Cu-catalyzed Azide-alkyne Cycloaddition Research Articles

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 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 peroxide, acid, base, and 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 CuSO-4-THPTA standard.

Reactivity Profiling for High-Yielding Ynamine-Tagged Oligonucleotide Click Chemistry Bioconjugations.

The Cu-catalyzed azide-alkyne cycloaddition (CuAAC) reaction is a key ligation tool used to prepare bioconjugates. Despite the widespread utility of CuAAC to produce discrete 1,4-triazole products, the requirement of a Cu catalyst can result in oxidative damage to these products. Ynamines are superior reactive groups in CuAAC reactions and require lower Cu loadings to produce 1,4-triazole products. This study discloses a strategy to identify optimal reaction conditions for the formation of oligodeoxyribonucleotide (ODN) bioconjugates. First, the surveying of reaction conditions identified that the ratio of Cu to the choice of reductant (i.e., either sodium ascorbate or glutathione) influences the reaction kinetics and the rate of degradation of bioconjugate products. Second, optimized conditions were used to prepare a variety of ODN-tagged products and ODN-protein conjugates and compared to conventional CuAAC and Cu-free azide-alkyne (3 + 2)cycloadditions (SPAAC), with ynamine-based examples being faster in all cases. The reaction optimization platform established in this study provides the basis for its wider utility to prepare CuAAC-based bioconjugates with lower Cu loadings while maintaining fast reaction kinetics.

Water-Soluble Aromatic Nanobelt with Unique Cellular Internalization.

A water-soluble aromatic nanobelt was synthesized, and its cellular uptake behavior in HeLa cells was investigated. The late-stage functionalization of the parent methylene-bridged [6]cycloparaphenylene ([6]MCPP) provided an easily accessible alkyne-functionalized nanobelt in a single reaction step. The alkyne-substituted [6]MCPP was subjected to Cu-catalyzed azide-alkyne cycloaddition by using a dye-attached azide to obtain a water-soluble aromatic nanobelt. Cell-imaging experiments on the synthesized nanobelt in HeLa cells revealed stop-and-go cellular uptake dynamics. Similar experiments with control molecules and theoretical studies indicated that the unique dynamics of the nanobelt was derived from the belt-shaped structure.

Synthesis of Carbosilane Dendrimers with an Amine-Containing Shell Using CuAAC

A new synthetic approach to carbosilane dendrimers bearing amino-containing functional groups in the shell is proposed based on the Cu-catalyzed azide–alkyne cycloaddition. The structures of the resulting compounds are confirmed using 1H, 13C, and 29Si NMR spectroscopy, as well as gel permeation chromatography.

Radiosynthesis and Preclinical Evaluation of 18F-Labeled Estradiol Derivatives with Different Lipophilicity for PET Imaging of Breast Cancer.

About 75% of breast tumors show an overexpression of the estradiol receptor (ER), making it a valuable target for tumor diagnosis and therapy. To date, 16α-[18F]fluoroestradiol (FES) is the only FDA-approved imaging probe for the positron emission tomography (PET) imaging of ER-positive (ER+) breast cancer. However, FES has the drawback of a high retention in the liver. Therefore, the aim of this study was the development and preclinical evaluation of estradiol (E2) derivatives with different lipophilicity. Three 18F-labeled prosthetic groups (two glycosyl and one PEG azide) were chosen for conjugation with ethinyl estradiol (EE) by 18F-CuAAC (Cu-catalyzed azide-alkyne cycloaddition). The cellular uptake in ER+ MCF-7 tumor cells was highest for the less hydrophilic derivative (18F-TA-Glyco-EE). In nude mice bearing different breast tumors (ER+ MCF-7 and T47D versus ER- MDA-MB-231), 18F-TA-Glyco-EE revealed a high uptake in the liver (13%ID/g, 30 min p.i.), which decreased over 90 min to 1.2%ID/g, indicating fast hepatobiliary clearance. The statistically significant difference of 18F-TA-Glyco-EE uptake in T47D compared to MDA-MB-231 tumors at 60-90 min p.i. indicated ER-specific uptake, whereas in vivo PET imaging did not provide evidence for specific uptake of 18F-TA-Glyco-EE in MCF-7 tumors, probably due to ER occupation by E2 after E2-dependent MCF-7 tumor growth in mice. However, in vitro autoradiography revealed a high specific binding of 18F-TA-Glyco-EE to ER+ tumor slices. We conclude that 18F-TA-Glyco-EE, with its increased hydrophilicity after deacetylation in the blood and thus rapid washout from non-target tissues, may be a viable alternative to FES for the PET imaging of breast cancer.

