Organoselenium-Catalyzed Synthesis of Oxygen- and Nitrogen-Containing Heterocycles

Mechanistic Analysis Reveals Key Role of Interchalcogen Multicatalysis in Photo-Aerobic 3-Pyrroline Syntheses by Aza-Wacker Cyclizations

A light-driven dual and ternary catalytic aza-Wacker protocol for the construction of 3-pyrrolines by partially disulfide-assisted selenium-π-acid multicatalysis is reported. A structurally diverse array of sulfonamides possessing homopolar mono-, di- and trisubstituted olefinic double bonds is selectively converted to the corresponding 3-pyrrolines in up to 95 % isolated yield and with good functional group tolerance. Advanced electrochemical mechanistic investigations of the protocol suggest a dual role of the disulfide co-catalyst. On the one hand, the disulfide serves as an electron hole shuttle between the excited photoredox catalyst and the selenium co-catalyst. On the other hand, the sulfur species engages in the final, product releasing step of the catalytic cycle by accelerating the β-elimination of the selenium moiety, which was found in many cases to lead to considerably improved product yields.

Catalytic 1,1-diazidation of alkenes

Compared to well-developed catalytic 1,2-diazidation of alkenes to produce vicinal diazides, the corresponding catalytic 1,1-diazidation of alkenes to yield geminal diazides has not been realized. Here we report an efficient approach for catalytic 1,1-diazidation of alkenes by redox-active selenium catalysis. Under mild conditions, electron-rich aryl alkenes with Z or E or Z/E mixed configuration can undergo migratory 1,1-diazidation to give a series of functionalized monoalkyl or dialkyl geminal diazides that are difficult to access by other methods. The method is also effective for the construction of polydiazides. The formed diazides are relatively safe by TGA-DSC analysis and impact sensitivity tests, and can be easily converted into various valuable molecules. In addition, interesting reactivity that geminal diazides give valuable molecules via the geminal diazidomethyl moiety as a formal leaving group in the presence of Lewis acid is disclosed. Mechanistic studies revealed that a selenenylation-deselenenylation followed by 1,2-aryl migration process is involved in the reactions, which provides a basis for the design of new reactions.

Pd-catalyzed divergent regioselective annulation of phosphinyl allenes accessing polyarylfurans and 2H-chromenes

A novel and efficient regioselective annulation of phosphinyl allenes with 2-bromophenol or 1-bromo-2-naphthol is achieved by palladium catalysis. The divergent pathway delivers structurally diverse polyarylfurans and 2H-chromene skeletons via two different Heck-type annulations. This protocol represents regioselectivity-tunable transformation of allenes into functionalized O-containing heterocycles with excellent group compatibility.

Asymmetric Photoaerobic Lactonization and Aza-Wacker Cyclization of Alkenes Enabled by Ternary Selenium-Sulfur Multicatalysis

An adaptable, sulfur-accelerated photoaerobic selenium-π-acid ternary catalyst system for the enantioselective allylic redox functionalization of simple, nondirecting alkenes is reported. In contrast to related photoredox catalytic methods, which largely depend on olefinic substrates with heteroatomic directing groups to unfold high degrees of stereoinduction, the current protocol relies on chiral, spirocyclic selenium-π-acids that covalently bind to the alkene moiety. The performance of this ternary catalytic method is demonstrated in the asymmetric, photoaerobic lactonization and cycloamination of enoic acids and unsaturated sulfonamides, respectively, leading to an averaged enantiomeric ratio (er) of 92:8. Notably, this protocol provides for the first time an asymmetric, catalytic entryway to pharmaceutically relevant 3-pyrroline motifs, which was used as a platform to access a 3,4-dihydroxyproline derivative in only seven steps with a 92:8 er.

