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

Dual-function advanced magnetic bacterial cellulose materials: From enhanced adsorption phenomena to an unprecedented circular green catalytic strategy

With the growing emphasis on circular catalysis principles and green chemistry, addressing the dual challenge of wastewater treatment and sustainable catalysis has become increasingly critical. Although the adsorption of copper ions using magnetic biomaterials has been widely investigated, its full potential is still not fully understood. In particular, the reutilization of Cu(II)-loaded magnetic bacterial cellulose in circular green catalytic reactions remains underexplored. This study presents a novel magnetic bacterial cellulose-based material, designated as (BC-BPEM)@Fe3O4NPs, engineered through advanced chemical modifications to address these challenges. The adsorption kinetics followed a pseudo-second-order model, indicating chemisorption as the predominant mechanism. A key challenge addressed in this study was the efficient reuse of Cu(II)-loaded magnetic bacterial cellulose-based material. The recovered material was successfully employed as a catalyst in the synthesis of novel 1,4-disubstituted bis-1,2,3-triazoles under green conditions. Notably, the reaction achieved an impressive rate of 0.219 ± 0.006 mmol.gcat-1.min-1 and a 99 % yield within 15 min, using green deep eutectic solvents (ChCl/Gly) and glutathione as a reducing agent. Remarkably, the catalyst retained its high catalytic performance over 20 cycles, maintaining yields consistently between 99 % and 97 %. This study not only emphasizes the seamless integration of adsorption and catalytic recycling but also highlights the sustainability of the approach. Environmental metrics revealed an E-factor of 0.442 kg waste/kg product, a PMI of 1.442 kg materials/kg product, and an RME of 99.83 %, reinforcing the potential of catalyst in both sustainable catalysis and environmental remediation.

Flavanone-enabled CuAAC Reaction: Noninnocent Reagents Driving a Mononuclear Mechanism Over the Dinuclear Paradigm

In this work, we report the use of the CuAAC (copper-catalyzed azide-alkyne cycloaddition) reaction to obtain different triazole derivatives bridged to the naringenin skeleton, leading to the combination of a triazole pharmacophoric group with a bioactive flavanone. The generation of Cu(I) active species was done using CuSO4 salts and sodium ascorbate, resulting in moderate to high yields when the DMSO-water binary system was used as solvents. Mechanistic studies were done using density functional theory calculations and high-resolution mass spectrometry (HRMS). We investigated the reduction process of Cu(II) to Cu(I), and the role of mononuclear and dinuclear copper species in the catalysis of the cycloaddition reaction. Our combined theoretical and experimental results indicate that the mechanism involving a single copper species is taking place, with the cycloaddition step being the rate-determining step. The calculations indicate that the mechanism involving two copper species has the deprotonation of the coordinated terminal alkyne as the rate-determining step.

Peptide Multifunctionalization via Modular Construction of Trans-AB2C Porphyrin on Resin

Peptide multifunctionalization is a crucial technique to develop peptide-based agents for various purposes. Porphyrin-peptide conjugates are a class of popular multifunctional peptides renowned for their multifunctional and multimodal properties. However, the tedious synthetic works for porphyrin building blocks are involved in most previous studies. In this work, a modular solid-phase synthetic approach is reported to construct trans-AB2C porphyrin on peptide chains without presynthesized porphyrin building blocks. The products from this approach, which inherit both functionalities from the porphyrins and the modules employed for constructing porphyrins, show potential in biomedical and biomaterial applications. Furthermore, by extending this synthetic approach, the first example of "resin-to-resin" reaction is reported to link two peptides together along the construction of porphyrin motifs to give porphyrin-peptide conjugates with two different peptide chains.

Electron-Deficient Alkyne Lipids Enable Efficient Synthesis of Comparable Polymer Lipids via Copper-Free Azide-Alkyne Cycloaddition

Polymer lipids (PLs) are essential components of liposomes and lipid nanoparticles (LNPs) for drug and gene delivery, providing colloidal stabilization and defining the biological interface. While poly(ethylene glycol) (PEG)-based PLs are the current standard, they are suspected to be responsible for rare adverse reactions, e. g. to LNP-based Covid-19 vaccines. Therefore, PLs based on alternative stealth polymers are being intensively investigated for their use in LNPs. However, alternative PLs often lack comparability due to different synthesis protocols and are often not easily accessible. Herein we present a catalyst-free, efficient and versatile coupling procedure for PL synthesis based on azide-functionalized polymers and electron-deficient acetylene dicarboxylate lipids. To highlight the versatility of this approach, we prepared PLs based on PEG and 4 alternative stealth polymers with quantitative coupling efficiencies. The linker structure showed appropriate pH stability and biocompatibility. All PLs enabled the preparation of well-defined liposomes with excellent stability. Our facile and versatile approach yields comparable PLs with minimized linker size, making them promising candidates for future comparative studies, and biomedical applications.

