Iodine/Oxone® oxidative system for the synthesis of selenylindoles bearing a benzenesulfonamide moiety as carbonic anhydrase I, II, IX, and XII inhibitors

A wide range of 3-selenylindoles were synthesized via an eco-friendly approach that uses Oxone® as the oxidant in the presence of a catalytic amount of iodine. This mild and economical protocol showed broad functional group tolerance and operational simplicity. A series of novel selenylindoles bearing a benzenesulfonamide moiety were also synthesized and evaluated as carbonic anhydrase inhibitors of the human (h) isoforms hCa I, II, IX, and XII, which are involved in pathologies such as glaucoma and cancer. Several derivatives showed excellent inhibitory activity towards these isoforms in the nanomolar range, lower than that shown by acetazolamide.

Visible-light induced selenocyclization of 2-ethynylanilines under ambient conditions: simple FeBr3 as a dual-functional catalyst

We report herein a visible-light induced, Fe-catalyzed selenocyclization of 2-ethynylanilines with diselenides under ambient conditions, employing ethyl acetate as a benign solvent with no stoichiometric additive required. The simple iron salt FeBr3 serves as both a photo-induced LMCT (Ligand-to-Metal Charge Transfer) catalyst and a Lewis acid catalyst to promote the desired transformation in a sustainable manner, enabling the facile synthesis of diverse 3-selenylindoles with extended substitution patterns. Moreover, gram-scale reactions and late-stage functionalization of bioactive molecules further highlight the synthetic practicality of this method.

Bismuth(III)-Catalyzed Regioselective Selenation of Indoles with Diaryl Diselenides: Synthesis of 3-Selanylindoles

Heterocyclic aryl selenides have recently attracted considerable research interest owing to their applications in biological and pharmaceutical fields. Herein, we describe a simple and general synthesis of 3-selanylindoles via a novel regioselective C-H selenation of indoles using a bismuth reagent as a catalyst. The reactions of indoles with diselenides in the presence of 10 mol% BiI3 at 100 °C in DMF afforded the corresponding 3-selanylindoles in moderate-to-excellent yields. The reaction proceeded efficiently under aerobic conditions by adding only a catalytic amount of BiI3, which was non-hygroscopic and less toxic, and both selanyl groups of the diselenide were transferred to the desired products.

Reaction Pattern and Mechanistic Aspects of Iodine and Iodine-Based Reagents in Selenylation of Aliphatic, Aromatic, and (Hetero)Cyclic Systems

Organoselenium compounds have been the subject of extensive research since the discovery of the biologically active compound ebselen. Ebselen has recently been found to show activity against the main protease of the virus responsible for COVID-19. Other organoselenium compounds are also well-known for their diverse biological activities, with such compounds exhibiting interesting physical properties relevant to the fields of electronics, materials, and polymer chemistry. In addition, the incorporation of selenium into various organic molecules has garnered significant attention due to the potential of selenium to enhance the biological activity of these molecules, particularly in conjunction with bioactive heterocycles. Iodine and iodine-based reagents play a prominent role in the synthesis of organoselenium compounds, being valued for their cost-effectiveness, non-toxicity, and ease of handling. These reagents efficiently selenylate a broad range of organic substrates, encompassing alkenes, alkynes, and cyclic, aromatic, and heterocyclic molecules. They serve as catalysts, additives, inducers, and oxidizing agents, facilitating the introduction of different functional groups at alternate positions in the molecules, thereby allowing for regioselective and stereoselective approaches. Specific iodine reagents and their combinations can be tailored to follow the desired reaction pathways. Here, we present a comprehensive review of the progress in the selenylation of organic molecules using iodine reagents over the past decade, with a focus on reaction patterns, solvent effects, heating, microwave, and ultrasonic conditions. Detailed discussions on mechanistic aspects, such as electrophilic, nucleophilic, radical, electrochemical, and ring expansion reactions via selenylation, multiselenylation, and difunctionalization, are included. The review also highlights the formation of various cyclic, heterocyclic, and heteroarenes resulting from the in situ generation of selenium intermediates, encompassing cyclic ketones, cyclic ethers, cyclic lactones, selenophenes, chromones, pyrazolines, pyrrolidines, piperidines, indolines, oxazolines, isooxazolines, lactones, dihydrofurans, and isoxazolidines. To enhance the reader's interest, the review is structured into different sections covering the selenylation of aliphatic sp2/sp carbon and cyclic sp2 carbon, and then is further subdivided into various heterocyclic molecules.

