PCy3-assisted Ag(I)-catalyzed click reaction for regioselective synthesis of 1,4-disubstituted 1,2,3-triazoles at room temperature

An approach towards Cu-free click chemistry has been developed in this work. Silver-catalyzed PCy3-ligand-assisted synthesis of 1,4-disubstituted 1,2,3-triazoles at room temperature has been developed. Regioselectivity of the reaction was confirmed from the results of single-crystal X-ray diffraction (SC-XRD) of one of the products. SC-XRD of ex situ-generated Ag-PCy3 complex helped us propose a plausible mechanism for the reaction. This reaction was indicated to exhibit a catalytic activity level similar to that for the in situ-generated complex. The methodology was found to work well with benzyl azides, phenyl azides, terminal alkynes and internal alkynes in aqueous medium. The one-pot three-component reaction leading to 1,2,3-triazole synthesis also proceeded well.

Isostructural Series of Ni(II), Pd(II), Pt(II), and Au(III) Azido Complexes with a N^C^N Pincer Ligand to Elucidate Trends in the iClick Reaction Kinetics and Structural Parameters of the Triazolato Products

An isoelectronic and isostructural series of cyclometalated azido complexes [M(N3)(dpb)] with M = Ni(II), Pd(II), Pt(II), and Au(III) based on the N^C^N pincer ligand 1,3-di(2-pyridyl)phenide (dpb) was characterized by X-ray diffraction analysis and investigated for reactivity in the iClick reaction with a wide range of internal and terminal alkynes by using 1H and 19F NMR spectroscopy. Reaction rate constants were found to increase with greater charge density in the order Ni(II) > Pd(II) > Pt(II) > Au(III). Terminal alkynes R-C≡C-R' with strongly electron-withdrawing groups R and R' exhibited faster kinetics than those with electron-donating substituents in the order CF3 > ketone > ester > H > phenyl ≫ amide, while R = CH3 resulted in complete loss of reactivity. Four symmetrical triazolato complexes [M(triazolatoCOOCH3,COOCH3)(dpb)] with M = Ni(II), Pd(II), Pt(II), and Au(III) as well as four nonsymmetrically substituted triazolato complexes [Pt(triazolatoR,R')(dpb)] originating from terminal and internal alkynes were shown by X-ray crystal structure analysis to exclusively feature N2-coordination of the five-membered ring ligand. However, the Pt(II) triazolato complexes exist as a mixture of N1- and N2-coordinated species in solution. Torsion angles between the mean planes of the N^C^N pincer and the triazolato ligand increase from a nearly coplanar to a perpendicular arrangement when going from Au(III)/Pt(II)/Pd(II) to Ni(II), while different substituents R and R' on the alkyne have no influence on the torsion angle and were rationalized by DFT calculations. Finally, a carbohydrate derivative obtained by glucuronic acid conjugation to methyl propiolate demonstrates the facile biofunctionalization of metal complexes via the iClick reaction.

Mechanochemical Approach towards Multi-Functionalized 1,2,3-Triazoles and Anti-Seizure Drug Rufinamide Analogs Using Copper Beads

Highly regiospecific, copper-salt-free and neat conditions have been demonstrated for the 1,3-dipolar azide-alkyne cycloaddition (AAC) reactions under mechanochemical conditions. A group of structurally challenging alkynes and heterocyclic derivatives was efficiently implemented to achieve highly functionalized 1,4-disubstituted-1,2,3-triazoles in good to excellent yield by using the Cu beads without generation of unwanted byproducts. Furthermore, the high-speed ball milling (HSBM) strategy has also been extended to the synthesis of the commercially available pharmaceutical agent, Rufinamide, an antiepileptic drug (AED) and its analogues. The same strategy was also applied for the synthesis of the Cl-derivative of Rufinamide. Analysis of the single crystal XRD data of the triazole was also performed for the final structural confirmation. The Cu beads are easily recoverable from the reaction mixture and used for the further reactions without any special treatment.

