C-centered radicals under mild conditions. This developed method provides a straightforward approach to utilize arylsulfonium salts as sulfur sources for the synthesis of aryl alkyl thioethers.