Glutathione Peroxidase-like Antioxidant Activity of 1,3-Benzoselenazoles: Synthesis and In Silico Molecular Docking Studies as Pancreatic Lipase Inhibitors

The synthesis of 1,3-benzoselenazoles was achieved by the reaction of corresponding bis[3-amino-N-(p-tolyl)benzamide-2-yl] diselenide, bis[3-amino-N-(4-methoxyphenyl)benzamide-2-yl] diselenide, and bis[3-amino-N-(4-(dimethylamino)phenyl) benzamide-2-yl] diselenide with aryl aldehydes. The 1,3-benzoselenazoles continued to exist as planar molecules due to the presence of secondary Se···O interactions as revealed by the single-crystal X-ray analysis. The presence of secondary Se···O interactions in 1,3-benzoselenazoles was confirmed using natural bond orbital (NBO) and atoms in molecules (AIM) calculations. Nucleus-independent chemical shift (NICS) values suggested the presence of aromatic character in a five-membered benzoselenazole heterocyclic ring. The glutathione peroxidase (GPx)-like antioxidant activity of all 1,3-benzoselenazoles was assessed using a thiophenol assay, exhibiting greater antioxidant activity than Ph2Se2 used as a reference. The most active catalyst carrying a strong electron-donating group (-NMe2) at the ortho-position to the benzoselenazole ring was further investigated at different concentrations of thiophenol, H2O2, and 1,3-benzoselenazoles as catalyst for determining their catalytic parameters. Moreover, the potential applications of all 1,3-benzoselenazoles against pancreatic lipase (PL) have been identified using in silico interactions between the active sites of the 1LPB protein as evaluated using a molecular docking study.

Synthesis of Selenocyanates and Selenoethers of Amino Pyrazoles and Amino Uracils by In Situ Triselenium Dicyanide from Malononitrile and Selenium Dioxide

Herein, we report an efficient method for synthesis of novel selenocyanates of amino pyrazole, amino uracil, and amino isoxazole derivatives using in situ triselenium dicyanide from the combination of malononitrile and selenium dioxide in DMSO medium. Using the same combination but changing the stoichiometry of reagents and sequence of addition and temperature, symmetrical selenoethers of amino pyrazoles and amino uracils were prepared in good yields. Furthermore, selenocyanates of amino pyrazoles were utilized for the synthesis of corresponding alkynyl selenides in the presence of CuI and Cs₂CO₃. The salient features of this methodology are inexpensive starting materials, short reaction time, and good to very good yields. This method is also applicable for the gram-scale synthesis of selenocyanates of amino pyrazoles and amino uracils.

A fruitful century for the scalable synthesis and reactions of biphenyl derivatives: applications and biological aspects

This review provides recent developments in the current status and latest synthetic methodologies of biphenyl derivatives. Furthermore, this review investigates detailed discussions of several metalated chemical reactions related to biphenyl scaffolds such as Wurtz-Fittig, Ullmann, Bennett-Turner, Negishi, Kumada, Stille, Suzuki-Miyaura, Friedel-Crafts, cyanation, amination, and various electrophilic substitution reactions supported by their mechanistic pathways. Furthermore, the preconditions required for the existence of axial chirality in biaryl compounds are discussed. Furthermore, atropisomerism as a type of axial chirality in biphenyl molecules is discussed. Additionally, this review covers a wide range of biological and medicinal applications of the synthesized compounds involving patented approaches in the last decade corresponding to investigating the crucial role of the biphenyl structures in APIs.

Radical annulation of a designed diene system: access to nitro-benzo[b]azepines

Herein, we describe a novel O₂N˙-triggered ordered addition 7-endo cyclization reaction with excellent chemo- and regioselectivity. With such a strategy, structurally diverse nitro-benzo[b]azepines were prepared with 28 examples. Large-scale operation and handy N-Ts and N-Cbz deprotection reveal the promising utility of this methodology. Mechanistic studies suggest that the reaction proceeds through a radical pathway.

