Base Dependence in Copper-Catalyzed Huisgen Reactions: Efficient Formation of Bistriazoles

Direct Organocatalytic Chemoselective Synthesis of Pharmaceutically Active 1,2,3-Triazoles and 4,5'-Bitriazoles

Carbonyl-containing 1,4,5-trisubstituted- and 1,4-disubstituted-1,2,3-triazoles are well-known for their wide range of applications in pharmaceutical and medicinal chemistry, but their high-yielding metal-free synthesis has always remained challenging, as no comprehensive protocol has been outlined to date. Owing to their structural and medicinal importance, herein, we synthesized various carbonyl-containing 1,4,5-trisubstituted- and 1,4-disubstituted-1,2,3-triazoles and unsymmetrical 4,5'-bitriazoles with high yields and chemo-/regioselectivity from the library of 2,4-diketoesters and azides in a sequential one-pot manner through the combination of organocatalytic enolization, in situ [3 + 2]-cycloaddition, and hydrolysis reactions. The commercial availability of the starting materials/catalysts, diverse substrate scope, performance in a one-pot manner, chemo-/regioselectivity of organo-click reaction, quick synthesis of unsymmetrical 4,5'-bitriazoles, a large number of synthetic applications, and numerous medicinal applications of carbonyl-containing 1,2,3-triazoles are the key attractions of this metal-free organo-click work.

Regiospecific Synthesis of 1,4-Diaryl-5-cyano-1,2,3-triazoles and Their Photoconversion to 2- or 3-Cyanoindoles

We report the synthesis of 1,4-diaryl-5-cyano-1,2,3-triazoles from azides and alkynes via two copper-mediated steps. Aryl-substituted cyanotriazoles are emissive in nonpolar solvents. When the N1-aryl group is electron-donating, the photoconversion of a cyanotriazole to a cyanoindole is efficient. Each of the seven pairs of 4- and 5-cyanotriazole isomers is photoconverted to either distinctive cyanoindoles without rearrangement or a major cyanoindole product via the presumed common intermediate azirine. The resulting cyanoindoles appear to be stronger emitters in polar solvents than the parent cyanotriazoles.

Double Regioselective Thermal Azide-Alkyne Cycloaddition of 1,3-Diynes

A double regioselective cycloaddition reaction of 1,3-diynes with azide is reported to synthesize fully substituted 5-alkynyl-1,2,3-triazoles without any catalyst, metal, or other factor. Computational studies revealed that the 5-alkynyl-1,2,3-triazole derivative is both kinetically and thermodynamically favorable irrespective of the nature of the substituents at the alkyne termini in 1,3-diynes. The simplicity of the reaction, extremely high regioselectivity under metal-free conditions, wide substrate scope, and good to excellent yields might inspire further studies of the cycloaddition of 1,3-diynes in addition to click chemistry.

Each Interruption is an Opportunity: Novel Synthetic Strategies Explored Through Interrupted Click Reactions

The particular and unique mechanism of the copper-catalyzed reaction between azides and alkynes (CuAAC) has not only allowed for the efficient synthesis of 1,2,3-trisubstituted 1,4-triazoles in excellent yields and under mild conditions, becoming the quintessential click reaction, but it has also enabled the straightforward formation of a metallocycle intermediate, the copper triazolyl. This, under suitable reaction conditions able to suppress its protonolysis, can be used either for the creation of new bicyclic triazolyl structures or for the generation of novel three or four-component reactions. The aim of this review is to rationalize and unify all these transformations, which are collectively referred to as "interrupted click reactions".

One-pot nucleophilic substitution-double click reactions of biazides leading to functionalized bis(1,2,3-triazole) derivatives

The nucleophilic substitution of benzylic bromides with sodium azide was combined with a subsequent copper-catalyzed (3 + 2) cycloaddition with terminal alkynes. This one-pot process was developed with a simple model alkyne, but then applied to more complex alkynes bearing enantiopure 1,2-oxazinyl substituents. Hence, the precursor compounds 1,2-, 1,3- or 1,4-bis(bromomethyl)benzene furnished geometrically differing bis(1,2,3-triazole) derivatives. The use of tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (TBTA) as ligand for the click step turned out to be very advantageous. The compounds with 1,2-oxazinyl end groups can potentially serve as precursors of divalent carbohydrate mimetics, but the reductive cleavage of the 1,2-oxazine rings to aminopyran moieties did not proceed cleanly with these compounds.

