Recent Advances in Electrochemical Chalcogen (S/Se)‐Functionalization of Organic Molecules

Electrosynthesis of Cyclic Isoureas and Ureas Through Contiguous Heterofunctionalizations

An efficient synthetic protocol for the selenylated cyclic isoureas was developed using electrochemical activation of diselenides. This sustainable approach permitted transition metal and chemical oxidant-free difunctionalization of olefins and overall access to distinct 1,2,3 triheterofunctionalized carbon skeletons. Excellent functional group tolerance was noticed, allowing the synthesis of a series of cyclic isourea derivatives. In addition, an acid-triggered skeletal isomerization facilitated the synthesis of cyclic urea derivatives from the corresponding cyclic isoureas. Mechanistic investigations, along with voltammetric studies, enabled the postulation of the reaction mechanism.

Electrochemical synthesis of sulfinic and sulfonic esters from sulfonyl hydrazides

An electrochemical synthetic method for the synthesis of sulfinic esters and sulfonic esters from sulfonyl hydrazides was developed. Alkyl sulfinic esters were synthesized by treating sulfonyl hydrazides with trialkyl orthoformate in a DMF solvent at a constant current of 5 mA and then optimizing the reaction conditions. Conversely, alkyl sulfonic esters were exclusively obtained when the reaction was conducted in alkyl alcohol solvents at a constant current of 15 mA. The various substituted arylsulfonyl hydrazides afforded moderate to good yields of the desired sulfinic esters and sulfonic esters. Mechanistic investigations revealed that sulfonyl radicals were formed through electrochemical oxidation and that they react with alkyl radicals or alkoxy radicals to generate the respective ester products.

Electrochemical selective divergent C-H chalcogenocyanation of N-heterocyclic scaffolds

An electrochemical direct selective C-H chalcogenocyanation approach for indolizine derivatives under mild conditions has been described. Cyclic enone-fused, chromone-fused and 2-substituted indolizines possessing EDGs (electron donating groups) and EWGs (electron withdrawing groups) were successfully reacted with NH4SCN and KSeCN under electrochemical conditions to provide a wide array of mono and bis-chalcogenocyanate-indolizines in 75-94% yields. In addition, 1-substituted imidazo[1,5-a]quinolines were also successfully chalcogenocyanated under the optimized reaction conditions providing a platform for the synthesis of pharmaceutically privileged molecules. By switching the reaction conditions, the developed protocol offers selective synthesis of C-3 thiocyanate and 1,3 bis-thiocyanate indolizines in good to excellent yields under catalyst-free conditions. On the basis of control experiments and cyclic voltammetry data, a plausible reaction pathway is also presented.

Palladium-Catalyzed C3-Carbaldehyde Directed Regioselective C2-Thioarylation of Indoles

Palladium-catalyzed thioarylation of indoles by diaryl disulfides in the presence of phenyliododiacetate is reported. The directing potential of weakly coordinating aldehyde group present at 3-position of indole was exploited for regioselective C2-H thioarylation over the possible C4-H functionalization. Mechanistic studies reveal that the process involves initial generation of thioaryl radical followed by sequential C-H activation, thiolate transfer, and reductive elimination.

Continuous flow reactions in the preparation of active pharmaceutical ingredients and fine chemicals

Herein, we present an overview of continuous flow chemistry, including photoflow and electroflow technologies in the preparation of active pharmaceutical ingredients (APIs) and fine chemical intermediates. Examples highlighting the benefits and challenges associated with continuous flow processes, mainly involving continuous thermal, photo- and electrochemical transformations, are drawn from the relevant literature, especially our experience and collaborations in this area, with emphasis on the synthesis and prospective scale-up.

Electrochemical Diselenation of BODIPY Fluorophores for Bioimaging Applications and Sensitization of 1 O2

We report a rapid, efficient, and scope-extensive approach for the late-stage electrochemical diselenation of BODIPYs. Photophysical analyses reveal red-shifted absorption - corroborated by TD-DFT and DLPNO-STEOM-CCSD computations - and color-tunable emission with large Stokes shifts in the selenium-containing derivatives compared to their precursors. In addition, due to the presence of the heavy Se atoms, competitive ISC generates triplet states which sensitize 1 O2 and display phosphorescence in PMMA films at RT and in a frozen glass matrix at 77 K. Importantly, the selenium-containing BODIPYs demonstrate the ability to selectively stain lipid droplets, exhibiting distinct fluorescence in both green and red channels. This work highlights the potential of electrochemistry as an efficient method for synthesizing unique emission-tunable fluorophores with broad-ranging applications in bioimaging and related fields.