New reactivity of late 3d transition metal complexes in catalytic reactions of alkynes

Late 3d metals such as iron, cobalt, nickel, and copper are abundantly present in the Earth's crust and they are produced in huge quantities in the mining industry. Often, these inexpensive metals exhibit unique or special reactivities in catalytic reactions as compared with expensive noble metals such as palladium, iridium, and rhodium. The novel reactivities of 3d metal complexes originate from their unique physical and atomic properties as compared with heavier 4d/5d congeners: smaller ionic and covalent radii, contracted 3d orbitals of smaller sizes and lower energies, lower values of Pauli electronegativity, etc. This review summarizes the recent progress in late 3d transition metal-catalyzed transformations of alkynes. We organize catalytic examples according to each type of novel elementary reactivity exhibited by 3d metal complexes. Each section includes a description of the unique reactivity of the 3d metals, the atomic and theoretical basis of the reactivity and illustrations of catalytic examples: (1) single electron transfer from low-valent metal complexes to alkyl halides, (2) facile reductive elimination from nickel(III), (3) facile reductive elimination from copper(III), (4) cis-to-trans isomerization of alkenyl metal complexes after syn-insertion, (5) ligand-to-ligand hydrogen transfer, (6) hydrogen atom transfer from hydride complexes and (7) protonation of nickel metallacyclopropenes.

Synthesis of 1,2,3-Triazole-Methyl-Menadione Derivatives: Evaluation of Electrochemical and Antiparasitic Properties against two Blood-Dwelling Parasites

This study explores the synthesis and evaluation of novel 1,2,3-triazole-methyl-1,4-naphthoquinone hybrids, focusing on their electrochemical properties and antiparasitic efficacies against two human blood-dwelling parasites Plasmodium falciparum and Schistosoma mansoni. Using copper-catalyzed azide-alkyne cycloaddition (CuAAC), a well-established tool in click chemistry, two synthetic routes were assessed to develop α- and β-[triazole-methyl]-menadione derivatives. By optimizing the CuAAC reaction conditions, yields were significantly improved, reaching up to 94 % for key intermediates and resulting in the formation of a library of approximately 30 compounds. Biological evaluation of the compounds in antiparasitic drug assays demonstrated notable antischistosomal potencies, while no significant activity was observed for the same series against P. falciparum parasites. Electrochemical and 'benzylic' oxidation studies confirmed that the active 'benzoyl' metabolite responsible for the antiplasmodial activity of plasmodione cannot be generated. These findings highlight the potential of triazole-linked menadione hybrids as promising early candidates for antischistosomal drug development, and provides insights into structure-activity relationships crucial for future therapeutic strategies.

The effects of buffer, pH, and temperature upon SPAAC reaction rates

This study investigates the effects of buffer type, pH, and temperature on the kinetics of strain-promoted alkyne-azide cycloaddition (SPAAC) reactions. Using 3-azido-L-alanine and 1-azido-1-deoxy-β-D-glucopyranoside as model azides and sulfo DBCO-amine as the alkyne, we examined reaction rates in a series of buffers, including PBS, HEPES, MES, borate buffer, and cell culture media (DMEM and RPMI), with pH values ranging from 5 to 10 and temperatures of 25 and 37 °C. Absorbance spectrophotometric data revealed that PBS (pH 7) exhibited among the lowest rate constants (0.32-0.85 M-1 s-1), whereas HEPES (pH 7) had the highest (0.55-1.22 M-1 s-1). Additionally, reactions in DMEM were faster than in RPMI (0.59-0.97 vs. 0.27-0.77 M-1 s-1). We observed that higher pH values generally increased reaction rates, except in HEPES buffer. Notably, 1-azido-1-deoxy-β-D-glucopyranoside reacted faster than 3-azido-L-alanine, highlighting the importance of considering the electron-donating capacity of azides in the optimisation of SPAAC reactions. Additional experiments with DBCO-modified antibodies (DBCO-trastuzumab and DBCO-PEG5-trastuzumab) corroborated the trends related to buffer and azide selection. The presence of a PEG linker notably enhanced reaction rates (0.18-0.37 M-1 s-1) by 31 ± 16%. This study offers useful insights into the factors affecting SPAAC kinetics, facilitating the development of optimised bioconjugation strategies.

Evaluation and In Situ Library Expansion of Small Molecule MHC-I Inducers

Immunotherapy has emerged as a powerful strategy for combating cancer by harnessing the patient immune system to recognize and eliminate malignant cells. The major histocompatibility complex class I (MHC-I) has a pivotal role in the recognition step. These surface proteins present cancer-specific neoantigens to CD8+ T cells, which triggers activation and T cell-mediated killing. However, cancer cells can often evade immune detection by downregulating MHC-I surface expression, which renders the immune response less effective. In turn, this resistance mechanism offers an opportunity to bolster MHC-I surface expression via therapeutic interventions. Here, we conducted an initial comprehensive evaluation of previously purported small molecule MHC-I inducers and identified heat shock protein 90 (Hsp90) inhibitors as privileged inducers of MHC-I surface expression. With a core scaffold in hand, we employed an in situ click chemistry-based derivatization strategy to generate 380 novel compounds in the same family. New agents from this library showed high levels of induction, with one of the triazole-based analogs, CliMB-325, also enhancing T cell activation and exhibiting lower toxicity, which could potentiate some immunotherapeutic modalities. Moreover, we demonstrated the potential of a click chemistry-based diversification strategy for the discovery of small molecules to counter immune evasion.

Mixed-valence metal-organic frameworks: concepts, opportunities, and prospects

Owing to increasing global demand for the development of multifunctional advanced materials with various practical applications, great attention has been paid to metal-organic frameworks due to their unique properties, such as structural, chemical, and functional diversity. Several strategies have been developed to promote the applicability of these materials in practical fields. The induction of mixed-valency is a promising strategy, contributing to exceptional features in these porous materials such as enhanced charge delocalization, conductivity, magnetism, etc. The current review provides a detailed study of mixed-valence MOFs, including their fundamental properties, synthesis challenges, and characterization methods. The outstanding applicability of these materials in diverse fields such as energy storage, catalysis, sensing, gas sorption, separation, etc. is also discussed, providing a roadmap for future design strategies to exploit mixed valency in advanced materials. Interestingly, mixed-valence MOFs have demonstrated fascinating features in practical fields compared to their homo-valence MOFs, resulting from an enhanced synergy between mixed-valence states within the framework.

