Transition-Metal-Free Diarylannulated Sulfide and Selenide Construction via Radical/Anion-Mediated Sulfur–Iodine and Selenium–Iodine Exchange

Access to Valuable Chalcogen-Containing Biaryl Derivatives via Regioselective 2,2'-Dichalcogenation of 2-Bromobiaryls

The regioselective installation of chalcogen atoms into biaryl scaffolds is an important synthetic task due to the great value of chalcogen-containing biaryl derivatives in many fields. Here we undertake this task by developing a regioselective 2,2'-dichalcogenation of 2-bromobiaryls with common chalcogen sources using an organolanthanum-mediated one-pot, two-step protocol. This strategy features high regioselectivity, readily available substrates, transition-metal-free conditions, and performance superior to those of previous methods, thereby demonstrating the unique advantages of organolanthanum reagents in organic synthesis.

Base- and sulfur-promoted oxidative lactonization of chalcone-acetate Michael adducts: access to pyran-2-ones

A cost-effective, practical, straightforward and scalable synthesis of α-pyrones via base- and sulfur-promoted annulation of phenylacetates and chalcones is reported. Generated in situ from the starting components by using dbu as a base catalyst, the Michael adducts underwent a smooth oxidative cyclization into 3,4,6-triaryl-2-pyranones upon heating with DABCO and sulfur in DMSO. Extension to malonate in place of phenylacetates led to 4,6-diaryl-2-pyranone-2-carboxylates.

Visible-Light-Promoted Cascade Coupling of 2-Isocyanonaphthalenes with Elemental Sulfur and Amines to Construct Naphtho[2,1-d]thiazol-2-Amines

A visible-light-induced strategy has been explored for the synthesis of naphtho[2,1-d]thiazol-2-amines through ortho-C-H sulfuration of 2-isocyanonaphthalenes with elemental sulfur and amines under external photocatalyst-free conditions. This three-component reaction, which utilizes elemental sulfur as the odorless sulfur source, molecular oxygen as the clean oxidant, and visible light as the clean energy source, provides a mild and efficient approach to construct a series of naphtho[2,1-d]thiazol-2-amines. Preliminary mechanistic studies indicated that visible-light-promoted photoexcitation of reaction intermediates consisting of thioureas and DBU might be involved in this transformation.

Functionalization of Chlorotonils: Dehalogenil as Promising Lead Compound for In Vivo Application

The natural product chlorotonil displays high potency against multidrug-resistant Gram-positive bacteria and Plasmodium falciparum. Yet, its scaffold is characterized by low solubility and oral bioavailability, but progress was recently made to enhance these properties. Applying late-stage functionalization, we aimed to further optimize the molecule. Previously unknown reactions including a sulfur-mediated dehalogenation were revealed. Dehalogenil, the product of this reaction, was identified as the most promising compound so far, as this new derivative displayed improved solubility and in vivo efficacy while retaining excellent antimicrobial activity. We confirmed superb activity against multidrug-resistant clinical isolates of Staphylococcus aureus and Enterococcus spp. and mature transmission stages of Plasmodium falciparum. We also demonstrated favorable in vivo toxicity, pharmacokinetics and efficacy in infection models with S. aureus. Taken together, these results identify dehalogenil as an advanced lead molecule.

Multimodal Acridine Photocatalysis Enables Direct Access to Thiols from Carboxylic Acids and Elemental Sulfur

Development of photocatalytic systems that facilitate mechanistically divergent steps in complex catalytic manifolds by distinct activation modes can enable previously inaccessible synthetic transformations. However, multimodal photocatalytic systems remain understudied, impeding their implementation in catalytic methodology. We report herein a photocatalytic access to thiols that directly merges the structural diversity of carboxylic acids with the ready availability of elemental sulfur without substrate preactivation. The photocatalytic transformation provides a direct radical-mediated segue to one of the most biologically important and synthetically versatile organosulfur functionalities, whose synthetic accessibility remains largely dominated by two-electron-mediated processes based on toxic and uneconomical reagents and precursors. The two-phase radical process is facilitated by a multimodal catalytic reactivity of acridine photocatalysis that enables both the singlet excited state PCET-mediated decarboxylative carbon-sulfur bond formation and the previously unknown radical reductive disulfur bond cleavage by a photoinduced HAT process in the silane-triplet acridine system. The study points to a significant potential of multimodal photocatalytic systems in providing unexplored directions to previously inaccessible transformations.