Expedient synthesis of imino-C-nucleoside fleximers featuring a one-pot procedure to prepare aryl triazoles

Nucleoside analogues such as the antiviral agents galidesivir and ribavirin are of synthetic interest. This work reports a "one-pot" preparation of similar fleximers using a bifunctional copper catalyst that generates the aryl azide in situ, which is captured by a terminal alkyne to effect triazole formation.

Glycopolymers Decorated with 3-O-Substituted Thiodigalactosides as Potent Multivalent Inhibitors of Galectin-3

Galectin-3 (Gal-3) participates in many cancer-related metabolic processes. The inhibition of overexpressed Gal-3 by, e.g., β-galactoside-derived inhibitors is hence promising for cancer treatment. The multivalent presentation of such inhibitors on a suitable biocompatible carrier can enhance the overall affinity to Gal-3 and favorably modify the interaction with Gal-3-overexpressing cells. We synthesized a library of C-3 aryl-substituted thiodigalactoside inhibitors and their multivalent N-(2-hydroxypropyl)methacrylamide (HPMA)-based counterparts with two different glycomimetic contents. Glycopolymers with a higher content of glycomimetic exhibited a higher affinity to Gal-3 as assessed by ELISA and biolayer interferometry. Among them, four candidates (with 4-acetophenyl, 4-cyanophenyl, 4-fluorophenyl, and thiophen-3-yl substitution) were selected for further evaluation in cancer-related experiments in cell cultures. These glycopolymers inhibited Gal-3-induced processes in cancer cells. The cyanophenyl-substituted glycopolymer exhibited the strongest antiproliferative, antimigratory, antiangiogenic, and immunoprotective properties. The prepared glycopolymers appear to be prospective modulators of the tumor microenvironment applicable in the therapy of Gal-3-associated cancers.

Synthesis of Diethoxy Arylphosphoryl Functionalized, Fully Substituted 5-Triazenyl-1,2,3-triazoles via Chelation-Assisted Interrupted Domino Reaction of ortho-Azidophosphonates with Copper(I) Alkynes

We describe the synthesis of 1,4-disubstituted 5-triazenyl-1,2,3-triazoles bearing diethoxy arylphosphoryl moieties via a domino reaction between ortho-azidophosphonates and premade copper(I) alkynides involving chelation-assisted [3+2] cycloaddition followed by interception of the copper(I) triazolide generated by the azide. A resulting dicopper(I) triazoletriazenide complex has been characterized through X-ray diffraction.

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

Copper-Mediated Synthesis of (E)-1-Azido and (Z)-1,2-Diazido Alkenes from 1-Alkene-1,2-diboronic Esters: An Approach to Mono- and 1,2-Di-(1,2,3-Triazolyl)-Alkenes and Fused Bis-(1,2,3-Triazolo)-Pyrazines

A stereoselective and convenient route has been demonstrated to access (Z)-1,2-diazido alkenes from the corresponding 1,2-diboronic esters via a copper-mediated reaction with sodium azide. Alternately, mono-functionalization was regioselectively carried out with trimethylsilyl azide as an azidation reactant. The in situ conversion of bis-azides to the corresponding bis-triazoles can be readily achieved in the presence of copper sulfate and sodium ascorbate, while the modification of the catalytic system opened a new convenient route to bis-triazolo-pyrazines, a new class of fused heterocycles.

5-Formyltriazoles as Valuable Starting Materials for Unsymmetrically Substituted Bi-1,2,3-Triazoles

Herein, we present the first synthetic methodologies toward non-symmetrical 5,5′-C, C-linked bi-1,2,3-triazoles starting from 5-formyl-1,2,3-triazole via two related pathways. In a first reaction, 5-formyl-1,2,3-triazole is successfully reacted with a variety of nitroalkanes and organic azides in a one-pot three-component fashion resulting in tetra-ortho-substituted bi-1,2,3-triazoles. In the second, closely related reaction, 5-formyl-1,2,3-triazole is initially converted with nitromethane to the corresponding nitroalkene, and then subsequently oxidatively cyclized with a number of organic azides toward 4-nitro substituted non-symmetrical tetra-ortho-substituted 5,5′-bi-1,2,3-triazoles. The scope of both reactions and furtherr post-functionalizations are examined, and the atropisomeric properties of the obtained bi-1,2,3-triazoles are evaluated.

