Rhodium Thiavinyl Carbenes from 1,2,3-Thiadiazoles Enable Modular Synthesis of Multisubstituted Thiophenes

Synthesis of γ-Thiapyrones by Diels-Alder/Retro-Diels-Alder Reaction of α-Pyrones with 5-H-1,2,3-Thiadiazoles

The efficient synthesis of γ-thiapyrones by a base-mediated Diels-Alder/retro-Diels-Alder reaction of α-pyrones with 5-H-1,2,3-thiadiazoles is reported herein. Thioketenes in situ generated from thiadiazoles as electron-poor dienophile and electron-rich 4-hydroxy-2-pyrones as dienes are conjunctively transformed into a series of γ-thiapyrones with broad functional group compatibility in good to excellent yields (35 examples, 67% average yield).

1,2,3-Thiadiazole as a Modifiable and Scalable Directing Group for ortho-C-H Functionalization

In the last few decades, directed C-H bond functionalization has had enormous applicability in academia and industry. The development of a novel, readily accessible, and scalable directing group with modifiable ability is highly desirable in C-H functionalization. Herein, we report the 1,2,3-thiadiazole as a modifiable directing group for C-H amidation and alkynylation with dioxazolones, p-toluenesulfonyl azide, and bromoalkynes in high yield. The densely functionalized 1,2,3-thiadiazole products are modified into thioamide, multisubstituted furan, γ-thiapyrone, thiazole, and various alkynyl sulfides through simple and one-step reactions. The competition experiments reveal that the directing ability of 1,2,3-thiadiazole is slightly weaker than pyridine and bidentate amide but stronger than the widely used carboxylate.

Advances in Selected Heterocyclization Methods

This Account summarizes efforts in our group toward synthesis of heterocycles in the past decade. Selected examples of transannulative heterocyclizations, intermediate construction of reactive compounds en route to these important motifs, and newer developments of a radical approach are outlined.

Construction of pyrroles, furans and thiophenes via intramolecular cascade desulfonylative/dehydrogenative cyclization of vinylidenecyclopropanes induced by NXS (X = I or Br)

Pyrroles, furans, and thiophenes are important structural motifs in biologically active substances, pharmaceuticals and functional materials. In this paper, we disclose an efficient synthetic strategy for the rapid construction of multisubstituted pyrroles, furans, and thiophenes via NXS mediated desulfonylative/dehydrogenative cyclization of vinylidenecyclopropanes (VDCPs). The advantages of this method include wide substrate range, high efficiency and synthetic usefulness of the heterocyclic products under metal-free and mild conditions. The derivatization of pyrrole products and the preparation of functional molecules successfully demonstrated the synthetic potential of the products as platform molecules. The reaction mechanism has been investigated on the basis of control experiments and DFT calculations.

Recent Advances in Catalytic Alkyne Transformation via Copper Carbene Intermediates

As one of the abundant and inexpensive metals on the earth, copper has demonstrated broad applications in synthetic chemistry and catalysis. Among these copper-catalyzed advances, copper carbenes are versatile and reactive intermediates that can mediate a variety of transformations, which have attracted much attention in the past decades. The present review summarizes two different reaction models that take place between a copper carbene intermediate and alkyne species, including the cross-coupling reaction of copper carbene intermediate with terminal alkyne, and the addition of copper carbene intermediate onto the C–C triple bond. This article will cover the profile from 2010 to 2021 by placing emphasis on the detailed catalytic models and highlighting the synthetic applications offered by these practical and mild methods.

Catalytic Diastereospecific and Enantioselective (3 + 2) Transannulations of 1,2,3-Thiadiazoles with Strained Norbornene Derivatives

With the adoption of a ring-strain-release strategy, iridium-catalyzed transannulations with norbornene derivatives are achieved in a diastereospecific and enantioselective manner. The first asymmetric transannulations of 1,2,3-thiadiazoles are reported.

Synthesis of Substituted Thiophenes through Dehydration and Heterocyclization of Alkynols

A protocol was described for obtaining a variety of substituted thiophenes with functional potential via metal-free dehydration and sulfur cyclization of alkynols with elemental sulfur (S₈) or EtOCS₂K in moderate-to-good yields. The method provides the base-free generation of a trisulfur radical anion (S₃^•-) and its addition to alkynes as an initiator. This research broadens the applications of S₃^•- in the synthesis of sulfur-containing heterocycles.

