Polysubstituted 2-Aminopyrrole Synthesis via Gold-Catalyzed Intermolecular Nitrene Transfer from Vinyl Azide to Ynamide: Reaction Scope and Mechanistic Insights

Gold-Catalyzed Cyclization of Yndiamides with Isoxazoles via α-Imino Gold Fischer Carbenes

Gold catalysis is an important method for alkyne functionalization. Here we report the gold-catalyzed formal [3+2] aminative cyclization of yndiamides and isoxazoles in a direct synthesis of polysubstituted diaminopyrroles, which are important motifs in drug discovery. Key to this process is the formation, and subsequent cyclization, of an α-imino gold Fischer carbene, which represents a new type of gold carbene intermediate. The reaction proceeds rapidly under mild conditions, with high regioselectivity being achieved by introducing a subtle steric bias between the nitrogen substituents on the yndiamide. DFT calculations revealed that the key to this regioselectivity was the interconversion of isomeric gold keteniminiun ions via a low-barrier π-complex transition state, which establishes a Curtin-Hammett scenario for isoxazole addition. By using benzisoxazoles as substrates, the reaction outcome could be switched to a formal [5+2] cyclization, leading to 1,4-oxazepines.

Triflamides and Triflimides: Synthesis and Applications

Among the variety of sulfonamides, triflamides (CF₃SO₂NHR, TfNHR) occupy a special position in organic chemistry. Triflamides are widely used as reagents, efficient catalysts or additives in numerous reactions. The reasons for the widespread use of these compounds are their high NH-acidity, lipophilicity, catalytic activity and specific chemical properties. Their strong electron-withdrawing properties and low nucleophilicity, combined with their high NH-acidity, makes it possible to use triflamides in a vast variety of organic reactions. This review is devoted to the synthesis and use of N-trifluoromethanesulfonyl derivatives in organic chemistry, medicine, biochemistry, catalysis and agriculture. Part of the work is a review of areas and examples of the use of bis(trifluoromethanesulfonyl)imide (triflimide, (CF₃SO₂)₂NH, Tf₂NH). Being one of the strongest NH-acids, triflimide, and especially its salts, are widely used as catalysts in cycloaddition reactions, Friedel–Crafts reactions, condensation reactions, heterocyclization and many others. Triflamides act as a source of nitrogen in C-amination (sulfonamidation) reactions, the products of which are useful building blocks in organic synthesis, catalysts and ligands in metal complex catalysis, and have found applications in medicine. The addition reactions of triflamide in the presence of oxidizing agents to alkenes and dienes are considered separately.

Gold(I)-Catalyzed Synthesis of Heterocycles via Allene Oxide from Propargylic Alcohols

A new mechanistic pathway of propargylic alcohol activation by gold(I) catalysis has been proposed toward the efficient synthesis of N-protected pyrroles, 5,6-dihydropyridin-3(4H)-ones from N-protected 5-aminopent-2-yn-1-ol, and 5-aminopent-2-yn-1-ol. Control experiments support that the reaction proceeded via the neighboring group participation of the oxygen atom of propargylic alcohol to form an allene oxide intermediate where the nucleophilic heteroatom attacks intramolecularly. Further, this methodology is successfully extrapolated toward the atom-economic synthesis of hydroxyalkyl indoles and benzofurans. The short reaction time of 30 s, low catalyst loading of 0.5 mol %, high yield, variation in the substrate scope, and procedurally simple open-flask reaction conditions make this methodology highly applied.