Electrochemical selenocyclization of 2-ethynylanilines with diselenides: facile and efficient access to 3-selenylindoles

An efficient electrochemical selenocyclization strategy for the synthesis of 3-selenylindoles from 2-ethynylanilines and diselenides has been developed in simple tube- or beaker-type undivided cells under ambient conditions. Notably, these sustainable transformations are completed within a short time with low equivalents of charges, diselenides and electrolytes, exhibiting a broad substrate scope with excellent functional group compatibility. Moreover, a gram-scale electrosynthesis and late-stage functionalization of complex molecules further demonstrate the practical synthetic potential of this facile electrochemical system.

Regioselective intramolecular cyclization of o-alkynyl arylamines with the in situ formation of ArXCl to construct poly-functionalized 3-selenylindoles

In this article, a practical and metal-free method for the synthesis of poly-functionalized 3-selenyl/sulfenyl/telluriumindoles from o-alkynyl arylamines has been achieved. In this protocol, the in situ formation of selenenyl chloride, sulfenyl chloride or tellurenyl chloride is considered as the key intermediate and the 3-selenyl/sulfenyl/telluriumindoles can be obtained in good to excellent yields. Furthermore, the product 2-phenyl-3-(phenylselanyl)-1-tosyl-1H-indole can be selectively oxidized to compounds 2-phenyl-3-(phenylseleninyl)-1-tosyl-1H-indole and 2-phenyl-3-(phenylselenonyl)-1-tosyl-1H-indole in good yields.

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.

Electrochemical organic reactions: A tutorial review

Although the core of electrochemistry involves simple oxidation and reduction reactions, it can be complicated in real electrochemical organic reactions. The principles used in electrochemical reactions have been derived using physical organic chemistry, which drives other organic/inorganic reactions. This review mainly comprises two themes: the first discusses the factors that help optimize an electrochemical reaction, including electrodes, supporting electrolytes, and electrochemical cell design, and the second outlines studies conducted in the field over a period of 10 years. Electrochemical reactions can be used as a versatile tool for synthetically important reactions by modifying the constant electrolysis current.

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.

Iodine pentoxide-mediated oxidative selenation and seleno/thiocyanation of electron-rich arenes

A simple and efficient method for the regioselective selenation of electron-rich arenes by employing non-metal inorganic iodine pentoxide (I₂O₅) as a reaction promoter under ambient conditions has been developed. The present protocol showed broad functional group tolerance and easy-to-operate and time-economical features. Additionally, this protocol also allows access to 3-seleno and 3-thiocyanoindoles by the use of readily available selenocyanate and thiocyanate salts. A mechanistic study indicated that the transformation operated through selenenyl iodide-induced electrophilic substitution processes.

Synthesis of 1,3-diselenyl-dihydroisobenzofurans via electrochemical radical selenylation with substituted o-divinylbenzenes and diselenides

An efficient electrochemical method for the direct synthesis of complicated 1,3-diselenyl-dihydroisobenzofurans was developed under external oxidant free conditions at room temperature from substituted o-divinylbenzenes and diselenides. A radical mechanism is proposed for this novel and practical transformation.

Visible-Light-Induced, Graphene Oxide-Promoted C3-Chalcogenylation of Indoles Strategy under Transition-Metal-Free Conditions

An efficient and general method for the synthesis of 3-sulfenylindoles and 3-selenylindoles employing visible-light irradiation with graphene oxide as a promoter at room temperature has been achieved. The reaction features are high yields, simple operation, metal-free and iodine-free conditions, an easy-to-handle oxidant, and gram-scalable synthesis. This simple protocol allows one to access a wide range of 3-arylthioindoles, 3-arylselenylindoles, and even 3-thiocyanatoindoles with good to excellent yields.

Regio- and stereoselective electrochemical selenoalkylation of alkynes with 1,3-dicarbonyl compounds and diselenides

A regio- and stereoselective electrochemical approach for the selenoalkylation of alkynes with 1,3-dicarbonyl compounds and diselenides has been developed.

Electrochemical oxidative selenocyclization of olefinic amides towards the synthesis of iminoisobenzofurans

We introduced an electrochemical oxidative radical cascade cyclization of olefinic amides and diselenides without a transition-metal catalyst and external oxidant. This selenocyclization reaction provided a facile method to construct C–Se and C–O bonds in one step.

