Recent advances in organic synthesis applying elemental selenium

Assembly of Selenadiazine Scaffolds via Rh(III)-Catalyzed Amidine-Directed Cascade C-H Selenylation/[5 + 1] Annulation with Elemental Selenium

By employing elemental selenium as the selenium source, we have realized the amidine-directed Rh(III)-catalyzed cascade C-H selenylation/[5 + 1] annulation for the direct construction of structurally novel selenadiazine, benzoselenadiazine, and benzoselenazol-3-amine frameworks with specific site selectivity and good functional group tolerance. Besides, the obtained products can serve as fundamental platforms for subsequent chemical transformations, and thus, the feasible SeNEx reaction, SeNEx/Michael addition, and simple conversion of the selenadiazine product into diverse other organoselenium molecules were demonstrated accordingly. Taken together, the developed methodology efficiently expands the chemical space of organoselenium species.

A general approach to construct selenopheno[3,2-b]indole-cored molecules using Fischer indolization

A wide series of various selenopheno[3,2-b]indole-based compounds, including 2-aryl-substituted selenopheno[3,2-b]indoles as well as derivatives of benzo[4,5]selenopheno[3,2-b]indole, pyrido[3',2':4,5]selenopheno[3,2-b]indole, pyrazino[2',3':4,5]selenopheno[3,2-b]indole and chromeno[3',4':4,5]selenopheno[3,2-b]indol-6-one, were prepared from the appropriate 3-aminoselenophen-2-carboxylates via a one-pot two-step procedure based on the Fischer indole synthesis. The present synthetic strategy includes the conversion of 3-aminoselenophen-2-carboxylates into 2-unsubstituted 3-aminoselenophenes, their C-2 protonation to form selenophen-3(2H)-iminium cations, and the reaction of these iminium intermediates with arylhydrazines to obtain arylhydrazones of selenophen-3(2H)-ones followed by their Fischer indolization affording selenopheno[3,2-b]indole molecules.

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.

Copper-Catalyzed Tandem Cyclization/Chalcogenation with Elemental S8/Se: Concise Synthesis of 3-(β-Hydroxychalcogen)benzofurans

A novel copper-catalyzed cyclization/chalcogenation of o-alkynylphenols with epoxides and elemental S8/Se was developed for the synthesis of a 3-chalcogen-benzofuran architecture in a domino process with no intermediate isolation or purification. Various sensitive functional groups were compatible at room temperature and furnished chalcogenation derivatives in moderate to good yields.

Electrochemical Multicomponent Cascade Reaction for the Synthesis of Selenazol-2-amines with Elemental Selenium

The first example of an electrochemical multicomponent synthesis of selenium-containing compounds with inexpensive and abundant elemental selenium as the selenating reagent was developed. A variety of selenazol-2-amines were constructed in high yields with good functional group tolerance under metal-free and chemical oxidant-free conditions.

Sodium Carbonate-Promoted Formation of 5-Amino-1,2,4-thiadiazoles and 5-Amino-1,2,4-selenadiazoles with Elemental Sulfur and Selenium

Sodium carbonate-promoted facile synthesis of 5-amino-1,2,4-thiadiazoles and 5-amino-1,2,4-selenadiazoles with elemental sulfur and selenium, respectively, was developed. This method was carried out with O2 in the air as the green oxidant, and it has several advantages, including low cost, low toxicity, and stable sulfur and selenium sources, good to excellent yields with water as the sole byproduct, simple operation, and a broad substrate scope. Preliminary mechanistic studies indicate that the formation of the 1,2,4-thiadiazole ring and the 1,2,4-selenadiazole ring undergoes different processes.

Synthesis of Benzoselenophenes via TMSCN-Enabled Radical-Mediated Tandem Reaction Involving Enamides and Elemental Selenium

This study presents a TMSCN-enabled tandem reaction involving enamides and elemental selenium to access a diverse array of benzoselenophenes. Notably, this methodology involves the direct 2-fold C(sp2)-H bond activation without the need for preinstalled halides or boronic acids as reaction handles. The protocol offers several noteworthy features, including the absence of transition metals and strong oxidants, high reaction efficiency, broad substrate scopes, and the use of stable elemental selenium as a selenium source.

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.

Synthesis of novel alkynyl imidazopyridinyl selenides: copper-catalyzed tandem selenation of selenium with 2-arylimidazo[1,2-a]pyridines and terminal alkynes

Alkynyl selenides have attracted considerable research interest recently, owing to their applications in the biological and pharmaceutical fields. The Cu-catalyzed tandem reaction for the synthesis of novel alkynyl imidazopyridinyl selenides is presented. A one-pot synthesis route afforded alkynyl imidazopyridinyl selenides in moderate to good yields. This was achieved by a two-step reaction between terminal alkynes and diimidazopyridinyl diselenides, generated from imidazo[1,2-a]pyridines and Se powder, using 10 mol % of CuI and 1,10-phenanthroline as the catalytic system under aerobic conditions. The C(sp²)–Se and C(sp)–Se bond-formation reaction can be performed in one-pot by using inexpensive and easy to handle Se powder as the Se source. The reaction proceeded with terminal alkynes having various substitutions, such as aryl, vinyl, and alkyl groups. The obtained alkynyl imidazopyridinyl selenide was found to undergo nucleophilic substitution reaction on Se atom using organolithium reagents and 1,3-dipolar azide–alkyne cycloaddition based on the alkyne moiety.