Mechanochemical Solvent-Free One-Pot Synthesis of Poly-Functionalized 5-(Arylselanyl)-1H-1,2,3-triazoles Through a Copper(I)-Catalyzed Click Reaction

A mechanochemistry-driven practical and efficient synthetic protocol for accessing diverse series of biologically relevant poly-functionalized 5-(arylselanyl)-1H-1,2,3-triazoles through copper(I)-catalyzed click reaction between aryl/heteroaryl acetylenes, diaryl diselenides, benzyl bromides, and sodium azide has been accomplished under high-speed ball-milling. Advantages of this method include operational simplicity, avoidance of using solvent and external heating, one-pot synthesis, short reaction time in minutes, good to excellent yields, broad substrate scope, and gram-scale applications. Furthermore, synthesized organoselenium compounds were synthetically diversified to biologically promising selenones.

Electro-oxidative Synthesis of Unsymmetrical Alkyne-1,4-diones via Sequential Ring Formation and Cleavage

Synthesis of unsymmetrical but-2-yne-1,4-diones is reported through oxidative alkyne translocation of readily accessible homopropargylic alcohols. The developed method consists of an unprecedented one-pot sequential electro-oxidative annulation-fragmentation-chemical selenoxide elimination process. Excellent functional group compatibility was observed, and an array of yne-1,4-diones were synthesized. Derivatization of the alkyne gave access to other valuable scaffolds. Detailed mechanistic studies through the isolation of key intermediates clarified the cascade transformation.

Electrochemical direct C-H mono and bis-chalcogenation of indolizine frameworks under oxidant-free conditions

Herein, we disclosed a sustainable electrochemical approach for site-selective C-H mono and bis-chalcogenation (sulfenylation or selenylation) of indolizine frameworks. Diversely functionalized disulfides and diselenides possessing EDGs and EWGs were successfully reacted with a variety of indolizines to directly access sulfenylated/selenylated indolizines in 40-96% yields. A mechanistic radical pathway was also validated with control experiments and cyclic voltammogram data.

Recent Advances in the Use of Diorganyl Diselenides as Versatile Catalysts

The importance of organoselenium compounds has been increasing in synthetic chemistry. These reagents are well-known as electrophiles and nucleophiles in many organic transformations, and in recent years, their functionality as catalysts has also been largely explored. The interest in organoselenium-based catalysts is due to their high efficacy, mild reaction conditions, strong functional compatibility, and great selectivity. Allied to organoselenium catalysts, the use of inorganic and organic oxidants that act by regenerating the catalytic species for the reaction pathway is common. Here, we provide a comprehensive review of the last five years of organic transformations promoted by diorganyl diselenide as a selenium-based catalyst. This report is divided into four sections: (1) cyclisation reactions, (2) addition reactions and oxidative functionalisation, (3) oxidation and reduction reactions, and (4) reactions involving phosphorus-containing starting materials.

Electrochemical Organoselenium Catalysis for the Selective Activation of Alkynes: Easy Access to Carbonyl-pyrroles/oxazoles from N-Propargyl Enamines/Amides

Intramolecular electro-oxidative addition of enamines or amides to nonactivated alkynes was attained to access carbonyl-pyrroles or -oxazoles from N-propargyl derivatives. Organoselenium was employed as the electrocatalyst, which played a crucial role as a π-Lewis acid and selectively activated the alkyne for the successful nucleophilic addition. The synthetic strategy permits a wide range of substrate scope up to 93% yield. Several mechanistic experiments, including the isolation of a selenium-incorporated intermediate adduct, enlighten the electrocatalytic pathway.

