A Metal- and Azide-Free Multicomponent Assembly toward Regioselective Construction of 1,5-Disubstituted 1,2,3-Triazoles

Recent Progress on Synthesis of Functionalized 1,5-Disubstituted Triazoles

Immediately after the invention of 'Click Chemistry' in 2002, the regioselective 1,2,3- triazole scaffolds resulted from respective organic azides and terminal alkynes under Cu(I) catalysis have been well recognized as the functional heterocyclic core at the centre of modern organic chemistry, medicinal chemistry, and material sciences. This CuAAC reaction has several notable features including excellent regioselectivity, high-to-excellent yields, easy to execute, short reaction time, modular in nature, mild condition, readily available starting materials, etc. Moreover, the resulting regioselective triazoles can serve as amide bond isosteres, a privileged functional group in drug discovery and development. More than hundreds of reviews had been devoted to the 'Click Chemistry' in special reference to 1,4-disubstituted triazoles, while only little efforts were made for an opposite regioisomer i.e., 1,5-disubstituted triazole. Herein, we have presented various classical approaches for an expeditious synthesis of a wide range of biologically relevant 1,5- disubstituted 1,2,3-triazole analogues. The syntheses of such a class of diversly functionalized triazoles have emerged as a crucial investigation in the domain of chemistry and biology. This tutorial review covers the literature assessment on the development of various synthetic protocols for the functionalized 1,5-disubstituted triazoles reported during the last 12 years.

Organocatalyzed Regioselective Synthesis of 1,5-Disubstituted 1,2,3-Triazolyl Glycoconjugates

A novel organocatalyzed [3+2] cycloaddition reaction of nitroolefins with glycosyl azides as well as organic azides has been developed for successful construction of 1,5-disubstituted triazolyl glycoconjugates. This metal-free and acid-free, regioselective synthetic protocol proceeds in the presence of only Schreiner thiourea organocatalysts, which enable the required activation of nitroolefins through double hydrogen bonding. The straightforward, operationally simple, and regioselectivity of this methodology, complementing to the classical RuAAC catalyzed synthesis of 1,5-disubstituted 1,2,3-triazoles. In the presence of catalytic amount of Schreiner thiourea organocatalyst, organic azides react with a broad array of nitroolefins producing a series of diverse 1,5-disubstituted 1,2,3- triazoles in good yields with excellent regioselectivity.

Versatile Synthetic Platform for 1,2,3-Triazole Chemistry

1,2,3-Triazole scaffolds are not obtained in nature, but they are still intensely investigated by synthetic chemists in various fields due to their excellent properties and green synthetic routes. This review will provide a library of all synthetic routes used in the past 21 years to synthesize 1,2,3-triazoles and their derivatives using various metal catalysts (such as Cu, Ni, Ru, Ir, Rh, Pd, Au, Ag, Zn, and Sm), organocatalysts, metal-free as well as solvent- and catalyst-free neat syntheses, along with their mechanistic cycles, recyclability studies, solvent systems, and reaction condition effects on regioselectivity. Constant developments indicate that 1,2,3-triazoles will help lead to future organic synthesis and are useful for creating molecular libraries of various functionalized 1,2,3-triazoles.

Synthesis methods of 1,2,3-/1,2,4-triazoles: A review

Triazole, comprising three nitrogen atoms and two carbon atoms, is divided into two isomers 1,2,3-triazole and 1,2,4-triazole. Compounds containing a triazole are one of the significant heterocycles that exhibit broad biological activities, such as antimicrobial, analgesic, anti-inflammatory, anticonvulsant, antineoplastic, antimalarial, antiviral, antiproliferative, and anticancer activities. A great quantity of drugs with a triazole structure has been developed and proved, for example, ketoconazole and fluconazole. Given the importance of the triazole scaffold, its synthesis has attracted much attention. This review summarizes the synthetic methods of triazole compounds from various nitrogen sources in the past 20 years.

