Improved Copper(I)-NHC Catalytic Efficiency on Huisgen Reaction by Addition of Aromatic Nitrogen Donors

Active-metal template clipping synthesis of novel [2]rotaxanes

Mechanically interlocked molecules (MIMs) have been important synthetic targets in supramolecular chemistry due to their beautiful structures and intriguing properties. We present herein a new synthetic strategy to access [2]rotaxanes, namely active-metal template clipping. We discuss the design of the target [2]rotaxanes, synthesis and characterization of the axle, macrocycle precursors and macrocycles as well as preparation of the final [2]rotaxanes by active template copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC) as key step of the synthesis. HRMS and NMR experiments have been performed to confirm the formation of the interlocked structures.

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.

Mononuclear Cu Complexes Based on Nitrogen Heterocyclic Carbene: A Comprehensive Review

During the last decade, organometallic, coordination, and catalytic chemistry of the three-dimensional metals such as copper (Cu) has been greatly affected by the emergence of nitrogen heterocyclic carbene (NHC) complexes. The NHCs, and in particular the mononuclear Cu^I-based ones, have been proven vastly useful in several applications such as in biosynthesis, catalysis, photochemistry, etc. This review tries to thoroughly describe a series of mononuclear Cu^I NHC complexes and their subcategories such as heteroleptics, and bidentate and tridentate heteroatom complexes, and give some detailed insights on their development, emergence, and applications. A brief outlook is also disclosed to enable other researchers to further develop a platform for future advances and studies in the field of Cu^I-based NHCs.

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.

Green Methodologies for Copper(I)-Catalyzed Azide-Alkyne Cycloadditions: A Comparative Study

Successful copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reactions may be achieved by several methods. In this paper, four synthetic protocols were performed for direct comparison of time required for the synthesis, yield, and purity of the 1H-1,2,3-triazole products. The methods with Cu(I) catalysts were conventional, microwave heating, solvent-free, and a method using glycerol solvent. The compounds synthesized in this paper were known non-fluorinated triazoles and new fluorinated triazoles. The results lead to the conclusion that the microwave method should be strongly considered for CuAAC syntheses.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Cu–NHC azide complex: synthesis and reactivity

The first example of a NHC-based copper azide complex is reported. Its reactivity was investigated with various reagents affording 8 new complexes.

A dicopper(i)-dimesoionic carbene complex as a click catalyst: mechanistic implications

A dicopper(i) complex comprising a dimesoionic carbene and an acetate-bridge is synthesized and fully characterized.

Use of an Air-Stable Cu(I)-NHC Catalyst for the Synthesis of Peptidotriazoles

We report the use of air-stable Cu(I)-NHC complex 4a as a catalyst for the efficient microwave-assisted synthesis of peptidotriazoles on solid phase. Compared with the usual conditions (CuI or CuSO₄/NaAsc), catalyst 4a allowed the preparation of a series of peptidomimetic compounds containing a 1,2,3-triazole ring in their backbone without the oxidation of common side-chains. Overall, the peptidotriazoles were obtained in good yields (61-87%), in excellent purity (higher than 94%) and with low copper contamination.

A Highly Efficient Single-Chain Metal-Organic Nanoparticle Catalyst for Alkyne-Azide "Click" Reactions in Water and in Cells

We show that copper-containing metal-organic nanoparticles (MONPs) are readily synthesized via Cu(II)-mediated intramolecular cross-linking of aspartate-containing polyolefins in water. In situ reduction with sodium ascorbate yields Cu(I)-containing MONPs that serve as highly efficient supramolecular catalysts for alkyne-azide "click chemistry" reactions, yielding the desired 1,4-adducts at low parts per million catalyst levels. The nanoparticles have low toxicity and low metal loadings, making them convenient, green catalysts for alkyne-azide "click" reactions in water. The Cu-MONPs enter cells and perform efficient, biocompatible click chemistry, thus acting as intracellular nanoscale molecular synthesizers.

Exploring potential cooperative effects in dicopper(i)-di-mesoionic carbene complexes: applications in click catalysis

Dicopper(i) dimesoionic carbene complexes are active catalysts for the azide–alkyne cycloaddition reaction and are more active than their mononuclear counterparts.

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.

The nature of Pd–carbene and Pd–halogen bonds in (bisNHC)PdX2 type catalysts: insights from density functional theory

Pd–carbene and Pd–halogen play a key role in the catalytic activity of (bisNHC)PdX₂ complexes and they can be fine-tuned with proper substitution in the carbene moiety and choosing a weakly coordinating halide ion.

