Photoinduced Cyclizations of o-Diisocyanoarenes with Organic Diselenides and Thiols that Afford Chalcogenated Quinoxalines

Efficacy of radical reactions of isocyanides with heteroatom radicals in organic synthesis

Isocyanide is a promising synthetic reagent not only as a one-carbon homologation reagent but also as a nitrogen source for nitrogen-containing molecules. Because of their isoelectronic structure with carbon monoxide, isocyanides also react with nucleophiles, electrophiles, carbon radicals, and transition metal reagents, and are widely used in organic synthesis. On the other hand, the use of isocyanides in reactions with heteroatom radicals is limited. However, the reaction of isocyanides with heteroatom radicals is a promising synthetic tool for the construction of nitrogen-containing organic molecules modified with a variety of heteroatoms. In this Perspective, we review the addition and cyclization reactions of heteroatom radicals with isocyanides and discuss the synthetic prospects of the reaction of isocyanides with heteroatom radicals.

Sustainable Synthesis through Catalyst-Free Photoinduced Cascaded Bond Formation

The beginning of photochemical reactions revolutionized synthetic chemistry through sustainable practices. This review explores cutting-edge developments in leveraging light-induced processes for generating cascaded C-C and C-hetero bonds without catalysts. Significantly, catalyst-free photoinduced methodologies have garnered considerable attention, especially in the creation of varied heterocyclic frameworks for drug design and the synthesis of natural products. The article delves into underlying mechanisms, addresses limitations, and evaluates various methodologies, emphasizing the potential of photocatalyst and transition metal-free photochemical reactions to enhance sustainability. Divided into two sections, it covers recent strides in C-C and C-heteroatom and multiple C-heteroatom bond formation reactions.

Recent Progress in Synthetic and Biological Application of Diorganyl Diselenides

Diorganyl diselenides have emerged as privileged structures because they are easy to prepare, have distinct reactivity, and have broad biological activity. They have also been used in the synthesis of natural products as an electrophile in the organoselenylation of aromatic systems and peptides, reductions of alkenes, and nucleophilic substitution. This review summarizes the advancements in methods for the transformations promoted by diorganyl diselenides in the main functions of organic chemistry. Parallel, it will also describe the main findings on pharmacology and toxicology of diorganyl diselenides, emphasizing anti-inflammatory, hypoglycemic, chemotherapeutic, and antimicrobial activities. Therefore, an examination detailing the reactivity and biological characteristics of diorganyl diselenides provides valuable insights for academic researchers and industrial professionals.

Nucleophilic Aromatic Substitution of Heteroaryl Halides with Thiols

The nucleophilic aromatic substitution (SNAr) between heteroaryl halides (Cl, Br) and thiols proceeds smoothly in DMAc under the action of K2CO3 at rt-100 °C. For most electron-deficient heteroarenes, reaction takes place without introducing an additional electron-withdrawing group. For electron-rich heteroarenes, an additional electron-withdrawing group such as a simple ester, keto, cyano, and nitro group is required to ensure the reaction completes. The reactivity trend of heteroaryl halides is highly dependent on the electronic nature of the heteroarenes and orientation of halogens. Besides thiols, a couple of functionalized thioureas and thioamides are compatible with these conditions, providing the corresponding heteroaryl thioethers in good yields.

Organoselenium Compounds: Chemistry and Applications in Organic Synthesis

Selenium, originally described as a toxin, turns out to be a crucial trace element for life that appears as selenocysteine and its dimer, selenocystine. From the point of view of drug developments, selenium-containing drugs are isosteres of sulfur and oxygen with the advantage that the presence of the selenium atom confers antioxidant properties and high lipophilicity, which would increase cell membrane permeation leading to better oral bioavailability. In this article, we have focused on the relevant features of the selenium atom, above all, the corresponding synthetic approaches to access a variety of organoselenium molecules along with the proposed reaction mechanisms. The preparation and biological properties of selenosugars, including selenoglycosides, selenonucleosides, selenopeptides, and other selenium-containing compounds will be treated. We have attempted to condense the most important aspects and interesting examples of the chemistry of selenium into a single article.

Domino Cyclization Reaction of o-Diisocyanoarenes for the Synthesis of Imidazo[1,2-a]pyridinobenzimidazole Backbones

An efficient procedure to access a variety of connected imidazo[1,2-a]pyridine and benzimidazole skeletons through the C-N bond was described as a new type of Buchwald-Hartwig reaction. Furthermore, the bis(imidazo[1,2-a]pyridin-3-yl)aryl-1,2-diamine scaffolds were obtained by changing the equivalent ratio of the starting materials. Some advantages of the protocol are the formation of four new bonds (C═C, C-N), a transition-metal-free reaction, a broad substrate scope, high yields, and mild reaction conditions. The reaction mechanism was confirmed on the basis of DFT calculations.

