Assembly of 3-Sulfenylbenzofurans and 3-Sulfenylindoles by Palladium-Catalyzed Cascade Annulation/Arylthiolation Reaction

Reactions of alkynes with C-S bond formation: recent developments

Alkynes are important in organic synthesis. This review mainly focuses on recent advances (2013-2023) on alkynes with C-S bond formation, based on more than 30 types of sulfur reagents. The reactions of alkynes with various sulfur-containing compounds including RSSR (disulfides), RSH (thiols), S8 (elemental sulphur), alkynyl thioethers, RSCN, AgSCF3, K2S, Na2S, dithiane, RSCl, NFSI, RNCS, EtOCS2K, thiocarbamate, RSONH2, thiourea, sulfoxide, RSO2N3, CS2, RSO2NH2, RSO2NHNH2, RSO2Cl, RSO2Oar, RSO2SR', DABCO·(SO2)2, Na2S2O5, K2S2O5, RSO2H, RSO2Na and related compounds are discussed. Diverse mechanisms such as radical, electrophilic/nucleophilic addition, rearrangement, C-C bond cleavage, and CuAAC are discussed. The content is organized by substrates and reactivity patterns. We hope it will help in future research in this area.

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.

Expedient Synthesis of Alkyl and Aryl Thioethers Using Xanthates as Thiol-Free Reagents

Thioethers are critical in the fields of pharmaceuticals and organic synthesis, but most of the methods for synthesis alkyl thioethers employ foul-smelling thiols as starting materials or generate them as by-products. Additionally, most thiols are air-sensitive and are easily oxidized to produce disulfides under atmospheric conditions; thus, a novel method for synthesizing thioethers is necessary. This paper reports a simple, effective, green method for synthesizing dialkyl or alkyl aryl thioether derivatives using odorless, stable, low-cost ROCS2K as a thiol surrogate. This transformation offers a broad substrate scope and good functional group tolerance with excellent selectivity. The reaction likely proceeds via xanthate intermediates, which can be readily generated via the nucleophilic substitution of alkyl halides or aryl halides with ROCS2K under transition-metal-free and base-free conditions.

Gold self-relay catalysis enabling annulative oxygenation of propargylic alcohols with O-nucleophiles

A new gold(I) self-relay catalysis reaction enabling the annulative oxygenation of propargylic alcohols with various O-nucleophiles, such as carboxylic acids, alcohols and TBHP, is reported, producing a series of functionalized benzofurans in moderate to good yields under mild conditions. This protocol benefits from the π- and σ-Lewis acid capability of gold complexes, demonstrating high molecular convergence, broad substrate flexibility, high functional group compatibility and mild conditions.

One-pot C-C, C-N, and C-S bond construction for synthesis of 3-sulfenylindoles directly from unactivated anilines involving dual palladium catalysis and mechanistic insights from DFT

An efficient dual Pd-catalytic system was developed for one-pot synthesis of 3-sulfenylindoles via C-C, C-N and C-S bond construction directly from unactivated 2-iodo(NH)anilines under mild reaction conditions. Furthermore, 3-selenyl/halo/carbon-functionalized indoles were synthesized in good yields and a short reaction time. The synthetic utility of 3-sulfenylindole was also demonstrated. The key role of solvent in palladium catalysis was unravelled by DFT.

Copper catalyzed reaction of alcohols, alkyl halides and Na2S2O3: An odorless and ligand-free rout to unsymmetrical thioether synthesis

A CuI-catalyzed condensation reaction between alcohols, alkyl halides, and Na2S2O3 leading to structurally diverse thioethers is reported. A variety of alkyl halides and electron-rich benzyl alcohols were employed successfully in...

Synthesis of Indoles via Intermolecular and Intramolecular Cyclization by Using Palladium-Based Catalysts

As part of natural products or biologically active compounds, the synthesis of nitrogen-containing heterocycles is becoming incredibly valuable. Palladium is a transition metal that is widely utilized as a catalyst to facilitate carbon-carbon and carbon-heteroatom coupling; it is used in the synthesis of various heterocycles. This review includes the twelve years of successful indole synthesis using various palladium catalysts to establish carbon-carbon or carbon-nitrogen coupling, as well as the conditions that have been optimized.