Electrochemical Regioselective C(sp2)-H Selenylation and Sulfenylation of Substituted 2-Amino-1,4-naphthoquinones

A straightforward and efficient electrochemical method for regioselective C(sp²)-H selenylation and sulfenylation of substituted 2-amino-1,4-naphthoquinones has been unearthed. This oxidative cross-coupling reaction avoids using transition metal catalysts, oxidants, and high temperatures. The other notable advantages of this protocol are the tolerance of diverse functional groups, mild reaction conditions at ambient temperature, energy efficiency, good to excellent yields, short reaction times (in minutes), gram-scale applicability, and eco-friendliness.

Silver-catalyzed direct selanylation of indoles: synthesis and mechanistic insights

Herein we describe the Ag(i)-catalyzed direct selanylation of indoles with diorganoyl diselenides. The reaction gave 3-selanylindoles with high regioselectivity and also allowed direct access to 2-selanylindoles when the C3 position of the indole ring was blocked via a process similar to Plancher rearrangement. Experimental analyses and density functional theory calculations were carried out in order to picture the reaction mechanism. Among the pathways considered (via concerted metalation-deprotonation, Ag(iii), radical, and electrophilic aromatic substitution), our findings support a classic electrophilic aromatic substitution via Lewis adducts between Ag(i) and diorganoyl diselenides. The results also afforded new insights into the interactions between Ag(i) and diorganoyl diselenides.

Electrochemical Organoselenium-Catalyzed Intermolecular Hydroazolylation of Alkenes with Low Catalyst Loadings

The organoselenium-catalyzed amination of alkenes is a promising way to construct functionalized amines. However, the use of chemical oxidants and the unavoidable formation of allylic amine or enamine are the two main limitations of these methodologies. Against this background, we herein report an electro-selenocatalytic regime for the hydroazolylation of alkenes with azoles under external oxidant-free conditions with low catalyst loadings. Moreover, this protocol enables the generation of amines without vinyl substituents.

Anodic dearomatization of 2-alkynylanilines for the synthesis of multi-functionalized indoles

An anodic oxidative dearomatization reaction of 2-alkynylanilines was developed. The formed dearomatized compounds were used as versatile precursors in the synthesis of a variety of benzenoid ring multi-functionalized indoles through simple conversions.

Versatile Electrochemical Synthesis of Selenylbenzo[b]Furan Derivatives Through the Cyclization of 2-Alkynylphenols

We report an electrochemical oxidative intramolecular cyclization reaction between 2-alkynylphenol derivatives and different diselenides species to generate a wide variety of substituted-benzo[b]furans. Driven by the galvanostatic electrolysis assembled in an undivided cell, it provided efficient transformation into oxidant-, base-, and metal-free conditions in an open system at room temperature. With satisfactory functional group compatibility, the products were obtained in good to excellent yields.

Electrochemical thiocyanation of barbituric acids

The electrochemical thiocyanation of barbituric acids with NH₄SCN was disclosed in an undivided cell under constant current conditions. The electrosynthesis is the most efficient at a record high current density (j_anode ≈50-70 mA cm^-2). NH₄SCN has a dual role as the source of the SCN group and as the electrolyte. Electrochemical thiocyanation of barbituric acids starts with the generation of (SCN)₂ from the thiocyanate anion. The addition of thiocyanogen to the double bond of the enol tautomer of barbituric acid gives thiocyanated barbituric acid. A variety of thiocyanated barbituric acids bearing different functional groups were obtained in 18-95% yields and were shown to exhibit promising antifungal activity.

Direct Synthesis of α-Sulfenylated Ketones under Electrochemical Conditions

We investigated the electrochemical sulfenylation reaction in both batch and continuous flow regimes, involving thiophenols/thiols and enol-acetates to yield α-sulfenylated ketones, without using additional oxidants or catalysts. Studies with different electrolytes were also performed, revealing that quaternary ammonium salts are the best mediators for this reaction. Notably, during the study of the reaction scope, a Boc-cysteine proved to be extremely tolerant to our protocol, thus increasing its relevance. The methodology also proved to be scalable in both batch and continuous flow conditions, opening up possibilities for further studies since these relevant functional groups are important moieties in organic synthesis.

