Rhodium(iii)-catalyzed C–H annulation of 2-acetyl-1-arylhydrazines with sulfoxonium ylides: synthesis of 2-arylindoles

An efficient Rh(iii)-catalyzed synthesis of 2-arylindole derivatives via intermolecular C–H annulation of arylhydrazines with sulfoxonium ylides was accomplished. A variety of 2-acetyl-1-arylhydrazines with sulfoxonium ylides were converted into 2-arylindoles in satisfactory yields. Excellent selectivity and good functional group tolerance of this transformation were also observed.

Rh(iii)-catalyzed intermolecular C–H annulation of arylhydrazines with sulfoxonium ylides for synthesis of 2-arylindole derivatives was well established.     

DABCO bond cleavage for the synthesis of piperazine derivatives

The applications of DABCO (1,4-diazabicyclo[2.2.2]octane) in the synthesis of piperazine derivatives including biologically active compounds via C–N bond cleavage are investigated in this review. Different reagents such as alkyl halides, aryl(heteroary) halides, carboxylic acids, diaryliodonium salts, tosyl halides, activated alkynes, benzynes etc. were applied for the preparation of the corresponding quaternary ammonium salts of DABCO, which are very good electrophiles for various nucleophiles such as phenols, thiophenols, thiols, alcohols, aliphatic and aromatic amines, sulfinates, phthalimide, indoles, NaN₃, triazole and terazoles, NaCN, enols and enolates, halides, carboxylic acid salts etc. Besides preactivated DABCO salts, the in situ activation of DABCO in multicomponent reactions is also an efficient tactic in synthetic organic chemistry for the diversity oriented synthesis of drug-like piperazine derivatives.

The applications of DABCO (1,4-diazabicyclo[2.2.2]octane) in the synthesis of piperazine derivatives including biologically active compounds via C–N bond cleavage are investigated in this review.     

Rh(iii)-catalyzed spiroannulation of 3-arylquinoxalin-2(1H)-ones with alkynes: practical access to spiroquinoxalinones

The Rh(iii)-catalyzed synthesis of spiroquinoxalinone derivatives from 3-arylquinoxalin-2(1H)-ones and alkynes via a C–H functionalization/[3 + 2] annulation sequence has been developed. This method, featuring low catalyst loading, was amenable to Gram scale synthesis and tolerated a variety of functional groups and substitution patterns on the aryl rings, providing the target products in good to excellent yields.

The use of imines as a H acceptor for Rh(iii)-catalyzed spirocyclization of 3-arylquinoxalinones and alkynes via a C–H functionalization/[3 + 2] annulation sequence has been achieved.     

Direct intramolecular double cross-dehydrogentive-coupling (CDC) cyclization of N-(2-pyridyl)amidines under metal-free conditions

A facile transition-metal-free protocol to form 2-iminoimidazo[1, 2-a]-pyridines bearing a –CHBr₂ group and an aza-quaternary carbon center at the 3 position from N-(2-pyridyl)amidines substrates, in which the new heterocyclic skeletons constructed from amidines via radical reactions or nucleophilic substitution reactions are promoted only by CBr₄ under mild conditions, is demonstrated. The reactions were realized by intramolecular CDC reaction involving C–N and C–C bond formation via the sequential C(sp³)–H bifunctionalization mode on the same carbon atom under mild conditions. Moreover, this work also provides an excellent and representative example for CBr₄ as an efficient reagent to initiate radical reactions under initiator-free conditions or to give rise to nucleophilic substitution reactions only by base.

A metal-free protocol to form 2-iminoimidazo[1,2-a]-pyridines from N-(2-pyridyl)amidines, in which the constructed heterocyclic skeletons are promoted by CBr₄via radical or nucleophilic substitution reactions under mild conditions, is demonstrated.     