A Review on the Synthetic Methods towards Benzothienobenzothiophenes

Benzothienobenzothiophenes (BTBTs) are a class of heteroacenes for which two distinct isomers have been identified depending on the locations of the fused benzothiophene motifs. Benzothienobenzothiophenes represent a class of heteroacenes demonstrating remarkable electronic properties that make them prominent in the realm of organic semiconductors. The structure of BTBTs, incorporating two sulfur atoms, contributes to their unique electronic characteristics, including narrow bandgaps and effective charge transport pathways. These compounds have gained attention for their high charge carrier mobility, making them desirable candidates for application in organic field-effect transistors (OFETs) and other electronic devices. Researchers have explored various synthetic strategies to design and tailor the properties of BTBT derivatives, leading to advancements in the development of high-performance organic semiconductors. Various synthetic techniques for benzothienobenzothiophenes have been reported in the literature including multistep synthesis, tandem transformations, electrochemical synthesis, and annulations. This review investigates the generality of each synthetic methodology by highlighting its benefits and drawbacks, and it analyses all synthetic approaches described for the creation of the two isomers. For the advantage of the readers, we have delved upon every mechanism of the reactions that are known. Finally, we have also summarized the synthetic methodologies that are used for making benzothienobenzothiophene analogues for material applications.

Synthesis of highly substituted 1,3-dienes through halonium promoted 1,2-sulfur migration of propargylic thioethers

Conjugated 1-bromo or 1-iodo-1,3-dienes bearing a sulfide substituent have been synthesized via 1,2-sulfur migration from propargylic thioethers upon activation with NIS or NBS. The reaction generally proceeds with high control over the regio- and diastereoselectivity. Highly substituted thiophenes and selenophenes are easily obtained from the generated dienes.

DABCO-Catalyzed DMSO-Promoted Sulfurative 1,2-Diamination of Phenylacetylenes with Elemental Sulfur and o-Phenylenediamines: Access to Quinoxaline-2-thiones

The oxidative amination of alkynes typically requires transition metal catalysts and strong oxidants. Herein, we alternatively utilize DABCO as a sulfur-activating catalyst to achieve the sulfurative 1,2-diamination of phenylacetylenes with elemental sulfur and o-phenylenediamines. DMSO was found to be particularly suitable for use as a terminal oxidant for this three-component process. A mechanistic study has shown that this cascade reaction is triggered by the addition of active sulfur species to the triple bond of phenylacetylenes.

Sulfur-Promoted Oxidative Cyclization of Pentan-1-ones: Direct Access to Tetrasubstituted Furans from Deoxybenzoins and Chalcones

Furan is an important heterocyclic scaffold in natural product, bioorganic, and medicinal chemistry as well as in materials science. The system S8/DABCO/DMSO was found to efficiently mediate the oxidative cyclization of 1,2,3,5-tetraarylpentan-1-ones A, which were obtained in situ as the Michael adducts of chalcones 1 and deoxybenzoins 2, to furan 3. The strategy provided convenient and direct access to tetrasubstituted furans 3 from readily available starting materials with high functional group tolerance.

Metal-free and atom-efficient protocol for diarylation of selenocyanate by diaryliodonium salts

We developed an atom- and reaction mass efficient strategy for the preparation of diarylselenides using iodonium salts as reactants. The developed approach allows the obtaining of diarylselenides from the corresponding trimethoxyphenyl-substituted iodonium salts via a two-step one-pot reaction sequence. The proposed metal-free methodology is based on the involvement of both iodonium aryl groups for diarylation.