Synthesis, DFT calculations, and investigation of catalytic and biological activities of back‐bond functionalized re‐NHC‐Cu complexes

Five copper complexes (3a–e) stabilized by ring‐expanded back‐bond functionalized N‐heterocyclic carbene ligands (re‐NHCs) were produced in the glovebox by reacting free re‐NHC with CuI precursor. The potential of these re‐NHC‐Cu complexes as catalysts on the synthesis of mono‐ and di‐(1,4‐disubstituted‐1,2,3‐triazoles) by Cu‐catalyzed azide–alkyne cycloaddition (CuAAC) reactions was investigated. Various spectroscopic approaches were utilized to completely characterize the structures of the re‐Cu‐NHC complexes. Furthermore, density functional theory (DFT) calculations were carried out to get further insights into their molecular geometry and CuAAC reaction mechanism. The re‐NHC‐Cu complexes showed high activity on the CuAAC reaction in an open‐air atmosphere at rt. The Gibbs free energies as well as the optimized geometries of the intermediates and the transition states of the determining step of the reactions catalyzed by 3a, 3e, and 3b complexes were computed. Complex 3e was found to be the most efficient catalyst among these re‐NHC‐Cu complexes. Additionally, re‐Cu‐NHC complexes were investigated for their biological activities, including antiproliferative, antioxidant, AChE and TyrE inhibition, and antiparasitic activity. The results showed that the 3b, 3d, and 3e complexes possessed strong antiproliferative activity against human colon carcinoma (HCT‐116) and moderate cytotoxic activity against hepatocellular carcinoma (HepG‐2) cell lines. In addition, effective selective antileishmanial effects were observed for the 3e compound against both promastigotes and amastigotes stages of L. major with an IC50 value of 0.027 and 0.39 μM mL−1, respectively. These results demonstrate that these compounds are promising candidates for the treatment of colorectal cancer and leishmaniasis.

Mechanistic Basis of the Cu(OAc)2 Catalyzed Azide-Ynamine (3 + 2) Cycloaddition Reaction.

The Cu-catalyzed azide-alkyne cycloaddition (CuAAC) reaction is used as a ligation tool throughout chemical and biological sciences. Despite the pervasiveness of CuAAC, there is a need to develop more efficient methods to form 1,4-triazole ligated products with low loadings of Cu. In this paper, we disclose a mechanistic model for the ynamine-azide (3 + 2) cycloadditions catalyzed by copper(II) acetate. Using multinuclear nuclear magnetic resonance spectroscopy, electron paramagnetic resonance spectroscopy, and high-performance liquid chromatography analyses, a dual catalytic cycle is identified. First, the formation of a diyne species via Glaser-Hay coupling of a terminal ynamine forms a Cu(I) species competent to catalyze an ynamine-azide (3 + 2) cycloaddition. Second, the benzimidazole unit of the ynamine structure has multiple roles: assisting C-H activation, Cu coordination, and the formation of a postreaction resting state Cu complex after completion of the (3 + 2) cycloaddition. Finally, reactivation of the Cu resting state complex is shown by the addition of isotopically labeled ynamine and azide substrates to form a labeled 1,4-triazole product. This work provides a mechanistic basis for the use of mixed valency binuclear catalytic Cu species in conjunction with Cu-coordinating alkynes to afford superior reactivity in CuAAC reactions. Additionally, these data show how the CuAAC reaction kinetics can be modulated by changes to the alkyne substrate, which then has a predictable effect on the reaction mechanism.

Arylazopyrazoles for Conjugation by CuAAC Click Chemistry.

Molecular photoswitches are increasingly being implemented in bioactive compounds and responsive materials. For this purpose, photoswitches must be coupled to a wide variety of substrates and scaffolds. We present a library of "clickable" arylazopyrazoles (AAPs), which can be conjugated by Cu-catalyzed alkyne azide cycloaddition (CuAAC). All synthesized AAP alkynes show good photostationary states (at least 88%) and long half-life times of the Z-isomer (18 to 108 h). The AAP azides decompose upon exposure to ultraviolet (UV) irradiation, but after CuAAC, all AAPs exhibit good photophysical properties.