Synthesis of tetrahydro-β-carbolines from 2-indolylmethyl azides and propargylic alcohols

A facile and efficient route to tetrahydro-β-carbolines from 2-indolylmethyl azides and propargylic alcohols via acid-catalyzed dehydrative annulation reactions is described. This reaction proceeds through a cascade sequence of Friedel–Crafts-type alkylation followed by intramolecular “Click” reaction, involving the formation of multiple chemical bonds in a single operation with excellent atom-economy and broad functional group tolerance.

The synthesis of tetrahydro-β-carbolines from 2-indolylmethyl azides and propargylic alcohols via acid-catalyzed dehydrative [4 + 2]-annulation reaction is described.     

Rhodium(iii)-catalyzed annulation of enamides with sulfoxonium ylides toward isoquinolines

An efficient rhodium(iii)-catalyzed C–H activation followed by intermolecular annulation between enamides and sulfoxonium ylides has been developed. The transformation proceeds smoothly with a broad range of substrates, affording a series of isoquinoline derivatives in moderate to good yields under additive-free conditions.

An efficient rhodium(iii)-catalyzed C–H activation followed by intermolecular annulation between enamides and sulfoxonium ylides has been developed.     

Copper-catalyzed tandem reaction of 2-alkynylanilines with benzoquinones: efficient access to 3-indolylquinones

A simple, mild, catalytic and efficient method for the straightforward synthesis of an interesting class of 2-aryl/alkyl-substituted-3-indolyl quinones in good to high yields is reported for the first time. This atom-efficient method proceeds via copper-catalyzed one-pot sequential intramolecular hydroamination (C–N bond formation) of 2-alkynylanilines followed by oxidative C–C coupling with benzoquinones.

Copper-catalyzed domino cyclization of 2-alkynylanilines followed by C–C bond formation with quinones is an efficient method of accessing 3-indolyl quinones.     

Copper-mediated construction of benzothieno[3,2-b]benzofurans by intramolecular dehydrogenative C–O coupling reaction

An efficient method to synthesize benzothieno[3,2-b]benzofurans via intramolecular dehydrogenative C–H/O–H coupling has been developed. Good to excellent yields (64–91%) could be obtained no matter if the substituted group is electron-donating or electron-withdrawing. Notably, three-to-six fused ring thienofuran compounds could be constructed using this method. A reaction mechanism study showed that 1,1-diphenylethylene can completely inhibit the reaction. Therefore, it is a radical pathway initiated by single electron transfer between the hydroxyl of the substrate and the copper catalyst.

The construction of benzothieno[3,2-b]benzofurans via novel dehydrogenative C–H/O–H coupling reaction has been developed with excellent yields. Furthermore, three-to-six fused ring thienofuran compounds could be constructed.     

One-pot construction of diverse and functionalized isochromenoquinolinediones by Rh(iii)-catalyzed annulation of unprotected arylamides with 3-diazoquinolinediones and their application for fluorescence sensor

A facile and efficient Rh(iii)-catalyzed annulation of arylamides with 3-diazoquinolinediones for the construction of diverse and highly functionalized isochromenoquinolinediones is described. Furthermore, the methodology is applicable for delivering various relevant molecules such as pyridopyranoquinolindiones, thienopyranoquinolinones, and indolopyranoquinolinone. The reaction proceeds via cascade C–H activation, carbene insertion, and intramolecular lactonization. The reaction exhibits high atom economy, good functional group tolerance, and high regioselectivity. The synthesized compound can also behave as a potent fluorescence sensor for Fe³⁺ ion.

An efficient Rh(iii)-catalyzed annulation of arylamides with 3-diazoquinolinediones for the construction of diverse and highly functionalized isochromenoquinolinedione derivatives is described.     

