Three-Coordinate Copper(II) Aryls: Key Intermediates in C–O Bond Formation

C-N Bond Formation at Discrete CuIII-Aryl Complexes

Copper(III) aryl species are widely proposed as intermediates in Cu-catalyzed C-C and C-heteroatom bond formation reactions. Despite their wide utility, mechanistic aspects of C-heteroatom formation at CuIII centers as well as factors that lead to byproducts, e.g., Ar-H, Ar-Ar, remain elusive due to the rarity of discrete CuIII-Ar complexes. Herein, we report the synthesis and reactivity of a series of CuII and CuIII aryl complexes that closely mimic the intermediates in Cu-catalyzed C-N coupling reactions. Copper(II) aryl complexes [TBA][LCuII-ArR] were synthesized via the treatment of CuII with a range of aryl donors, such as ZnAr2R, TMS-ArR, and ArR-Bpin. Oxidation of [TBA][LCuII-ArR] produces formal copper(III) aryl complexes LCuIII-ArR. Treatment of copper(III) aryl complexes with neutral nitrogen nucleophiles produces the C-N coupling product in up to 64% yield, along with commonly observed byproducts, such as Ar-H and Ar-Ar. Hammett analysis of the C-N bond formation performed with various N-nucleophiles shows a ρ value of -1.66, consistent with the electrophilic character of LCuIII-ArR. We propose mechanisms for common side reactions in Cu-catalyzed coupling reactions that lead to the formation of Ar-Ar and Ar-H.

Selective Radical Transfer in a Series of Nonheme Iron(III) Complexes

A series of nonheme iron complexes, FeIII(BNPAPh2O)(Lax)(Leq) (Lax/eq = N3-, NCS-, NCO-, and Cl-) have been synthesized using the previously reported BNPAPh2O- ligand. The ferrous analogs FeII(BNPAPh2O)(Lax) (Lax = N3-, NCS-, and NCO-) were also prepared. The complexes were structurally characterized using single crystal X-ray diffraction, which shows that all the FeIII complexes are six-coordinate, with one anionic ligand (Lax) in the H-bonding axial site and the other anionic ligand (Leq) in the equatorial plane, cis to the Lax ligand. The reaction of FeIII(BNPAPh2O-)(Lax)(Leq) with Ph3C• shows that one ligand is selectively transferred in each case. A selectivity trend emerges that shows •N3 is the most favored for transfer in each case to the carbon radical, whereas Cl• is the least favored. The NCO and NCS ligands showed an intermediate propensity for radical transfer, with NCS > NCO. The overall order of selectivity is N3 > NCS > NCO > Cl. In addition, we also demonstrated that H-bonding has a small effect on governing product selectivity by using a non-H-bonded ligand (DPAPh2O-). This study demonstrates the inherent radical transfer selectivity of nonhydroxo-ligated nonheme iron(III) complexes, which could be useful for efforts in synthetic and (bio)catalytic C-H functionalization.

Anaerobic photoinduced Cu(0/I)-mediated Glaser coupling in a radical pathway

The reaction mechanism of the historic copper-catalyzed Glaser coupling has been debated to be based on redox cycles of Cu ions in specific oxidation states or on a radical mechanism based on Cu(0)/Cu(I). Here, the authors demonstrate two coexisting Glaser coupling pathways which can be differentiated by anaerobic/irradiation or aerobic reaction conditions. Without O2, copper(I) acetylides undergo a photo-excited pathway to generate highly reactive alkynyl radicals, which combine together to form a homo-coupling product or individually react with diverse X-H (X = C, N, O, S and P) substrates via hydrogen atom transfer. With O2, copper(I) acetylides are oxidized to become a Cu-acetylide/Cu-O merged Cu(I/II) intermediate for further oxidative coupling. This work not only complements the radical mechanism for Glaser coupling, but also provides a mild way to access highly energetic alkynyl radicals for efficient organic transformations.

Comproportionation and disproportionation in nickel and copper complexes

Disproportionation and comproportionation reactions have become increasingly important electron transfer events in organometallic chemistry and catalysis. The renewed interest in these reactions is in part attributed to the improved understanding of first-row metals and their ability to occupy odd and even oxidation states. Disproportionation and comproportionation reactions enable metal complexes to shuttle between various oxidation states, a matter of utmost relevance for controlling the speciation and catalytic turnover. In addition, these reactions have a direct impact in the thermodynamic and kinetic stability of the corresponding metal complexes. This review covers the relevance and impact of these processes in electron transfer reactions and provides valuable information about their non-negligible influence in Ni- and Cu-catalysed transformations.