Palladium-catalyzed bisthiolation of terminal alkynes for the assembly of diverse (Z)-1,2-bis(arylthio)alkene derivatives

An efficient and straightforward palladium-catalyzed three-component cascade bisthiolation of terminal alkynes and arylhydrazines with sodium thiosulfate (Na₂S₂O₃) as the sulfur source for the assembly of functionalized (Z)-1,2-bis(arylthio)alkene derivatives is described. Using 0.5 mol% IPr–Pd–Im–Cl₂ as the catalyst, a wide range of terminal alkynes and arylhydrazines are well tolerated, thus producing the desired products in good yields with good functional group tolerance and excellent regioselectivity. Moreover, this protocol could be readily scaled up, showing potential applications in organic synthesis and material science.

An efficient palladium-catalyzed bisthiolation of terminal alkynes and arylhydrazines with Na₂S₂O₃ as the sulfur source for the assembly of (Z)-1,2-bis(arylthio)alkene derivatives is described.

Synthetic developments on the preparation of sulfides from thiol-free reagents

This critical review covers the main thiolating reagents with respect to their characteristics and reactivities. In fact, they are complementary to each other and bring different thiolation strategies, avoiding the hazardous thiol derivatives.

Palladium-catalyzed aerobic oxyarylthiolation of alkynone O-methyloximes with arylhydrazines and elemental sulfur

A novel and practical palladium-catalyzed aerobic oxyarylthiolation of alkynone O-methyloximes for the assembly of 4-sulfenylisoxazole derivatives using S8 and arylhydrazines as the S-aryl sources is accomplished. In the presence of 0.1 mol% of IPr-Pd-allyl-Cl as the catalyst and O2 (1 atm) as the sole oxidant, both alkynone O-methyloximes and arylhydrazines are suitable substrates, delivering diverse 4-sulfenyl isoxazoles in moderate to good yields with good functional group tolerance. Notably, the phenyl diazonium salt and sodium phenyl sulfinate are also suitable arylation reagents, providing an alternative synthetic strategy to access structurally diverse 4-sulfenyl isoxazoles.

Benzofuran: A Key Heterocycle - Ring Closure And Beyond

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The benzofuranyl motif present in compounds exhibits various medicinal properties and non-drug applications. These derivatives are naturally occurring compounds or synthetic materials, which cover a broad spectrum of pharmacological activities like anti-inflammatory, anti-diabetic, anti- depressant, anti-HIV, anti-microbial, anti-proliferative, anti-convulsant, cytotoxic, analgesic, etc. Few of the commercially interesting compounds from this class are, ailanthoidol (anti-inflammatory), amiodarone, dronedarone, celivarone (anti-arrhythmic), bufuralol (muscular airways relaxant), morphine, 5-(2-aminopropyl)benzofuran; 5-APB, 6-(2-aminopropyl)benzofuran; 6-APB (CNS), rifampicin (antibiotic), etc., whereas, some of the non-drug applications are in perfumery industry (bergapten) and as tannin activators in sunscreen preparations (psoralen, 8-methoxypsoralen, and angelicin). Considering these interesting biological activities and commercial utilities, a review on the synthetic aspects of this privileged scaffold was attempted. For the benefit of natural product-based drug discovery, available sources of these derivatives, extraction process and reported biological activities have also been outlined in this review.

Palladium-catalyzed three-component cascade arylthiolation with aryldiazonium salts as S-arylation sources

A novel palladium-catalyzed three-component cascade arylthiolation for the assembly of 3-sulfenylindoles and 3-sulfenylbenzofurans is described, with aryldiazonium salts as the ideal S-arylation sources.

Recent advances in the synthesis of indoles from alkynes and nitrogen sources

This review highlights ten years of success in the synthesis of indoles using alkynes and nitrogen sources as substrates.

Recent developments in palladium-catalyzed C–S bond formation

This review summarized the recent developments in palladium-catalyzed C–S bond formation involving sulfenylation and sulfonylation reactions.