Dichotomy of platinum(II) and gold(III) carbene intermediates switching from N- to O-selectivity

Pt(II) and Au(III)-mediated intermolecular divergent annulations of benzofurazans and ynamides highlighted the N- to O-selectivity of tunable metal carbene intermediates. PtCl₂ with a bulky phosphite ligand resulted in the specific synthesis of six-membered quinoxaline N-oxides and successfully suppressed the in-situ deoxygenation of N-oxides. On the other hand, an unique gold(III) catalyst (2,6-di-MeO-PyrAuCl₃) led to the five-membered ring products, benzimidazoles. A broad scope of functional groups was well compatible, delivering better yields and selectivities in contrast to conventional gold(I) catalysts. The different behavior of presumed platinum(II) and gold(III) carbenes with respect to chemoselectivity was intensively examined by experiments and DFT calculations. A detailed mechanistic study, based on DFT calculations, revealed that the highly electrophilic carbocation-like gold(III) carbene triggers an oxophilic cyclization, followed by a cascade ring contraction and acyl migration. On the contrary, the Pt carbene species is less cationic, favoring the formation of the six-membered ring via N-attack.

Benzofurazan, a cyclic heterocycle, can form open-chain metal carbene species in the presence of suitable catalysts. Here the authors show divergent reactivity when using gold(III) and platinum(II) catalysts, and perform computational and experimental mechanistic studies to explain the differing reactivity

Efficient access to 1,3,4-trisubstituted pyrroles via gold-catalysed cycloisomerization of 1,5-diynes

A gold-catalysed cycloisomerization of 1,5-diynes is described, which offers a selective approach to access 1,3,4-trisubstituted pyrroles. In this reaction, the cationic gold catalyst activates the ynamide moiety, initiating the cycloisomerization to produce the pyrrole core, and H₂O acts as an external nucleophile to trap the vinyl cationic species, thus leading to the formation of 1,3,4-trisubstituted pyrroles with high selectivity.

Insights into gold-catalyzed formation of aza-heterocycles using benzofuroxans as nitrene transfer reagents: mechanism and origins of chemoselectivity

Density functional theory (DFT) calculations have been performed to reveal the mechanism of gold(i)-catalyzed annulation of N-allylynamides and benzofuroxans as nitrene transfer reagents to construct azaheterocyclic compounds. The calculated results revealed that the reaction mechanism mainly undergoes eight processes. Among the reaction steps, intramolecular nucleophilic attack of the imino N atom on the α-position of activated gold keteniminium is a rate-determining process, which is different from that proposed previously by experiment. The chemoselectivity of the products is controlled by competition between the cyclopropanation of α-imino gold carbenes and intramolecular nucleophilic attack of the phenyl ring on α-imino gold carbenes, and could be explained by NPA charge. The different yields of cyclopropanated product in different solvents are dictated by the relative polarity leading to the different energy barriers of the rate-determining steps. The present work expounds the experimental observation at the molecular level and is informative for exploring efficient syntheses of 3-azabicyclo[3.1.0]hexanes.

DFT calculations were employed to study the mechanism and origin of chemoselectivity of gold-catalyzed annulation of N-allylynamides and benzofuroxans as nitrene transfer reagents.

Tandem Synthesis of Tetrasubstituted NH Pyrroles from Simple Nitriles

An efficient tandem route to obtain tetrasubstituted NH pyrroles in a one-pot manner has been developed, staring from simple nitriles, ethyl bromoacetates, and zinc. This reaction involves oxidative dimerization of the zinc bromide complex of β-enaminoesters using cerium ammonium nitrate (CAN) as an oxidant, affording 2,3,4,5-tetrasubstituted pyrroles in yields up to 91%.

Gold-Catalyzed Annulations with Nucleophilic Nitrenoids Enabled by Heteroatom-Substituted Alkynes

The combination of a nucleophilic nitrene equivalent, a triple bond and a π-acid catalyst has underpinned numerous efficient transformations for the preparation of azacycles. This personal account details our efforts in developing an annulation strategy. Adding a nucleophilic nitrenoid to an activated alkyne can generate carbenoid character that is then quenched by a cyclisation onto the nitrenoid substituent. The use and development of N-acyl and N-heterocyclic pyridinium-N-aminides as 1,3-N,O and 1,3-N,N-dipole equivalents is discussed in the context of oxazole and heterocycle-fused imidazole formation, respectively. The resulting processes are highly efficient, practically straightforward, and tolerate considerable structural and functional group variation. Our use of heteroatom-substituted alkynes as enabling tools for reaction discovery is discussed. The reactivity accessed from the strong donor-like properties of ynamides is complemented by that obtained from alkynyl thioethers, which are emerging as interesting substrates for π-acid catalysis.