Triazoles and Their Derivatives: Chemistry, Synthesis, and Therapeutic Applications

Among the nitrogen-containing heterocyclic compounds, triazoles emerge with superior pharmacological applications. Structurally, there are two types of five-membered triazoles: 1,2,3-triazole and 1,2,4-triazole. Due to the structural characteristics, both 1,2,3- and 1,2,4-triazoles are able to accommodate a broad range of substituents (electrophiles and nucleophiles) around the core structures and pave the way for the construction of diverse novel bioactive molecules. Both the triazoles and their derivatives have significant biological properties including antimicrobial, antiviral, antitubercular, anticancer, anticonvulsant, analgesic, antioxidant, anti-inflammatory, and antidepressant activities. These are also important in organocatalysis, agrochemicals, and materials science. Thus, they have a broad range of therapeutic applications with ever-widening future scope across scientific disciplines. However, adverse events such as hepatotoxicity and hormonal problems lead to a careful revision of the azole family to obtain higher efficacy with minimum side effects. This review focuses on the structural features, synthesis, and notable therapeutic applications of triazoles and related compounds.

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.

Silver incorporated into g-C3N4/Alginate as an efficient and heterogeneous catalyst for promoting click and A3 and KA2 coupling reaction

Fe3O4/g-C3N4/Alginate-Ag nanocomposite as a novel and effective nanocatalyst was successfully prepared. This nanocomposite was fully characterized using several techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy with energy dispersive spectroscopy (FESEM-EDS), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR). In addition, the catalytic activity of this novel and characterized nanocatalyst was investigated in the regioselective synthesis of 1,4-disubstituted 1,2,3-triazoles via click reaction and A3 and KA2 coupling reaction in aqueous media. The prepared nanocatalyst was simply recovered by using an external magnet and reused for several times with a slight loss of catalytic activity.

DFT and AFIR study on the copper(i)-catalyzed mechanism of 5-enamine-trisubstituted-1,2,3-triazole synthesis via C-N cross-coupling and the origin of ring-opening of 2H-azirines

Understanding the synthesis mechanism of substituted 1,2,3-triazoles is an important and state-of-the-art research area of contemporary copper(i)-catalyzed terminal alkyne and organic azide click reaction (CuAAC), which has invoked increasing close collaborations between experiment and theory including copper catalyzed interrupted click reaction. In this study, the mechanism of Cu(i)-catalyzed 5-enamine-functionalized fully substituted 1,2,3-triazole synthesis was rationalized via density functional theory (DFT) and multicomponent artificial force-induced reaction (MC-AFIR) methods. The reasonable reaction route consists of (a) di-copper catalyzed ring-opening of 2H-azirines, (b) alkyne hydrogen atom transfer, (c) [3 + 2] ring cycloaddition, and (d) C-N bond formation through reductive elimination. The MC-AFIR method was used for the systematic determination of transition states for the C/N-Cu bond formation, C-N bond coupling and crossing points between singlet and triplet states. Our survey on the prereactant complexes suggested that the dicopper-catalyzed 2H-azirine ring-opening and alkyne hydrogen activation are both thermodynamically feasible via a singlet/triplet crossing point. This explains why Et3N is critical for alkyne hydrogen transfer (HT) before the [3 + 2] cycloaddition reaction, and the C-N cross-coupling product instead of the click product (byproduct). Our DFT results indicate that the transmetalation process is the rate determination step along the triplet state potential energy surface. This study provides important mechanistic insights for the interrupted CuAAC reaction to form 5-enamine-fully-substituted-1,2,3-triazoles. Further insight prediction interprets that solvent and extra strong ligand coordination play a certain role in competitive reactions.

Magnetic AgNPs/Fe3O4@chitosan/PVA nanocatalyst for fast one-pot green synthesis of propargylamine and triazole derivatives

A new green magnetic nanocatalyst was introduced for one-pot fast synthesis of propargylamine and triazole derivatives.