Characterization and Reactivity of Copper(II) and Copper(III) σ-Aryl Intermediates in Aminoquinoline-Directed C-H Functionalization

Over the past decade, numerous reports have focused on the development and applications of Cu-mediated C-H functionalization reactions; however, to date, little is known about the Cu intermediates involved in these transformations. This paper details the observation and characterization of CuII and CuIII intermediates in aminoquinoline-directed C(sp2)-H functionalization of a fluoroarene substrate. An initial C(sp2)-H activation at CuII occurs at room temperature to afford an isolable anionic cyclometalated CuII complex. This complex undergoes single-electron oxidation with ferrocenium or AgI salts under mild conditions (5 min at room temperature) to afford C(sp2)-C(sp2) or C(sp2)-NO2 coupling products. Spectroscopic studies implicate the formation of a transient diamagnetic CuIII-σ-aryl intermediate that undergoes either (i) a second C(sp2)-H activation at CuIII followed by C-C bond-forming reductive elimination or (ii) reaction with a NO2- nucleophile and C(sp2)-NO2 coupling.

Unraveling the Chemistry of High Valent Arylcopper Compounds and Their Roles in Copper-Catalyzed Arene C-H Bond Transformations Using Synthetic Macrocycles

Recent decades have witnessed a resurgence of the study of copper-catalyzed organic reactions. As the surrogate of noble metal catalysts, copper salts have been shown to exhibit remarkable versatility in activating various C-H bonds enabling the construction of diverse carbon-carbon and carbon-heteroatom bonds. Advantageously, copper salts are also naturally abundant, inexpensive, and less toxic in comparison to precious metals. Despite significant developments in synthesis, the mechanism of copper catalysis remains elusive. Hypothetical pathways such as the two-electron Cu(III)/Cu(I) and Cu(II)/Cu(0) catalytic cycles and the one-electron Cu(II)/Cu(I) catalytic cycle have been invoked to diagram C-H bond transformations because of the formidable challenges to isolate and characterize transient high valent organocopper intermediates. In fact, organocopper chemistry has been dominated for a long time by the acknowledged nucleophilic organocopper(I) compounds. Since the beginning of the new millennium, we have been systematically studying the supramolecular chemistry of heteracalix[n]aromatics. Owing to the ease of their synthesis and selective functionalizations, self-tunable conformation and cavity structures resulting from the interplay of heteroatoms with aromatic subunits, and outstanding properties in molecular recognition and self-assembly, heteracalix[n]aromatics have become a class of privileged synthetic macrocyclic hosts. Our journey to the chemistry of high valent organocopper compounds started with a serendipitous discovery of the facile formation of a stable organocopper compound, which contains astonishingly a Ph-Cu(III) σ-bond under very mild aerobic conditions. When we examined routinely the effect of the macrocyclic structures on noncovalent complexation properties, titration of tetraazacalix[1]arene[3]pyridine with Cu(ClO₄)₂·6H₂O resulted in the precipitation of dark-purple crystals of phenylcopper(III) diperchlorate. Our curiosity about the transformation of an arene C-H bond into an Ar-Cu(III) bond prompted us to conduct an in-depth investigation of the reaction of macrocyclic arenes with copper(II) salts, leading to the isolation of arylcopper(II) compounds which are unprecedented and the missing link in organocopper chemistry. With structurally well-defined organometallics in hand, we have explored extensively the reactivities of both arylcopper(II) and arylcopper(III) compounds, demonstrating their versatility and uniqueness in chemical synthesis. Novel and fascinating arene C-H transformations under copper catalysis have been developed. Using acquired high valent arylcopper compounds as molecular probes, and employing the functionalizations of tetraazacalix[1]arene[3]pyridines as model reactions, we have revealed the diverse mechanisms of copper-promoted arene C-H bond reactions. Elusive reaction pathways of some copper-catalyzed C-X bond activations have also been unraveled. In the meantime, we have also witnessed pleasingly the rapid development of field with the advent of new high valent organocopper compounds. Without any doubt, studies of the synthesis, reactivity, and catalysis of high valent organocopper compounds have been reshaping the field of organocopper chemistry. This Account summarizes our endeavors to explore the chemistry of structurally well-defined arylcopper(II) and arylcopper(III) compounds and the mechanisms of copper-catalyzed arene C-H and C-X bond transformations. We hope this Account will inspire chemists to study thoroughly the fundamentals and the cutting-edge catalysis of high valent organocopper compounds advancing and redefining the discipline of organocopper chemistry.