Copper-Promoted One-Pot Sandmeyer-Type Reaction for the Synthesis of N-Aryltriazoles

Herein, we report the copper-catalyzed one-pot Sandmeyer-type reaction of aromatic amines with triazoles to afford N-aryl-1,2,3-triazoles. Diazonium salts, formed from aromatic amines and tert-butyl nitrite in the presence of fluoroboric acid, reacted with triazoles in a copper-catalyzed Sandmeyer-type reaction. The reaction proceeded under mild conditions to afford N-aryltriazoles in moderate to good yields. This method is amenable to a wide range of aromatic amines and triazoles and shows diverse functional group tolerance. Inhibition of the reaction upon the addition of free radical scavengers suggested a radical pathway, in which the aryl radical, copper, and triazole formed a complex that underwent reductive elimination to give aryltriazole compounds; this is consistent with the mechanism underlying the Sandmeyer reaction. Thus, we demonstrate a new effective strategy for the construction of C-N bonds via Sandmeyer-type reactions and a valuable alternative approach for the synthesis of aryltriazole derivatives.

A magnetic porous organic polymer: catalytic application in the synthesis of hybrid pyridines with indole, triazole and sulfonamide moieties

Herein, the synthesis and characterization of a triazine-based magnetic ionic porous organic polymer are reported. The structure, morphology, and components of the prepared structure have been investigated with several spectroscopic and microscopic techniques such as FT-IR, EDX, elemental mapping, TGA/DTA, SEM, TEM, VSM, and BET analysis. Also, catalytic application of the prepared triazine-based magnetic ionic porous organic polymer was investigated for the synthesis of hybrid pyridine derivatives bearing indole, triazole and sulfonamide groups. Furthermore, the prepared hybrid pyridine systems were characterized by FT-IR, 1H NMR, 13C NMR and mass analysis. A cooperative vinylogous anomeric-based oxidation pathway was suggested for the synthesis of target molecules.

Recent advances in transition metal-free annulation toward heterocycle diversity based on the C-N bond cleavage of enaminone platform

The enaminones and analogous stable enamines are well known as platform building blocks in organic synthesis for construction of heterocyclic compounds, especially N-heterocycles. To date, especially enaminones have been successfully...

Metal-Free Synthesis of 1,5-Disubstituted 1,2,3-Triazoles

A three-component synthesis of 1,5-disubstituted 1,2,3-triazoles from α-acetyl-α-diazomethane sulfonamides, primary aliphatic amines, and aromatic aldehydes is presented. The 1,2,3-triazoles can be accessed in two alternative variants, depending on the substitutions in the sulfonamide portion of the diazo reagent. In one variant, intermediate 1,2,3-triazoline-4-sulfonamides are isolated chromatographically and then subjected to thermally promoted aromatization with elimination of sulfur(IV) oxide and amine. In the other variant, both chemical transformations take place in a single step conducted at room temperature.

Recent Progress in Catalytic Synthesis of 1,2,3-Triazoles

1,2,3-triazoles represent a functional heterocyclic core that has been at the center of modern organic chemistry since the beginning of click chemistry. Being a versatile framework, such an aromatic ring can be observed in uncountable molecules useful in medicine and photochemistry, just to name a few. This review summarizes the progress achieved in their synthesis from 2015 to today, with particular emphasis on the development of new catalytic and eco-compatible approaches. In doing so, we subdivided the report based on their degree of functionalization and, for each subparagraph, we outlined the role of the catalyst employed.

Substituted 1,2,3-triazoles: a new class of nitrification inhibitors

Nitrogen (N) fertilisers amended with nitrification inhibitors can increase nitrogen use efficiencies in agricultural systems but the effectiveness of existing commercial inhibitor products, including 3,4-dimethylpyrazole phosphate (DMPP), is strongly influenced by climatic and edaphic factors. With increasing pressure to reduce the environmental impact of large-scale agriculture it is important to develop new nitrogen-stabilising products that can give reliable and consistent results, particularly for warmer climatic conditions. We synthesised a library of 17 compounds featuring a substituted 1,2,3-triazole motif and performed laboratory incubations in two south-eastern Australian soils. In the neutral (pH 7.3) soil, the compounds N002, N013, N016 and N017, which possess short non-polar alkyl or alkynyl substituents at the triazole ring, retained NH₄⁺-N concentrations at 35 °C soil temperature to a better extent (P < 0.001) than DMPP. In the alkaline soil (pH 8.8) N013 performed better with regards to NH₄⁺-N retention (P = 0.004) than DMPP at 35 °C soil temperature. Overall, our data suggest that substituted 1,2,3-triazoles, which can be synthesized with good yields and excellent atom economy through 1,3-dipolar cycloaddition from readily available starting materials, are promising nitrification inhibitors performing similar to, or better than DMPP, particularly at elevated soil temperatures.