Advances in Triazole Synthesis from Copper-catalyzed Azide-alkyne Cycloadditions (CuAAC) Based on Eco-friendly Procedures

Background:

1,2,3-triazoles are an important class of organic compounds and because of their aromatic stability, they are not easily reduced, oxidized or hydrolyzed in acidic and basic environments. Moreover, 1,2,3-triazole derivatives are known by their important biological activities and have drawn considerable attention due to their variety of properties. The synthesis of this nucleus, based on the click chemistry concept, through the 1,3-dipolar addition reaction between azides and alkynes is a well-known procedure. This reaction has a wide range of applications, especially on the development of new drugs.

Methods:

The most prominent eco-friendly methods for the synthesis of triazoles under microwave irradiation published in articles from 2012-2018 were reviewed.

Results:

In this review, we cover some of the recent eco-friendly CuAAC procedures for the click synthesis of 1,2,3-triazoles with remarks to new and easily recoverable catalysts, such as rhizobial cyclic β-1,2 glucan; WEB (water extract of banana); biosourced cyclosophoraose (CyS); egg shell powder (ESP); cyclodextrin (β- CD); fish bone powder; nanoparticle-based catalyst, among others.

Conclusion:

These eco-friendly procedures are a useful tool for the synthesis of 1,2,3-triazoles, providing many advantages on the synthesis of this class, such as shorter reaction times, easier work-up and higher yields when compared to classical procedures. Moreover, these methodologies can be applied to the industrial synthesis of drugs and to other areas.

Surface Plasmon-Polariton: A Novel Way To Initiate Azide-Alkyne Cycloaddition

Plasmon catalysis has recently generated tremendous interest in the field of modern chemistry. Application of plasmon introduces the principally new stimulus for the activation of organic reactions, keeping the optical energy concentrated in the vicinity of plasmonic structure, creating an optical near-field enhancement as well as hot electron injection. In this work, for the first time, we presented a new way for the initiation of the azide-alkyne cycloaddition (AAC) using the surface plasmon-polariton wave, supported by the gold grating. With this concept in hand, the plasmon-active gold grating was functionalized with 4-ethynylbenzenediazonium compound. Then, surface-grafted 4-ethynylphenyl groups were plasmon activated and clicked with 4-azidobenzoic acid. Additional experiments allowed to exclude the potential effect of photon, heating, and metal impurities confirmed the key role of surface plasmon-polariton AAC activation. For the investigation of plasmon-induced AAC mechanism, 4-azidophenyl groups (instead of 4-ethynylphenyl groups) were also grafted to the grating surface. Further careful evaluation of reaction kinetics demonstrates that the AAC reaction rate is significantly higher in the case of acetylene activation than in the case of azide activation.

Arylhydrazone ligands as Cu-protectors and -catalysis promoters in the azide-alkyne cycloaddition reaction

A series of water soluble copper(ii) complexes, [Cu(κO¹O²N-H₂L¹)(H₂O)₂]·2H₂O (2), [Cu(κO-H₃L¹)₂(H₂O)₄] (3), [Cu(κO-H₄L²)₂(H₂O)₄] (5) and [Cu(H₂O)₆]·2H₂L³·2(CH₃)₂NCHO (7), were prepared by the reaction of Cu(NO₃)₂·3H₂O with sodium (Z)-2-(2-(1-amino-1,3-dioxobutan-2-ylidene)hydrazineyl)benzenesulfonate, [Na(μ₄-1:2κO¹,2κO²,3κO³,4κO⁴-H₃L¹)]_n (1; for 2 and 3), sodium (Z)-3-(2-(1-amino-1,3-dioxobutan-2-ylidene)hydrazineyl)-4-hydroxybenzene-sulfonate, [Na(μ-1κO¹,2κO²-H₄L²)]₂ (4; for 5) or sodium (Z)-2-(2-(1,3-dioxo-1-(phenylamino)butan-2-ylidene)hydrazineyl)naphthalene-1-sulfonate, [Na(μ-1κO¹O²,2κO³-H₂L³)(CH₃OH)₂]₂ (6; for 7). Compounds 1-7 were fully characterized, also by single-crystal X-ray diffraction analysis, and applied as homogeneous catalysts for the azide-alkyne cycloaddition (AAC) reaction to afford 1,4-disubstituted 1,2,3-triazoles. A structure-catalytic activity relationship has been recognized for the first time on the basis of the occurrence of resonance- and charge-assisted hydrogen bond interactions (RAHB and CAHB), in charge and ligand binding modes, enabling the catalytic activity of the compounds to be ordered as follows: Cu(NO₃)₂≪7 (complex salt with RAHB and CAHB) < 3 (with RAHB and CAHB) < 5 (with RAHB) < 2 (neither RAHB nor CAHB). Complex 2, without such non-covalent interactions, was found to be the most efficient catalyst for the AAC reaction, affording up to 98% product yield after being placed for 15 min, at 125 °C, in a water/acetonitrile mixture under low power (10 W) MW irradiation.