CuII -Catalyzed Oxidative Formation of 5-Alkynyltriazoles

In an alcoholic solvent under the catalysis of Cu(OAc)₂ ⋅H₂ O, organic azide and terminal alkyne could oxidatively couple to afford 5-alkynyl-1,2,3-triazole (alkynyltriazole) at room temperature under an atmosphere of O₂ in a few hours. The involvement of 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) is essential, without which the redox neutral coupling instead proceeds to produce 5-H-1,2,3-triazole (protiotriazole) as the major product. Therefore, DBN switches the redox neutral coupling between terminal alkyne and organic azide, the copper-catalyzed "click" reaction to afford protiotriazole, to an oxidation reaction that results in alkynyltriazole. The organic base DBN is effective in accelerating the copper(II)-catalyzed oxidation of terminal alkyne or copper(I) acetylide, which is intercepted by an organic azide to produce alkynyltriazole. The proposed mechanistic model suggests that the selectivity between alkynyl- and protiotriazole, and other acetylide or triazolide oxidation products is determined by the competition between copper(I)-catalyzed redox neutral cycloaddition and copper(II)/O₂ -mediated acetylide oxidation after the formation of copper(I) acetylide.

The CuAAC: Principles, Homogeneous and Heterogeneous Catalysts, and Novel Developments and Applications

The copper-catalyzed azide/alkyne cycloaddition reaction (CuAAC) has emerged as the most useful "click" chemistry. Polymer science has profited enormously from CuAAC by its simplicity, ease, scope, applicability and efficiency. Basic principles of the CuAAC are reviewed with a focus on homogeneous and heterogeneous catalysts, ligands, anchimeric assistance, and basic chemical principles. Recent developments of ligand design and acceleration are discussed.

Exploring Approaches for the Synthesis of Few-Layered Graphdiyne

Graphdiyne (GDY) is an emerging carbon allotrope in the graphyne (GY) family, demonstrating extensive potential applications in the fields of electronic devices, catalysis, electrochemical energy storage, and nonlinear optics. Synthesis of few-layered GDY is especially important for both electronic applications and structural characterization. This work critically summarizes the state-of-art of GDY and focuses on exploring approaches for few-layered GDY synthesis. The obstacles and challenges of GDY synthesis are also analyzed in detail. Recently developed synthetic methods are discussed such as i) the copper substrate-based method, ii) the chemical vapor deposition (CVD) method, iii) the interfacial construction method, and iv) the graphene-templated method. Throughout the discussion, the superiorities and limitations of different methods are analyzed comprehensively. These synthetic methods have provided considerable inspiration approaching synthesis of few-layered or single-layered GDY film. The work concludes with a perspective on promising research directions and remaining barriers for layer-controlled and morphology-controlled synthesis of GDY with higher crystalline quality.

Controlled ATRP Synthesis of Novel Linear-Dendritic Block Copolymers and Their Directed Self-Assembly in Breath Figure Arrays

Herein, we report the formation and characterization of novel amphiphilic linear-dendritic block copolymers (LDBCs) composed of hydrophilic dendritic poly(ether-ester), PEE, blocks and hydrophobic linear poly(styrene), PSt. The LDBCs are synthesized via controlled atom transfer radical polymerization (ATRP) initiated by a PEE macroinitiator. The copolymers formed have narrow molecular mass distributions and are designated as LGn-PSt Mn, in which LG represents the PEE fragment, n denotes the generation of the dendron (n = 1⁻3), and Mn refers to the average molecular mass of the LDBC (Mn = 3.5⁻68 kDa). The obtained LDBCs are utilized to fabricate honeycomb films by a static "breath figure" (BF) technique. The copolymer composition strongly affects the film morphology. LDBCs bearing acetonide dendron end groups produce honeycomb films when the PEE fraction is lower than 20%. Pore uniformity increases as the PEE content decreases. For LDBCs with hydroxyl end groups, only the first generation LDBCs yield BF films, but with a significantly smaller pore size (0.23 μm vs. 1⁻2 μm, respectively). Although higher generation LDBCs with free hydroxyl end groups fail to generate honeycomb films by themselves, the use of a cosolvent or addition of homo PSt leads to BF films with a controllable pore size (3.7⁻0.42 μm), depending on the LDBC content. Palladium complexes within the two triazole groups in each of the dendron's branching moieties can also fine-tune the morphology of the BF films.