Photo- or Electrochemical Cyclization of Dienes with Diselenides to Access Seleno-Benzo[b]azepines

A cascade selenylation/cyclization of dienes with diselenides has been realized under visible-light irradiation or electrolysis conditions. Employing O₂ or electricity as a "green" oxidant, this protocol provides a green and efficient method for an array of biologically important seleno-benzo[b]azepine derivatives in moderate to good yields. The direct sunlight irradiation and gram-scale reaction render the approach practical and attractive.

Synthesis and Application Dichalcogenides as Radical Reagents with Photochemical Technology

Dichalcogenides (disulfides and diselenides), as reactants for organic transformations, are important and widely used because of their potential to react with nucleophiles, electrophilic reagents, and radical precursors. In recent years, in combination with photochemical technology, the application of dichalcogenides as stable radical reagents has opened up a new route to the synthesis of various sulfur- and selenium-containing compounds. In this paper, synthetic strategies for disulfides and diselenides and their applications with photochemical technology are reviewed: (i) Cyclization of dichalcogenides with alkenes and alkynes; (ii) direct selenylation/sulfuration of C-H/C-C/C-N bonds; (iii) visible-light-enabled seleno- and sulfur-bifunctionalization of alkenes/alkynes; and (iv) Direct construction of the C(sp)-S bond. In addition, the scopes, limitations, and mechanisms of some reactions are also described.

Photoinduced Synthesis of Bisphosphinated Quinoxalines via Radical Cyclization of o-Diisocyanoarenes with Diphosphines

The cycloaddition reaction of o-diisocyanoarenes with interelement compounds under light is a very important reaction system to clarify whether this reaction proceeds by radical cyclization or by aza-Bergman cyclization. In this study, a series of diphosphines with phosphorus-phosphorus single bonds were selected as interelement compounds, and their cycloaddition reactions with o-diisocyanoarenes under light were investigated in detail to achieve a novel photoinduced synthesis of bisphosphinated quinoxalines via the radical cyclization pathway. In addition, the photoinduced reaction of diphosphines with isocyanides having o-functional groups such as cyano and ethenyl groups allowed us to elucidate the reaction pathway and product selectivity of this bisphosphination. Furthermore, the one-pot synthesis of Pd^II -quinoxaline complex was successfully achieved by applying the developed reaction.

Metal-Free One-Pot Multi-Functionalization of Unsaturated Compounds with Interelement Compounds by Radical Process

In recent years, the importance of "environmentally friendly manufacturing" has been increasing toward the establishment of a resource-recycling society. In organic synthesis, as well, it is becoming increasingly important to develop new synthetic strategies with resource conservation and the recycling of elemental resources in mind, rather than just only synthesis. Many studies on the construction of frameworks of functional molecules using ionic reactions and transition-metal-catalyzed reactions have been reported, but most of them have focused on the formation of carbon-carbon bonds. However, it is essential to introduce appropriate functional groups at appropriate positions in molecules in order for the molecules to express their functions, and furthermore, the highly selective preparation of multiple functional groups is considered important for the creation of new functional molecules. In this review, we focus on radical reactions with high functional group selectivity and overview the recent progress in practical methods for the simultaneous introduction of multiple functional groups and propose future synthetic strategies that emphasize the recycling of elemental resources and environmental friendliness.

Stereoselective Reduction of Alkynes: Synthesis of 4-Organoselenyl Quinolines

This study describes the reaction of 2-amino arylalkynyl ketones with organoselenolates to form (Z)-vinyl selenides, which lead to 4-organoselenyl quinolines via an intramolecular condensation. Using the optimized reaction conditions, the generality of this cyclization was studied with various arylalkynyl ketones and diorganyl diselenides. The study of the reaction mechanisms led to the isolation and identification of a vinyl selenide, which was the key intermediate for this cyclization. To expand the structural diversity and to demonstrate the applicability of the 4-organoselenyl quinolines prepared, we studied their application as substrates in the cleavage of the carbon-selenium bond using n-butyllithium followed by the capture of the lithium intermediate by electrophiles and Suzuki and Sonogashira cross-coupling reactions.

Recent Advances in Light-Induced Selenylation

Selenium-containing organic molecules have recently found a plethora of applications, ranging from organic synthesis to pharmacology and material sciences. In view of these concepts, the development of mild, efficient, and general protocols for the formation of C-Se bonds is desirable, and light induced approaches are appealing ways. The aim of this Review is to provide the reader with the most recent examples of light promoted selenylation 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.