Ag–NHC anchored on silica: an efficient ultra-low loading catalyst for regioselective 1,2,3-triazole synthesis

A silica-supported silver complex, Ag–NHC@SiO₂, was prepared by an anchoring coordination technique, which was successfully employed for the click reaction under mild reaction conditions.

Mechanistic insights into N-Bromosuccinimide-promoted synthesis of imidazo[1,2-a]pyridine in water: Reactivity mediated by substrates and solvent

The mechanism of N-Bromosuccinimide (NBS) promoted synthesis of imidazo[1,2-a]pyridine in water as well as the effective activation modes of NBS was investigated by Density Functional Theory (DFT) calculations. Two main mechanisms that differ in the reaction sequence of substrate were explored: styrene with NBS then followed by 2-aminopyridine (M1) or simultaneously with NBS and 2-aminopyridine (M2), and water-assisted M2 is the more favored one. We found that the adding sequence of 2-aminopyridine affects profoundly on the title reaction. Moreover, upon the assistance of water and NBS, the preferential mechanistic scenario involves three major processes: nucleophilic addition, stepwise H-shift and intramolecular cyclization, three-step deprotonation, rather than a classical bromonium ion species. Specifically, the cooperative interaction of NBS and water plays a critical role in the title reaction. Water acts as solvent, reactant, anchoring, stabilizer, and catalyst. NBS promotes the above three processes by the effective forms of Br⁺ /Br^- , succinimide, and its ethanol isomer. Furthermore, noncovalent interactions between catalysts and substrates are responsible for the different reactive activities of M1 and M2. Our results indicate that simultaneous adding of all reactants is recommended toward economical synthesis. © 2018 Wiley Periodicals, Inc.

Copper(ii)-benzotriazole coordination compounds in click chemistry: a diagnostic reactivity study

This diagnostic study aims to shed light on the catalytic activity of a library of Cu(ii) based coordination compounds with benzotriazole-based ligands. We report herein the synthesis and characterization of five new coordination compounds formulated as [Cu^II(L⁴)(MeCN)₂(CF₃SO₃)₂] (1), [Cu^II(L⁵)₂(CF₃SO₃)₂] (2), [Cu^II(L⁶)₂(MeCN)(CF₃SO₃)]·(CF₃SO₃) (3), [Cu^II(L⁶)₂(H₂O)(CF₃SO₃)]·(CF₃SO₃)·2(Me₂CO) (4), and [Cu(L¹)₂(L^1')₂(CF₃SO₃)₂]₂·4(CF₃SO₃)·8(Me₂CO) (5), derived from similar nitrogen-based ligands. The homogeneous catalytic activity of these compounds along with our previously reported coordination compounds (6-13), derived from similar ligands, is tested against the well-known Cu(i)-catalysed azide-alkyne cycloaddition reaction. The optimal catalyst [Cu^II(L¹)₂(CF₃SO₃)₂] (10) activates the reaction to afford 1,4-disubstituted 1,2,3-triazoles with yields up to 98% and without requiring a reducing agent. Various control experiments are performed to optimize the method and examine parameters such as ligand variation, metal coordination geometry and environment, in order to elucidate the behaviour of the catalytic system.