A High-Yielding Active Template Click Reaction (AT-CuAAC) for the Synthesis of Mechanically Interlocked Nanohoops.

Mechanically interlocked molecules (MIMs) represent an exciting yet underexplored area of research in the context of carbon nanoscience. Recently, work from our group and others has shown that small carbon nanotube fragments-[n]cycloparaphenylenes ([n]CPPs) and related nanohoop macrocycles-may be integrated into mechanically interlocked architectures by leveraging supramolecular interactions, covalent tethers, or metal-ion templates. Still, available synthetic methods are typically difficult and low yielding, and general methods that allow for the creation of a wide variety of these structures are limited. Here we report an efficient route to interlocked nanohoop structures via the active template Cu-catalyzed azide-alkyne cycloaddition (AT-CuAAC) reaction. With the appropriate choice of substituents, a macrocyclic precursor to 2,2'-bipyridyl embedded [9]CPP (bipy[9]CPP) participates in the AT-CuAAC reaction to provide [2]rotaxanes in near-quantitative yield, which can then be converted into the fully π-conjugated catenane structures. Through this approach, two nanohoop[2]catenanes are synthesized which consist of a bipy[9]CPP catenated with either Tz[10]CPP or Tz[12]CPP (where Tz denotes a 1,2,3-triazole moiety replacing one phenylene ring in the [n]CPP backbone).

Clickable Polymerization-Induced Emission Luminogens Toward Color-Tunable Modification of Non-Traditional Intrinsic Luminescent Polymers.

Non-traditional intrinsic luminescent (NTIL) polymer is an emerging field, and its color-tunable modification is highly desirable but still rarely investigated. Here, a click chemistry approach for the color-tunable modifications of NTIL polymers by introducing clickable polymerization-induced emission luminogen (PIEgen), is demonstrated. Through Cu-catalyzed azide-alkyne cycloaddition click chemistry, a series of PIEgens is successful prepared, which is further polymerized via reversible addition-fragmentation chain transfer (RAFT) polymerization. Interestingly, after clickable modification, these monomers are nonemissive in both solution and aggregation states; while, the corresponding polymers exhibit intriguing aggregation-induced emission (AIE) characteristics, confirming their PIEgen characteristics. By varying alkynyl substitutions, color-tunable NTIL polymers are achieved with emission wavelength varying from 448 to 498nm, revealing a series of PIEgens and verifying the importance of modification of NTIL polymers. Further luminescence energy transfer application is carried out as well. This work therefore designs a series of clickable PIEgens and opens a new avenue for the modification of NTIL polymers via click chemistry, which may cause inspirations to the research fields including luminescent polymer, NTIL, click chemistry, AIE and modification.

Cu-catalyzed cycloaddition of aryl azides to 1-iodobuta-1,3-diynes: an experimental and quantum chemical study of unusual regiochemistry

The nontrivial CuAAC regiochemistry of aryl azides and 1-iodobutadiynes helped to establish the binuclear character of the CuAAC mechanism for iodoalkynes.

Synthesis of Bioactive 1,2,3-Triazole-Fused Macrocycles via Azide-Alkyne Cycloaddition

AbstractA systematic highlight of syntheses reported since 2006 of 1,2,3-triazole-fused macrocycles possessing biological activities such as anticancer, antibacterial, antiviral, anti-inflammatory and antilarval action, is presented in this review. The well-renowned Cu-catalyzed azide-alkyne cycloaddition reaction was noted to be highly efficient and is one the most common methods utilized by scientists for the synthesis of 1,4-disubstituted triazole-fused macrocycles, whereas Ru-catalyzed cycloaddition is common for the formation of 1,5-disubstituted bioactive triazoles. This review would thus be extremely beneficial for both synthetic organic and medicinal chemists.1 Introduction2 Anticancer Derivatives3 Antibacterial Derivatives4 Derivatives with Dual Activity5 Antilarval Derivatives6 Anti-inflammatory Derivatives7 Antiviral Derivatives8 Anti-trypanosomal Derivatives9 Derivatives with Miscellaneous Activities10 Conclusion

Pullulan-Graft-Polyoxazoline: Approaches from Chemistry and Physics.