Metal-free syntheses of N-functionalized and NH-1,2,3-triazoles: an update on recent developments

An overview of the latest developments in the metal-free synthesis of non-benzo-fused N-functionalized and NH-1,2,3-triazoles is provided in this feature article. Synthetic studies that appeared from 2016 until August 2020 are organized according to a wide-ranging classification, comprising oxidative and eliminative azide-dipolarophile cycloadditions, diazo transfer reactions and N-tosylhydrazone-mediated syntheses. The newly developed methods constitute a significant contribution to the field of 1,2,3-triazole synthesis in terms of structural variation via either the exploration of novel reactions, or the exploitation of existing methodologies.

Synthesis of (E)-Quinoxalinone Oximes through a Multicomponent Reaction under Mild Conditions

Herein, a novel method for the gram-scale synthesis of (E)-quinoxalinone oximes through a multicomponent reaction under mild conditions is described. Such a transformation was performed under transition-metal-free conditions, affording (E)-oximes in a moderate-to-good yield through recrystallization. Our methodology demonstrates a successful combination of a Mannich-type reaction and radical coupling, providing a green and practical approach for the synthesis of potentially bioactive quinoxalinone-containing molecules.

Triazolization of Enolizable Ketones with Primary Amines: A General Strategy toward Multifunctional 1,2,3-Triazoles

The development of metal-free syntheses toward 1,2,3-triazoles has been a burgeoning research area throughout the past decade. Despite the numerous advances, the scarceness of methods for the preparation of 1,5-disubstituted 1,2,3-triazoles from readily available substrates remained a challenge that was addressed by our group in 2016. A metal-free three-component reaction, which we have dubbed the triazolization reaction, was established for the rapid synthesis of 1,5-disubstituted, fully functionalized and NH-1,2,3-triazoles. This novel approach stands out because it utilizes widely available starting materials, namely primary amines and enolizable ketones. Furthermore, the broad substrate scope is a major advantage, and was further expanded by the number of modified protocols that have been reported. Triazolization products have successfully found utility as intermediates in various synthetic transformations, and were the subject of a few interesting biological activity studies.

Recent Advances in Organic Synthesis Based on N,N-Dimethyl Enaminones

Enaminones are gaining increasing interest because of their unique properties and their importance in organic synthesis as versatile building blocks. N,N-Dimethyl enaminones offer a better leaving group (a dimethylamine group) than other enaminones, and allow further elaboration via a range of facile chemical transformations. Over the past five years, there have been an increasing number of reports describing the synthetic applications of N,N-dimethyl enaminones. This review provides a comprehensive overview on the synthetic applications of N,N-dimethyl enaminones that have been reported since 2016.

1 Introduction

2 Direct C(sp2)–H α-Functionalization

2.1 Synthesis of α-Sulfenylated N,N-Dimethyl Enaminones

2.2 Synthesis of α-Thiocyanated N,N-Dimethyl Enaminones

2.3 Synthesis of α-Acyloxylated N,N-Dimethyl Enaminones

3 Functionalization Reactions via C=C Double Bond Cleavage

3.1 Synthesis of Functionalized Methyl Ketones

3.2 Synthesis of α-Ketoamides, α-Ketoesters and 1,2-Diketones

3.3 Synthesis of N-Sulfonyl Amidines

4 Construction of All-Carbon Aromatic Scaffolds

4.1 Synthesis of Benzaldehydes

4.2 Synthesis of the Naphthalenes

5 Construction of Heterocyclic Scaffolds

5.1 Synthesis of Five-Membered Heterocycles

5.2 Synthesis of Six-Membered Heterocycles

5.3 Synthesis of Quinolines

5.4 Synthesis of Functionalized Chromones

5.5 Synthesis of Other Fused Polycyclic Heterocycles

6 Conclusions and Perspectives

A simple route towards the synthesis of 1,4,5-trisubstituted 1,2,3-triazoles from primary amines and 1,3-dicarbonyl compounds under metal-free conditions

