Selenium-π-Acid Catalyzed Oxidative Functionalization of Alkynes: Facile Access to Ynones and Multisubstituted Oxazoles

Organoselenium-Catalyzed C2,3-Diarylation of N-H Indoles

An organoselenium-catalyzed C2,3-diarylation of unprotected N-H indoles with electron-rich aromatics has been developed. This one-pot multicomponent tandem cross-dehydrogenation coupling reaction allows for the incorporation of two different aromatic groups to indoles. More importantly, this approach offers significant advantages, including a high atom and step economy, eliminating the need for prepreparation of the reaction substrates, streamlining the synthetic process and enhancing its practicality. Overall, this organoselenium-catalyzed C2,3-diarylation reaction presents an efficient and versatile strategy for the functionalization of indole derivatives.

Regioselective Radical Cascade Cyclizations of Alkyne-Tethered Cyclohexadienones with Chalcogenides under Visible-Light Catalysis

Herein, we demonstrated a silver/K2S2O8-mediated highly regio- and diastereoselective 6/5-exo trig radical cascade cyclization of alkyne-tethered cyclohexadienones with sulfonyl hydrazides or sodium sulfinates and subsequent selenation to access 6,6-dihydrochromenone and 6,5-fused tetrahydro benzofuranone derivatives. This reaction protocol features high functional group compatibility and has a wide substrate scope providing a variety of dihydrochromenones and tetrahydro benzofuranone derivatives in good to excellent yields. The reaction proceeds via the attack of a sulfonyl radical to alkyne over the activated Michael acceptor. The TEMPO quenching experiment implies the presence of a radical intermediate. Further synthetic versatility of 6,6- and 5,6-fused derivatives is also showcased.

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.

Photocatalytic Benzylic C-H Oxidation/Cyclization of Enaminones to the Synthesis of Polysubstituted Oxazoles

Photocatalytic benzylic C-H oxidation/cyclization of enaminones was efficiently achieved to afford oxazole derivatives under mild conditions. The oxygen in the oxazole ring originated from green oxidant molecular oxygen. The synthetic protocol features broad substrate scopes and good functional group tolerance. The combined experimental and theoretical studies reveal that in suit benzylic C-H oxidation/cyclization is involved in the reaction transformations.

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.

Synthesis of Phosphorus(V)-Substituted Six-Membered N-Heterocycles: Recent Progress and Challenges

Heterocycles functionalized with pentavalent phosphorus are of great importance since they include a great variety of biologically active compounds and pharmaceuticals, advanced materials, and valuable reactive intermediates for organic synthesis. Significant progress in synthesis of P(O)R2-substituted six-membered heterocycles has been made in the past decade. This review covers the synthetic strategies towards aromatic monocyclic six-membered N-heterocycles, such as pyridines, pyridazines, pyrimidines, and pyrazines bearing phosphonates and phosphine oxides, which were reported from 2012 to 2022.

Regiodivergent Metal-Controlled Synthesis of Multiply Substituted Quinolin-2-yl- and Quinolin-3-ylphosphonates

In this study, we developed a selective method for synthesis of multi-substituted quinoline-2-ylphosphonates and quinoline-3-ylphosphonates by copper- or gold-catalyzed reactions of phosphoryl-substituted conjugated ynones with 2'-amino-2,2,2-trifluoroacetophenones. The approach proposed makes it possible to obtain various substituted quinolines in good yields. It is also shown that (4,4,4-trifluoro-3-oxobut-1-yn-1-yl)phosphonate reacts with 2-aminoaryl ketones under non-catalytic conditions with formation of 4-substituted quinoline-2-ylphosphonates in high yields.

Electrochemical Selenium-Catalyzed N,O-Difunctionalization of Ynamides: Access to Polysubstituted Oxazoles

A green and efficient approach for the difunctionalization of ynamides by merging the electrochemical and organoselenium-catalyzed processes is described. This strategy features mild reaction conditions, broad functional group tolerance and high atom-economy, and requires no external chemical oxidant. Hence, we provide a sustainable alternative for the synthesis of polysubstituted oxazoles.

Oxidative [3+2]Cycloaddition of Alkynylphosphonates with Heterocyclic N-Imines: Synthesis of Pyrazolo[1,5-a]Pyridine-3-phosphonates

A series of pyrazolo[1,5-a]pyridine-3-ylphosphonates were prepared with moderate to good yields by the oxidative [3+2]cycloaddition of 2-subtituted ethynylphosphonates with in situ generated pyridinium-N-imines and their annulated analogs. 2-Aliphatic and 2-Ph acetylenes demonstrate low activity, and the corresponding pyrazolopyridines were achieved with a moderate yield in the presence of 10 mol% Fe(NO₃)₃·9H₂O. At the same time, tetraethyl ethynylbisphosphonate, diethyl 2-TMS- and 2-OPh-ethynylphosphonates possess much greater reactivity and the corresponding pyrazolo[1,5-a]pyridines, and their annulated derivatives were obtained with good to excellent yields without any catalyst. 2-Halogenated ethynylphosphonates also readily reacted with pyridinium-N-imines, forming complex mixtures containing poor amounts of 2-halogenated pyrazolopyridines.

