Cu(I)-amido complexes in the Ullmann reaction: reactions of Cu(I)-amido complexes with iodoarenes with and without autocatalysis by CuI

Copper-Mediated Cyanodifluoromethylation of (Hetero)aryl Iodides and Activated (Hetero)aryl Bromides with TMSCF2CN

Molecules bearing fluorine are increasingly prevalent in pharmaceuticals, agrochemicals, and functional materials. The cyanodifluoromethyl group is unique because its size is closer than that of any other substituted difluoromethyl group to the size of the trifluoromethyl group, but its electronic properties are distinct from those of the trifluoromethyl group. In addition, the presence of the cyano group provides synthetic entry to a wide range of substituted difluoromethyl groups. However, the synthesis of cyanodifluoromethyl compounds requires multiple steps, highly reactive reagents (such as DAST, NSFI, or IF5), or specialized starting materials (such as α,α-dichloroacetonitriles or α-mercaptoacetonitriles). Herein, we report a copper-mediated cyanodifluoromethylation of aryl and heteroaryl iodides and activated aryl and heteroaryl bromides with TMSCF2CN. This cyanodifluoromethylation tolerates an array of functional groups, is applicable to late-stage functionalization of complex molecules, yields analogues of FDA-approved pharmaceuticals and fine chemicals, and enables the synthesis of a range of complex molecules bearing a difluoromethylene unit by transformations of the electron-poor CN unit. Calculations of selected steps of the reaction mechanism by Density Functional Theory indicate that the barriers for both the oxidative addition of iodobenzene to [(DMF)CuCF2CN] and the reductive elimination of the fluoroalkyl product from the fluoroalkyl copper intermediate lie in between those of [(DMF)CuCF3] and [(DMF)CuCF2C(O)NMe2].

Cross-coupling by a noncanonical mechanism involving the addition of aryl halide to Cu(II)

Copper complexes are widely used in the synthesis of fine chemicals and materials to catalyze couplings of heteroatom nucleophiles with aryl halides. We show that cross-couplings catalyzed by some of the most active catalysts occur by a mechanism not previously considered. Copper(II) [Cu(II)] complexes of oxalamide ligands catalyze Ullmann coupling to form the C-O bond in aryl ethers by concerted oxidative addition of an aryl halide to Cu(II) to form a high-valent species that is stabilized by radical character on the oxalamide ligand. This mechanism diverges from those involving Cu(I) and Cu(III) intermediates that have been posited for other Ullmann-type couplings. The stability of the Cu(II) state leads to high turnover numbers, >1000 for the coupling of phenoxide with aryl chloride electrophiles, as well as an ability to run the reactions in air.

Copper-Catalyzed Hydroamination: Enantioselective Addition of Pyrazoles to Cyclopropenes

Chiral N-cyclopropyl pyrazoles and structurally related heterocycles are prepared using an earth-abundant copper catalyst under mild reaction conditions with high regio-, diastereo-, and enantiocontrol. The observed N2:N1 regioselectivity favors the more hindered nitrogen of the pyrazole. Experimental and DFT studies support a unique mechanism that features a five-centered aminocupration.

Oxalamide/Amide Ligands: Enhanced and Copper-Catalyzed C-N Cross-Coupling for Triarylamine Synthesis

Triarylamines are privileged core structures that are found in versatile optoelectronic materials. New methods are constantly being sought for their preparation. Herein, a new protocol for triarylamine synthesis is presented where a wide range of diarylamines couple smoothly with aryl bromides mediated by a copper oxalamide (or amide) catalytic system. Notably, a new non-C₂-symmetric 1-isoquinolinamide-based N,N-/N,O-bidentate ligand was introduced that could tolerate bulky diarylamines. Plenty of known optoelectronic functional molecules could be synthesized in good to excellent yields. The practicality of this C-N cross-coupling was illustrated by the gram-scale synthesis of a patented thermally activated delayed fluorescence emitter for organic light-emitting diodes.

Synthesis of unsymmetrical 1,3-substituted-1,3-dihydro-benzimidazolones via copper-catalyzed C–N coupling under visible light

Although photoinduced copper catalysis for carbon–nitrogen (C–N) amine bond formation with alkyl/aryl halides has been developed, the potential of copper photocatalysis for the synthesis of 1,3-substituted benzimidazolones remains mostly unexplored.