Silver-mediated annulation between 5-H-1,2,3-thiadiazoles and 1,3-dicarbonyl compounds to construct polysubstituted furans

An efficient synthesis of polysubstituted furans by Ag(i)-mediated annulation between 5-H-1,2,3-thiadiazoles and 1,3-dicarbonyl compounds is reported.

Catalytic (3 + 2) umpolung annulations of α-thioacyl carbenes with aryl isothiocyanates

As masked S-electrophilic thia-1,3-dipoles, 1,2,3-thiadiazoles undergo denitrogenative (3 + 2) umpolung transannulations with aryl isothiocyanates, regio- and stereoselectively.

Thioamide synthesis via copper-catalyzed C–H activation of 1,2,3-thiadiazoles enabled by slow release and capture of thioketenes

A copper-catalyzed coupling of 1,2,3-thiadiazoles with various amines under base-free conditions was developed as a robust protocol for the synthesis of thioamide derivatives via C–H activation/Cu coordination strategy.

Visible-Light-Induced Formation of Thiavinyl 1,3-Dipoles: A Metal-Free [3+2] Oxidative Cyclization with Alkynes as Easy Access to Thiophenes

A visible-light-induced [3+2] oxidative cyclization of various alkynes with easily available ketene dithioacetals as the previously unknown thiavinyl 1,3-dipoles in the presence of an acridine photosensitizer is reported. A series of multisubstituted thiophenes were achieved regioselectively in ≤98% yields under very mild metal-free conditions without other additives. This reaction could tolerate a wide range of substrates and achieve good efficiency in large-scale syntheses. The reaction mechanism and their applications are described in detail to reveal the reactivity of the new 1,3-dipoles and the selectivity of the reactions.

Sulfur-controlled and rhodium-catalyzed formal (3 + 3) transannulation of thioacyl carbenes with alk-2-enals and mechanistic insights

A rhodium-catalyzed denitrogenative formal (3 + 3) transannulation of 1,2,3-thiadiazoles with alk-2-enals is achieved, producing 2,3-dihydrothiopyran-4-ones in moderate to excellent yields. An inverse KIE of 0.49 is obtained, suggesting the reversibility of the oxidative addition of thioacyl Rh(i) carbenes to alk-2-enals. The late-stage structural modifications of steroid compounds are realized. Moreover, our studies show that thioacyl carbenes have different reactivities to those of α-oxo and α-imino carbenes, and highlight the importance of heteroatoms in deciding the reactivities of heterovinyl carbenes.

Access to thiazoles via [3 + 2] cycloaddition of 1,2,3-thiadiazoles with isonitriles

The formal [3 + 2] cycloaddition of 1,2,3-thiadiazoles with isonitriles is developed to access structurally diverse 4,5-disubstituted thiazoles, in which the thioketene as a sulfo-2C-synthon is first disclosed.

Visible-light photocatalytic preparation of alkenyl thioethers from 1,2,3-thiadiazoles and Hantzsch esters: synthetic and mechanistic investigations

A visible-light-promoted S-alkylation of 1,2,3-thiadiazoles with C-radical precursors, 4-alkyl-1,4-dihydropyridines (DHPs), to produce alkenyl thioethers is disclosed.

Mechanistic insights into the Rh(i)-catalyzed transannulation of 1,2,3-thiadiazoles with alkenes, alkynes, and nitriles: Does the intermediacy of α-thiavinyl Rh-carbenoids play an important role?

Density functional theory (DFT) calculations were performed to gain an in-depth mechanistic understanding of the Rh(i)-catalyzed transannulation of 1,2,3-thiadiazoles with alkenes, alkynes, and nitriles.

Dual roles of bisphosphines and epoxides: Rh-catalyzed highly chemoselective and diastereoselective (3 + 2) transannulations of 1,2,3-thiadiazoles with cyanoepoxides

Dual roles of bisphosphines and epoxides are demonstrated in highly chemoselective and diastereoselective (3 + 2) transannulations.

Csp–Csp bond cleavage and fragment coupling: a transition metal-free “extrusion and recombination” approach towards synthesis of 1,2-diketones

A metal-free strategy for C–C bond activation of 1,3-diynes has been established via an “extrusion and recombination” approach to derive structurally important 1,2-diketones in good yields with excellent selectivity.