Stable Cu (I) Complexes for Intracellular Cu-Catalyzed Azide Alkyne Cycloaddition

The copper-catalyzed azide-alkyne cycloaddition (CuAAC) has heralded a new era of chemical biology and biomedicine. However, caveats of the CuAAC include formation of reactive oxygen species (ROS) and other copper-related toxicity. This limits utility in sensitive biological samples and matrices. Towards addressing these caveats, we synthesized and fully characterized two air and water stable trinuclear Cu(I) dimer complexes. The complexes were stable to oxidation in the presence of hydrogen peroxideand other chelators, which was reasoned to be due to the linear benzimidazole-Cu-benzimidazole geometry. Computational investigations of the catalytic cycle implicated two of the three coppers in the trimer complex as the active metal centers. The complexes were shown to catalyze the reaction at far below sub-toxic concentrations for intracellular click reactions to label triple negative breast cancer cells and compared to the current CuSO4-THPTA standard.

Compartmentalised single-chain nanoparticles and their function

Single-chain nanoparticles (SCNPs) are generated by intramolecular collapse and crosslinking of single polymer chains, thus conceptually resembling the structures of folded proteins. Their chemical flexibility and ability to form compartmentalised nanostructures sized ∼1 nm make them perfect candidates for numerous applications, such as in catalysis and drug delivery. In this review we discuss principles for the design, synthesis and analysis of SCNPs, with a focus on their compartmentalised structures, highlighting our own previous work. As such compartments offer the potential to generate a specific nanoenvironment e.g. for the covalent and non-covalent encapsulation of catalysts or drugs, they represent a novel, exciting, and expanding research area. Starting from the architectural and chemical design of the starting copolymers by controlling their amphiphilic profile, the embedding of blocks-, or secondary-structure-mimetic arrangements, we discuss design principles to form internal compartments inside the SCNPs. While the generation of compartments inside SCNPs is straightforward, their analysis is still challenging and often demands special techniques. We finally discuss applications of SCNPs, also linked to the compartment formation, predicting a bright future for these special nanoobjects.

Spot the difference in reactivity: a comprehensive review of site-selective multicomponent conjugation exploiting multi-azide compounds

Going beyond the conventional approach of pairwise conjugation between two molecules, the integration of multiple components onto a central scaffold molecule is essential for the development of high-performance molecular materials with multifunctionality. This approach also facilitates the creation of functionalized molecular probes applicable in diverse fields ranging from pharmaceuticals to polymeric materials. Among the various click functional groups, the azido group stands out as a representative click functional group due to its steric compactness, high reactivity, handling stability, and easy accessibility in the context of multi-azide scaffolds. However, the azido groups in multi-azide scaffolds have not been well exploited for site-specific use in molecular conjugation. In fact, multi-azide compounds have been well used to conjugate to the same multiple fragments. To circumvent problems of promiscuous and random coupling of multiple different fragments to multiple azido positions, it is imperative to distinguish specific azido positions and use them orthogonally for molecular conjugation. This review outlines methods and strategies to exploit specific azide positions for molecular conjugation in the presence of multiple azido groups. Illustrative examples covering di-, tri- and tetraazide click scaffolds are included.

Mechanochemistry for Organic and Inorganic Synthesis

In recent years, mechanochemistry has become an innovative and sustainable alternative to traditional solvent-based synthesis. Mechanochemistry rapidly expanded across a wide range of chemistry fields, including diverse organic compounds and active pharmaceutical ingredients, coordination compounds, organometallic complexes, main group frameworks, and technologically relevant materials. This Review aims to highlight recent advancements and accomplishments in mechanochemistry, underscoring its potential as a viable and eco-friendly alternative to conventional solution-based methods in the field of synthetic chemistry.

Click Chemistry: Reaction Rates and Their Suitability for Biomedical Applications

Click chemistry has become a commonly used synthetic method due to the simplicity, efficiency, and high selectivity of this class of chemical reactions. Since their initial discovery, further click chemistry methods have been identified and added to the toolbox of click chemistry reactions for biomedical applications. However, selecting the most suitable reaction for a specific application is often challenging, as multiple factors must be considered, including selectivity, reactivity, biocompatibility, and stability. Thus, this review provides an overview of the benefits and limitations of well-established click chemistry reactions with a particular focus on the importance of considering reaction rates, an often overlooked criterion with little available guidance. The importance of understanding each click chemistry reaction beyond simply the reaction speed is discussed comprehensively with reference to recent biomedical research which utilized click chemistry. This review aims to provide a practical resource for researchers to guide the selection of click chemistry classes for different biomedical applications.