Catalytic Reactions Directed by a Structurally Well-Defined Aminomethyl Cyclopalladated Complex

The introduction of N-containing moieties into feedstock molecules to build nitrogenated functional molecules has always been widely studied by the organic chemistry community. Progress in this field paves new roads to the synthesis of N-containing molecules, which are of significant importance in biological activities and play vital roles in pharmaceuticals and functional materials. Remarkable progress has been achieved in the field of transition metal-catalyzed C-N bond-forming reactions, typified by alkene hydroamination and the aza-Wacker reaction. However, the poisoning effect of electron-donating amine substrates on late transition metal catalysts presents a key impediment to these reactions, thus limiting the scope of amine substrates to electron-deficient amide derivatives. To address this problem, our group developed a palladium-aminomethyl complex with a three-membered palladacycle structure that allowed for the incorporation of electron-rich amine building blocks via C-C bond instead of C-N bond construction. This Account details the discovery of the well-defined aminomethyl cyclopalladated complex and recapitulates its applications for the catalysis of a series of aminomethylation reactions. We highlight how the understanding of the fundamental structural properties of the defined complex guided us toward tuning the reactivity of nucleophiles to initiate aminomethylation in different modes. Moreover, principles of designing and establishing further cascade reactions are also described.Aminomethyl cyclopalladated complexes can be prepared via the oxidative addition of aminals or N,O-acetals to Pd⁰ species. Thorough structural investigations by single-crystal X-ray diffraction analysis of the cyclopalladated complex suggest the presence of both aminomethylene-Pd^II (3-membered-ring) and Pd⁰-iminium (π-ligated) resonance forms, which indicates that both the palladium center and the methylene site are electrophilic. This is further verified by analysis of charge distribution. Two general types of reactions can be established, differing by the selective affinity of the nucleophiles to the two electrophilic positions, which is relevant to the "hardness suitability" of the nucleophiles with each electrophilic site. Softer nucleophiles such as alkenes prefer to attack the palladium center to initiate the reaction, mainly via migratory insertion into the Pd-C bond on the 3-membered ring with high strain. Through tandem β-hydride or reductive elimination, the Heck-type aminomethylation of styrenes, the aminomethylalkoxylation of electron-rich olefins, and even the aminomethylamination of allenes, dienes, enynes, and carbenoids with full atom-economy have been realized in line with this reaction mode. In contrast, harder nucleophiles tend to attack the harder electrophilic methylene site, leading to the aminomethylation of electron-deficient dienes. For secondary amines, a "C-N bond metathesis" process would be furnished through a reductive elimination, 1,3-proton transfer, and oxidative addition sequence. More intriguingly, when using appropriate "dinucleophile" substrates such as electron-rich amine-tethered dienes, sequential C-N bond metathesis and intramolecular insertion would occur to furnish Pd-catalyzed annulation reactions, which exhibits both the hard and soft nucleophile reactivities mentioned above. These transformations provide convenient methods for the preparation of N-containing molecules, such as amines, diamines, amino acetals, and multiple types of N-heterocycles.

Transmetalation Reactions Triggered by Electron Transfer between Organocopper Complexes

[Cu(bipy)(C₆F₅)] reacts with most aryl iodides to form heterobiphenyls by cross-coupling, but when Rf–I is used (Rf = 3,5-dicholoro-2,4,6-trifluorophenyl), homocoupling products are also formed. Kinetic studies suggest that, for the homocoupling reaction, a mechanism based on transmetalation from [Cu(bipy)(C₆F₅)] to Cu(III) intermediates formed in the oxidative addition step is at work. Density functional theory calculations show that the interaction between these Cu(III) species and the starting Cu(I) complex involves a Cu(I)–Cu(III) electron transfer concerted with the formation of an iodine bridge between the metals and that a fast transmetalation takes place in a dimer in a triplet state between two Cu(II) units.

In copper-catalyzed cross-coupling reactions, electron-transfer processes between Cu(I) and Cu(III) species are overlooked behind RDS (C−X activation). Density functional theory studies considering two molecules of the catalyst and two spin states throughout the course of the reaction have revealed the feasibility of such a process and the transmetalation between Cu(II) species, justifying in this way the formation of homocoupling products.