Progress in the Electrochemical Reactions of Sulfonyl Compounds

Electrosynthesis has recently attracted more and more attention due to its great potential to replace chemical oxidants or reductants in molecule-electrode electron transfer. Sulfonyl compounds such as sulfonyl hydrazides, sulfinic acids (and their salts), sulfonyl halides have been discovered as practical precursors of several radicals. As electrochemical redox reactions can provide green and efficient pathways for the activation of sulfonyl compounds, studies for electrosynthesis have rapidly increased. Several types of radicals can be generated from anodic oxidation or cathodic reduction of sulfonyl compounds and can initiate fluoroalkylation, benzenesulfonylation, cyclization or rearrangement. In this Review, we summarize the electrosynthesis developments involving sulfonyl compounds mainly in the last decade.

"Green Is the Color": An Update on Ecofriendly Aspects of Organoselenium Chemistry

Organoselenium compounds have been successfully applied in biological, medicinal and material sciences, as well as a powerful tool for modern organic synthesis, attracting the attention of the scientific community. This great success is mainly due to the breaking of paradigm demonstrated by innumerous works, that the selenium compounds were toxic and would have a potential impact on the environment. In this update review, we highlight the relevance of these compounds in several fields of research as well as the possibility to synthesize them through more environmentally sustainable methodologies, involving catalytic processes, flow chemistry, electrosynthesis, as well as by the use of alternative energy sources, including mechanochemical, photochemistry, sonochemical and microwave irradiation.

The Synthesis of α-Keto Acetals from Terminal Alkynes and Alcohols via Synergistic Interaction of Organoselenium Catalysis and Electrochemical Oxidation

Herein, an unprecedented electrochemical approach for the synthesis of α-keto acetals has been established from readily available terminal alkynes and alcohols. By merging the electrochemical and organoselenium-catalyzed processes, the desired products are obtained at room temperature in the absence of basic or metallic additives, with carbonyl and acetal motifs incorporated simultaneously across the triple bonds in a single operation.

Electrochemical Synthesis of Sulfonyl Fluorides from Sulfonyl Hydrazides

The synthesis of sulfonyl fluorides via the reaction of sulfonyl hydrazides and Et3N3HF under electrochemical conditions is reported. Various sulfonyl fluorides were obtained in good yields under a constant current...

Strategies for the direct oxidative esterification of thiols with alcohols

This review paper provides an overview of the main strategies for the oxidative esterification of thiols with alcohols. The review is divided into two major parts according to final products. The first includes the methods for the synthesis of sulfinic esters, while the second contains the procedures for the fabrication of sulfonic ester derivatives.

This review paper provides an overview of the main strategies for the oxidative esterification of thiols with alcohols.

Electrochemical oxidative cyclization of alkenes, boronic acids, and dichalcogenides to access chalcogenated boronic esters and 1,3-diols

A sustainable, environmentally benign electrochemical oxidative three-component cyclization of allylic alcohols, boronic acids, and dichalcogenides under metal-free and oxidant-free conditions has been developed.

Synthesis of Polysubstituted Furans through Electrochemical Selenocyclization of Homopropargylic Alcohols

The current method represents an electrochemically driven synthetic route to access polysubstituted selenofuran derivatives through the diselenide-promoted cyclization of homopropargyl alcohols. The tandem electro-oxidative transformation operates at ambient temperature and in the absence of an external oxidant. This mild and efficient methodology exhibits good functional group compatibility, providing a broad range of substrate scopes up to 84% isolated yield. Further conversion of the seleno-functionality afforded other valuable furan derivatives.

Electrochemical Radical Selenylation of Alkenes and Arenes via Se-Se Bond Activation

A novel electrochemical radical selenylation of alkenes and activated arenes without external oxidants is reported. The diselenide was fully transformed into Se-centered radicals through electrochemical Se-Se bond activation. Three-component radical carbonselenation was successfully realized using styrenes to trap the RSe radical. Besides, the direct coupling of RSe radicals with activated arenes was further developed. Using this atom-economic protocol, diversity of unsymmetric aryl-aryl, aryl-alkyl, and alkyl-alkyl selenoethers was obtained regioselectively, which has potential application in biological chemistry.

Electrochemical sulfonylation of enamides with sodium sulfinates to access β-amidovinyl sulfones

The electrochemical sulfonylation of enamides with sodium sulfinates was developed in an undivided cell in constant current mode, leading to the formation of β-amidovinyl sulfones in moderate to good yields. The catalyst-, electrolyte- and oxidant-free protocol features good functional group tolerance and employs electric current as a green oxidant. Mechanistic insights into the reaction indicate that the reaction may proceed via a radical mechanism.