Copper Dispersed Covalent Organic Framework for Azide-Alkyne Cycloaddition and Fast Synthesis of Rufinamide in Water

A crystalline porous bipyridine-based Bpy-COF with a high BET surface area (1864 m2 g-1) and uniform mesopore (4.0 nm) is successfully synthesized from 1,3,5-tris-(4'-formyl-biphenyl-4-yl)triazine and 5,5'-diamino-2,2'-bipyridine via a solvothermal method. After Cu(I)-loading, the resultant Cu(I)-Bpy-COF remained the ordered porous structure with evenly distributed Cu(I) ions at a single-atom level. Using Cu(I)-Bpy-COF as a heterogeneous catalyst, high conversions for cycloaddition reactions are achieved within a short time (40 min) at 25 °C in water medium. Moreover, Cu(I)-Bpy-COF proves to be applicable for aromatic and aliphatic azides and alkynes bearing various substituents such as ester, hydroxyl, amido, pyridyl, thienyl, bulky triphenylamine, fluorine, and trifluoromethyl groups. The high conversions remain almost constant after five cycles. Additionally, the antiepileptic drug (rufinamide) is successfully prepared by a simple one-step reaction using Cu(I)-Bpy-COF, proving its practical feasibility for pharmaceutical synthesis.

Clicking in harmony: exploring the bio-orthogonal overlap in click chemistry

In the quest to scrutinize and modify biological systems, the global research community has continued to explore bio-orthogonal click reactions, a set of reactions exclusively targeting non-native molecules within biological systems. These methodologies have brought about a paradigm shift, demonstrating the feasibility of artificial chemical reactions occurring on cellular surfaces, in the cell cytosol, or within the body - an accomplishment challenging to achieve with the majority of conventional chemical reactions. This review delves into the principles of bio-orthogonal click chemistry, contrasting metal-catalyzed and metal-free reactions of bio-orthogonal nature. It comprehensively explores mechanistic details and applications, highlighting the versatility and potential of this methodology in diverse scientific contexts, from cell labelling to biosensing and polymer synthesis. Researchers globally continue to advance this powerful tool for precise and selective manipulation of biomolecules in complex biological systems.

Preparation of Protein A Membranes Using Propargyl Methacrylate-Based Copolymers and Copper-Catalyzed Alkyne-Azide Click Chemistry

The development of convective technologies for antibody purification is of interest to the bioprocessing industries. This study developed a Protein A membrane using a combination of graft polymerization and copper(I)-catalyzed alkyne-azide click chemistry. Regenerated cellulose supports were functionalized via surface-initiated copolymerization of propargyl methacrylate (PgMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMEMA300), followed by a reaction with azide-functionalized Protein A ligand. The polymer-modified membranes were characterized using attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR), gravimetric analysis, and permeability measurements. Copolymer composition was determined using the Mayo-Lewis equation. Membranes clicked with azide-conjugated Protein A were evaluated by measuring static and dynamic binding (DBC10) capacities for human immunoglobulin G (hIgG). Copolymer composition and degree of grafting were found to affect maximum static binding capacities, with values ranging from 5 to 16 mg/mL. DBC10 values did not vary with flow rate, as expected of membrane adsorbers.

CuAAC-Based Synthesis, Copper-Catalyzed Aldehyde-Forming Hydrolytic Fission and Antiproliferative Evaluation of Novel Ferrocenoylamino-Substituted Triazole-Tethered Quinine-Chalcone Hybrids

A series of novel triazole-tethered ferrocenoylamino-substituted cinchona-chalcone hybrids along with two representative benzoylamino-substituted reference compounds were prepared by three methods of CuAAC chemistry. In line with the limited success or complete failure of attempted conversions with low catalyst loadings, by means of DFT modeling studies, we demonstrated that a substantial part of the Cu(I) ions can be chelated and thus trapped in the aroylamino-substituted cinchona fragment and all of the accessible coordinating sites of the chalcone residues. Accordingly, increased amounts of catalysts were used to achieve acceptable yields; however, the cycloadditions with para-azidochalcones were accompanied by partial or complete aldehyde-forming hydrolytic fission of the enone C=C bond in a substituent-, solvent- and copper load-dependent manner. The experienced hydrolytic stability of the hybrids obtained by cycloadditions with ortho-azidochalcones was interpreted in terms of relative energetics, DFT reactivity indices and MO analysis of simplified models of two isomer copper-enone complexes. The novel hybrids were evaluated on HeLa, MDA-MB-231 and A2780 cell lines and showed substantial activity at low-to-submicromolar concentrations. An organometallic model carrying 3,4,5-trimethoxyphenyl residue in the enone part with a para-disubstituted benzene ring in the central skeletal region was identified as the most potent antiproliferative lead, characterized by submicromolar IC50 values measured on the three investigated cells. The biological assays also disclosed that this ferrocenoylamino-containing lead compound displays a ca. two- to five-fold more substantial antiproliferative effect than its benzoylamino-substituted counterpart.

Click chemistry as a method for the synthesis of steroid bioconjugates of bile acids derivatives and sterols

Six steroid conjugates of bile acids and sterol derivatives have been synthesized using the click chemistry method. The azide-alkyne Huisgen cycloaddition of the propionyl ester of lithocholic, deoxycholic and cholic acid with azide derivatives of cholesterol and cholestanol gave new bile acid-sterol conjugates linked with a 1,2,3-triazole ring. Previously, sterols were converted to bromoacetate substituted derivatives by reaction with bromoacetic acid bromide in anhydrous dichloromethane. These compounds were then converted to azide derivatives using sodium azide. The propiolic esters of lithocholic, deoxycholic and cholic acids were obtained by reaction with propiolic acid in the presence of p-toluenesulfonic acid. Additionally, two of these steroids: methyl 3α-propynoyloxy-12α-acetoxy-5β-cholane-24-oate and methyl 3α-propynoyloxy-7 α,12α-diacetoxy-5β-cholane-24-oate were also obtained and characterized for the first time. All conjugates were obtained in good yields using an efficient synthesis method. The structures of all conjugates and the four substrates were confirmed by spectral (1H- and 13C NMR, FT-IR) analysis, mass spectrometry (ESI-MS), and PM5 semiempirical methods. The pharmacotherapeutic potential of the synthesized compounds was estimated based on the in silico Prediction of Activity Spectra for Substances (PASS) method. The cytotoxicity of the compounds was in vitro evaluated in a hemolytic assay using human erythrocytes as a cell model. The in silico and in vitro study results indicate that the selected compound possesses an interesting biological activity and can be considered as potential drug design agent. Additionally, molecular docking was performed for the selected conjugate.