Cyclic diaryliodonium salts: applications and overview

Owing to the recent renewed interest and groundbreaking advances in hypervalent chemistry, cyclic diaryliodonium salts have had a myriad of unique applications in the past decade. Their numerous properties, such as an efficient dual arylation mechanism, straightforward one-pot synthesis compatibility, wide substrate scope, and functionalization tolerance, have made them appropriate starting materials for many bioactive compounds. Fluorenes, thiophenes, carbazoles, phenanthrenes, and many other useful cyclic bioactive molecules that are essential for pharmaceutical synthesis can be readily accessed from cyclic diaryliodonium salts. Particular focus has been given to the high optical activity and good enantiomeric excess of the products that facilitate the easy formation of many difficult-to-obtain optical isomers, such as atropisomers. This review aims to compile and summarize all the recent advances in synthesizing methodologies to prepare the important compounds where cyclic diaryliodonium salt is an integral part of the methodologies and would hopefully provide a good foundation for further research on this topic.

Three-component selective synthesis of phenothiazines and bis-phenothiazines under metal-free conditions

An iodine-containing reagent promoted three-component method for the selective synthesis of phenothiazines and bis-phenothiazines has been developed. The present protocol starts from simple and easily available cyclohexanones, elemental sulfur, and inorganic ammonium salts, selectively producing phenothiazines and bis-phenothiazines in satisfactory yields under aerobic conditions. This method has the advantages of simple and readily available starting materials and metal-free conditions, affording a facile and practical approach for the preparation of phenothiazines and bis-phenothiazines.

Transition-Metal-Free Difunctionalization of Sulfur Nucleophiles

Efficient protocols for accessing iodo-substituted diaryl and aryl(vinyl) sulfides have been developed using iodonium salts as reactive electrophilic arylation and vinylation reagents. The reactions take place under transition-metal-free conditions, employing odorless and convenient sulfur reagents. A wide variety of functional groups are tolerated in the S-diarylation, enabling the regioselective late-stage application of several heterocycles and drug molecules under mild reaction conditions. A novel S-difunctionalization pathway was discovered using vinyliodonium salts, which proceeds under additive-free reaction conditions and grants excellent stereoselectivity in the synthesis of aryl(vinyl) sulfides. A one-pot strategy combining transition-metal-free diarylation and subsequent reduction provided facile access to electron-rich thioanilines and a direct synthesis of a potential drug candidate derivative. The retained iodo group allows a wide array of further synthetic transformations. Mechanistic insights were elucidated by isolating the key intermediate, and the relevant energy profile was substantiated by DFT calculations.

Photoinduced Construction of Thieno[3,4-c]quinolin-4(5H)-ones/Selenopheno[3,4-c]quinolin-4(5H)-ones Using Diphenyl Disulfide or Diphenyl Diselenide as Sulfur or Selenium Sources

A photocatalytic synthesis of thieno[3,4-c]quinolin-4(5H)-ones/selenopheno[3,4-c]quinolin-4(5H)-ones using diphenyl disulfide or diphenyl diselenide as sulfur or selenium sources was developed. Two C-S/Se bonds and one C-C bond were constructed simultaneously without transition metals and other additives.

Reaction of 1-acetonaphthones with anilines and elemental sulfur: rapid construction of 1-anilinonaphtho[2,1-b]thiophenes

1-Anilinonaphtho[2,1-b]thiophenes could be conveniently synthesized from a three-component reaction of 1-acetonaphthones with anilines and elemental sulfur under catalyst-free simple heating conditions.