Fragmentation behavior of EDTA complexes under different activation conditions

Ethylenediaminetetraacetic acid (EDTA) is an aminopolycarboxylic acid and complexation agent that is able to bind a large variety of metals. The formation of highly stable metal-EDTA complexes is generally very quick. This has led to the use of EDTA in a variety of applications, including food, medical, and household applications. In the current study, we have investigated the fragmentation behavior of EDTA and various metal complexes under collision-induced dissociation (CID), infrared-multiphoton dissociation (IRMPD), and higher-energy collisional dissociation (HCD) activation conditions. Both, positive and negative mode electrospray ionization (ESI) were applied. The metals used to complex with EDTA ranged from alkaline earth metals, such as sodium and cesium, via calcium, nickel, zinc, aluminum, copper, iron, and indium to yttrium and several lanthanides. Furthermore, the protonated and deprotonated species of EDTA, as well as disodium and trisodium species, have been subjected to fragmentation. The results show that characteristic fragmentations were obtained for EDTA and the metal complexes under the investigated conditions. The use of an ion cyclotron resonance (ICR) and an Orbitrap mass spectrometer, as high resolution-accurate mass instruments, allowed the assignment of elemental compositions undoubtedly for the vast majority of fragments. Certain trends were observed that trend correlated with the size of the metal and the location within the periodic table.

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

Copper-catalyzed, N-auxiliary group-controlled switchable transannulation/nitration initiated by nitro radicals: selective synthesis of pyridoquinazolones and 3-nitroindoles

Within the scope of nitration reactions, the efficiency of sensitive heteroaromatics such as indoles is often eroded by various competitive oxidative decomposition pathways.

Reshaping Ullmann Amine Synthesis in Deep Eutectic Solvents: A Mild Approach for Cu-Catalyzed C-N Coupling Reactions With No Additional Ligands

The CuI-catalyzed Ullmann amine cross-coupling between (hetero)aryl halides (Br, I) and aromatic and aliphatic amines has been accomplished in deep eutectic solvents as environmentally benign and recycling reaction media. Under optimized conditions, the reaction proceeds smoothly under mild conditions (60–100°C) in air, in the absence of ligands, with a catalyst (CuI) loading of 10 mol% and K₂CO₃ (aliphatic primary and secondary amines) or t-BuOK (aromatic amines) as the base. A variety of amines have been synthesized in yields up to 98% with a broad substrate scope.

l -( -) -Quebrachitol as a Ligand for Selective Copper(0)-Catalyzed N-Arylation of Nitrogen-Containing Heterocycles

l-(-)-Quebrachitol (QCT) has been found as a ligand of copper powder for selective N-arylation of nitrogen-containing heterocycles with aryl halides. Furthermore, another potential catalytic system (copper powder/QCT/ t-BuOK) was successfully adapted to unactivated aryl chlorides.

Copper-Promoted Functionalization of Organic Molecules: from Biologically Relevant Cu/O2 Model Systems to Organometallic Transformations

Copper is one of the most abundant and less toxic transition metals. Nature takes advantage of the bioavailability and rich redox chemistry of Cu to carry out oxygenase and oxidase organic transformations using O₂ (or H₂O₂) as oxidant. Inspired by the reactivity of these Cu-dependent metalloenzymes, chemists have developed synthetic protocols to functionalize organic molecules under enviormentally benign conditions. Copper also promotes other transformations usually catalyzed by 4d and 5d transition metals (Pd, Pt, Rh, etc.) such as nitrene insertions or C-C and C-heteroatom coupling reactions. In this review, we summarized the most relevant research in which copper promotes or catalyzes the functionalization of organic molecules, including biological catalysis, bioinspired model systems, and organometallic reactivity. The reaction mechanisms by which these processes take place are discussed in detail.

On the preferences of five-membered chelate rings in coordination chemistry: insights from the Cambridge Structural Database and theoretical calculations

The purpose of this review is to give an overview of three important N-bidentate ligands: 1,10-phenanthroline (phen), 2,2'-bipyridine (bpy), and ethylenediamine (en). We have not attempted to be comprehensive because of the huge amount of activity being done in coordination chemistry using these ligands. Instead we present a full structural and geometrical study by using the Cambridge Structural Database (CSD) combined with theoretical calculations that allow us to parameterize their coordinating properties and ability to coordinate to transition and non-transition metals. More importantly, we illustrate that upon coordination and formation of the five-membered chelate ring, these ligands are able to adapt themselves to the requirements of the different metals by changing the MN distances and NMN angles. Therefore, a redefinition of the preferences of these ligands to metals with large ionic radii is needed. Finally, we will present some facts about the participation of these ligands in inorganic-organic hybrids (IOHs) based on Keggin polyoxometalates (POMs).

Ligand-free Cu(ii)-catalyzed aerobic etherification of aryl halides with silanes: an experimental and theoretical approach

Owing to their wide occurrence in nature and immense applications in various fields, the synthesis of aryl alkyl ethers has remained a focus of interest.

Recyclable bimetallic CuMoO4 nanoparticles for C–N cross-coupling reaction under mild conditions

New bimetallic CuMoO₄ nanocatalysts for C–N cross-coupling.