Photoswitchable Azoheteroarene-Based Chelating Ligands: Light Modulation of Properties, Aqueous Solubility and Catalysis

We report the design and synthesis of eight photoswitchable phenylazopyridine- and phenylazopyrazole-based molecular systems as chelation-type light-controllable ligands. Besides the studies on fundamental photoisomerization behaviour, the ligands were also screened for light-tuneable properties such as photochromism, phase transition, and solubility, especially in the aqueous medium. This investigation demonstrates how the modulation of aqueous solubility can be achieved through photoisomerization and can further be utilized towards controlling the amount of catalytically active Cu(I) species in the aqueous conditions. Through this approach, light control over the catalytic activity was achieved for Cu-catalyzed azide-alkyne cycloaddition (CuAAC) reactions, along with a partial recovery of the catalytically active species.

Click Chemistry and Targeted Degradation: A Winning Combination for Medicinal Chemists?

Click chemistry is universally recognized as a powerful strategy for the fast and precise assembly of diverse building blocks. Targeted Protein Degradation (TPD) is a new therapeutic modality based on heterobifunctional small-molecule degraders that provides new opportunities to medicinal chemists dealing with undruggable targets and incurable diseases. Here, we highlight how very recently the TPD field and that of click chemistry have merged, opening up the possibility for fine-tuning the properties of a degrader, chemically assembled through a "click" synthesis. By reviewing concrete examples, we want to provide the reader with the insight that the application of click and bioorthogonal chemistry in the TDP field may be a winning combination.

Copper and Silver Catalysis in the (3 + 2) Cycloaddition of Neutral Three-Atom Components with Terminal Alkynes

The introduction of the copper-catalyzed azide-alkyne coupling (CuAAC) to 1,3-dipolar cycloadditions was pivotal to their popularization in synthetic chemistry and to their application to multiple other domains of science. The reaction rate enhancement observed when coinage metal acetylide intermediates are involved in the cyclization process greatly expanded the structural and conditional range in which (3 + 2) cycloadditions may take place with terminal alkynes. Herein, we report that comparable rate enhancements, in nature and level, are induced by copper and silver catalysts in the intramolecular (3 + 2) cycloaddition of terminal alkynes with "neutral" three-atom components (TACs), specifically alkynyl sulfides. Through careful observations amidst reaction optimization, experimental, and DFT mechanistic studies, a pathway involving a proton-coupled cyclometallation key step is proposed. The sets of catalytic conditions that have been developed allow us to overcome several scope limitations previously presented by the thermally promoted (3 + 2) cycloaddition of "neutral" TACs, thus expanding their synthetic and applicative potential.

CuAAC-methacrylate interpenetrating polymer network (IPN) properties modulated by visible-light photoinitiation

Interpenetrating polymer networks (IPNs) are a class of materials with interwoven polymers that exhibit unique blended or enhanced properties useful to a variety of applications, ranging from restorative protective materials to conductive membranes and hydrophobic adhesives. The IPN formation kinetics can play a critical role in the development of the underlying morphology and in turn the properties of the material. Dual photoinitiation of copper-catalyzed azide-alkyne (CuAAC) and radical mediated methacrylate polymerization chemistries enable the manipulation of IPN microstructure and properties by controlling the kinetics of IPN formation via the intensity of the initiating light. Specifically, azide and alkyne-based polyethylene glycol monomers and tetraethylene glycol dimethacrylate (TEGDMA) were polymerized in a single pot to form IPNs and the properties were evaluated as a function of the photoinitiating light intensity. Morphological differences as a function of intensity were observed in the IPNs as determined by thermomechanical properties and phase-contrast imaging in tapping mode atomic force microscopy (AFM). At moderate intensities (20 mW/cm2) of visible light (470 nm), the TEGDMA polymerization gels first and therefore forms the underlying network scaffold. At low intensities (0.2 mW/cm2), the CuAAC polymerization can gel first. The ability to switch sequence of gelation and IPN trajectory (simultaneous vs. sequential), affords control over phase separation behavior. Thus, light not only allows for spatial and temporal control over the IPN formation but also provides control over their thermomechanical properties, representing a route for facile IPNs design, synthesis, and application.

The Certainty of a Few Good Reactions

The impact of click chemistry was recently recognized with the 2022 Nobel Prize in Chemistry. The breadth of areas where click chemistry has accelerated discovery is prodigal. In one of the most written about subjects in chemistry over recent years, this short perspective zones in on a small fragment of what we, the authors, consider are some of the most critical developments in synthetic chemistry, which have expanded access to the click chemistry toolbox. In addition, we touch upon areas within medicinal chemistry and novel approaches to drug discovery enabled by click chemistry, where we believe there is untapped potential for biological function to be found and exploited.