An acetic acid-promoted approach that enables the synthesis of 1,4,5-trisubstituted 1,2,3-triazole derivatives has been achieved. This transformation employs readily available primary amines, 1,3-dicarbonyls and tosyl azide as the starting materials via a cycloaddition reaction under metal-free conditions. The reaction provides a simple access to fully substituted 1,2,3-triazoles from commercial substrates in moderate to excellent yields.

In-Water Synthesis of 5-Thiolated 1,2,3-Triazoles from β-Thioenaminones by Diazo Transfer Reaction

The synthesis of 1,2,3-triazoles with a sulfur-based side chain has been accessed with the metal-free annulation reactions of readily available β-thiolated enaminones and tosyl hydrazine. By these reactions with water as the only medium, a broad array of 5-thiolated 1,2,3-triazoles have been synthesized with generally good to excellent yields. Except using TMEDA (N,N,N',N'-tetramethylethylenediamine) as the only base promoter, not any other catalyst or additive is required, thus providing an efficient and environmentally benign method for useful 1,2,3-triazole synthesis.

Transition-metal-free regioselective construction of 1,5-diaryl-1,2,3-triazoles through dehydrative cycloaddition of alcohols with aryl azides mediated by SO2F2

A novel, simple and practical method for mild, efficient, cost-effective and regioselective synthesis of highly valuable 1,5-diaryl-1,2,3-triazoles was achieved through dehydrative annulation of readily available alcohols with aryl azides. The reaction proceeded at room temperature, without any metal catalysts, exhibiting excellent compatibility to a large variety of functional groups (>50 examples), resulting in up to quantitative yields.

Domino C-H Sulfonylation and Pyrazole Annulation for Fully Substituted Pyrazole Synthesis in Water Using Hydrophilic Enaminones

The cascade reactions between NH₂-functionalized enaminones and sulfonyl hydrazines have been developed for the synthesis of fully substituted pyrazoles. By making use of the hydrophilic primary amino group in the enaminones, the reactions proceed well in the medium of pure water in the presence of molecular iodine, TBHP, and NaHCO₃ via cascade C-H sulfonylation and pyrazole annulation. The cleavage of the C-N bond in enaminones is confirmed by the experiment using ¹⁵ N-labeled enaminone.

Direct Transformation of Terminal Alkynes into Amidines by a Silver-Catalyzed Four-Component Reaction

An unprecedented conversion of terminal alkynes into N-sulfonimidamides (amidines) is reported by a silver-catalyzed, one-pot, four-component reaction with TMSN₃, sodium sulfinate, and sulfonyl azide. The reaction scope includes both aromatic and aliphatic alkynes. A possible cascade reaction mechanism, consisting of alkyne hydroazidation, sulfonyl radical addition, 1,3-dipolar cycloaddition by TMSN₃, and retro-1,3-dipolar cycloaddition, is proposed. TMSN₃ is found to play an essential role in each step of the reaction.

Catalyst-free synthesis of 4-acyl-NH-1,2,3-triazoles by water-mediated cycloaddition reactions of enaminones and tosyl azide

The synthesis of 4-acyl-NH-1,2,3-triazoles has been accomplished with high efficiency through the cycloaddition reactions between N,N-dimethylenaminones and tosyl azide. This method is featured with extraordinary sustainability by employing water as the sole medium, free of any catalyst or additive, authentically mild conditions (40 °C stirring) as well as practical scalability.

An efficient synthesis of 4,5-disubstituted-2H-1,2,3-triazoles from nitroallylic derivatives via a cycloaddition–denitration process

A variety of nitroallylic derivatives were smoothly reacted with sodium azide in the presence of p-TsOH to form synthetically-viable triazoles.

Iodine/DMSO oxidations: a contemporary paradigm in C–N bond chemistry

A new era in the organic synthetic world is demanding greener protocols for the execution of reactions.