NO Coupling at Copper to cis-Hyponitrite: N2O Formation via Protonation and H-Atom Transfer

Copper nitrite reductases (CuNIRs) convert NO2- to NO as well as NO to N2O under high NO flux at a mononuclear type 2 Cu center. While model complexes illustrate N-N coupling from NO that results in symmetric trans-hyponitrite [CuII]-ONNO-[CuII] complexes, we report NO assembly at a single Cu site in the presence of an external reductant Cp*2M (M = Co, Fe) to give the first copper cis-hyponitrites [Cp*2M]{[CuII](κ2-O2N2)[CuI]}. Importantly, the κ1-N-bound [CuI] fragment may be easily removed by the addition of mild Lewis bases such as CNAr or pyridine to form the spectroscopically similar anion {[CuII](κ2-O2N2)}-. The addition of electrophiles such as H+ to these anionic copper(II) cis-hyponitrites leads to N2O generation with the formation of the dicopper(II)-bis-μ-hydroxide [CuII]2(μ-OH)2. One-electron oxidation of the {[CuII](κ2-O2N2)}- core turns on H-atom transfer reactivity, enabling the oxidation of 9,10-dihydroanthracene to anthracene with concomitant formation of N2O and [CuII]2(μ-OH)2. These studies illustrate both the reductive coupling of NO at a single copper center and a way to harness the strong oxidizing power of nitric oxide via the neutral cis-hyponitrite [Cu](κ2-O2N2).

Regioselective Alkylpolyfluoroarylation of Styrenes by Copper-Catalyzed C(sp3)-H and C(sp2)-H Double Activation

A novel dehydrogenative dicarbofunctionalization of vinyl arenes with polyfluoroarenes and unactivated alkanes enabled by copper catalysis has been accomplished under mild conditions. This transformation provides a regioselective route to highly functionalized polyfluoroaryl compounds that occur as structural scaffolds in a variety of pharmaceuticals and materials. Preliminary mechanistic studies indicate that the carbon-based radical and copper intermediate are involved in the reaction, and the reaction pathway is dominated by the bond dissociation energy (BDE) of C(sp³)-H bonds.

Room-Temperature Cu(II) Radical-Triggered Alkyne C-H Activation

A dimeric Cu(II) complex [Cu(II)2L2(μ2-Cl)Cl] (1) built from an asymmetric tridentate ligand (2-(((2-aminocyclohexyl)imino)methyl)-4,6-di-tert-butylphenol) and weakly coordinating anions has been synthesized and structurally characterized. In dichloromethane solution, 1 exists in a monomeric [Cu(II)LCl] (1') (85%)-dimeric (1) (15%) equilibrium, and cyclic voltammetry (CV) and electron paramagnetic resonance (EPR) studies indicate structural stability and redox retention. Addition of phenylacetylene to the CH2Cl2 solution populates 1' and leads to the formation of a transient radical species. Theoretical studies support this notion and show that the radical initiates an alkyne C-H bond activation process via a four-membered ring (Cu(II)-O···H-Calkyne) intermediate. This unusual C-H activation method is applicable for the efficient synthesis of propargylamines, without additives, within 16 h, at low loadings and in noncoordinating solvents including late-stage functionalization of important bioactive molecules. Single-crystal X-ray diffraction studies, postcatalysis, confirmed the framework's stability and showed that the metal center preserves its oxidation state. The scope and limitations of this unconventional protocol are discussed.

Copper(II) mediated ortho C-H alkoxylation of aromatic amines using organic peroxides: efficient synthesis of hindered ethers

Synthesis of hindered alkyl aryl ether derivatives (R-O-Ar) remains a huge challenge and highly desirable in organic and medicinal chemistry because extensive substitution on the ether bond prevents the undesired metabolic process and thus avoids rapid degradation in vivo. Herein, we report an unprecedented hindered alkoxylation of picolinamide attached aromatic amines using economic copper salt and organic peroxide to get highly desirable α-tertiary alkyl aryl ethers.