Gold-Catalyzed Transformation of Ynamides

Ynamide, a unique species with inherited polarization of nitrogen lone pair electron to triple bond, has been largely used for the developement of novel synthetic methods and the construction of unusual N-bearing heterocycles. The reaction versatility of ynamide on umpolung reactivity, radical reactions and asymmetric synthesis have been recently reviewed. This review provides an overall scenic view into the gold catalyzed transformation of ynamides. The ynamides reactivity towards nitrogen-transfer reagents, such as azides, nitrogen ylides, isoxazoles, and anthranils; oxygen atom-transfer reagents, like nitrones, sulfoxides, and pyridine N-oxides; and carbon nucleophiles under gold catalysis are herein uncovered. The scope as well the mechanistic insights of each reaction is also briefed.

Direct synthesis of pentasubstituted pyrroles and hexasubstituted pyrrolines from propargyl sulfonylamides and allenamides

Multisubstituted pyrroles are important fragments that appear in many bioactive small molecule scaffolds. Efficient synthesis of multisubstituted pyrroles with different substituents from easily accessible starting materials is challenging. Herein, we describe a metal-free method for the preparation of pentasubstituted pyrroles and hexasubstituted pyrrolines with different substituents and a free amino group by a base-promoted cascade addition-cyclization of propargylamides or allenamides with trimethylsilyl cyanide. This method would complement previous methods and support expansion of the toolbox for the synthesis of valuable, but previously inaccessible, highly substituted pyrroles and pyrrolines. Mechanistic studies to elucidate the reaction pathway have been conducted.

Recent progress towards the transition-metal-catalyzed Nazarov cyclization of alkynes via metal carbenes

In recent years, transition-metal-catalyzed tandem cyclization reactions of alkynes, especially those involving a metal carbene intermediate, have received worthwhile interest, as this type of reaction does not require the use of risky and potentially explosive diazo compounds as starting materials for carbene generation. A significant and general strategy for the stereospecific synthesis of 5-membered cycles is Nazarov cyclization based on the 4π-conrotatory electrocyclization of a conjugated pentadienyl cation to afford a cyclopentenyl cation. In this review, we introduce an overview of recent advances in the transition-metal-catalyzed Nazarov cyclization of alkynes via a metal carbene intermediate, and categorize these reactions according to the structure of the metal carbene. Our aim is to accelerate advancements in this enchanting area of research.

Gold(I)-Catalyzed Synthesis of 3-Sulfenyl Pyrroles and Indoles by a Regioselective Annulation of Alkynyl Thioethers

The combination of nucleophilic nitrenoids and π-acid catalysis has emerged as a powerful tool in heterocycle synthesis. Accessing more varied heterocycle-substitution patterns by maintaining the same reaction pathways across different alkynes remains a challenge. Here we show that Au(I) catalysis of isoxazole-based nitrenoids with alkynyl thioethers provides controlled access to (3 + 2) annulation by a regioselective addition β to the sulfenyl group. The reaction with isoxazole-containing nitrenoids delivers sulfenylated pyrroles and indoles as single regioisomers bearing useful functional groups and structural variety.

Short and Modular Synthesis of Substituted 2-Aminopyrroles

We herein describe a simple and metal-free domino methodology to synthesize 2-aminopyrroles from alkynyl vinyl hydrazides. The domino reaction involves a novel propargylic 3,4-diaza-Cope rearrangement and a tandem isomerization/5-exo-dig N-cyclization reaction. By using this approach, a number of 2-aminopyrroles with diverse substituents have been prepared.