Novel synthesis of 1,5-disubstituted-1,2,3-triazolines catalysed by Zepto magnetic microspheres under the influence of a rotating magnetic field

The novel reactor has been designed to perform chemical reactions under the influence of a magnetic field generated by alternating magnetic poles as a function of time. The system was successfully employed to synthesize a series of 1,5-disubstituted-1,2,3-triazolines via the regioselective [3 + 2] cycloaddition reactions between alkyl/aryl azides and nitroolefins catalysed by Zepto (para magnetic ultra-blue carboxy functionalized) microspheres (bead diameter 2.5 μm). All of the reactions went smoothly without any adverse effect on nitro, cyano, thienyl, hydroxy, halogens, and ether functions at 25 ± 2 °C and afforded 82%–99% pure products at a magnetic field of 18.99 mT and an exposure time of 180–240 min. The influence of the magnetic force exerted on the magnetic materials was found to enhance the catalytic activity of microspheres. The catalyst could easily be separated by simple centrifugation, which could be reused for at least 15 runs with no loss in activity.

Medium Rings Bearing Bitriazolyls: Easily Accessible Structures with Superior Performance as Cu Catalyst Ligands

Benefiting from their unique properties, the development of structurally novel and easily accessible medium rings is of significant interest in the pharmaceutical industry and academic research. However, synthetic access to medium-ring scaffolds is very difficult due to their rigid skeleton and large-angle strains. In this paper, a new class of medium rings bearing bitriazolyls (MRBTs) was designed, synthesized, identified as a promising new skeleton ligand for the Cu(I)-catalyzed click reaction, and used in site-special modification of protein. One of the MRBTs, 3aa, exhibited a turnover number (TON) as high as 55 000 and dramatic accelerating effects ( k_obs = 1.95 M^-1 s^-1) and ranked among the most efficient ligands for copper-catalyzed alkyne and azide cycloaddition. Unlike the difficult access to other known medium rings, these 7-12-membered MRBTs can be prepared in straightforward, one-step manner from structurally diverse linear terminal diynes and azides. The unique accessibility and intriguing properties therefore imply their broad application perspectives.

Synthesis and characterization of new H-shaped triphenylene discotic room-temperature liquid crystal tetramers by a copper-free click reaction

A series of new H-shaped triphenylene discotic liquid crystal tetramers has been designed and synthesized using a copper-free [3+2] cycloaddition reaction.

Adapting the Glaser Reaction for Bioconjugation: Robust Access to Structurally Simple, Rigid Linkers

Copper-mediated coupling between alkynes to generate a structurally rigid, linear 1,3-diyne linkage has been known for over a century. However, the mechanistic requirement to simultaneously maintain CuI and an oxidant has limited its practical utility, especially for complex functional molecules in aqueous solution. We find that addition of a specific bpy-diol ligand protects unprotected peptides from CuII -mediated oxidative damage through the formation of an insoluble CuII gel which solves the critical challenge of applying Glaser coupling to substrates that are degraded by CuII . The generality of this method is illustrated through the conjugation of a series of polar and nonpolar labels onto a fully unprotected GLP-1R agonist through a linear 7 Å diynyl linker.

5,5′-Bistriazoles as axially chiral, multidentate ligands: synthesis, configurational stability and catalytic application of their scandium(iii) complexes

The design and development of 5,5′-bistriazoles featuring aminomethyl substituents is discussed.

Cu-Catalyzed Click Reaction in Carbohydrate Chemistry

Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC), popularly known as the "click reaction", serves as the most potent and highly dependable tool for facile construction of simple to complex architectures at the molecular level. Click-knitted threads of two exclusively different molecular entities have created some really interesting structures for more than 15 years with a broad spectrum of applicability, including in the fascinating fields of synthetic chemistry, medicinal science, biochemistry, pharmacology, material science, and catalysis. The unique properties of the carbohydrate moiety and the advantages of highly chemo- and regioselective click chemistry, such as mild reaction conditions, efficient performance with a wide range of solvents, and compatibility with different functionalities, together produce miraculous neoglycoconjugates and neoglycopolymers with various synthetic, biological, and pharmaceutical applications. In this review we highlight the successful advancement of Cu(I)-catalyzed click chemistry in glycoscience and its applications as well as future scope in different streams of applied sciences.