Mediator-Enabled Electrocatalysis with Ligandless Copper for Anaerobic Chan-Lam Coupling Reactions

Simple copper salts serve as catalysts to effect C-X bond-forming reactions in some of the most utilized transformations in synthesis, including the oxidative coupling of aryl boronic acids and amines. However, these Chan-Lam coupling reactions have historically relied on chemical oxidants that limit their applicability beyond small-scale synthesis. Despite the success of replacing strong chemical oxidants with electrochemistry for a variety of metal-catalyzed processes, electrooxidative reactions with ligandless copper catalysts are plagued by slow electron-transfer kinetics, irreversible copper plating, and competitive substrate oxidation. Herein, we report the implementation of substoichiometric quantities of redox mediators to address limitations to Cu-catalyzed electrosynthesis. Mechanistic studies reveal that mediators serve multiple roles by (i) rapidly oxidizing low-valent Cu intermediates, (ii) stripping Cu metal from the cathode to regenerate the catalyst and reveal the active Pt surface for proton reduction, and (iii) providing anodic overcharge protection to prevent substrate oxidation. This strategy is applied to Chan-Lam coupling of aryl-, heteroaryl-, and alkylamines with arylboronic acids in the absence of chemical oxidants. Couplings under these electrochemical conditions occur with higher yields and shorter reaction times than conventional reactions in air and provide complementary substrate reactivity.

Copper-Catalyzed Formal Dehydrogenative Coupling of Carbonyls with Polyfluoroarenes Leading to β-C-H Arylation

We herein communicate a formal dehydrogenative coupling of carbonyls with polyfluoroarenes enabled by Cu catalysis. Silyl enol ethers initially prepared from carbonyls are postulated to undergo the copper-mediated oxidative dehydrogenative coupling with polyfluoroarenes via a radical pathway. Including cyclic and linear ketones, aldehydes, and esters, a broad range of β-aryl carbonyl products were efficiently obtained in high regio- and stereoselectivity with excellent functional group tolerance.

Three-Coordinate Copper(II) Alkynyl Complex in C-C Bond Formation: The Sesquicentennial of the Glaser Coupling

Copper(II) alkynyl species are proposed as key intermediates in numerous Cu-catalyzed C-C coupling reactions. Supported by a β-diketiminate ligand, the three-coordinate copper(II) alkynyl [Cu^II]-C≡CAr (Ar = 2,6-Cl₂C₆H₃) forms upon reaction of the alkyne H-C≡CAr with the copper(II) tert-butoxide complex [Cu^II]-O^tBu. In solution, this [Cu^II]-C≡CAr species cleanly transforms to the Glaser coupling product ArC≡C-C≡CAr and [Cu^I](solvent). Addition of nucleophiles R'C≡C-Li (R' = aryl, silyl) and Ph-Li to [Cu^II]-C≡CAr affords the corresponding C_sp-C_sp and C_sp-C_sp2 coupled products RC≡C-C≡CAr and Ph-C≡CAr with concomitant generation of [Cu^I](solvent) and {[Cu^I]-C≡CAr}^-, respectively. Supported by density functional theory (DFT) calculations, redox disproportionation forms [Cu^III](C≡CAr)(R) species that reductively eliminate R-C≡CAr products. [Cu^II]-C≡CAr also captures the trityl radical Ph₃C· to give Ph₃C-C≡CAr. Radical capture represents the key C_sp-C_sp3 bond-forming step in the copper-catalyzed C-H functionalization of benzylic substrates R-H with alkynes H-C≡CR' (R' = (hetero)aryl, silyl) that provide C_sp-C_sp3 coupled products R-C≡CR via radical relay with ^tBuOO^tBu as oxidant.

Reactions of a pincer proligand with copper iodide: bridging instead of C–H metalation

Various precursors of divalent copper have been treated with the meta-disubstituted phenylene-based proligand POC(H)OP (1,3-(i-Pr2PO)2C6H4) with the objective of preparing classical pincer complexes (POCOP)CuX. However, in no case was such species obtained, presumably owing to the difficult C–H metallation step. Analogous reactions of monovalent precursors were also unsuccessful, whereas reaction of POC(H)OP with CuI under different conditions gave the non-metallated adducts {(μ, κP, [Formula: see text]-POC(H)OP)Cu(μ-Ι)}2, 1, {(μ, κP, [Formula: see text]-POC(H)OP)Cu2(μ-Ι)2(DMAP)2}, 2 (DMAP = 4-dimethylaminopyridine), and {(μ, κP, [Formula: see text]-POC(H)OP)Cu2(μ3-Ι)2}2, 3. Treating 1 with DMAP gave the adduct 2, whereas 3 could be obtained by treating 1 with BuLi or by sublimation of 1. The solid state structures of these complexes revealed the tetrahedral geometry that might be anticipated for the d10 Cu(I) centers, in addition to fairly close I–H distances; on the other hand, no C–H interaction (agostic or otherwise) was observed with the Cu centers in any of these structures. The unsuccessful metallation of the C(2)–H moiety is thought to be a result of the strong preference of monovalent copper center to form bridging interactions with iodide and the POC(H)OP ligand; this appears to prevent the approach of the central carbon of the ligand to the Cu centers.