Ultrasensitive and Specific Phage@DNAzyme Probe-Triggered Fluorescent Click Chemistry for On-Site Detection of Foodborne Pathogens Using a Smartphone

Rapid, specific, and on-site detection of virulent foodborne pathogenic strains plays a key role in controlling food safety. In this work, an ultrasensitive and specific Phage@DNAzyme signal probe was designed to detect foodborne pathogens. The proposed sensing probe was composed of the selected phage and functionalized DNAzyme, which realized the specific recognition of target foodborne pathogens at the strain level and the efficient catalysis of copper(II) based azide-alkyne cycloaddition (CuAAC) click reaction with fluorescent signal, respectively. As a proof of concept, the virulent Escherichia coli O157:H7 (E. coli O157:H7) as the representative analyte was first enriched and purified from the complex food samples by a 4-mercaptophenylboronic acid-modified gold slide. Following, the Phage@DNAzyme probes were specifically combined with the captured E. coli O157: H7 and catalyzed the click reaction between 3-azido-7-hydroxycoumarin and 3-butyn-1-ol with the assistance of Cu(II) to generate a visual fluorescent signal. Finally, the corresponding fluorescent signals were measured by a smartphone to quantify the target concentrations. Under optimized conditions, the bioassay exhibited a wide linear range from 10² to 10⁸ CFU/mL and the detection limit was 50 CFU/mL (S/N = 3). It was further extended to the detection of another foodborne pathogen Salmonella typhimurium with satisfying sensing performances. This work gives a new path for developing rapid, specific, and on-site detection methods for trace levels of pathogenic strains in foods.

Bioorthogonal Fluoride-Responsive Azide and Alkynyl Pyridinium Click Cycloaddition in Vitro and in Live Cells

The copper-free azide-alkyne cycloaddition was broadly applied in numerous research fields. Herein, we report a facile Cu-free click reaction utilizing fluoride-responsive azide and alkynyl pyridinium cycloaddition at ambient temperatures in aqueous media. The reactivity of alkynyl pyridinium was successfully masked by a silyl-protecting group at the alkyne group, and the deprotection could be readily achieved with the addition of F-, which renders the reactivity. The substrates were readily synthesized and proven to be stable at the bench. This bioorthogonal fluoride-responsive click reaction was then successfully employed in peptide modification, protein labeling, and cell imaging, suggesting its potential in various applications.

Polymer Coated Functional Catalysts for Industrial Applications

Surface engineering of conventional catalysts using polymeric coating has been extensively explored for producing hybrid catalytic material with enhanced activity, high mechanical and thermal stability, enhanced productivity, and selectivity of the desired product. The present review discusses in detail the state-of-the-art knowledge on surface modification of catalysts, namely photocatalysts, electrocatalysts, catalysts for photoelectrochemical reactions, and catalysts for other types of reactions, such as hydrodesulfurization, carbon dioxide cycloaddition, and noble metal-catalyzed oxidation/reduction reactions. The various techniques employed for the polymer coating of catalysts are discussed and the role of polymers in enhancing the catalytic activity is critically analyzed. The review further discusses the applications of biodegradable and biocompatible natural polysaccharide-based polymers, namely, chitosan and polydopamine as prospective coating material.

Advances in the Synthesis of Fused 1,2,3-Triazoles via a MCR-Intramolecular Azide-Alkyne Cycloaddition Approach

The present review narrates several reports which deal with the synthesis of fused 1,2,3-triazole containing scaffolds following a sequential multicomponent reaction (MCR)-intramolecular azide-alkyne cycloaddition (IAAC) approach. The reviewed reactions were cleverly designed so as to incorporate azide and alkyne functionalities in the MCR product which was then subjected to IAAC. The review is divided into two sections based on the number of components in the multicomponent reaction. We have aimed at a critical discussion and also have highlighted either advantages or disadvantages of each methodology.

ortho-Substituted Aryldiazonium Design for the Defect Configuration-Controlled Photoluminescent Functionalization of Chiral Single-Walled Carbon Nanotubes

Defect functionalization of single-walled carbon nanotubes (SWCNTs) by chemical modification is a promising strategy for near-infrared photoluminescence (NIR PL) generation at >1000 nm, which has advanced telecom and bio/medical applications. The covalent attachment of molecular reagents generates sp³-carbon defects in the sp²-carbon lattice of SWCNTs with bright red-shifted PL generation. Although the positional difference between proximal sp³-carbon defects, labeled as the defect binding configuration, can dominate NIR PL properties, the defect arrangement chemistry remains unexplored. Here, aryldiazonium reagents with π-conjugated ortho-substituents (phenyl and acetylene groups) were developed to introduce molecular interactions with nanotube sidewalls into the defect-formation chemical reaction. The functionalized chiral SWCNTs selectively emitted single defect PL in the wavelength range of ∼1230-1270 nm for (6,5) tubes, indicating the formation of an atypical binding configuration, different from those exhibited by typical aryl- or alkyl-functionalized chiral tubes emitting ∼1150 nm PL. Moreover, the acetylene-based substituent design enabled PL brightening and a subsequent molecular modification of the doped sites using click chemistry.

Synthesis of push-pull-activated ynol ethers and their evaluation in the bioorthogonal hydroamination reaction

A new class of push-pull-activated alkynes featuring di- and trifluorinated ynol ethers was synthesized. The difluorinated ynol ether exhibited an optimal balance of stability and reactivity, displaying a substantially improved half-life in the presence of aqueous thiols over the previously reported 1-haloalkyne analogs while reacting just as fast in the hydroamination reaction with N,N-diethylhydroxylamine. The trifluorinated ynol ether reacted significantly faster, exhibiting a second order rate constant of 0.56 M-1 s-1 in methanol, but it proved too unstable toward thiols. These fluorinated ynol ethers further demonstrate the importance of the hyperconjugation-rehybridization effect in activating alkynes and demonstrate how substituent effects can both activate and stabilize alkynes for bioorthogonal reactivity.