When CuAAC 'Click Chemistry' goes heterogeneous

Within the green chemistry context, heterogeneous catalysis is more and more applied to organic synthesis. The well known ‘click chemistry’ and especially its flagship, the copper-catalyzed azide–alkyne cycloaddition reaction (CuAAC), is now catch up by such heterogenisation process and copper ions or metals have been grafted or deposited on or into various solids, such as (bio)polymers, charcoal, silica, zeolites, POM or MOF.

Synthesis of bi- and bis-1,2,3-triazoles by copper-catalyzed Huisgen cycloaddition: A family of valuable products by click chemistry

The Cu(I)-catalyzed azide-alkyne cycloaddition reaction, also known as click chemistry, has become a useful tool for the facile formation of 1,2,3-triazoles. Specifically, the utility of this reaction has been demonstrated by the synthesis of structurally diverse bi- and bis-1,2,3-triazoles. The present review focuses on the synthesis of such bi- and bistriazoles and the importance of using copper-promoted click chemistry (CuAAC) for such transformations. In addition, the application of bitriazoles and the related CuAAAC reaction in different fields, including medicinal chemistry, coordination chemistry, biochemistry, and supramolecular chemistry, have been highlighted.

Copper-catalysed azide-alkyne cycloadditions (CuAAC): an update

The reactions of organic azides and alkynes catalysed by copper species represent the prototypical examples of click chemistry. The so-called CuAAC reaction (copper-catalysed azide-alkyne cycloaddition), discovered in 2002, has been expanded since then to become an excellent tool in organic synthesis. In this contribution the recent results described in the literature since 2010 are reviewed, classified according to the nature of the catalyst precursor: copper(I) or copper(II) salts or complexes, metallic or nano-particulated copper and several solid-supported copper systems.

Double [3 + 2]-dimerisation cascade synthesis of bis(triazolyl)bisphosphanes, a new scaffold for bidentate bisphosphanes

A highly convergent synthesis of bis(triazolylphosphane oxides) was developed by a tandem copper-mediated Huisgen reaction-oxidative coupling. The phosphane oxides were reduced by trichlorosilane and the coordination of the resulting bisphosphanes was studied with various transition metals.

Mechanism of samarium-catalyzed 1,5-regioselective azide-alkyne [3 + 2]-cycloaddition: a quantum mechanical investigation

The mechanism of the samarium-catalyzed 1,5-regioselective azide-alkyne [3 + 2]-cycloaddition (SmAAC) reaction has been examined with quantum mechanical calculations at the B3LYP/6-31+G(d,p) level of theory with ECP51MWB on Sm. Four stepwise pathways were located, with two leading to the 5-endocyclic 1,5-disubstituted 1,2,3-triazole product PSmL2 (paths 1 and 2) and the other two to the exocyclic product ExoPSmCl2 (path 3) as well as 1,4-disubstituted 1,2,3-triazole RegPSmL2 (path 4), respectively. Among them, path 2 (R-COM1-TS12-COM2-TS23-COM3-TS3P-PSmL2) is the most favored one both in the gas phase and in toluene solution, which is in good agreement with the experimental data. Moreover, 1,1-insertion forming COM2 in path 2 is the rate-determining step. The computational results also infer that the participation of samarium catalyst changes the distribution of the electrostatic potential on the reactants' surface, which determines the polarization direction of the reactants and formation of different intermediates (COM1 and RegCOM1), and finally affects the regioselectivity. When solvent corrections for toluene are considered, the 1,1-insertion process is discouraged, while the intramolecular [1,3]-shift reaction is facilitated.

Facile purification and click labeling with 2-[18F]fluoroethyl azide using solid phase extraction cartridges

A facile method was developed to purify 2-[¹⁸F]fluoroethyl azide ([¹⁸F]FEA) using a C18 cartridge and an Oasis^® HLB cartridge in series, in which [¹⁸F]FEA was exclusively trapped on the HLB cartridge. [¹⁸F]FEA can be eluted for reactions in solution; alternatively click labeling can be carried out on the HLB cartridge itself by loading an alkyne substrate and copper (I) catalyst dissolved in DMF onto the cartridge. This solid phase extraction methodology for purification and click labeling with [¹⁸F]FEA, either in solution or on the cartridge, is safe, simple, reproducible in high yield, and compatible with automated synthesis of ¹⁸F-labeled PET tracers.

BINOL-linked 1,2,3-triazoles: an unexpected fluorescent sensor with anion–π interaction for iodide ions

BINOL-derived triazoles could be used in organocatalytic silylation and unexpectedly as fluorescent sensors for the recognition of I⁻.