Design, synthesis and anticancer activity of 5-aryl-4-(4-arylpiperazine-1-carbonyl)-1,2,3-thiadiazoles as microtubule-destabilizing agents

Hereby, we report our efforts on discovery and optimization of a new series of 5-aryl-4-(4-arylpiperazine-1-carbonyl)-1,2,3-thiadiazoles as new microtubule-destabilizing agents along our previous study. Guided by docking model analysis, we introduced the 1,2,3-thiadiazole moiety containing the hydrogen-bond acceptors as B-ring of XRP44X analogues. Extensive structure modifications were performed to investigate the detailed structure and activity relationships (SARs). Some compounds exhibited potent antiproliferative activities against three human cancer cell lines (SGC-7901, A549 and HeLa). The compound 5m exhibited the highest potency against the three cancer cell lines. The tubulin polymerization experiments indicated that compound 5m effectively inhibited the tubulin polymerization, and immunostaining assay revealed that it significantly disrupted microtubule dynamics. Moreover, cell cycle studies revealed that compound 5m dramatically arrested cell cycle progression at G2/M phase.

Recent strategies in the synthesis of thiophene derivatives: highlights from the 2012-2020 literature

Thiophene-based analogs have been fascinated by a growing number of scientists as a potential class of biologically active compounds. Furthermore, they play a vital role for medicinal chemists to improve advanced compounds with a variety of biological effects. The current review envisioned to highlight some recent and particularly remarkable examples of the synthesis of thiophene derivatives by heterocyclization of various substrates from 2012 on.

Ruthenium-Catalyzed Tandem Carbene/Alkyne Metathesis/N-H Insertion: Synthesis of Benzofused Six-Membered Azaheterocycles

The Cp*RuCl-based catalyst enables expedient access to a variety of benzofused six-membered azaheterocycles from unprotected o-alkynylanilines and trimethylsilyldiazomethane through an unprecedent tandem carbene/alkyne metathesis/N-H insertion reaction. The transformation takes place under mild reaction conditions (room temperature, <15 min) and with excellent functional group tolerance. The synthetic utility of the final products and a mechanistic rationale are also discussed.

Synthesis of tetrasubstituted thiophenes via a [3+2] cascade cyclization reaction of pyridinium 1,4-zwitterionic thiolates and activated allenes

A [3+2] cascade cyclization reaction of pyridinium 1,4-zwitterionic thiolates and activated allenes was developed to access tetrasubstituted thiophenes.

Water/Alkali-Catalyzed Reactions of Azides with 2-Cyanothioacetamides. Eco-Friendly Synthesis of Monocyclic and Bicyclic 1,2,3-Thiadiazole-4-carbimidamides and 5-Amino-1,2,3-triazole-4-carbothioamides

The reactions of thioamides with azides in water were studied. It was reliably shown that the reaction of 2-cyanothioacetamides 1 with various types of azides 2 in water in the presence of alkali presents an efficient, general, one-step, atom-economic, and eco-friendly method for the synthesis of 1,2,3-thiadiazol-4-carbimidamides 5 and 1,2,3-triazole-4-carbothioamides 4. This method can be extended to the one-pot reaction of sulfonyl chlorides and 6-chloropyrimidines 2'o with sodium azide, leading to final products in higher yields, that is, avoiding the isolation of unsafe sulfonyl azides. The method was furthermore applied to the reaction of N,N'-bis-(2-cyanothiocarbonyl)pyrazine 1h with sulfonyl azides to afford bicyclic 1,2,3-thiadiazoles 8 and 1,2,3-triazoles 9 connected via a 1,1'-piperazinyl linker. 2-Cyanothioacetamides 1 were also shown to react with aromatic azides in water in the presence of alkali to afford 1-aryl-5-amino-1,2,3-triazole-4-carbothioamides 11. In contrast to aromatic azides and similarly to sulfonyl azides, 6-azidopyrimidine-2,4-diones 2o-q react with cyanothioacetamides to form N-pyrimidin-6-yl-5-dialkylamino-1,2,3-thiadiazole-4-N-l-carbimidamides 12. A mechanism was proposed to rationalize the role of water in changing the reactivity of azides toward 2-cyanothioacetamides.