Copper-Catalyzed Direct C-H Alkylation of Polyfluoroarenes by Using Hydrocarbons as an Alkylating Source

Construction of carbon-carbon bonds is one of the most important tools in chemical synthesis. In the previously established cross-coupling reactions, prefunctionalized starting materials were usually employed in the form of aryl or alkyl (pseudo)halides or their metalated derivatives. However, the direct use of arenes and alkanes via a 2-fold oxidative C-H bond activation strategy to access chemoselective C(sp²)-C(sp³) cross-couplings is highly challenging due to the low reactivity of carbon-hydrogen (C-H) bonds and the difficulty in suppressing side reactions such as homocouplings. Herein, we present the new development of a copper-catalyzed cross-dehydrogenative coupling of polyfluoroarenes with alkanes under mild conditions. Relatively weak sp³ C-H bonds at the benzylic or allylic positions, and nonactivated hydrocarbons could be alkylated by the newly developed catalyst system. A moderate-to-high site selectivity was observed among various C-H bonds present in hydrocarbon reactants, including gaseous feedstocks and complex molecules. Mechanistic information was obtained by performing combined experimental and computational studies to reveal that the copper catalyst plays a dual role in activating both alkane sp³ C-H bonds and sp² polyfluoroarene C-H bonds. It was also suggested that the noncovalent π-π interaction and weak hydrogen bonds formed in situ between the optimal ligand and arene substrates are key to facilitating the current coupling reactions.

Radical Reactivity, Catalysis, and Reaction Mechanism of Arylcopper(II) Compounds: The Missing Link in Organocopper Chemistry

Organocopper(I) compounds are recognized as carbon nucleophiles, while organocopper(III) complexes are involved in copper catalysis as intermediates to undergo a cross-coupling reaction with various anionic nucleophiles. In contrast to the chemistry of organocopper(I) and (III) compounds, organocopper(II) chemistry is virtually a missing link in integral organocopper chemistry because structurally well-defined organocopper(II) compounds have barely been isolated or studied. We report in this Article an investigation of the radical reactions of stable and structurally well-defined arylcopper(II) compounds, obtained readily from the arene C-H bond reaction of macrocyclic azacalix[1]arene[3]pyridines and Cu(ClO₄)₂. We have found that arylcopper(II) compounds acted as essentially radical species to undergo an efficient three-component reaction with radical initiators 2,2'-azobis(isobutyronitrile) (AIBN) or 2,2'-azobis(2,4-dimethylvaleronitrile) (ABVN) and α,β-unsaturated compounds CH₂═CHX (X = CO₂CH₃, CN, CONH₂, COCH₃, and SO₂Ph) to afford polyfunctionalized products. Combined experimental and theoretical studies revealed that radicals couple directly with the C_aryl atom of arylcopper(II) compounds to form C_alkyl-C_aryl bonds through a Cu(II)/Cu(I) mechanism. Comprehension of the formation and radical reactivity of arylcopper(II) compounds has allowed the development of a copper-catalyzed three-component radical reaction for arene C-H bond functionalization.

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.

Controllable chemoselectivity in the coupling of bromoalkynes with alcohols under visible-light irradiation without additives: synthesis of propargyl alcohols and α-ketoesters

The chemoselectivity of visible-light-induced coupling reactions of bromoalkynes with alcohols can be controlled by simple changes to the reaction atmosphere.

N,O-Bidentate ligand-tunable copper(ii) complexes as a catalyst for Chan-Lam coupling reactions of arylboronic acids with 1H-imidazole derivatives

An efficient procedure for Chan-Lam coupling reactions of arylboronic acids with 1H-imidazole derivatives using N,O-bidentate ligand-tunable copper(ii) complexes as a catalyst under base-free conditions has been developed. This protocol features mild reaction conditions, high yields and compatibility with different functional groups, providing a direct and facile strategy for the construction of C-N bonds and synthesis of heterocyclic compounds.

Ligand and electronic effects on copper–arylnitroso self-assembly

The topology and degree of electron transfer in self-assembled redox reactions between copper(i) species and nitrosoarenes are controlled by ligand properties.

Halodifluoroacetates as formylation reagents for various amines via unprecedented quadruple cleavage

Unprecedented quadruple cleavage of halodifluoromethyl compounds has been disclosed for the first time, resulting in carbonyl synthons for reaction with various amines.