Calix[4]arene Polyamine Triazoles: Synthesis, Aggregation and DNA Binding

Artificial gene delivery systems are in great demand from both scientific and practical biomedical points of view. In this paper, we present the synthesis of a new click chemistry calix[4]arene precursor with free lower rim and new water-soluble calixarene triazoles with 12 amino-groups on the upper rim (one with free phenol hydroxyl groups and two another containing four butyl or tetradecyl fragments). Aggregation in the series of amino-triazole calixarenes of different lipophilicity (calixarene with free phenol hydroxyl groups or butyl and tetradecyl fragments on the lower rim) was studied using dynamic light scattering and fluorescent pyrene probe. It was found that calix[4]arene with a free lower rim, like alkyl-substituted butyl calix[4]arene, forms stable submicron aggregates 150-200 nm in size, while the more lipophilic tetradecyl -substituted calix[4]arene forms micellar aggregates19 nm in size. Using UV-Vis spectroscopy, fluorimetry and CD, it was shown that amino-triazole calix[4]arenes bind to calf thymus DNA by classical intercalation. According to DLS and TEM data, all studied macrocycles cause significant DNA compaction, forming stable nanoparticles 50-20 nm in size. Among all studied calix[4]arenes the most lipophilic tetradecyl one proved to be the best for both binding and compaction of DNA.

Synthesis of Novel Ferrocene-Benzofuran Hybrids via Palladium- and Copper-Catalyzed Reactions

The combination of the ferrocene skeleton with pharmacophores often leads to molecules with interesting biological properties. Five ferrocene-benzofuran hybrids of different structures were synthesized by transition metal catalyzed reactions. The efficiency of both homogeneous and heterogeneous catalytic methods was tested. The products were characterized using 1H, 13C NMR and FTIR spectroscopy, HRMS and cyclic voltammetry. The structure of one of the new compounds was also proved with X-ray crystallography. The new hybrids showed moderate cytotoxicity on MCF-7 and MDA-MB-231 cell lines. It is remarkable that the less curable MDA-MB-231 cell line was more sensitive to treatment with three ferrocene derivatives.

Synthesis of Radioiodinated Compounds. Classical Approaches and Achievements of Recent Years

This review demonstrates the progress in the synthesis of radioiodinated compounds over the past decade. The possibilities and limitations of radiopharmaceuticals with different iodine isotopes, as well as the synthesis of low and high molecular weight compounds containing radioiodine, are discussed. An analysis of synthesis strategies, substrate frameworks, isolation methods, and metabolic stability, and the possibility of industrial production of radioiodinated organic derivatives which can find applications in the synthesis of drugs and diagnostics are presented.

Bio-Vitrimers for Sustainable Circular Bio-Economy

The aim to achieve sustainable development goals (SDG) and cut CO2-emission is forcing researchers to develop bio-based materials over conventional polymers. Since most of the established bio-based polymeric materials demonstrate prominent sustainability, however, performance, cost, and durability limit their utilization in real-time applications. Additionally, a sustainable circular bioeconomy (CE) ensures SDGs deliver material production, where it ceases the linear approach from production to waste. Simultaneously, sustainable circular bio-economy promoted materials should exhibit the prominent properties to involve and substitute conventional materials. These interceptions can be resolved through state-of-the-art bio-vitrimeric materials that display durability/mechanical properties such as thermosets and processability/malleability such as thermoplastics. This article emphasizes the current need for vitrimers based on bio-derived chemicals; as well as to summarize the developed bio-based vitrimers (including reprocessing, recycling and self-healing properties) and their requirements for a sustainable circular economy in future prospects.

A practical flow synthesis of 1,2,3-triazoles

A robust and versatile protocol for synthesis of 1-monosubstituted and 1,4-disubstituted 1H-1,2,3-triazoles was established under continuous flow conditions using copper-on-charcoal as a heterogeneous catalyst. This methodology allowed for the synthesis of a diverse set of substituted 1,2,3-triazoles with good functional group tolerance and high yields. 2-Ynoic acids were also used as small-chain alkyne donors in a decarboxylation/cycloaddition cascade, allowing gaseous reagents to be bypassed, delivering desired triazoles in high yields. The developed methodology was used to synthesize an antiepileptic agent, rufinamide, which was obtained in 96% isolated yield.

Copper-on-charcoal is an excellent heterogeneous catalyst for the alkyne–azide cycloaddition reaction performed under continuous flow conditions. 2-Ynoic acids undergo decarboxylation/cycloaddition cascade giving triazoles bearing small alkyl chains.

In Vitro Antioxidant and Pancreatic Anticancer Activity of Novel 5-Fluorouracil-Coumarin Conjugates

Molecular hybridization consists of the combination of two or more non-identical pharmacophores in a single molecule. It has emerged as a promising strategy that allows the design of molecular frameworks with enhanced activity and affinity compared to their parent drugs. In this work, two novel hybrids that combine the well-known anticancer chemotherapeutic agent 5-fluorouracil with antioxidant coumarin derivatives have been synthesized and characterized by means of a copper-catalyzed azide-alkyne cycloaddition (CuAAC). The conjugates showed good antioxidant properties and a high tendency to aggregate and form stable nanoparticles in aqueous media, with regular shape and uniform size. These materials have proven to be preferential cytotoxic agents in vitro against human pancreatic cancer cells PANC-1, with an activity superior to free 5-fluorouracil. These results open up the possibility of exploiting the synergistic combination between 5-fluorouracil and coumarin derivatives and warrant further investigation of these hybrids as promising pancreatic anticancer agents.