Tunable Synthesis of 2-Ene-1,4-diones, 4-Hydroxycyclopent-2-en-1-ones, and 2-(Furan-3-yl)acetamides via Palladium-Catalyzed Cascade Reactions of Allenols

An efficient and regioselective synthesis of 2-ene-1,4-diones, 4-hydroxycyclopent-2-en-1-ones, or 2-(furan-3-yl)acetamides is successfully realized through palladium-catalyzed one-pot multicomponent reactions of allenols with aryl iodides and carbon monoxide in the presence of tertiary amines. Interestingly, the selectivity depends on the substitution patterns of the allenol substrates. To be specific, from the reaction of allenols with no substituent attached on the internal position of the allenic moiety, 2-ene-1,4-diones or 4-hydroxycyclopent-2-en-1-ones were formed selectively through carbonylation of aryl iodide followed by acylation of allenol with the in situ formed acyl palladium species, β-hydride elimination of the in situ formed allyl palladium complex, and further tautomerization or intramolecular aldol reaction. From the reaction of allenols bearing a substituent at the internal position of the allenic unit, on the other hand, diversely substituted 2-(furan-3-yl)acetamides were formed through a cascade process combining carbonylation of aryl iodide, acylation, and carbonylation of allenol followed by intramolecular condensation and amination by tertiary amine featuring an oxidant-free C-N bond cleavage.

Copper-Catalyzed Aerobic Oxidative [3+2] Annulation for the Synthesis of 5-Amino/Imino-Substituted 1,2,4-Thiadiazoles through C-N/N-S Bond Formation

A copper-catalyzed aerobic oxidative annulation reaction of 2-aminopyridine/amidine with isothiocyanate has been reported. This strategy involving C-N/N-S bond formations provides various 5-amino/imino-substituted 1,2,4-thiadiazole derivatives under a Cu/O₂ catalytic system. This method has demonstrated high reactivity, mild reaction conditions, and a broad substrate scope. Furthermore, the synthetic utilities of the approach are demonstrated by further modifications.

Highly Efficient and Diastereoselective Construction of Trifluoromethyl-Containing Spiro[pyrrolidin-3,2'-oxindole] by a Catalyst-free Mutually Activated [3+2] Cycloaddition Reaction

A catalyst-free self-catalyzed [3+2] cycloaddition reaction of isatin-derived α-(trifluoromethyl)imines with vinylpyridines is reported. The reaction offers a straightforward and atom-economical procedure for the preparation of a series of 5'-trifluoromethyl-spiro[pyrrolidin-3,2'-oxindoles] in excellent yields and diastereoselectivities. The reaction mechanism has been investigated by control experiments, DFT calculation of pK_a values and the kinetic profiles, revealing that this reaction featured the mutual activation between isatin-derived α-(trifluoromethyl)imines and vinylpyridine to generate the reactive species.

Copper(I)-USY as a Ligand-Free and Recyclable Catalyst for Ullmann-Type O-, N-, S-, and C-Arylation Reactions: Scope and Application to Total Synthesis

The copper(I)-doped zeolite Cu^I-USY proved to be a versatile, efficient, and recyclable catalyst for various Ullmann-type coupling reactions. Easy to prepare and cheap, this catalytic material enables the arylation and heteroarylation of diverse O-, N-, S-, and C-nucleophiles under ligand-free conditions while exhibiting large functional group compatibility. The facility of this catalyst to promote C-O bond formation was further demonstrated with the total synthesis of 3-methylobovatol, a naturally occurring diaryl ether of biological relevance. From a mechanistic viewpoint, two competitive pathways depending on the nature of the nucleophile and consistent with the obtained results have been proposed.

Mechanistic Insight into the Cu-Catalyzed C-S Cross-Coupling of Thioacetate with Aryl Halides: A Joint Experimental-Computational Study

The mechanism of the Ullmann-type reaction between potassium thioacetate (KSAc) and iodobenzene (PhI) catalyzed by CuI associated with 1,10-phenanthroline (phen) as a ligand was explored experimentally and computationally. The study on C-S bond formation was investigated by UV-visible spectrophotometry, cyclic voltammetry, mass spectrometry, and products assessment from radical probes. The results indicate that under experimental conditions the catalytically active species is [Cu(phen)(SAc)] regardless of the copper source. An examination of the aryl halide activation mechanism using radical probes was undertaken. No evidence of the presence of radical species was found during the reaction process, which is consistent with an oxidative addition cross-coupling pathway. The different reaction pathways leading to the experimentally observed reaction products were studied by DFT calculation. The oxidative addition-reductive elimination mechanism via an unstable Cu^III intermediate is energetically more feasible than other possible mechanisms such as single electron transfer, halogen atom transfer, and σ-bond methatesis.