Tunable Electrocatalytic Annulations of o-Arylalkynylanilines: Green and Switchable Syntheses of Skeletally Diverse Indoles

Tunable electrocatalytic annulation reactions of o-arylalkynylanilines have been established, leading to green and divergent syntheses of skeletally diverse indoles by adjusting the electrolytes and the solvents. The presence of ammonium halides as the electrolytes enabled the halogenation of o-arylalkynylanilines to give C3-halogenated indoles whereas naphtho[1',2':4,5]furo[3,2-b]indoles could be obtained by changing the electrolyte from ammonium halides to KI. Interestingly, by combining acetone as the solvent and both NH₄I and NH₄Cl as the electrolytes, the reaction worked through an intramolecular annulation and [5 + 1] cyclization cascade to form naphtho[1',2':5,6][1,3]oxazino[3,4-a]indoles.

Diorganyl diselenides: a powerful tool for the construction of selenium containing scaffolds

Organoselenium compounds find versatile applications in organic synthesis, materials synthesis, and ligand chemistry. Organoselenium heterocycles are widely studied agents with diverse applications in various biological processes. This review highlights the recent progress in the synthesis of selenium heterocycles using diorganyl diselenides with keen attention on green synthetic approaches, scopes, C-H selanylation, the mechanisms of different reactions and insights into the formation of metal complexes. The C-H selanylation using diorganyl diselenides with different catalysts, bases, transition metals, iodine salts, NIS, hypervalent iodine, and other reagents is summarised. Finally, the diverse binding modes of bis(2/4-pyridyl)diselenide with different metal complexes are also summarised.

Synthesis of 3-nitroindoles by sequential paired electrolysis

3-Nitroindoles are synthetically versatile intermediates but current methods for the preparation hinder their widespread application. Herein, we report that nitroenamines undergo electrochemical cyclisation to 3-nitroindoles in the presence of potassium iodide. Detailed control experiments and cyclic voltammogram studies infer the reaction proceeds via a sequential paired electrolysis process, beginning with anodic oxidation of iodide (I^-) to the iodine radical (I˙), which facilitates cyclisation of the nitroenamine to give a 3-nitroindolinyl radical. Cathodic reduction and protonation generates a 3-nitroindoline that upon oxidation forms the 3-nitroindole.

Transition-Metal-Free HFIP-Mediated Organo Chalcogenylation of Arenes/Indoles with Thio-/Selenocyanates

We have developed a protocol for the synthesis of diaryl thio-/selenoethers by the reaction of aryl chalcogenocyanates with electron rich arenes/hetero arenes via HFIP promoted C-H activation. The reaction produces chalcogenides in good to excellent yields under mild conditions without the need of a transition metal as a catalyst. The HFIP-mediated reactions tolerated a wide range of functional groups and set the stage for the synthesis of diversely decorated chalcogenides. A mechanism involving activation of the C-H bond through hydrogen bonding is proposed.

Meyer-Schuster-Type Rearrangement of Propargylic Alcohols into α-Selenoenals and -enones with Diselenides

We describe a mild and broadly applicable protocol for the preparation of a diverse array of multisubstituted α-selenoenals and -enones from readily accessible propargylic alcohols and diselenides. The transformation proceeds via the Selectfluor-promoted selenirenium pathway, which enables selenenylation/rearrangement of a variety of propargylic alcohols. Gram-scale experiments showed the potential of this synergistic protocol for practical application.

Synthetic strategies for aryl/heterocyclic selenides and tellurides under transition-metal-catalyst free conditions

Aryl and heteroaryl selenides and tellurides are found to have broad applications in the diverse fields such as medicine, biology, materials science, pharmaceutical etc. and thus their synthesis remains a challenging field for synthetic chemists in last decade. Although a large no of methodologies have been developed based on metal catalyzed C-Se/Te coupling, a large number of researches has been focused on developing metal catalyst free protocols due to their sustainability in recent times. This review covers all the recent developments in last decade on their synthesis under metal catalyst free conditions by using different sustainable techniques e.g. greener reagents and solvents, ball milling, visible light photocatalysis, microwave, ultrasound etc.