Tandem cyclization of o-alkynylanilines with isocyanides triggered by intramolecular nucleopalladation: access to heterocyclic fused 2-aminoquinolines

Herein, a novel strategy for the synthesis of various heterocyclic fused 2-aminoquinolines via palladium-catalyzed tandem cyclization of o-alkynylanilines with isocyanides has been developed. This process includes trans-oxy/aminopalladation, isocyanide insertion, elimination and 1,3-hydrogen migration. Besides high atom and step economy, this method shows good functional group compatibility with excellent chemo- and regioselectivities under mild reaction conditions.

Synthesis of 2,3-Difunctionalized Benzofuran Derivatives through Palladium-Catalyzed Double Isocyanide Insertion Reaction

A novel palladium-catalyzed tandem cyclization of 1-(allyloxy)-2-ethynylbenzene derivatives with isocyanides in the presence of water has been developed. The key intermediates, benzofuran-3-α-carbonyl aldehydes, were obtained through a simple acid hydrolysis process and could serve as precursors for structurally diverse 2,3-difunctionalized benzofuran derivatives such as important 2-benzofurylquinoxalines, benzofuran-3-α-ketoesters and benzofuryl ynediones. This transformation features convenient operation, simple and commercially available starting materials, broad functional-group compatibility, and moderate to good reaction yields.

Stereoselective and regioselective 5-exo-dig cyclization of 8-alkynylnaphthalen-1-amines for the synthesis of (E)-2-(arylthio)alkylene-1,2-dihydrobenzo[cd]indoles

A palladium and iodine-cocatalyzed 5-exo-dig aza-thiocyclization of 8-alkynylnaphthalen-1-amines for the synthesis of (E)-2-alkylene-1,2-dihydrobenzo[cd]indole thioethers is reported. As a result of broad reaction scope, simple operation, mild conditions, and high stereoselectivity and regioselectivity, this reaction should have potential utility in organic synthesis.

N-Heterocyclic carbene palladium-catalyzed cascade annulation/alkynylation of 2-alkynylanilines with terminal alkynes

A straightforward and highly effective N-heterocyclic carbene-palladium catalyzed cascade annulation/alkynylation of 2-alkynylanilines with terminal alkynes has been enabled to afford free (NH)-3-alkynylindole derivatives in moderate to good yields. This protocol features mild conditions, broad substrate scope, and high atom- and step-economy. Notably, the resultant 3-alkynylindoles could be conveniently transformed into a variety of functionalized indole scaffolds, thus illustrating their potential applications in synthetic and pharmaceutical chemistry.

Hydrazone–Pd-catalyzed direct intermolecular reaction of o-alkynylphenols with allylic acetates

The hydrazone–palladium catalyzed direct intermolecular reaction of o-alkynylphenols with allylic acetates gave the corresponding 2-substituted-3-allylbenzofuran derivatives at room temperature.

DDQ-mediated regioselective C–S bond formation: efficient access to allylic sulfides

A novel metal-free dehydrocoupling approach for constructing allylic sulfides via oxidative allylic C–H functionalization with high atom- and step-economy has been developed.

Synthesis of Functionalized Benzo[b]furans via Oxidative Cyclization of o-Cinnamyl Phenols

Disclosed herein is an efficient synthetic route for the synthesis of functionalized 2-benzyl benzo[b]furans via a regioselective 5-exo-trig intramolecular oxidative cyclization of ortho-cinnamyl phenols using [PdCl₂(CH₃CN)₂] as catalyst and benzoquinone as an oxidant. Further, a sequential ortho-cinnamylation of phenols using cinnamyl alcohols catalyzed by Re₂O₇, followed by an oxidative cyclization using the above Pd catalyst, is performed. The reaction showed broad substrate scope with good to excellent yields.

Iodine-catalyzed cascade annulation of alkynes with sodium arylsulfinates: assembly of 3-sulfenylcoumarin and 3-sulfenylquinolinone derivatives

An efficient and environmentally benign iodine-catalyzed cascade cyclization has been achieved to afford functionalized 3-sulfenylcoumarin and 3-sulfenylquinolinone derivatives.

Palladium-catalyzed C–S bond activation and functionalization of 3-sulfenylindoles and related electron-rich heteroarenes

A novel palladium-catalyzed approach for constructing functionalized heteroarenes via C–S bond cleavage and C–C bond formation has been demonstrated.