Mechanism of the Ullmann Biaryl Ether Synthesis Catalyzed by Complexes of Anionic Ligands: Evidence for the Reaction of Iodoarenes with Ligated Anionic CuI Intermediates

A series of experimental studies, along with DFT calculations, are reported that provide a detailed view into the mechanism of Ullmann coupling of phenols with aryl halides in the presence of catalysts generated from Cu(I) and bidentate, anionic ligands. These studies encompass catalysts containing anionic ligands formed by deprotonation of 8-hydroxyquinoline, 2-pyridylmethyl tert-butyl ketone, and 2,2,6,6-tetramethylheptane-3,5-dione. Three-coordinate, heteroleptic species [Cu(LX)OAr]^- were shown by experiment and DFT calculations to be the most stable complexes in catalytic systems containing 8-hydroxyquinoline or 2-pyridylmethyl tert-butyl ketone and to be generated reversibly in the system containing 2,2,6,6-tetramethylheptane-3,5-dione. These heteroleptic complexes were characterized by a combination of ¹⁹F NMR, ¹H NMR, and UV-vis spectroscopy, as well as ESI-MS. The heteroleptic complexes generated in situ react with iodoarenes to form biaryl ethers in high yields without evidence for an aryl radical intermediate. Measurements of ¹³C/¹²C isotope effects showed that oxidative addition of the iodoarene occurs irreversibly. This information, in combination with the kinetic data, shows that oxidative addition occurs to the [Cu(LX)OAr]^- complexes and is turnover-limiting. A Hammett analysis of the effect of phenoxide electronic properties on the rate of the reaction of [Cu(LX)OAr]^- with iodotoluene also is consistent with oxidative addition of the iodoarene to an anionic phenoxide complex. Calculations by DFT suggest that this oxidative addition is followed by dissociation of I^- and reductive elimination of the biaryl ether from the resulting neutral Cu(III) complex.

Accelerated multi-reagent copper catalysed coupling reactions in micro droplets and thin films

Electrospray (ES) and paper spray (PS) mass spectrometry were used to create confined liquid volumes in which accelerated air and water sensitive, heterogeneous, copper catalysed C–O and C–N coupling reactions occur.

Palladium-Catalyzed Hydroaminocarbonylation of Alkynes with Tertiary Amines via C-N Bond Cleavage

An efficient strategy for the cleavage of the C-N bond of tertiary amines was developed with DTBP as an oxidant, in which the cleaved H atom and amine moiety were successfully transferred to the desired products. This strategy has enabled an efficient palladium-catalyzed hydroaminocarbonylation of alkynes with tertiary amines. Notably, the catalyst loading could be lowered from 5 to 0.1 mol %, which represents the lowest catalyst loading among the reported work on carbonylation via C-N bond activation.

Origins of high catalyst loading in copper(i)-catalysed Ullmann-Goldberg C-N coupling reactions

A mechanistic investigation of Ullmann-Goldberg reactions using soluble and partially soluble bases led to the identification of various pathways for catalyst deactivation through (i) product inhibition with amine products, (ii) by-product inhibition with inorganic halide salts, and (iii) ligand exchange by soluble carboxylate bases. The reactions using partially soluble inorganic bases showed variable induction periods, which are responsible for the reproducibility issues in these reactions. Surprisingly, more finely milled Cs2CO3 resulted in a longer induction period due to the higher concentration of the deprotonated amine/amide, leading to suppressed catalytic activity. These results have significant implications on future ligand development for the Ullmann-Goldberg reaction and on the solid form of the inorganic base as an important variable with mechanistic ramifications in many catalytic reactions.

Room-Temperature CuI-Catalyzed Amination of Aryl Iodides and Aryl Bromides

A general and effective CuI/N',N'-diaryl-1H-pyrrole-2-carbohydrazide catalyst system was developed for the amination of aryl iodides and bromides at room temperature with good chemoselectivity between -OH and -NH₂ groups. Only 5 mol % of CuI and ligands was needed in this protocol to effect the amination of various aryl bromides and aryl iodides with a wide range of aliphatic and aryl amines (1.3 equiv).

Copper and silver benzoate and aryl complexes and their implications for oxidative decarboxylative coupling reactions

A copper(ii) benzoate complex is a viable intermediate for copper-catalyzed oxidative decarboxylative coupling reactions.