Electrochemically facilitated oxidative C–H amination enables access to fluorescent N9-fused tricyclic xanthines

An electrochemically enabled intramolecular C−H amination route for accessing a broad range of fluorescent N9-fused tricyclic xanthines with various substitution patterns under simple, green, and mild condition is developed.

Synthesis of 3-Selenylindoles through Organoselenium-Promoted Selenocyclization of 2-Vinylaniline

A novel metal-free one-pot protocol for the synthesis of potential biologically active molecules 3-selenylindoles via intramolecular cyclization/selenylation with simple 2-vinylaniline has been developed with moderate to good yield, thus representing it as a facile route to diverse substitution patterns around the indole core. The reaction proceeded smoothly with a broad substrate scope and excellent functional group tolerance. Moreover, the present synthetic route could be readily scaled up to gram quantity without difficulty. Mechanistic studies have revealed that in situ formed selenium electrophile species may be the key intermediate for the selenocyclization process.

Accelerating Electrochemical Synthesis through Automated Flow: Efficient Synthesis of Chalcogenophosphites

Integrated electrochemical reactors in automated flow systems have been utilised for chalcogenophosphite formations. Multiple electrochemical reactions can be performed using a programmed sequence in a fully autonomous way. Differently functionalised chalcogenophosphites have been efficiently synthesised in short reaction times.

Three-Component Reactions of Alkynone o-Methyloximes, Element Selenium, and Boronic Acids Leading to 4-Organoselenylisoxazoles

We report an Ag-catalyzed one-pot three-component reaction of alkynone o-methyloxime, element selenium, and boronic acid, providing a facile route to selenated isoxazole product. This protocol features high efficiency, wide substrate scope, and the use of selenium powder as the selenium source.

Electrochemical regioselective selenylation/oxidation of N-alkylisoquinolinium salts via double C(sp2)–H bond functionalization

An efficient and environmentally friendly electrochemical regioselective selenylation/oxidation of N-alkylisoquinolinium salts via double C(sp²)–H bond functionalization has been developed.

The direct C3 chalcogenylation of indolines using a graphene-oxide-promoted and visible-light-induced synergistic effect

A green methodology for the construction of carbon–chalcogen (S and Se) bonds via a GO-promoted and metal-free light-induced synergistic effect is demonstrated.

Metal-free synthesis of 3-chalcogenyl chromones from alkynyl aryl ketones and diorganyl diselenides/disulfides mediated by PIFA

3-Selenyl/sulfenyl chromones/thiochromones were conveniently synthesized from the PIFA-mediated reactions between alkynyl aryl ketones bearing an ortho-methoxy/methylthio group and diorganyl diselenides/disulfides.

Arylseleninic acid as a green, bench-stable selenylating agent: synthesis of selanylanilines and 3-selanylindoles

C–Se bonds in electron-rich arenes are easily formed by the reaction of bench-stable arylseleninic acids as an electrophilic selenium source. The only waste in the reaction is water.

Electrochemical diselenylation of indolizines via intermolecular C-Se formation with 2-methylpyridines, α-bromoketones and diselenides

An efficient electrochemical system for the construction of diselenytlated indolizines from available pyridines, ketones and diselenides under undivided electrolytic conditions was developed. No external oxidants and transition-metal catalysts are needed for achieving this three-component tandem reaction realizing C-C, C-N and C-Se bond formations.

Regioselective deoxygenative chalcogenation of 7-azindole N-oxides promoted by I2/PEG-200

We developed a general and sustainable approach for the regioselective deoxygenative chalcogenation of 7-azindole N-oxides; the combination of an internal oxidant and a green solvent has been used successfully for the synthesis of mono- and dichalcogenyl 7-azaindoles which are of pharmaceutical interest. The regioselectivity is tunable by the variation of the reaction conditions. I2/PEG was established as an efficient and reusable catalytic system for C-H chalcogenation. This developed methodology has great potential for practical utility, with a broad substrate scope, green reaction conditions, and operational simplicity.

Electrochemical trifluoromethylation/semipinacol rearrangement sequences of alkenyl alcohols: synthesis of β-CF3-substituted ketones

Electrochemical oxidative radical trifluoromethylation/semipinacol rearrangement sequences of alkenyl alcohols were developed in this study. This approach is environmentally benign and uses the shelf-stable Langlois reagent as a trifluoromethyl radical precursor and electrons as the oxidizing reagents. The present protocol offers a facile route to prepare β-trifluoromethylated ketone derivatives.