Reactivity of ynamides in catalytic intermolecular annulations

Ynamides are unique alkynes with a carbon-carbon triple bond directly attached to the nitrogen atom bearing an electron-withdrawing group. The alkyne is strongly polarized by the electron-donating nitrogen atom, but its high reactivity can be finely tempered by the electron-withdrawing group. Accordingly, ynamides are endowed with both nucleophilic and electrophilic properties and their chemistry has been an active research field. The catalytic intermolecular annulations of ynamides, featuring divergent assembly of structurally important amino-heterocycles in a regioselective manner, have gained much attention over the past decade. This review aims to provide a comprehensive summary of the advances achieved in this area involving transition metal and acid catalysis. Moreover, the intermolecular annulations of ynamide analogs including ynol ethers and thioalkynes are also discussed, which can provide insights into the reactivity difference caused by the heteroatoms.

Recent Progress in the Synthesis of Pyrroles

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Pyrrole derivatives are nitrogen-containing heterocyclic compounds and widely distributed in a large number of natural and non-natural compounds. These compounds possess a broad spectrum of biological activities such as anti-infammatory, antiviral, antitumor, antifungal, and antibacterial activities. Besides their biological activity, pyrrole derivatives have also been applied in various areas such as dyes, conducting polymers, organic semiconductors.

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Due to such a wide range of applicability, access to this class of compounds has attracted intensive research interest. Various established synthetic methods such as Paal-Knorr, Huisgen, and Hantzsch have been modified and improved. In addition, numerous novel methods for pyrrole synthesis have been discovered. This review will focus on considerable studies on the synthesis of pyrroles, which date back from 2014.

Gold-Catalyzed Oxidative Cascade Cyclization of 1,3-Diynamides: Polycyclic N-Heterocycle Synthesis via Construction of a Furopyridinyl Core

A facile and practical approach to construct a furopyridinyl motif through a gold-catalyzed cascade cyclization of easily accessible diynamides is described. This strategy offers a straightforward approach to furo[2,3-c]isoquinoline and 6H-furo[3',2':5,6]pyrido[3,4-b]indole derivatives. The reaction could build up four new bonds and two additional heteroaromatic rings via a single operation. The heterocyclic products show promising blue luminous performance with fluorescence quantum yields up to 75%.

Acyl Migration versus Epoxidation in Gold Catalysis: Facile, Switchable, and Atom-Economic Synthesis of Acylindoles and Quinoline Derivatives

We report a switchable synthesis of acylindoles and quinoline derivatives via gold-catalyzed annulations of anthranils and ynamides. α-Imino gold carbenes, generated in situ from anthranils and an N,O-coordinated gold(III) catalyst, undergo electrophilic attack to the aryl π-bond, followed by unexpected and highly selective 1,4- or 1,3-acyl migrations to form 6-acylindoles or 5-acylindoles. With the (2-biphenyl)di-tert-butylphosphine (JohnPhos) ligand, gold(I) carbenes experienced carbene/carbonyl additions to deliver quinoline oxides. Some of these epoxides are valuable substrates for the preparation of 3-hydroxylquinolines, quinolin-3(4H)-ones, and polycyclic compounds via facile in situ rearrangements. The reaction can be efficiently conducted on a gram scale and the obtained products are valuable substrates for preparing other potentially useful compounds. A computational study explained the unexpected selectivities and the dependency of the reaction pathway on the oxidation state and ligands of gold. With gold(III) the barrier for the formation of the strained oxirane ring is too high; whereas with gold(I) this transition state becomes accessible. Furthermore, energetic barriers to migration of the substituents on the intermediate sigma-complexes support the observed substitution pattern in the final product.

The Fe(iii)-catalyzed decarboxylative cycloaddition of β-ketoacids and 2H-azirines for the synthesis of pyrrole derivatives

An Fe(iii)-catalyzed Mannich/decarboxylative ring expansion reaction to generate pyrrole derivatives has been achieved using Fe(OTf)₃ as the catalyst. A series of pyrrole derivatives was obtained in moderate to excellent yields.