Electrochemical selenium-π-acid promoted hydration of alkynyl phosphonates

An unprecedented electrochemical selenium-π-acid promoted hydration of internal alkynes bearing a phosphonate auxiliary was described. Thus, valuable (hetero)aryl and alkyl ketones could be accessed under mild, metal- and external oxidant-free conditions. This protocol features high atom-economy, good chemo- and regio-selectivity, excellent functional group tolerance and easily transformable products. Control experiments demonstrate that phosphonate assistance is essential for this transformation.

Synthesis of 2,5-disubstituted selenophenes via a copper-catalyzed regioselective [2+2+1] cyclization of terminal alkynes and selenium

Herein, a straightforward method for the synthesis of 2,5-disubstituted selenophenes via [2+2+1] cyclization of easily accessible terminal alkynes and elemental selenium has been developed. This reaction features high atom- and step-economy, excellent regioselectivity, good functional group tolerance and the use of stable and non-toxic selenium as a selenium source. A series of control experiments suggests that the reaction might undergo Glaser coupling reaction of two molecules of alkynes, followed by insertion of H₂Se and subsequent cyclization. Moreover, the newly formed products can be further converted to diverse conjugated selenophene-based derivatives, demonstrating their potential applications in organic synthesis and materials science.

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.

Electrochemically Enabled Selenium Catalytic Synthesis of 2,1-Benzoxazoles from o-Nitrophenylacetylenes

The study reported an electrochemically mediated method for the preparation of 2,1-benzoxazoles from o-nitrophenylacetylenes. Different from the traditional electrochemical reduction of nitro to nitroso, the nitro group directly underwent a cyclization reaction with the alkyne activated by selenium cation generated by the anodic oxidation of diphenyl diselenide and finally produced the desired products.

Synthesis of Aminoallenes via Selenium-π-Acid-Catalyzed Cross-Coupling of N-Fluorinated Sulfonimides with Simple Alkynes

The facile synthesis of aminoallenes, accomplished by a selenium-π-acid-catalyzed cross-coupling of an N-fluorinated sulfonimide with simple, non-activated alkynes, is reported. Until now, aminoallenes were difficult to be accessed by customary means, inasmuch as pre-activated and, in part, intricate starting materials were necessary for their synthesis. In sharp contrast, the current study shows that ordinary internal alkynes can serve as simple and readily available precursors for the construction of the aminoallene motif. The operating reaction conditions tolerate numerous functional groups such as esters, nitriles, (silyl)ethers, acetals, and halogen substituents, furnishing the target compounds in up to 86 % yield.

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.

Reactivity of ynamides in catalytic intermolecular annulations

Ynamides are unique alkynes with a carbon-carbon triple bond directly attached to the nitrogen atom bearing an electron-withdrawing group. The alkyne is strongly polarized by the electron-donating nitrogen atom, but its high reactivity can be finely tempered by the electron-withdrawing group. Accordingly, ynamides are endowed with both nucleophilic and electrophilic properties and their chemistry has been an active research field. The catalytic intermolecular annulations of ynamides, featuring divergent assembly of structurally important amino-heterocycles in a regioselective manner, have gained much attention over the past decade. This review aims to provide a comprehensive summary of the advances achieved in this area involving transition metal and acid catalysis. Moreover, the intermolecular annulations of ynamide analogs including ynol ethers and thioalkynes are also discussed, which can provide insights into the reactivity difference caused by the heteroatoms.

Iron-Catalyzed Cycloaddition of Amides and 2,3-Diaryl-2H-azirines To Access Oxazoles via C-N Bond Cleavage

A novel and efficient iron-catalyzed cycloaddition reaction using readily available 2,3-diaryl-2H-azirines and primary amides is reported. A wide range of trisubstituted oxazoles could be achieved in good yields with good functional group compatibility. In this transformation, two C-N bonds were cleaed and new C-N and C-O bonds were formed.

Electrochemical Desulfurative Cyclization Accessing Oxazol-2-amine Derivatives via Intermolecular C-N/C-O Bond Formation

A practical protocol has been established to access diverse oxazol-2-amine derivatives in one step via the electrochemical desulfurative cyclization of isothiocyanates and α-amino ketones. On the basis of the cycle of in situ generation of iodine/desulfurative cyclization/iodide anion regeneration, the reaction is performed under metal-free and external-oxidant-free electrolytic conditions to achieve the formation of intermolecular C-O and C-N bonds, providing oxazol-2-amines in moderate to excellent yields.

