Recent advances in click chemistry reactions mediated by transition metal based systems

Recent Advances in Copper-Based Solid Heterogeneous Catalysts for Azide-Alkyne Cycloaddition Reactions

The copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction is considered to be the most representative ligation process within the context of the "click chemistry" concept. This CuAAC reaction, which yields compounds containing a 1,2,3-triazole core, has become relevant in the construction of biologically complex systems, bioconjugation strategies, and supramolecular and material sciences. Although many CuAAC reactions are performed under homogenous conditions, heterogenous copper-based catalytic systems are gaining exponential interest, relying on the easy removal, recovery, and reusability of catalytically copper species. The present review covers the most recently developed copper-containing heterogenous solid catalytic systems that use solid inorganic/organic hybrid supports, and which have been used in promoting CuAAC reactions. Due to the demand for 1,2,3-triazoles as non-classical bioisosteres and as framework-based drugs, the CuAAC reaction promoted by solid heterogenous catalysts has greatly improved the recovery and removal of copper species, usually by simple filtration. In so doing, the solving of the toxicity issue regarding copper particles in compounds of biological interest has been achieved. This protocol is also expected to produce a practical chemical process for accessing such compounds on an industrial scale.

Ciprofloxacin-Tethered 1,2,3-Triazole Conjugates: New Quinolone Family Compounds to Upgrade Our Antiquated Approach against Bacterial Infections

A newer ciprofloxacin series containing 1,2,3-triazole conjugates of ciprofloxacin was designed, synthesized, and well characterized using modern analytical techniques by reacting diversified anilines with ciprofloxacin obtained from ciprofloxacin hydrochloride. The newer conjugates were evaluated for their antimicrobial activity against various strains, viz. Staphylococcus aureus (ATCC25923), Enterococcus faecalis (clinical isolate), Staphylococcus epidermidis (ATCC3594), Escherichia coli (ATCC25922), Pseudomonas aeruginosa (ATCC27853), Salmonella typhi (clinical isolate), Salmonella typhimurium (clinical isolate), Acinetobacter baumannii (ATCC19606), Aeromonas hydrophila (ATCC7966), Plesiomonas shigelloides (ATCC14029), and Sphingo biumpaucimobilis (MTCC6362) in vitro. Interestingly, some of the conjugates showed superior antimicrobial activity as compared to the control drug ciprofloxacin. The three compounds 4i, 4j, and 4n showed strong activity with minimum inhibitory concentration (MIC) 0.78 μM, while the compound 4g showed MIC 1.56 μM against S. typhi (clinical). The compound 4a showed good efficacy against S. aureus (ATCC25923) and S. typhi (clinical) with MIC 3.12 μM, while the compound 4b exhibited efficacy with MIC 3.12 μM against S. aureus (ATCC25923) and the control drug ciprofloxacin showed MIC 6.25 μM. Among all of the synthesized compounds, 4e, 4f, 4g, 4h, 4p, 4q, 4t, and 4u displayed less than 20% hemolysis, while the rest of the compounds showed hemolysis in the range of 21-48%. Moreover, the structure of compound 4b was also established by single-crystal X-ray diffraction studies.

Modified pyridine-triazole and 2,2'-bipyrimidine ligands generating robust titanium complexes constructed around a TiO4N2 core

The synthesis and characterisation of novel chelate nitrogen ligands with phasmidic tails (pyridine-triazole ligand 1b; 2,2'-bipyrimidine ligands 2b and 3b) as well as their titanium(IV) coordination complexes are reported. The analogous ligands 1a, 2a and 3a with methoxy substituents instead of the tails were also synthesized, together with titanium complexes that could be crystallographically characterised. A good agreement is noticed between analytical data of the complexes in solution (NMR) and in the solid state (X-ray diffraction). The complexes are overall robust on phases like alumina or silica, so that they could be characterised by TLC and sometimes chromatographied. Supramolecular architectures were generated from an equimolar solution of titanium(IV) isopropoxide, ligand 1a and a polyphenol ligand 5-H₄, leading to a double-stranded helicate characterised by MS, NMR and crystallography, which was then converted into a trinuclear complex as shown by MS and NMR DOSY data. The liquid-crystalline behaviour of the ligands 1b, 2b and 3b incorporating the long alkyl tails and that of the complexes derived from these ligands have been investigated.

Cross-linked polymer microparticles with tunable surface properties by the combination of suspension free radical copolymerization and Click chemistry

We propose a general, versatile and broad in scope two-steps approach for the elaboration of cross-linked polymer microparticles (µPs) with tunable functionalities and surface properties. Surface-functionalized cross-linked polymer µPs with diameter in the 80 μm range are prepared by the combination of: 1) suspension free radical copolymerization of styrene, propargyl methacrylate and 1,6-hexanediol dimethacrylate, 2) subsequent covalent tethering of a variety of azide-functionalized moieties (i.e. rhodamine B fluorescent dye or poly(ethylene glycol) (PEG) brush precursor) by copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) and, 3) optional N-alkylation of the 1,2,3-triazole groups followed by anion exchange reaction to afford covalently-tethered 1,2,3-triazolium ionic liquids with iodide or cresol red counter-anions. The resulting µPs are characterized by laser diffraction, differential scanning calorimetry, as well as by optical, confocal fluorescence, scanning electron and atomic force microscopies. Finally, the rheological properties of concentrated suspensions (volume fractions of 0.40 and 0.44) of the different synthesized µPs dispersed in a 1:1 (vol/vol) mixture of polyalkylene glycol and water are studied. The modification of µPs surface properties contributes not only to change the stability of the suspensions against flocculation, but also to significantly modify their rheological behavior at high shear stresses. This represents a clear experimental evidence of the importance of non-hydrodynamic contact forces in the rheology of non-Brownian suspensions (NBSs).