Cationic palladium(ii)-catalyzed synthesis of substituted pyridines from α,β-unsaturated oxime ethers

An efficient method for the synthesis of multi-substituted pyridines from β-aryl-substituted α,β-unsaturated oxime ethers and alkenes via Pd-catalyzed C–H activation has been developed. The method, using Pd(OAc)₂ and a sterically hindered pyridine ligand, provides access to various multi-substituted pyridines with complete regioselectivity. Mechanistic studies suggest that the pyridine products are formed by Pd-catalyzed electrophilic C–H alkenylation of α,β-unsaturated oxime followed by aza-6π-electrocyclization. The utility of this method is showcased by the synthesis of 4-aryl-substituted pyridine derivatives, which are difficult to synthesize efficiently using previously reported Rh-catalyzed strategies with alkenes.

An efficient method for the synthesis of multi-substituted pyridines from α,β-unsaturated oxime ethers via cationic Pd(ii)-catalyzed C–H activation has been developed.     

The C–H activated controlled mono- and di-olefination of arenes in ionic liquids at room temperature

In this study, controlled mono and di-olefination of arenes was first realized at room temperature via the C–H bond activation in ionic liquids, probably due to the positive effects of ionic liquids. It is an energy-saving routes in industrial production without the need for heating equipment. Different catalysts were screened, and it was found that [Ru(p-cymene)Cl₂]₂ generated mono-olefinated products predominantly while [Cp*RhCl₂]₂ selectively gave di-olefinated products. These catalysts ([BMIM]NTf₂ and [BMIM]PF₆) as green and recyclable reaction media are highly efficient under mild conditions. This reaction process can avoid any volatile and environmentally toxic organic solvents, and is much safer without the need for pressure-tight equipment. A wide substrate scope with good yields and satisfactory selectivity was achieved. The reactions can be scaled up to gram-scale. Furthermore, an expensive rhodium/ruthenium catalytic system was recycled for at least 6 times with consistently high catalytic activity, which was economical and environmental friendly from an industrial point of view. According to the mechanistic study, the C–H bond cleavage was probably achieved via the concerted metalation–deprotonation. This technique can be applied in the synthesis of various valuable unsaturated aromatic compounds and shows a great potential for industrial production.

The controlled mono- and di-olefination of arenes was first realized at room temperature through C–H bond activation in ionic liquids.     

Iodine-mediated C–N and N–N bond formation: a facile one-pot synthetic approach to 1,2,3-triazoles under metal-free and azide-free conditions

A novel strategy towards the synthesis of 1,4-disubstituted 1,2,3-triazoles via C–N and N–N bond formation has been demonstrated under transition metal-free and azide-free conditions. These 1,2,3-triazoles were obtained in a regioselective manner from commercially available anilines, aryl alkenes/aryl alkynes and N-tosylhydrazines using I₂ under O₂ atmosphere. Broad substrate scope, milder reaction conditions, good to moderate yields and clean protocol are the notable features of the method. Moreover, this protocol is amenable for the generation of a library of medicinally important key building blocks.

A novel strategy towards the synthesis of 1,4-disubstituted 1,2,3-triazoles via C–N and N–N bond formation has been demonstrated under transition metal-free and azide-free conditions.     

Oxidative radical coupling of hydroquinones and thiols using chromic acid: one-pot synthesis of quinonyl alkyl/aryl thioethers

An efficient, simple and practical protocol for one-pot sequential oxidative radical C–H/S–H cross-coupling of thiols with hydroquinones (HQs) and oxidation leading to the formation of quinonyl alkyl/aryl thioethers using H₂CrO₄ was developed. This cross-coupling of thiyl and aryl radicals offers mono thioethers in good to moderate yield and works well with a wide variety of thiols. Similarly, this method works well for coupling of 2-amino thiophenol and HQs to form phenothiazine-3-ones 5a–c. C–S bond formation via thioether synthesis was observed using a chromium reagent for the first time. Theoretical studies on the pharmacokinetic properties of compounds 5a–c revealed that due to drug-like properties, compound 5b strongly binds with Alzheimer''s disease (AD) associated AChE target sites.

Oxidative radical C–H/S–H cross coupling of hydroquinones and thiols and oxidation to quinone using a H₂CrO₄ system was developed.     