An approach to the preparation of pullulan-graft-poly(2-methyl-2-oxazoline)s based on Cu-catalyzed azide-alkyne cycloaddition with polyoxazoline-azide was applied. All of the obtained polymers were characterized through classical molecular hydrodynamic methods and NMR. The formation of graft copolymers was accomplished by oxidative degradation of pullulan chains. Nevertheless, graft copolymers were obtained as uniform products with varied side chain lengths and degrees of substitution.

Copper-Catalyzed Three-Component Tandem Reaction of Alkynes, α-Diazo Esters, and TMSN3 to Access N-Substituted 1,2,3-Triazoles.

An efficient copper-catalyzed three-component tandem reaction of alkynes, α-diazo esters, and TMSN3 to construct triazoles has been developed. Through this strategy, a number of diverse N-substituted 1,2,3-triazoles were conveniently obtained in moderate to good yields from simple and readily available starting materials using K2CO3 as the base. The mechanism of the tandem Cu-catalyzed azide-alkyne cycloaddition (CuAAC) and Cu-carbenoid-participated C-N coupling reaction has been investigated.

Terminus-Selective Covalent Immobilization of Heparin on a Thermoresponsive Surface Using Click Chemistry for Efficient Binding of Basic Fibroblast Growth Factor.

Cell therapy using endothelial cells (ECs) has great potential for the treatment of congenital disorders, such as hemophilia A. Cell sheet technology utilizing a thermoresponsive culture dish is a promising approach to efficiently transplant donor cells. In this study, a new method to prepare terminus-selective heparin-immobilized thermoresponsive culture surfaces is developed to facilitate the preparation of EC sheets. Alkynes are introduced to the reducing terminus of heparin via reductive amination. Cu-catalyzed azide-alkyne cycloaddition (CuAAC) facilitates efficient immobilization of the terminus of heparin on a thermoresponsive surface, resulting in a higher amount of immobilized heparin while preserving its function. Heparin-immobilized thermoresponsive surfaces prepared using CuAAC exhibit good adhesion to human endothelial colony-forming cells (ECFCs). In addition, upon further binding to basic fibroblast growth factor (bFGF) on heparin-immobilized surfaces, increased proliferation of ECFCs on the surface is observed. The confluent ECFC monolayer cultured on bFGF-bound heparin-immobilized thermoresponsive surfaces exhibits relatively high fibronectin accumulation and cell number and detaches at 22°C while maintaining the sheet-like structure. Because heparin has an affinity for several types of bioactive molecules, the proposed method can be applied to facilitate efficient cultures and sheet formations of various cell types.

Side-chain functionality driven thermoresponsive and semicrystalline poly(L-glutamate)s and self-assembly

The appropriate functionalization of synthetic polypeptides is the key to introduce stimuli-responsiveness and amphiphilicity for generating self-assembled nanostructures (micelles, vesicles, etc.) with promising applications in the areas of drug/gene delivery and tissue engineering. This work presents a way to functionalize poly(L-glutamate)s with long chain alkyl groups and to study the semicrystalline as well as solution-phase behavior such as thermoresponsiveness, self-aggregation etc. of the resultant polypeptides. The ring opening polymerization (ROP) of the as-synthesized γ-propargyl-L-glutamate N-carboxyanhydride produce poly(L-glutamate) (PGlu-Pr) with pendant propargyl group, which is clicked with long-chain alkyl azides via Cu-catalyzed alkyne-azide-cycloaddition (Cu-AAC) reaction to give alkyl-grafted poly(L-glutamate)s [PGlu–Cn(s), n = 10, 12, 16, 20]. The helical secondary conformation predominates for PGlu-Pr and PGlu–Cn(s) in both solution phase and in solid state and the extent of helicity decreases with increase in the length of alkyl chains. The corresponding control monomeric analogs, alkyl grafted L-glutamates (Glu–Cn)s, show formation of birefringent crystals due to the presence of the long alkyl chains as examined by differential scanning calorimetry (DSC) and polarized optical microscopy. Whereas PGlu–Cn(s) having n ≥ 12 show feeble crystallinity in their solid state. Interestingly, the solutions of PGlu–Cn(s) in CHCl3 and THF show upper critical solution temperature (UCST)-type thermoresponsive properties with tunable cloud point, but no such property is observed for Glu–Cn(s). Self-assembly of PGlu–Cn(s) in these solvents generates vesicular nanoaggregates as confirmed by the dynamic light scattering and transmission electron microscopy. These nanoaggregates can encapsulate organic dye (Eosin B) as studied by fluorescence and confocal microscopy.