Green synthesis of CuO- and Cu2O-NPs in assistance with high-gravity: The flowering of nanobiotechnology

This study, for the first time, reports the synthesis of CuO- and Cu2O nanoparticles (NPs) using the Salvia hispanica extract by a high-gravity technique. The original green synthesis procedure led to the formation of nanoparticles with promising catalytic and biological properties. The synthesized nanoparticles were fully characterized and their catalytic activity was evaluated through a typical Azide-Alkyne Cycloaddition (AAC) reaction. The potential antibacterial activity against gram positive (S. aureus) and gram negative (E. coli) bacteria were investigated. It was shown that the antibacterial properties were independent of the NP morphology as well as of the texture of the synthesis media. As a result, the presently synthesized nanoparticles showed very good photocatalytic and catalytic activities in comparison with the literature. From a biological perspective, they showed lower cytotoxicity in comparison with the literature, and also showed higher antioxidant and antibacterial activities. Thus, these present green CuO and Cu2O nanoparticles deserve further attention to improve numerous medical applications.

Supported Tris-Triazole Ligands for Batch and Continuous-Flow Copper-Catalyzed Huisgen 1,3-Dipolar Cycloaddition Reactions

The lack of supported versions of the tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (TBTA) ligand, suitable for flow-chemistry applications at scale, prompted us to develop a new route for the immobilization of such tris-triazole chelating units on highly cross-linked polystyrene resins. With this aim, the preparation of the known TBTA-type monomer 3 was optimized to develop a high-yield synthetic sequence, devoid of chromatographic purifications at any stage. Then, bead-type (P7) and monolithic (M7) functional resins were obtained by the easy and scalable suspension- or mold-copolymerization of 3 with divinylbenzene. Both types of materials were found to possess a highly porous morphology and specific surface area in the dry state and could be charged with substantial amounts of Cu(I) or Cu(II) salts. After treatment of the latter with a proper reducing agent, the corresponding supported Cu(I) complexes were tested in the copper-catalyzed alkyne-azide cycloaddition reaction (CuAAC). The immobilized catalysts proved active at room temperature and, in batch and with catalyst loadings as low as 0.6 mol%, afforded quantitative conversions within 20 h. Independent of the alkyne structure, extended use of the supported catalyst in flow was also possible. In the reaction of benzylazide and propargyl alcohol, this allowed a total turnover number larger than 400 to be reached.

Preparation, Characterization of Propargyl Terminal Polybutadiene and Its Crosslinked Elastomers Properties

Propargyl terminal Polybutadiene (PTPB) was successfully prepared through hydroxyl terminal polybutadiene (HTPB) end-capping modification. The FTIR and ¹³C NMR results indicated that the HTPB terminal hydroxyl was thoroughly replaced and yielded the target product, PTPB, with a theoretical propargyl content of 0.66 mmol g⁻¹. In comparison with HTPB, PTPB has a lower viscosity. Using 1,6-diazide hexane as a curing agent, polytriazole crosslinked polybutadiene (PT_riPB) elastomers with various functional molar ratios (R) were prepared by CuAAC reaction, and the glass transition temperatures of the resultant PT_riPB elastomers were approximately −75 °C, measured by differential scanning calorimetry (DSC), nearly independent of elastomer R values. Mechanical tests indicated, that with the increase in R, the mechanical properties of PT_riPB elastomers exhibit a parabolic dependence on R. In addition, the thermal stability of PT_riPB elastomers were also studied. The findings revealed some fundamental features of polytriazole crosslinking elastomer prepared by CuAAC reaction.

Antimicrobial Photodynamic Polymeric Films Bearing Biscarbazol Triphenylamine End-Capped Dendrimeric Zn(II) Porphyrin