Utilisation of water soluble iridium catalysts for signal amplification by reversible exchange

The catalytic hyperpolarisation of pyridine, 3-hydroxypyridine and oxazol by the Signal Amplification By Reversible Exchange (SABRE) process is achieved by a series of water soluble iridium phosphine and N-heterocyclic carbene dihydride complexes.

A simple access to transition metal cyclopropenylidene complexes

The first example of a BAC–Cu complex, its outstanding catalytic activity in Click chemistry and its use as a carbene-transfer reagent to easily access Au-, Pd–, Ir– and Rh–BAC compounds are reported.

Copper nanoparticles generated from aggregates of a hexarylbenzene derivative: a reusable catalytic system for ‘click’ reactions

Copper nanoparticles stabilized by aggregates of a hexaphenylbenzene derivative display excellent catalytic activity in ‘click’ reactions under solvent-free conditions.

Cellulose-immobilized NHC–Cu(i) complex: an efficient and reusable catalyst for multicomponent synthesis of 1,2,3-triazoles

A novel cellulose supported copper NHC complex has been prepared by the reaction of cellulose supported imidazolium salt with copper(i) iodide.

Recyclable catalytic dendrimer nanoreactor for part-per-million Cu(I) catalysis of "click" chemistry in water

Upon catalyst and substrate encapsulation, an amphiphilic dendrimer containing 27 triethylene glycol termini and 9 intradendritic triazole rings serves as a catalytic nanoreactor by considerably accelerating the Cu(I)-catalyzed alkyne-azide cycloaddition (CuAAC) "click" reactions of various substrates in water using the catalyst Cu(hexabenzyltren)Br (tren = triaminoethylamine). Moreover this recyclable nanoreactor with intradendritic triazole rings strongly also activates the simple Sharpless-Fokin catalyst CuSO4 + sodium ascorbate in water under ambient conditions leading to exceptional TONs up to 510,000. This fully recyclable catalytic nanoreactor allows to considerably decrease the amount of this cheap copper catalyst down to industrially tolerable residues, and some biomedical and cosmetic applications are exemplified.

Advancements in the mechanistic understanding of the copper-catalyzed azide-alkyne cycloaddition

The copper-catalyzed azide-alkyne cycloaddition (CuAAC) is one of the most broadly applicable and easy-to-handle reactions in the arsenal of organic chemistry. However, the mechanistic understanding of this reaction has lagged behind the plethora of its applications for a long time. As reagent mixtures of copper salts and additives are commonly used in CuAAC reactions, the structure of the catalytically active species itself has remained subject to speculation, which can be attributed to the multifaceted aggregation chemistry of copper(I) alkyne and acetylide complexes. Following an introductory section on common catalyst systems in CuAAC reactions, this review will highlight experimental and computational studies from early proposals to very recent and more sophisticated investigations, which deliver more detailed insights into the CuAAC's catalytic cycle and the species involved. As diverging mechanistic views are presented in articles, books and online resources, we intend to present the research efforts in this field during the past decade and finally give an up-to-date picture of the currently accepted dinuclear mechanism of CuAAC. Additionally, we hope to inspire research efforts on the development of molecularly defined copper(I) catalysts with defined structural characteristics, whose main advantage in contrast to the regularly used precatalyst reagent mixtures is twofold: on the one hand, the characteristics of molecularly defined, well soluble catalysts can be tuned according to the particular requirements of the experiment; on the other hand, the understanding of the CuAAC reaction mechanism can be further advanced by kinetic studies and the isolation and characterization of key intermediates.

Use of copper(I) catalyzed azide alkyne cycloaddition (CuAAC) for the preparation of conjugated pyrrolo[2,3-a]carbazole Pim kinase inhibitors

We have previously demonstrated that pyrrolo[2,3-a]carbazole-3-carbaldehydes are potent Pim kinase inhibitors with in vitro antiproliferative activities. In the present study, we report the synthesis of new pyrrolocarbazoles substituted at the N-10 position. When their ability to inhibit Pim kinase activities were evaluated in in vitro assays, we observed that this nitrogen atom can be substituted without loss of Pim-1 and Pim-3 inhibitory potencies. Moreover, when we added a fluorescent dansyl group (compound 13), we were able to show that 13 penetrates the plasma membrane and enters the cytoplasm.