Recent Progress in Cyclic Aryliodonium Chemistry: Syntheses and Applications

Hypervalent aryliodoumiums are intensively investigated as arylating agents. They are excellent surrogates to aryl halides, and moreover they exhibit better reactivity, which allows the corresponding arylation reactions to be performed under mild conditions. In the past decades, acyclic aryliodoniums are widely explored as arylation agents. However, the unmet need for acyclic aryliodoniums is the improvement of their notoriously low reaction economy because the coproduced aryl iodides during the arylation are often wasted. Cyclic aryliodoniums have their intrinsic advantage in terms of reaction economy, and they have started to receive considerable attention due to their valuable synthetic applications to initiate cascade reactions, which can enable the construction of complex structures, including polycycles with potential pharmaceutical and functional properties. Here, we are summarizing the recent advances made in the research field of cyclic aryliodoniums, including the nascent design of aryliodonium species and their synthetic applications. First, the general preparation of typical diphenyl iodoniums is described, followed by the construction of heterocyclic iodoniums and monoaryl iodoniums. Then, the initiated arylations coupled with subsequent domino reactions are summarized to construct polycycles. Meanwhile, the advances in cyclic aryliodoniums for building biaryls including axial atropisomers are discussed in a systematic manner. Finally, a very recent advance of cyclic aryliodoniums employed as halogen-bonding organocatalysts is described.

Synthesis and reactivity of azole-based iodazinium salts

A systematic investigation of imidazo- and pyrazoloiodazinium salts is presented. Besides a robust synthetic protocol that allowed us to synthesize these novel cyclic iodonium salts in their mono- and dicationic forms, we gained in-depth structural information through single-crystal analysis and demonstrated the ring opening of the heterocycle-bridged iodonium species. For an exclusive set of dicationic imidazoiodaziniums, we show highly delicate post-oxidation functionalizations retaining the hypervalent iodine center.

Dithieno[3,2-b:2',3'-d]thiophene (DTT): an emerging heterocyclic building block for future organic electronic materials & functional supramolecular chemistry

Heterocyclic compounds being potent biochemical materials are ubiquitous molecules in our life. Amongst, the five membered aromatic ring systems, thiophene has emerged as a remarkable entity in organic electronics owing to its (i) high resonance energy, (ii) more electrophilic reactivity than benzene, (iii) high π-electron density, (iv) planar structure and, (v) presence of vacant d-orbital in addition to the presence of loosely bind lone-pairs of electrons on sulfur atoms. In recent past, thiophene-fused molecule namely, dithienothiophene (DTT) has attracted a tremendous attention of the researchers worldwide due to their potential applicability in organic electronics such as in solar cells, electrochromic devices (ECDs), organic field effect transistors (OFETs), organic limiting diodes (OLEDs), fluorescent probes, redox switching and so forth because of their (i) higher charge mobility, (ii) extended π-conjugation, and (iii) better tuning of band gaps, etc. In this particular review article, we envisioned to report the recent advancements made on the DTT-based architectures not only because of the potential applicability of this valuable scaffold in organic electronic but also to motivate the young researchers worldwide to look for the challenging opportunities related to this privileged building block in both material sciences and functional supramolecular chemistry.

Electrochemical Synthesis of Dibenzothiophenes via Intramolecular C-S Cyclization with a Halogen Mediator

Electrochemical synthesis of dibenzothiophene derivatives was achieved. Several bis(biaryl) disulfides are efficiently converted to dibenzothiophenes by electrochemical oxidation. The use of Bu4NBr as a halogen mediator was essential, and wide varieties of dibenzothiophene derivatives were obtained in good yields.

Base-Promoted Reaction of 4-Chloro-3-vinyl Coumarins, Phenacylpyridinium Bromides, and Elemental Sulfur: A Designed Approach to Thiopyrano[4,3-c]chromen-5(1H)-ones

A designed synthesis of thiopyrano[4,3-c]chromen-5(1H)-ones was developed based on a base-promoted three-component reaction between 4-chloro-3-vinyl coumarins, phenacylpyridinium bromides, and elemental sulfur. Readily available starting materials, mild conditions, chemoselectivity, operational simplicity, and synthetically useful yields are some highlighted advantages of these transformations.