Fe3O4@Zein nanocomposites decorated with copper(II) as an efficient, durable, and biocompatible reusable catalyst for click synthesis of novel fluorescent 1,4-disubstituted-1,2,3-triazoles in water

In this paper, we have constructed Fe3O4@Zein-Cu(II) NPs as a novel reusable heterogeneous and superparamagnetic catalyst in Cu-catalysed azide-alkyne cycloaddition reactions (CuAAC). This magnetic Cu(II) nanocomposites were characterized by fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), thermogravimetric analysis (TGA), vibration sample magnetometer (VSM) and X-ray photoelectron spectroscopy (XPS). This biocompatible and inexpensive magnetic nanocomposite catalyses smoothly the regioselective click preparation of new fluorescent 1,4-disubstituted-1,2,3-triazoles of 1,8-naphthilimide and 3-nitrophthalimide with quantum yields 0.153 to 0.371 in good to excellent yields under mild conditions in H2O without using any ligands and reducing agents. The catalyst could be easily recovered with the aid of an external magnet and reused up to seven consecutive runs without significant reduction in its activity.

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.

Nitrogen-rich, click-based porous organic polymers featuring flexible amine cores for catalytic CO2/epoxide cycloaddition

CO2 utilization to produce value-added products such as cyclic carbonates, especially under mild catalytic conditions, is a promising strategy to reduce CO2 emissions to the environment. In this work, nitrogen-rich, click-based porous polymers (CPPs) were prepared from coupling of tripropargylamine and rigid diazido aromatic substrates, featuring 1,4-phenylene (TB), 1,5-naphthalene (TN), and 4,4′-benzanilide (TBA) linkers, in moderate to good yields (48–83%) using Cu-catalyzed azide–alkyne cycloaddition (CuAAC; TB-C, TN-C, TBA-C) and Huisgen cycloaddition (TB-H, TN-H, and TBA-H). The presence of the chelate tris(triazolylmethyl)amine ligands within the polymer's structure led to strong copper binding and consequently high copper loading ability of the CPPs. In addition, high contents of Lewis basic N donors (21–27 wt% N) of TB-C, TN-C, TBA-C are most likely attributed to strong interactions between CO2 and CPPs and exceptionally high CO2 selectivity over N2 at 273 K, based on gas sorption studies. The catalyst system TB-C/TBAB (TBAB = tetrabutylammonium bromide) promoted complete conversion of epoxides to cyclic carbonates under an atmospheric CO2 pressure, after 24 h or 48 h at 100 °C, with the exception of the epichlorohydrin substrate where 42% selectivity was observed by 1H NMR spectroscopy. The TB-C/TBAB catalyst system could be reused for five catalytic runs without a loss in catalytic activities and any pre-activation processes.

A Click Chemistry-Based Artificial Metallo-Nuclease.

Artificial metallo-nucleases (AMNs) are promising DNA damaging drug candidates. Here, we demonstrate how the 1,2,3-triazole linker produced by the Cu-catalysed azide-alkyne cycloaddition (CuAAC) reaction can be directed to build Cu-binding AMN scaffolds. We selected biologically inert reaction partners tris(azidomethyl)mesitylene and ethynyl-thiophene to develop TC-Thio, a bioactive C3 -symmetric ligand in which three thiophene-triazole moieties are positioned around a central mesitylene core. The ligand was characterised by X-ray crystallography and forms multinuclear CuII and CuI complexes identified by mass spectrometry and rationalised by density functional theory (DFT). Upon Cu coordination, CuII -TC-Thio becomes a potent DNA binding and cleaving agent. Mechanistic studies reveal DNA recognition occurs exclusively at the minor groove with subsequent oxidative damage promoted through a superoxide- and peroxide-dependent pathway. Single molecule imaging of DNA isolated from peripheral blood mononuclear cells shows that the complex has comparable activity to the clinical drug temozolomide, causing DNA damage that is recognised by a combination of base excision repair (BER) enzymes.