Synthesis of 1,2-diselenides via potassium persulfate-mediated diselenation of allenamides with diselenides

An efficient potassium persulfate-mediated radical addition of allenamides with diselenides was developed to create a workable route to 1,2-diselenide products. The reaction tolerates a wide spectrum of functional groups to deliver the products in good to excellent yields. Mechanistic investigations including a calculation study indicated that the radical cascade proceeds through a vinyl radical intermediate, which is formed via a selenium radical added to the terminal CC double bond of allenamides.

Regiocontrolled allylic functionalization of internal alkene via selenium-π-acid catalysis guided by boron substitution

The selenium-π-acid-catalysis has received increasing attention as a powerful tool for olefin functionalization, but the regioselectivity is often problematic. Reported herein is a selenium-catalyzed regiocontrolled olefin transpositional chlorination and imidation reaction. The reaction outcome benefits from an allylic B(MIDA) substitution. And the stabilization of α-anion from a hemilabile B(MIDA) moiety was believed to be the key factor for selectivity. Broad substrate scope, good functional group tolerance and generally good yields were observed. The formed products were demonstrated to be valuable precursors for the synthesis of a wide variety of structurally complex organoborons.

Recent Advances in Organoselenium Catalysis

Abstract:

: Organoselenium chemistry has developed as an important tool in the field of synthetic and medicinal chemistry. Various organoselenium reagents have been developed and used successfully to achieve different organic transformations such as the selenocyclizations, oxyselenenylations and selenoxide eliminations etc. Additionally, the potential of organoselenium reagents is not limited their use as stoichiometric reagents but they have successfully used as organocatalyst in number of synthetic transformations. Various organic and inorganic oxidants have been identified as terminal oxidants to regenerate the active catalytic specie. In this review article, the recent progress of organoselenium reagents in catalysis is being highlighted along with their asymmetric variants.

A Radical Addition/Cyclization and Se-Group Transfer Strategy for the Facile Synthesis of Se-Containing Cyclopentenes under Metal-Free and Peroxide-Free Conditions

A novel intermolecular radical addition/cyclization and Se-group transfer reaction of terminal alkynes and unsaturated alkyl selenide is presented which offers a straightforward and facile approach for the synthesis of valuable Se-containing cyclopentenes. Remarkable features of this strategy include easily accessible starting materials, metal-free and peroxide-free conditions, high atom economy, simple operation and broad substrate scope. More importantly, the reaction is easy to scale up and can be extended to the construction of six-membered carbon ring.

Gram-Scale Synthesis of Bufospirostenin A by a Biomimetic Skeletal Rearrangement Approach

Bufospirostenin A, which was the first spirostanol to be isolated from an animal, possesses an unprecedented 5/7/6/5/5/6 hexacyclic framework. Herein, we report two biomimetic syntheses of this natural product in just seven or nine steps from a readily available steroidal lactone. Key features of the syntheses include a photosantonin rearrangement and a Wagner-Meerwein rearrangement for rapid construction of the rearranged A/B ring system, as well as a cobalt-mediated olefin hydroselenylation and a selenide E2 reaction to accomplish a challenging olefin transposition. Our syntheses provide experimental support for the biogenetic pathway to 5(10→1)abeo-steroids that we have proposed.

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.

Rational Design 2-Hydroxypropylphosphonium Salts as Cancer Cell Mitochondria-Targeted Vectors: Synthesis, Structure, and Biological Properties

It has been shown for a wide range of epoxy compounds that their interaction with triphenylphosphonium triflate occurs with a high chemoselectivity and leads to the formation of (2-hydroxypropyl)triphenylphosphonium triflates 3 substituted in the 3-position with an alkoxy, alkylcarboxyl group, or halogen, which were isolated in a high yield. Using the methodology for the disclosure of epichlorohydrin with alcohols in the presence of boron trifluoride etherate, followed by the substitution of iodine for chlorine and treatment with triphenylphosphine, 2-hydroxypropyltriphenylphosphonium iodides 4 were also obtained. The molecular and supramolecular structure of the obtained phosphonium salts was established, and their high antitumor activity was revealed in relation to duodenal adenocarcinoma. The formation of liposomal systems based on phosphonium salt 3 and L-α-phosphatidylcholine (PC) was employed for improving the bioavailability and reducing the toxicity. They were produced by the thin film rehydration method and exhibited cytotoxic properties. This rational design of phosphonium salts 3 and 4 has promising potential of new vectors for targeted delivery into mitochondria of tumor cells.