Photoinduced Bisphosphination of Alkynes with Phosphorus Interelement Compounds and Its Application to Double-Bond Isomerization

The addition of interelement compounds with heteroatom-heteroatom single bonds to carbon-carbon unsaturated bonds under light irradiation is believed to be an atomically efficient method to procure materials with carbon-heteroatom bonds. In this study, we achieved the photoinduced bisphosphination of alkynes using the phosphorus interelement compound, tetraphenyldiphosphine monosulfide (1), to stereoselectively obtain the corresponding (E)-vic-1,2-bisphosphinoalkenes, which are important transition-metal ligands. The bisphosphination reaction was performed by mixing 1 and various alkynes and then exposing the mixture to light irradiation. Optimization of the conditions for the bisphosphination reaction resulted in a wide substrate range and excellent trans-selectivity. Moreover, the completely regioselective introduction of pentavalent and trivalent phosphorus groups to the terminal and internal positions of the alkynes, respectively, was achieved. We also found that the novel double-bond isomerization reaction of the synthesized bisphosphinated products occurred with a catalytic amount of a base under mild conditions. Our method for the photoinduced bisphosphination of carbon-carbon unsaturated compounds may have strong implications for both organic synthesis and organometallic and catalyst chemistry.

PhSe(O)OH/Al(NO3)3-Catalyzed selectivity controllable oxidation of sulphide owing to the synergistic effect of Se, Al3+ and nitrate

Catalyzed by PhSe(O)OH/Al(NO3)3, selective oxidation of sulphides to produce sulfoxides or sulphones could be achieved under mild conditions. The synergistic effect of Se, Al3+ and nitrate is the key factor for the reaction.

Catalyst-free aerobic radical cascade reactions of o-vinylphenylisocyanides with thiols to access 2-thio-substituted quinolines

We herein report an efficient and green aerobic radical cascade reaction of o-vinylphenylisocyanides with thiols to access a broad range of 2-thio-substituted quinolines without the need for additional catalysts or oxidants.

Recent advances in the transition-metal-free synthesis of quinoxalines

Quinoxalines, also known as benzo[a]pyrazines, constitute an important class of nitrogen-containing heterocyclic compounds as a result of their widespread prevalence in natural products, biologically active synthetic drug candidates, and optoelectronic materials. Owing to their importance and chemists' ever-increasing imagination of new transformations of these products, tremendous efforts have been dedicated to finding more efficient approaches toward the synthesis of quinoxaline rings. The last decades have witnessed a marvellous outburst in modifying organic synthetic methods to create them sustainable for the betterment of our environment. The exploitation of transition-metal-free catalysis in organic synthesis leads to a new frontier to access biologically active heterocycles and provides an alternative method from the perspective of green and sustainable chemistry. Despite notable developments achieved in transition-metal catalyzed synthesis, the high cost involved in the preparation of the catalyst, toxicity, and difficulty in removing it from the final products constitute disadvantageous effects on the atom economy and eco-friendly nature of the transformation. In this review article, we have summarized the recent progress achieved in the synthesis of quinoxalines under transition-metal-free conditions and cover the reports from 2015 to date. This aspect is presented alongside the mechanistic rationalization and limitations of the reaction methodologies. The scopes of future developments are also highlighted.

Clarification on the Reactivity of Diaryl Diselenides toward Hexacyclohexyldilead under Light

In this study, the reactivity of organochalcogen compounds toward a representative alkyl-lead bond compound under light was investigated in detail. Under light irradiation, the Cy-Pb bond of Cy6Pb2 (Cy = cyclohexyl) undergoes homolytic cleavage to generate a cyclohexyl radical (Cy•). This radical can be successfully captured by diphenyl diselenide, which exhibits excellent carbon-radical-capturing ability. In the case of (PhS)2 and (PhTe)2, the yields of the corresponding cyclohexyl sulfides and tellurides were lower than that of (PhSe)2. This probably occurred due to the low carbon-radical-capturing ability of (PhS)2 and the high photosensitivity of the cyclohexyl-tellurium bond.

Light-mediated Seleno-Functionalization of Organic Molecules: Recent Advances

Organoselenium compounds constitute an important class of substances with applications in the biological, medicinal and material sciences as well as in modern organic synthesis, attracting considerable attention from the scientific community. Therefore, the construction of the C-Se bond via facile, efficient and sustainable strategies to access complex scaffolds from simple substrates are an appealing and hot topic. Visible light can be regarded as an alternative source of energy and is associated with environmentally-friendly processes. Recently, the use of visible-light mediated seleno-functionalization has emerged as an ideal and powerful route to obtain high-value selenylated products, with diminished cost and waste. This approach, involving photo-excited substrates/catalyst and single-electron transfer (SET) between substrates in the presence of visible light has been successfully used in the versatile and direct insertion of organoselenium moieties in activated and unactivated C(sp³ )-H, C(sp² )-H, C(sp)-H bonds as well as C-heteroatom bonds. In most cases, ease of operation and accessibility of the light source (LEDs or commercial CFL bulbs) makes this approach more attractive and sustainable than the traditional strategies.