Multicomponent Cascade Reaction by Metal-Free Aerobic Oxidation for Synthesis of Highly Functionalized 2-Amino-4-coumarinyl-5-arylpyrroles

A novel approach has been constructed for the synthesis of two types of 2-amino-4-coumarinyl-5-arylpyrroles (ACAPs, 5 and 6) through a cascade reaction and a metal-free catalyzed aerobic oxidation reaction of arylglyoxal monohydrates 1, 1,1-enediamines (EDAMs) 2 and 3, and 4-hydroxy-2H-chromen-2-ones 4 via multicomponent reactions to produce the target compounds with good to excellent yields. Specifically, hydroxyl-substituted 2-amino-4-coumarinyl-5-arylpyrroles, that is, 2-amino-4-coumarinyl-5-aryl-6-hydroxylpyrroles (ACAHPs) 6, were obtained by metal-free aerobic oxidation in 1,4-dioxane at simple reflux for approximately 10 h. As a result, ACAHPs 6 have been produced without metal catalysts or traditional oxidizing agents. This method represents a route to obtain the novel ACAPs in an environmentally friendly, concise, rapid, and practical manner with potential biological activity of the product.

Switchable Reactivity between Vinyl Azides and Terminal Alkyne by Nano Copper Catalysis

A novel nano copper-catalyzed substrate-dependent chemodivergent transformation of vinyl azides with a terminal alkyne is disclosed. 2,5-Disubstituted pyrroles were selectively obtained in high yield with aryl alkynes and aliphatic alkynes, whereas 2,3,4-trisubstituted pyrroles were formed with silylated alkynes. This switchable method provides a controllable and facile access to both multisubstituted pyrrole scaffolds with high efficiency, excellent regioselectivity, and good functional group compatibility.

AuBr3 -Catalyzed Chemoselective Annulation of Isocyanates with 2H-Azirine

The chemoselective cyclization of isocyanates with 2H-azirine was achieved with AuBr₃ as catalyst. This transfer sets the stage for the synthesis of aromatic oxazole-ureas in a tandem process. The addition of a catalytic amount of phosphite enhances the process enormously. The reaction can also be performed in a one-pot process using benzoyl azide instead of isocyanate under the same conditions. A detailed study on the role of the phosphite that was applied as an additive revealed that only non-coordinated phosphite can reduce gold(III) and that gold(I) coordinated phosphite is not oxidized. Accompanied by the reduction of gold, HBr is generated in situ, which turned out to be the actual promotor in combination with the remaining AuBr₃ . The positive effect of acid can be explained by a strong N-Au coordination, which tends to break more easily in the presence of small amount of protic acid in the reaction solution.

Gold-catalyzed cyclization of 1-(2′-azidoaryl) propynols: synthesis of polysubstituted 4-quinolones

α-Imino gold carbene intermediate is produced from 1-(2′-azidoaryl) propynols, which triggers a 1,2-carbon migration and is converted to 2,3-disubstituted 4-quinolones. The reaction works under mild conditions and tolerates various functional groups.

Gold-catalyzed heterocyclic syntheses through α-imino gold carbene complexes as intermediates

This review reports a comprehensive compilation of gold-catalyzed heterocyclic synthesis by using α-imino gold carbene complexes as the proposed intermediates.

Chemodivergent reaction of azomethine imines and 2H-azirines for the synthesis of nitrogen-containing scaffolds

Metal-free reactions of 2H-azirines with C,N-cyclic azomethine imines were investigated.

Direct synthesis of aryl-annulated [c]carbazoles by gold(i)-catalysed cascade reaction of azide-diynes and arenes

The gold-catalysed annulation of conjugated alkynes bearing an azido group with arenes gave annulated [c]carbazoles. Using benzene, pyrrole, and indole derivatives as the nucleophiles, benzo[c]-, pyrrolo[2,3-c]-, and indolo[2,3-c]carbazoles were produced, respectively. The reaction proceeded through pyrrole and benzene ring construction accompanied by the formation of two carbon-carbon and one carbon-nitrogen bond and the cleavage of two aromatic C-H bonds. The mechanism of the reaction with pyrrole was investigated by density functional theory calculations. N,N'-dimethylated indolo[2,3-c]carbazole showed dual ultraviolet-visible-near-infrared and fluorescence spectral changes upon electrolysis.