The green synthesis of a palm empty fruit bunch-derived sulfonated carbon acid catalyst and its performance for cassava peel starch hydrolysis

A sulfonated carbon acid catalyst (C–SO₃H) was successfully generated from palm empty fruit bunch (PEFB) carbon via hydrothermal sulfonation via the addition of hydroxyethylsulfonic acid and citric acid. The C–SO₃H catalyst was identified as containing 1.75 mmol g⁻¹ of acid and 40.2% sulphur. The surface morphology of C–SO₃H shows pores on its surface and the crystalline index (CrI) of PEFB was decreased to 63.8% due to the change structure as it became carbon. The surface area of the carbon was increased significantly from 11.5 to 239.65 m² g⁻¹ after sulfonation via hydrothermal treatment. The identification of –SO₃H, COOH and –OH functional groups was achieved using Fourier-transform infrared spectroscopy. The optimal catalytic activity of C–SO₃H was achieved via hydrolysis reaction with a yield of 60.4% of total reducing sugar (TRS) using concentrations of 5% (w/v) of both C–SO₃H and cassava peel starch at 100 °C for 1 h. The stability of C–SO₃H shows good performance over five repeated uses, making it a good potential candidate as a green and sulfonated solid acid catalyst for use in a wide range of applications.

A sulfonated carbon acid catalyst (C–SO₃H) was successfully generated from palm empty fruit bunch (PEFB) carbon via hydrothermal sulfonation via the addition of hydroxyethylsulfonic acid and citric acid.     

Iron catalyzed C–C dehydrogenative coupling reaction: synthesis of arylquinones from quinones/hydroquinones

An atom-economical approach for the synthesis of arylquinones was achieved successfully via direct oxidative C–C dehydrogenative coupling reaction of quinones/hydroquinones with electron-rich arenes using an inexpensive Fe–I₂–(NH₄)₂S₂O₈ system. The efficiency of this catalytic approach was established with a broad scope of substrates involving quinones and hydroquinones to give high yields (60–89%) of several arylated quinones. The present protocol is simple, practical, and shows good functional group tolerance.

The synthesis of arylquinones was achieved via direct oxidative C–H/C–H cross-coupling of quinones/hydroquinones with electron-rich arenes using Fe–I₂–(NH₄)₂S₂O₈ system involving quinones/hydroquinones to give high yields (60–89%) of arylquinones.     

Site-specific role of bifunctional graphitic carbon nitride catalyst for the sustainable synthesis of 3,3-spirocyclic oxindoles in aqueous media

Functionalized graphitic carbon nitride (Sg-C₃N₄) has been manufactured and used as a reusable catalyst for the one-pot production of various spiro-pyrano chromenes and spiro indole-3,1′-naphthalene tetracyclic systems in aqueous media. An ultrasound-assisted method has been used for the functionalization of g-C₃N₄. The catalytic functionalities and the structural integrity of the catalyst were characterized via different analytical tools. The catalytic site-specific role of Sg-C₃N₄ was confirmed via various control experiments in one-pot reaction sequences. We recognized that Sg-C₃N₄ acts as a bifunctional acid–base catalyst for the first reaction sequence whereas it is an acidic catalyst for the second reaction sequence during the one-pot production of various spiro-pyrano chromenes. In addition, the bifunctional acid–base catalytic role of Sg-C₃N₄ has been confirmed for the first reaction sequence whereas it has a basic catalytic role for the second reaction sequence during the one-pot production of spiro indole-3,1′-naphthalene tetracyclic systems. Diverse C–C, C–O, and C–N bonds, six-membered cycles, stereogenic centers, and spiro frameworks were formed in a single reaction, enhancing the biocidal profile and possibly resulting in the discovery of new medicinal properties. The mild reaction environment, simple workup, easy separation, low cost, heterogeneity, and recyclability of Sg-C₃N₄ are some rewards of this approach.