A novel biscarbazol triphenylamine end-capped dendrimeric zinc(II) porphyrin (DP 5) was synthesized by click chemistry. This compound is a cruciform dendrimer that bears a nucleus of zinc(II) tetrapyrrolic macrocycle substituted at the meso positions by four identical substituents. These are formed by a tetrafluorophenyl group that possesses a triazole unit in the para position. This nitrogenous heterocyclic is connected to a 4,4'-di(N-carbazolyl)triphenylamine group by means of a phenylenevinylene bridge, which allows the conjugation between the nucleus and this external electropolymerizable carbazoyl group. In this structure, dendrimeric arms act as light-harvesting antennas, increasing the absorption of blue light, and as electroactive moieties. The electrochemical oxidation of the carbazole groups contained in the terminal arms of the DP 5 was used to obtain novel, stable, and reproducible fully π-conjugated photoactive polymeric films (FDP 5). First, the spectroscopic characteristics and photodynamic properties of DP 5 were compared with its constitutional components derived of porphyrin P 6 and carbazole D 7 moieties in solution. The fluorescence emissions of the dendrimeric units in DP 5 were more strongly quenched by the tetrapyrrolic macrocycle, indicating photoinduced energy transfer. In addition, FDP 5 film showed the Soret and Q absorption bands and red fluorescence emission of the corresponding zinc(II) porphyrin. Also, FDP 5 film was highly stable to photobleaching, and it was able to produce singlet molecular oxygen in both N,N-dimethylformamide (DMF) and water. Therefore, the porphyrin units embedded in the polymeric matrix of FDP 5 film mainly retain the photochemical properties. Photodynamic inactivation mediated by FDP 5 film was investigated in Staphylococcus aureus and Escherichia coli. When a cell suspension was deposited on the surface, complete eradication of S. aureus and a 99% reduction in E. coli survival were found after 15 and 30 min of irradiation, respectively. Also, FDP 5 film was highly effective to eliminate individual bacteria attached to the surface. In addition, photodynamic inactivation (PDI) sensitized by FDP 5 film produced >99.99% bacterial killing in biofilms formed on the surface after 60 min irradiation. The results indicate that FDP 5 film represents an interesting and versatile photodynamic active material to eradicate bacteria as planktonic cells, individual attached microbes, or biofilms.

Synthesis of Novel N-Heterocyclic Compounds Containing 1,2,3-Triazole Ring System via Domino, "Click" and RDA Reactions

An uncomplicated, high-yielding synthetic route has been developed to constitute complicated heterocycles, applying domino, click and retro-Diels–Alder (RDA) reaction sequences. Starting from 2-aminocarboxamides, a new set of isoindolo[2,1-a]quinazolinones was synthesized with domino ring closure. A click reaction was performed to create the 1,2,3-triazole heterocyclic ring, followed by an RDA reaction resulting in dihydropyrimido[2,1-a]isoindole-2,6-diones. The absolute configuration, concluded by the norbornene structure that served as a chiral source, remained constant throughout the transformations. The structure of the synthesized compounds was examined by ¹H and ¹³C Nuclear Magnetic Resonance (NMR) methods.

Site-Specific Surface Functionalization via Microchannel Cantilever Spotting (µCS): Comparison between Azide-Alkyne and Thiol-Alkyne Click Chemistry Reactions

Different types of click chemistry reactions are proposed and used for the functionalization of surfaces and materials, and covalent attachment of organic molecules. In the present work, two different catalyst-free click approaches, namely azide-alkyne and thiol-alkyne click chemistry are studied and compared for the immobilization of microarrays of azide or thiol inks on functionalized glass surfaces. For this purpose, the surface of glass is first functionalized with dibenzocyclooctyne-acid (DBCO-acid), a cyclooctyne with a carboxyl group. Then, the DBCO-terminated surfaces are functionalized via microchannel cantilever spotting with different fluorescent and nonfluorescent azide and thiol inks. Although both routes work reliably for surface functionalization, the protein binding experiments reveal that using a thiol-alkyne route will obtain the highest surface density of molecular immobilization in such spotting approaches. The obtained achievements and results from this work can be used for design and manufacturing of microscale patterns suitable for biomedical and biological applications.

Synthesis of heterogeneous Ru(ii)-1,2,3-triazole catalyst supported over SBA-15: application to the hydrogen transfer reaction and unusual highly selective 1,4-disubstituted triazole formation via multicomponent click reaction

Remarkable reactivity of solid catalyst in H₂O medium for the regioselective 1,4-disubstituted 1,2,3-triazole with excellent yield in one pot.

Synthesis, photophysical, antibacterial and larvicidal studies on triazolophanes with 5-nitro isophthalate functionality at the intraannular position

1 : 1 and 2 : 2 oligomeric triazolophanes have been successfully synthesized from the corresponding propargylic esters and various aryl alkyl azides.

Copper ferrite nanoparticle modified starch as a highly recoverable catalyst for room temperature click chemistry: multicomponent synthesis of 1,2,3-triazoles in water

Highly water dispersible CuFe₂O₄@Starch catalyzed click reaction.

Organometallic chemical biology: an organometallic approach to bioconjugation

This review summarizes the history and recent developments of the field of organometallic chemical biology with a particular emphasis on the development of novel bioconjugation approaches. Over the years, numerous transformations have emerged for biomolecule modification with the use of organometallic reagents; these include [3+2] cycloadditions, C–C, C–S, C–N, and C–O bond forming processes, as well as metal-mediated deprotection (“decaging”) reactions. These conceptually new additions to the chemical biology toolkit highlight the potential of organometallic chemistry to make a significant impact in the field of chemical biology by providing further opportunities for the development of chemoselective, site-specific and spatially resolved methods for biomolecule structure and function manipulation. Examples of these transformations, as well as existing challenges and future prospects of this rapidly developing field are highlighted in this review.