Chlorocyclization/cycloreversion of allylic alcohols to vinyl chlorides

Unprecedented chlorination of allylic alcohols: a simple mix-and-go procedure for the cyclization/cycloreversion of secondary and tertiary allylic alcohols with chloronium ions under mild conditions and practical access to remote carbonyl vinyl chlorides.

Catalytic Direct Regioselective Synthesis of Phosphonylated Tetrazoles from Aryl Diazonium Salts and Seyferth-Gilbert Reagent

Here we report a Ag-catalyzed [3 + 2] cycloaddition reaction between aryl diazonium salts and Seyferth-Gilbert reagent in a regioselective manner. This operationally simple transformation proceeds in an atom-economical way under mild reaction conditions, providing facile access to a broad set of phosphonylated tetrazoles. The synthetic utility of this method is further showcased by a straightforward procedure from upstream aromatic amines, denitrogenation transformations, and preparation of a phosphonylated drug scaffold.

Preparation and Utility of N-Alkynyl Azoles in Synthesis

Heteroatom-substituted alkynes have attracted a significant amount of interest in the synthetic community due to the polarized nature of these alkynes and their utility in a wide range of reactions. One specific class of heteroatom-substituted alkynes combines this utility with the presence of an azole moiety. These N-alkynyl azoles have been known for nearly 50 years, but recently there has been a tremendous increase in the number of reports detailing the synthesis and utility of this class of compound. While much of the chemistry of N-alkynyl azoles mirrors that of the more extensively studied N-alkynyl amides (ynamides), there are notable exceptions. In addition, as azoles are extremely common in natural products and pharmaceuticals, these N-alkynyl azoles have high potential for accessing biologically important compounds. In this review, the literature reports of N-alkynyl azole synthesis, reactions, and uses have been assembled. Collectively, these reports demonstrate the growth in this area and the promise of exploiting N-alkynyl azoles in synthesis.

Recent progress in selenium-catalyzed organic reactions

Selenium-based catalysts, including organo- and inorganoselenium ones, in organic synthesis in the recent decade are reviewed.

Bifunctional organocatalysts for the conversion of CO2, epoxides and aryl amines to 3-aryl-2-oxazolidinones

A route to synthesize 3-aryl-2-oxazolidinones is developed, which is achieved through a three component reaction between CO₂, aryl amines, and epoxides with a binary organocatalytic system composed of organocatalysts and DBU (1,8-diazabicyclo[5.4.0]undec-7-ene).

Metal-free difunctionalization of alkynes to access tetrasubstituted olefins through spontaneous selenosulfonylation of vinylidene ortho-quinone methide (VQM)

A metal-free protocol for difunctionalization of alkynes through spontaneous selenosulfonylation of vinylidene ortho-quinone methide (VQM) was developed with good yields and excellent stereoselectivities.

A selenium-catalysed para-amination of phenols

Antioxidant enzyme glutathione peroxidase (GPx) decomposes hydroperoxides by utilizing the different redox chemistry of the selenium and sulfur. Here, we report a Se-catalysed para-amination of phenols while, in contrast, the reactions with sulfur donors are stoichiometric. A catalytic amount of phenylselenyl bromide smoothly converts N-aryloxyacetamides to N-acetyl p-aminophenols. When the para position was substituted (for example, with tyrosine), the dearomatization 4,4-disubstituted cyclodienone products were obtained. A combination of experimental and computational studies was conducted and suggested the weaker Se−N bond plays a key role in the completion of the catalytic cycle. Our method extends the selenium-catalysed processes to the functionalisation of aromatic compounds. Finally, we demonstrated the mild nature of the para-amination reaction by generating an AIEgen 2-(2′-hydroxyphenyl)benzothiazole (HBT) product in a fluorogenic fashion in a PBS buffer.

Selenium has emerged as a metalloid for the catalytic construction of C–N bonds; however no functionalisation of aromatic compounds has been developed yet. Here, the authors report the para-amination of phenols via two successive sigmatropic rearrangements of a redox versatile Se–N bond.

Acid-catalyzed oxidative cleavage of S–S and Se–Se bonds with DEAD: efficient access to sulfides and selenides

An acid/DEAD system for oxidative cleavage of S–S and Se–Se bonds to generate chalcogenium ions has been developed. These ions could be captured by nucleophile-tethered alkenes and nucleophilic aryl reagents such as arylboronic acids and arenes to give chalcogenides.