Selenocyclization by formation of carbon-nitrogen bonds

Selenium promoted cyclization of unsaturated substrates containing internal nitrogen nucleophiles, such as different amines and amides, including the examples of its application in the synthesis of more complex polycyclic compounds is reviewed. Selenocyclization reactions of some more specific polyfunctional substrates, like Biginelli hybrids and hydantoins, are also covered.

Selenoxide Elimination Triggers Enamine Hydrolysis to Primary and Secondary Amines: A Combined Experimental and Theoretical Investigation

We discuss a novel selenium-based reaction mechanism consisting in a selenoxide elimination-triggered enamine hydrolysis. This one-pot model reaction was studied for a set of substrates. Under oxidative conditions, we observed and characterized the formation of primary and secondary amines as elimination products of such compounds, paving the way for a novel strategy to selectively release bioactive molecules. The underlying mechanism was investigated using NMR, mass spectrometry and density functional theory (DFT).

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.

Visible-Light Mediated Diarylselenylative Cyclization of 1,6-Enynes

We herein described a selenylative cyclization reaction of enynes by the utilization of diselenides as radical sources. The visible-light irradiation of the reaction mixture enables the generation of the selenium atom radical to trigger the radical addition/cyclization/selenation sequences. Both terminal alkyne and internal alkyne derived 1,6-enynes were tested and suitable for the current synthetic protocol, delivering various kinds of selenium-containing cycles in good yields.

Synergetic catalysis of Se and Cu allowing diethoxylation of halomethylene ketones using O2 as the mild oxidant

Catalyzed by PhSe(O)OH/Cu(OAc)₂, sp³-C–H alkylation of bromomethylene ketones produced useful α-carbonyl acetals under mild conditions. Bromo-containing substrates could release HBr during the reaction, avoiding the use of acidic additives.

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.

Synthesis of organoselenyl isoquinolinium imides via iron(III) chloride-mediated tandem cyclization/selenation of N'-(2-alkynylbenzylidene)hydrazides and diselenides

This report describes the synthesis of organoselenyl isoquinolinium imides through a tandem cyclization between N'-(2-alkynylbenzylidene)hydrazides and diselenides. The reaction was carried out at room temperature under an ambient atmosphere using cheap iron(iii) chloride as the metallic source. The strategy shows good tolerance to a broad range of N'-(2-alkynylbenzylidene)hydrazides and diselenides, and forms C-N and C-Se bonds in one step. The obtained product is further transformed into a bioactive H-pyrazolo[5,1-a]isoquinoline skeleton easily via a silver catalyzed [3 + 2] cycloaddition.

Catalytic Metal-free Allylic C-H Amination of Terpenoids

The selective replacement of C-H bonds in complex molecules, especially natural products like terpenoids, is a highly efficient way to introduce new functionality and/or couple fragments. Here, we report the development of a new metal-free allylic amination of alkenes that allows the introduction of a wide range of nitrogen functionality at the allylic position of alkenes with unique regioselectivity and no allylic transposition. This reaction employs catalytic amounts of selenium in the form of phosphine selenides or selenoureas. Simple sulfonamides and sulfamates can be used directly in the reaction without the need to prepare isolated nitrenoid precursors. We demonstrate the utility of this transformation by aminating a large set of terpenoids in high yield and regioselectivity.