Selective C-S Bond Constructions Using Inorganic Sulfurs via Photoinduced Electron Donor-Acceptor Activation

By complementing traditional transition metal catalysis, photoinduced catalysis has emerged as a versatile and sustainable way to achieve carbon-heteroatom bond formation. This work discloses a visible-light-induced reaction for the formation of a C-S bond from aryl halides and inorganic sulfuration agents via electron donor-acceptor (EDA) complex photocatalysis. Divergent formations of organic sulfide and disulfide have been demonstrated under mild conditions. Preliminary mechanistic studies suggest that visible-light-induced intracomplex charge transfer within the monosulfide-anion-containing EDA complex permits the C-S bond construction reactivity.

Metal-Free C-S Bond Formation in Elemental Sulfur and Cyclobutanol Derivatives: The Synthesis of Substituted Thiophenes

A general approach for the metal-free synthesis of thiophenes by tert-cyclobutanols and elemental sulfur has been developed. This protocol provides a strategy for constructing multisubstituted thiophene derivatives via C-S bond formation under air. This reaction shows good functionality tolerance under the reaction conditions, and the mechanism is validated by control experiments and density functional theory calculations.

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.

One-Pot Synthesis of Heteroatom-Bridged Cyclic Diaryliodonium Salts

Two one-pot procedures for the construction of O- and N-bridged diaryliodonium triflates are described. An effective aryne-mediated arylation of o-iodophenols and -sulfonamides provides diarylether and diarylamine intermediates, which are subsequently oxidized and cyclized to the corresponding diaryliodaoxinium and -iodazinium salts. Different derivatizations were applied to demonstrate their capacity as useful building blocks and gain a deeper understanding toward the general reactivity of these underdeveloped but potentially highly useful compounds.

Cu-catalyzed [2 + 2 + 1] cascade annulation of vinyl iodonium salts with elemental sulfur/selenium for the modular synthesis of thiophenes and selenophenes

The Cu-catalyzed [2 + 2 + 1] regioselective synthesis of thiophenes/selenophenes was realized from aryl-vinyl iodonium salts and elemental sulfur/selenium.

Tandem cyclization/arylation of diaryliodoniums via in situ constructed benzoxazole as a directing group for atom-economical transformation

Linear diaryliodoniums often undergo only single arylation and leave equivalent aryl iodide as waste.

Atom-Economical, Catalyst-Free Hydrosulfonation of Densely Functionalized Alkenes: Access to Oxindole Containing Sulfones

An atom-economical hydrosulfonation of densely functionalized alkenes under catalyst-free conditions is described. Alkenes possessing hydroxy-oxindole moiety underwent hydrosulfonation on treatment with arylsulfinic acids in green media to afford the resulting...

Direct Access to 2-Aryl-3-Cyanothiophenes by Base-Catalyzed One-pot Two-step Three-component Reaction of Chalcones with Benzoylacetonitriles and Elemental Sulfur

3-Cyanothiophene is an important heterocyclic scaffold in bioorganic and medicinal chemistry as a useful synthetic intermediate as well as in material sciences as a privilege motif for photovoltaic development. Herein,...

Benzyl thioether formation merging copper catalysis

A novel copper-catalyzed thioetherification reaction has been developed to afford benzyl thioethers in moderate to excellent yields. Under the mild and easy-to-operate conditions, a variety of thioethers are efficiently prepared from readily available benzyl alcohols (primary, secondary, and tertiary) and thiols in the presence of Cu(OTf)2 as the Lewis acid catalysis. This C-S bond formation protocol furnishes exceptional chemoselectivity, and the preliminary mechanism studies show that the reaction should proceed through a Lewis-acid-mediated SN1-type nucleophilic attack of the carbocations formed in situ.

α-Selective C(sp3)-H Thio/Selenocyanation of Ketones with Elemental Chalcogen

A facile method is disclosed for the synthesis of α-thio/selenocyanato ketones through regioselective C-H thio/selenocyanation of ketones. The advantages include the use of easily available starting materials, high efficiency, simple operation, and easy scale-up. Control experiments provide evidence that the reaction proceeded via a radical way, while kinetic isotope effect experiments reveal that the cleavage of the C-H bond serves as the rate-limiting step.

