Tetracopper(I) thiolate- and amido-(SNS) complexes and copper-catalyzed azide-alkyne cycloaddition in water

Two tetranuclear Cu(I) complexes bearing thiolate- and amido-SNS ligands were characterized by X-ray diffraction and mass spectrometry. Although the amido ligand undergoes irreversible N-protonation by the copper-bound alkyne, the thiolate complex demonstrates good activity in the copper-catalyzed azide-alkyne cycloaddition reaction with a variety of substrates. The base-free reactions are performed in water and afford excellent yields over 2 h at 70 °C. DFT calculations suggest a proton-shuttle role for the thiolate donor in formation of the initial dicopper σ,π-alkynyl intermediate.

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.

Efficient Copper-Catalyzed Multicomponent Synthesis of N-Acyl Amidines via Acyl Nitrenes

Direct synthetic routes to amidines are desired, as they are widely present in many biologically active compounds and organometallic complexes. N-Acyl amidines in particular can be used as a starting material for the synthesis of heterocycles and have several other applications. Here, we describe a fast and practical copper-catalyzed three-component reaction of aryl acetylenes, amines, and easily accessible 1,4,2-dioxazol-5-ones to N-acyl amidines, generating CO₂ as the only byproduct. Transformation of the dioxazolones on the Cu catalyst generates acyl nitrenes that rapidly insert into the copper acetylide Cu–C bond rather than undergoing an undesired Curtius rearrangement. For nonaromatic dioxazolones, [Cu(OAc)(Xantphos)] is a superior catalyst for this transformation, leading to full substrate conversion within 10 min. For the direct synthesis of N-benzoyl amidine derivatives from aromatic dioxazolones, [Cu(OAc)(Xantphos)] proved to be inactive, but moderate to good yields were obtained when using simple copper(I) iodide (CuI) as the catalyst. Mechanistic studies revealed the aerobic instability of one of the intermediates at low catalyst loadings, but the reaction could still be performed in air for most substrates when using catalyst loadings of 5 mol %. The herein reported procedure not only provides a new, practical, and direct route to N-acyl amidines but also represents a new type of C–N bond formation.

Ring-Expanded N-Heterocyclic Carbenes for Copper-Mediated Azide-Alkyne Click Cycloaddition Reactions

A series of well-defined copper(I) complexes bearing ring-expanded N-heterocyclic carbene (NHC) ligands has been applied to the azide-alkyne cycloaddition reaction. The obtained results notably showed that the six-membered NHC ligands outperform well-established five-membered ones. [CuI(Mes-6)] displayed a remarkable catalytic activity while respecting the strict criteria for click reactions.

Understanding the CO Dissociation in [Fe(CN)2(CO)2(dithiolate)]2- Complexes with Quantum Chemical Topology Tools

The active site of the [NiFe]-hydrogenase contains a pentacoordinated iron atom; therefore, a vacant coordination site is available for substrate binding. Nonetheless, most organometallic models of the [NiFe]-hydrogenase failed to reproduce this key feature of the active site. In order to rationalize such behavior, quantum chemical calculations were carried out on a series of [Fe(CN)₂(CO)_n(dithiolate)]^2- n = 1,2 complexes, where dithiolate denotes the ligands (CF₃)₂C₂S₂^2-, (CO₂Me)₂C₂S₂^2-, Ph₂C₂S₂^2-, C₆Cl₂H₂S₂^2-, C₆H₄S₂^2-, C₂H₄S₂^2-, and C₃H₆S₂^2-. Structural and energetic features are discussed, and a topological analysis based on two scalar fields, the one-electron density and the electron localization function (ELF), has been attempted to describe the nature of the metal-ligand bonds. The present approach contributes to better understand the ability of noninnocent dithiolene to strongly labilize one CO whereas innocent dithiolate cannot. The methodology developed throughout the paper could be useful in the field of the CO-releasing molecules.

Tripodal Amine Ligands for Accelerating Cu-Catalyzed Azide-Alkyne Cycloaddition: Efficiency and Stability against Oxidation and Dissociation

Ancillary ligands, especially the tripodal ligands such as tris(triazolylmethyl)amines, have been widely used to accelerate the Cu-catalyzed azide-alkyne cycloaddition (CuAAC, a "click" reaction). However, the relationship between the activity of these Cu(I) complexes and their stability against air oxidation and ligand dissociation/exchange was seldom studied, which is critical for the applications of CuAAC in many biological systems. In this work, we synthesized twenty-one Cu(I) tripodal ligands varying in chelate arm length (five to seven atoms), donor groups (triazolyl, pyridyl and phenyl), and steric hindrance. The effects of these variables on the CuAAC reaction, air oxidation, and ligand dissociation were evaluated. Reducing the chelate arm length to five atoms, decreasing steric hindrance, or using a relatively weakly-binding ligand can significantly increase the CuAAC reactivity of the Cu(I) complexes, but the concomitant higher degree of oxidation cannot be avoided, which leads to rapid degradation of a histidine-containing peptide as a model of proteins. The oxidation of the peptide can be reduced by attaching oligo(ethylene glycol) chains to the ligands as sacrificing reagents. Using electrospray ionization mass spectrometry (ESI-MS), we directly observed the tri- and di-copper(I)-acetylide complexes in CuAAC reaction in the [5,5,5] ligand system and a small amount of di-Cu(I)-acetylide in the [5,5,6] ligand system. Only the mono-Cu(I) ligand adducts were observed in the [6,6,6] and [5,6,6] ligand systems.

CuAAC Click Reactions in the Gas Phase: Unveiling the Reactivity of Bis-Copper Intermediates

Copper-catalysed azide alkyne cycloaddition (CuAAC) has been considered a breakthrough transformation over the last 15 years. Its debated mechanism arouses continuously growing interest. By means of a mass spectrometer modified ad hoc, the entire catalytic cycle of CuAAC reaction has been investigated in the gas phase. Ion-molecule reactions were performed inside the mass spectrometer to reproduce step-by-step, at a molecular level, the complete catalytic cycle of the click reaction. We successfully challenged the reactivity of elusive mono- and bis-copper intermediates by ion-molecule reactions leading to the production of mass-characterized triazole products, paving the way for detailed energetic studies to be performed in the gas phase. The structures of the relevant species, calculated at a DFT level, helped rationalise our experimental results.