Catalytic carbene/alkyne metathesis (CAM): a versatile strategy for alkyne bifunctionalization

Metal carbene, as a reactive intermediate, has shown versatile applications in modern organic synthesis. One of the priorities in this area is exploration of stable carbene precursors with structural diversity. Catalytic carbene/alkyne metathesis (CAM) with readily available and stable materials, such as α-carbonyl diazo compounds, provides an effective approach for the in situ generation of vinyl carbene intermediates, which is difficult to directly access with other carbene precursors. Thus, novel cascade transformations involving the CAM process for the straightforward construction of polycyclic frameworks have been well documented. Challenges including side reaction control and asymmetric catalysis in this area need to be explored. This review will summarize the recent advances in this field and be divided by the type of the terminating carbene reactions.

Ring-opening/annulation reaction of cyclopropyl ethanols: concise access to thiophene aldehydes via C–S bond formation

The synthesis of thiophene aldehydes from easily available cyclopropyl ethanol derivatives and potassium sulfide has been developed.

Thio-Michael addition of thioamides and allenes for the selective construction of polysubstituted 2-arylthiophenes via TBAI/H2O2 promoted tandem oxidative annulation and 1,2-sulfur migration

A novel TBAI-catalyzed tandem thio-Michael addition/oxidative annulation of allenes and thioamides for the construction of polysubstituted 2-arylthiophenes under a sulfur migration transformation protocol has been developed. The transition-metal-free protocol achieves the oxidative cyclization reaction of thioamides containing electron-rich substituents with allenes to construct polysubstituted thiophenes selectively by controlling oxidation conditions.

Rhodium(I)-Catalyzed Coupling-Cyclization of C═O Bonds with α-Diazoketones

An unprecedented intermolecular nucleophilic attack of C═X bonds (X = O and S) on the rhodium(I)-carbenes has been developed. This transformation allows for the coupling-cyclization of aroylamides with α-diazoketones and provides concise access to 2,4,5-trisubstituted 1,3-oxazoles and 1,3-thiazoles with a broad tolerance of functional groups.

Assembly of fully substituted 2,5-dihydrothiophenes via a novel sequential multicomponent reaction

A sequential multicomponent reaction between ketoesters, isothiocyanates and 1,2-diaza-1,3-dienes to create 2,5-dihydrothiophenes that can be converted into thiophenes.

Insight into the Activation of In Situ Generated Chiral RhI Catalysts and Their Application in Cyclotrimerizations

We report a detailed study concerning the efficient generation of highly active chiral rhodium complexes of the general structure [Rh(diphosphine)(solvent)₂ ]⁺ as well as their exemplary successful utilization as catalysts for cyclotrimerizations. Such solvent complexes could likewise be prepared from novel ammonia complexes of the type [Rh(diphosphine)(NH₃ )₂ ]⁺ . A valuable, feasible approach to generate novel chiral Rh^I complexes was found by in situ generation from Wilkinson's catalyst [RhCl(PPh₃ )₃ ] with chiral P,N ligands. The generated catalysts led to moderate to good enantioselectivities and excellent yields in the cyclotrimerizations of triynes, showcasing their usefulness in the synthesis of axially chiral benzene derivatives.

Synthesis of Bicyclic Isothiazoles through an Intramolecular Rhodium-Catalyzed Transannulation of Cyanothiadiazoles

An intramolecular rhodium-catalyzed transannulation of readily available cyanothiadiazoles containing an ester, amide, or ether as a linker is described. It provides a wide range of bicyclic isothiazoles in good to excellent yields together with the release of molecular nitrogen. These results indicate that the carbon atom in the α-thiavinyl carbene is nucleophilic and that the sulfur atom is electrophilic.

Synthesis of Imidazopyridines via Copper-Catalyzed, Formal Aza-[3 + 2] Cycloaddition Reaction of Pyridine Derivatives with α-Diazo Oxime Ethers

The Cu-catalyzed, formal aza-[3 + 2] cycloaddition reaction of pyridine derivatives with α-diazo oxime ethers in trifluoroethanol was used to synthesize imidazopyridines via the release of molecular nitrogen and elimination of alcohol. These methods enabled modular synthesis of a wide range of N-heterobicyclic compounds such as imidazopyridazines, imidazopyrimidines, and imidazopyrazines with an α-imino Cu-carbenoid generated from the α-diazo oxime ethers and copper.