Iron(iii) chloride-promoted cyclization of α,β-alkynic tosylhydrazones with diselenides: synthesis of 4-(arylselanyl)-1H-pyrazoles

A highly efficient iron(iii) chloride-promoted cyclization between α,β-alkynic tosylhydrazones and diselenides to form a 4-(arylselanyl)-1H-pyrazole skeleton is studied. This reaction forms C-N and C-Se bonds in one step by utilizing inexpensive iron(iii) chloride instead of expensive transition metal additives. This strategy features easily synthesized substrates, mild reaction conditions and high tolerance to functional groups.

An organoselenium-catalyzed N1- and N2-selective aza-Wacker reaction of alkenes with benzotriazoles

A novel and practical organoselenium-catalyzed, N¹- and N²-selective controllable aza-Wacker reaction is realized, which provides an easy access to N¹- and N²-olefinated benzotriazole derivatives.

Organoselenium chemistry-based polymer synthesis

Novel synthesis of selenium containing polymers with pre-determined structures and applications thereof.

Nitromethane as a nitrogen donor in Schmidt-type formation of amides and nitriles

The Schmidt reaction has been an efficient and widely used synthetic approach to amides and nitriles since its discovery in 1923. However, its application often entails the use of volatile, potentially explosive, and highly toxic azide reagents. Here, we report a sequence whereby triflic anhydride and formic and acetic acids activate the bulk chemical nitromethane to serve as a nitrogen donor in place of azides in Schmidt-like reactions. This protocol further expands the substrate scope to alkynes and simple alkyl benzenes for the preparation of amides and nitriles.

Metal-free aza-Claisen type ring expansion of vinyl aziridines: an expeditious synthesis of seven membered N-heterocycles

A metal-free approach for the synthesis of 7-membered aza-heterocycles has been developed by the intermolecular [5 + 2] cycloaddition of non-activated vinylaziridines and alkynes. This method has a broad substrate scope under mild reaction conditions to afford structurally diverse 7-membered N-heterocycles in high yield up to 92%.

Recent progress in selenium-catalyzed organic reactions

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

Organoselenium small molecules as catalysts for the oxidative functionalization of organic molecules

This perspective highlights the critical analysis of the challenges, in the past decade, which led to the development of organoselenium compounds and their use as versatile catalysts in organic synthesis towards the oxidation of olefins and C–H bonds. Furthermore, the emphasis here differs from previous reviews of the field by classifying the various types of catalyses and the diverse strategies.

Selenium reagents as catalysts

Organoselenium chemistry has become an important tool in synthetic and medicinal chemistry.

A radical-chain reaction of isocyanides with selenosulfonates and water: facile synthesis of selenocarbamates under metal-free conditions

A facile synthesis of secondary selenocarbamates through metal-free multicomponent reactions of isocyanides, selenosulfonates and water is reported here. The reaction is easy to handle and proceeds smoothly under mild conditions.

Enantioselective addition of selenosulfonates to α,β-unsaturated ketones

An organo-catalyzed enantioselective addition of selenosulfonates to α,β-unsaturated ketones was developed for the first time. With a chiral squaramide as an efficient catalyst, the desired α-selenylated ketones were obtained in a good yields with high enantioselectivity up to 89% ee, and good results could be obtained on a gram scale. The products could also be efficiently transformed into useful building blocks with a propenylic stereocenter; the strategy presented in this study may find further applications in organic synthesis.

Reaction mechanism of organoselenium-catalyzed syn-dichlorination of alkenes: a DFT study

A new method for the syn-dichlorination of alkenes at room temperature has been proposed by Denmark et al. The method uses diselenide (PhSeSePh) as the precatalyst, benzyltriethylammonium chloride (BnEt₃NCl) as the source of chlorine, and an N-fluoropyridinium salt as the oxidant to recover the catalyst. This approach has achieved exquisite diastereocontrol on a number of alkene substrates. In this paper, we report the results of DFT calculations we performed to study the mechanism of this reaction. We were able to identify a reasonable reaction path, including the intermediate and transition-state structures. The results also indicate that PhSeCl₃, rather than PhSeCl, is the active catalyst.