Construction of Thioamide Peptide via Sulfur-Involved Amino Acids/Amino Aldehydes Coupling

A sulfur-involved ligation for thioamide quasi-peptides was developed via amino acids and amino aldehydes coupling. The key to the transformation was the chelation of copper with imines for chiral activation and fixation. In this environment, linear polysulfur decreased the alkalinity of single sulfur anions to prevent racemization caused by the interaction between sulfur and sodium sulfide. Dipeptides, tripeptides, tetrapeptides, and the linkage between the drug and amino acids were successfully obtained.

Elemental Sulfur-Promoted Benzoxazole/Benzothiazole Formation Using a C═C Double Bond as a One-Carbon Donator

An efficient method to assemble diverse benzoxazoles/benzothiazoles in good yields was developed via oxidative cyclization with 2-aminothiophenols or 2-iodoanilines as raw materials. In this protocol, elemental sulfur was used as the effective oxidant and C atoms on the C═C double bond were introduced as a one-carbon donator.

Facile Synthesis of Thienoacenes via Transition-Metal-Free Ladderization

Herein, we report a facile transition-metal-free approach to sulfur-containing heteroacenes from fluorinated oligophenylenes. Unlike most existing methods, the presented approach is not restricted to simple dibenzothiophene derivatives and thus appears to be a useful tool for the synthesis of extended sulfur-containing heteroacenes. The incorporation of sulfur is unambiguously preprogrammed via the positions of fluorines in the precursors, allowing the selective synthesis of extended thienoacenes with up to 96% yield.

Fe/S-Catalyzed synthesis of 2-benzoylbenzoxazoles and 2-quinolylbenzoxazoles via redox condensation of o-nitrophenols with acetophenones and methylquinolines

An Fe/S catalyst generated in situ from FeCl₂·4H₂O and elemental sulfur S₈ in the presence of a tertiary amine as a base was found to catalyze efficiently a 6e^- redox condensation of o-nitrophenols with acetophenones and methylquinolines. The condensed products 2-benzoylbenzoxazoles and 2-quinolylbenzoxazoles were obtained in reasonable yields with water as the only byproduct at a temperature as low as 80 °C.

Fe-S Catalyst Generated In Situ from Fe(III)- and S3•--Promoted Aerobic Oxidation of Terminal Alkenes

An iron-sulfur complex formed by the simple mixture of FeCl₃ with S₃^•- generated in situ from K₂S is developed and applied to selective aerobic oxidation of terminal alkenes. The reaction was carried out under an atmosphere of O₂ (balloon) and could proceed on a gram scale, expanding the application of S₃^•- in organic synthesis. This study also encourages us to explore the application of an Fe-S catalyst in organic reactions.

NH4I-promoted oxidative formation of benzothiazoles and thiazoles from arylacetic acids and phenylalanines with elemental sulfur

A NH4I/K3PO4-based catalytic system has been established to enable oxidative formation of thiazole compounds from arylacetic acids and phenylalanines with elemental sulfur. While the three-component reaction of anilines or β-naphthylamines with arylacetic acids and elemental sulfur affords benzo[2,1-d]thiazoles and naphtho[2,1-d]thiazoles, the annulation of phenylalanines with elemental sulfur produces 2-benzyl and 2-benzoylthiazoles. This work well complements previous three-component annulations of benzothiazoles from other coupling partners.

Preparation and Synthetic Applicability of Imidazole-Containing Cyclic Iodonium Salts

A novel approach to the preparation of imidazole-substituted cyclic iodonium salts has been developed via the oxidative cyclization of 1-phenyl-5-iodoimidazole using a cheap and available Oxone/H₂SO₄ oxidative system. The structure of the new polycyclic heteroarenes has been confirmed by single-crystal X-ray diffractometry, revealing the characteristic structure features for cyclic iodonium salts. The newly produced imidazole-flanked cyclic iodonium compounds were found to readily engage in a heterocyclization reaction with elemental sulfur, affording benzo[5,1-b]imidazothiazoles in good yields.