Sulfur-Phenolate Exchange: SuFEx-Derived Dynamic Covalent Reactions and Degradation of SuFEx Polymers

The products of the SuFEx reaction between sulfonimidoyl fluorides and phenols, sulfonimidates, are shown to display dynamic covalent chemistry with other phenols. This reaction was shown to be enantiospecific, finished in minutes at room temperature in high yields, and useful for both asymmetric synthesis and sustainable polymer production. Its wide scope further extends the usefulness of SuFEx and related click chemistries.

Promoting Dinuclear-Type Catalysis in Cu1 -C3 N4 Single-Atom Catalysts

Reducing particle size in supported metal catalysts to single-atom level isolates the active metal sites and maximizes the atomic utilization efficiency. However, the large inter-atom distance, particularly in low-loading single-atom catalyst (SAC), is not favorable for a complex reaction where two (or more) reactants have to be activated. A key question is how to control the inter-atom distances to promote dinuclear-type coactivation at the adjacent metal sites. Here, it is reported that reducing the average inter-atom distance of copper SACs supported on carbon nitride (C₃ N₄ ) to 0.74 ± 0.13 nm allows these catalysts to exhibit a dinuclear-type catalytic mechanism in the nitrile-azide cycloaddition. Operando X-ray absorption fine structure study reveals a dynamic ligand exchange process between nitrile and azide, followed by their coactivation on dinuclear Cu SAC sites to form the tetrazole product. This work highlights that reducing the nearest-neighbor distance of SAC allows the mechanistic pathway to diversify from single-site to multisite catalysis.

Copper-Catalyzed Azide–Alkyne Cycloaddition (CuAAC) by Functionalized NHC-Based Polynuclear Catalysts: Scope and Mechanistic Insights

Copper(I) [Cu₂(μ-Br)₂(^tBuImCH₂pyCH₂L)]_n (L = OMe, NEt₂, NH^tBu) compounds supported by flexible functionalized NHC-based polydentate ligands have been prepared in a one-pot procedure by reacting the corresponding imidazolium salt with an excess of copper powder and Ag₂O. An X-ray diffraction analysis has revealed that [Cu₂(μ-Br)₂(^tBuImCH₂pyCH₂NEt₂)]_n is a linear coordination polymer formed by bimetallic [Cu(μ-Br)]₂ units linked by the lutidine-based NHC-py-NEt₂ ligand, which acts as a heteroditopic ligand with a 1κC-2κ²N,N′ coordination mode. We propose that the polymeric compounds break down in the solution into more compact tetranuclear [Cu₂(μ-Br)₂(^tBuImCH₂pyCH₂L)]₂ compounds with a coordination mode identical to the functionalized NHC ligands. These compounds have been found to exhibit high catalytic activity in the Cu-catalyzed azide–alkyne cycloaddition (CuAAC) reaction. In particular, [Cu₂(μ-Br)₂(^tBuImCH₂pyCH₂NEt₂)]₂ efficiently catalyzes the click reaction of a range of azides and alkynes, under an inert atmosphere at room temperature in neat conditions at a very low catalyst loading, to quantitatively afford the corresponding 1,4-disubstituted 1,2,3-triazole derivatives in a few minutes. The cycloaddition reaction of benzyl azide to phenylacetylene can be performed at 25–50 ppm catalyst loading by increasing the reaction time and/or temperature. Reactivity studies have shown that the activation of the polynuclear catalyst precursor involves the alkyne deprotonation by the NHC moiety of the polydentate ligand to afford a copper(I)-alkynyl species bearing a functionalized imidazolium ligand. DFT calculations support the participation of the dinuclear species [(CuBr)₂(μ-^tBuImCH₂pyCH₂NEt₂)], resulting from the fragmentation of the tetranuclear compound, as the catalytically active species. The proposed reaction pathway proceeds through zwitterionic dinuclear intermediates and entails the active participation of both copper atoms, as well as the NHC moiety as an internal base, which activates the reacting alkyne via deprotonation.

Green solvent-free synthesis of new N-heterocycle-L-ascorbic acid hybrids and their antiproliferative evaluation

Aim: The authors' aim was to improve the application of copper-catalyzed azide-alkyne cycloaddition in the synthesis of hybrids containing biologically significant nucleobases and L-ascorbic acid scaffolds by introducing an environmentally friendly and waste-free ball mill. Results: Two series of hybrids with a purine, pyrrolo[2,3- d]pyrimidine or 5-substituted pyrimidine attached to 2,3-dibenzyl-L-ascorbic acid via a hydroxyethyl- (15a–23a) or ethylidene-1,2,3-triazolyl (15b–23b) bridge were prepared by ball milling and conventional synthesis. The unsaturated 6-chloroadenine L-ascorbic acid derivative 16b can be highlighted as a lead compound and showed strong antiproliferative activity against HepG2 (hepatocellular carcinoma) and SW620 (colorectal adenocarcinoma) cells. Conclusion: Mechanochemical synthesis was superior in terms of sustainability, reaction rate and yield, highlighting the advantageous applications of ball milling over classical reactions.