Functionalized graphitic carbon nitride (Sg-C₃N₄) has been manufactured and used as a reusable catalyst for the one-pot production of various spiro-pyrano chromenes and spiro indole-3,1′-naphthalene tetracyclic systems in aqueous media.     

Palladium nanoparticles immobilized on a nano-silica triazine dendritic polymer: a recyclable and sustainable nanoreactor for C–S cross-coupling

Dendrimers are of great interest due to their special structural topology and chemical versatility. Owing to their properties, dendrimers have found practical applications in catalytic processes as efficient nanoreactors. Therefore, we herein report an environmentally attractive strategy and highly efficient route for the synthesis of a wide variety of diaryl sulfides using palladium nanoparticles immobilized on a nano-silica triazine dendritic polymer (Pd_np-nSTDP) as a nanoreactor. In this manner, different diaryl or aryl heteroaryl sulfides and bis(aryl/heteroarylthio)benzene/anthracene/pyridine derivatives were prepared via C–S cross-coupling reactions of aryl halides with diaryl/diheteroaryl disulfides under thermal conditions and microwave irradiation. The catalyst could be easily recovered and reused several times without any significant loss of its activity.

Efficient synthesis of diaryl sulfides via C–S cross-coupling of aryl halides with disulfides catalyzed by Pd_np-nSTDP is reported.     

Mechanistic insights into rare-earth-catalysed C–H alkylation of sulfides: sulfide facilitating alkene insertion and beyond

The catalytic C–H alkylation with alkenes is of much interest and importance, as it offers a 100% atom efficient route for C–C bond construction. In the past decade, great progress in rare-earth catalysed C–H alkylation of various heteroatom-containing substrates with alkenes has been made. However, whether or how a heteroatom-containing substrate would influence the coordination or insertion of an alkene at the catalyst metal center remained elusive. In this work, the mechanism of Sc-catalysed C–H alkylation of sulfides with alkenes and dienes has been carefully examined by DFT calculations, which revealed that the alkene insertion could proceed via a sulfide-facilitated mechanism. It has been found that a similar mechanism may also work for the C–H alkylation of other heteroatom-containing substrates such as pyridine and anisole. Moreover, the substrate-facilitated alkene insertion mechanism and a substrate-free one could be switched by fine-tuning the sterics of catalysts and substrates. This work provides new insights into the role of heteroatom-containing substrates in alkene-insertion-involved reactions, and may help guide designing new catalysis systems.

The alkene insertion via the heteroatom-containing substrate facilitated mechanism were computationally revealed in rare-earth-catalyzed C–H alkylation of sulfides and other heteroatom-containing substrates such as pyridines and anisoles.     

Metal-free site-selective C–H cyanoalkylation of 8-aminoquinoline and aniline-derived amides with azobisisobutyronitrile

Using K₂S₂O₈, an efficient and metal-free site-selective C–H cyanoalkylation of 8-aminoquinoline and aniline-derived amides with AIBN (azobisisobutyronitrile) was developed. Without any catalyst, various substrates and functional groups were compatible to afford corresponding products in moderate to high yields. A mechanism study displayed that a radical–radical coupling process was involved via the N-centered radical generation and delocalization of aryl amides.

An efficient metal-free cyanoalkylation of 8-aminoquinoline and aniline-derived amides was achieved in the presence of K₂S₂O₈. The method showed good substrate tolerance and also suitable for bromination and dimerization reactions.     

Mg3N2-assisted one-pot synthesis of 1,3-disubstituted imidazo[1,5-a]pyridine

A novel Mg₃N₂-assisted one-pot annulation strategy has been developed via cyclo-condensation reaction of 2-pyridyl ketones with alkyl glyoxylates or aldehydes, allowing the formation of imidazo[1,5-a]pyridines exclusively with an exellent yield.

A novel Mg₃N₂-assisted one-pot annulation strategy has been developed via cyclo-condensation reaction of 2-pyridyl ketones with alkyl glyoxylates or aldehydes, allowing the formation of imidazo[1,5-a]pyridines exclusively with an exellent yield.