Highly Diastereoselective Synthesis of Dihydro-benzoimidazo-[1,3]-thiazines via Electro-oxidative Selenocyclization of Thioallyl Benzoimidazoles

The current methodology reveals a green and proficient electro-oxidative tandem selenocyclization of thioallyl benzoimidazoles manufacturing selenylated dihydro-benzoimidazo-thiazine derivatives. Both C-Se and C-N bond formation were achieved via this mild protocol which exhibits good functional group tolerability affording an extensive range of substrate scope up to 96% isolated yields. Complete control over the regioselective formation of the six-membered heterocycle and stereoselective construction of the contiguous stereocenters was established. The practical electrochemical method operates in an undivided cell at ambient temperature without using any metal and external chemical oxidant.

Electrochemical synthesis of selenyl-dihydrofurans via anodic selenofunctionalization of allyl-naphthol/phenol derivatives and their anti-Alzheimer activity

Herein, we report an eco-friendly, electrosynthetic approach for the intramolecular oxyselenylation of allyl-naphthol/phenol derivatives. This reaction proceeds with 0.2 equiv. of nBu4NClO4 as an electrolyte and Pt working electrodes in an undivided cell, resulting in the selenyl-dihydrofurans in good to excellent yields. Furthermore, several of the synthesized products presented a high percentage of acetylcholinesterase (AChE) inhibition, highlighting their potential anti-Alzheimer activity.

Electro-Oxidative Coupling Reactions Leading to π-Conjugated Compounds

Electrochemical reactions are rapidly gaining attention today as a powerful and environmentally benign reaction processes for organic synthesis. We found that the electro-oxidation of palladium acetate afforded cationic palladium species and thus-generated cationic Pd species were efficient mediators for electro-oxidative coupling reactions. Homo-coupling of arylboronic acids and terminal alkynes proceeded efficiently to afford biaryls and butadiyne, respectively. Cross-coupling reactions between terminal alkynes and arylboronic acids were also achieved with the use of a Ag anode. As an advantage of electrochemical reactions, we developed a sequential reaction system switched between oxidative and neutral conditions by the on/off application of electricity, and several π-extended butadiynes were obtained in one-sequence by the system. Electrochemical intramolecular C-S coupling for the synthesis of thienoacene was also developed. The use of Bu₄ NBr as a halogen mediator was essential for the reaction.

Electrohalogenation of organic compounds

In this review we target sp, sp² and sp³ carbon fluorination, chlorination, bromination and iodination reactions using electrolysis as a redox medium. Mechanistic insights and substrate reactivity are also discussed.

Organochalcogen ligands in catalysis of oxidation of alcohols and transfer hydrogenation

Organochalcogen compounds have been used as the building blocks for the development of a variety of catalysts that have been studied comprehensively during the last two decades for several chemical transformations. Transfer hydrogenation (reduction of carbonyl compounds to alcohols) and oxidation of alcohols (conversion of alcohols to their respective ketones and aldehydes) are also among such chemical transformations. Some compilations are available in the literature on the development of catalysts, based on organochalcogen ligands, and their applications in Heck reaction, Suzuki reaction, and other related aspects. Some review articles have also been published on different aspects of oxidation of alcohols and transfer hydrogenation. However, no such article is available in the literature on the syntheses and use of organochalcogen ligated catalysts for these two reactions. In this perspective, a survey of developments pertaining to the synthetic aspects of such organochalcogen (S/Se/Te) based catalysts for the two reactions has been made. In addition to covering the syntheses of chalcogen ligands, their metal complexes and nanoparticles (NPs), emphasis has also been placed on the efficient conversion of different substrates during catalytic reactions, diversity in catalytic potential and mechanistic aspects of catalysis. It also includes the analysis of comparison (in terms of efficiency) between this unique class of catalysts and efficient catalysts without a chalcogen donor. The future scope of this area has also been highlighted.

Recent advances in tandem selenocyclization and tellurocyclization with alkenes and alkynes

This review highlights recent progress in tandem selenocyclization and tellurocyclization with alkenes and alkynes, with an emphasis on the scopes, limitations and mechanisms of these different reactions.