Copper-Carbon Homolysis Competes with Reductive Elimination in Well-Defined Copper(III) Complexes

SH2 Decarboxylative Monofluoromethylation of Aliphatic Carboxylic Acids Enabled by Nickel/Photoredox Dual Catalysis

Catalytic construction of alkyl fluorides through C─C bond-forming transformations represents a formidable challenge in modern organofluorine chemistry. In this work, we present a decarboxylative monofluoromethylation protocol enabled by synergistic nickel/photoredox dual catalysis for aliphatic carboxylic acid derivatives. A structurally diverse array of tertiary, secondary, and primary N-hydroxyphthalimide (NHP) esters underwent efficient conversion to their corresponding fluoromethylated products with excellent functional group tolerance (68 examples, up to 91% yield). The methodology's versatility was highlighted through late-stage monofluoromethylation of pharmaceutically relevant molecules. Mechanistic investigations reveal the involvement of a radical rebound pathway mediated by nickel(III) intermediate, with the homolytic substitution (SH2) process at sterically congested center representing a critical mechanistic distinction from conventional cross-coupling paradigms.

Competition between One- and Two-Electron Unimolecular Reactions of Late 3d-Metal Complexes [(Me3SiCH2)nM]-(M = Fe, Co, Ni, and Cu; n = 2-4)

Although organometallic complexes of the late 3d elements are known to undergo both one-and two-electron reactions, their relative propensities to do so remain poorly understood. To gain direct insight into the competition between these different pathways, we have analyzed the unimolecular gas-phase reactivity of a series of well-defined model complexes [(Me3SiCH2)nM]- (M = Fe, Co, Ni, and Cu; n = 2-4). Applying a combination of tandem-mass spectrometry, quantum-chemical computations, and statistical rate-theory calculations, we find several different fragmentation reactions, among which the homolytic cleavage of metal-carbon bonds and radical dissociations are particularly prominent. In all cases, these one-electron reactions are entropically favored. For the ferrate and cobaltate complexes, they are also energetically preferred, which explains their predominance in the corresponding fragmentation experiments. For [(Me3SiCH2)4Ni]- and, even more so, for [(Me3SiCH2)4Cu]-, a concerted reductive elimination as a prototypical two-electron reaction is energetically more favorable and gains in importance. [(Me3SiCH2)3Ni]- is special in that it has two nearly degenerate spin states, both of which react in different ways. A simple thermochemical analysis shows that the relative order of the first and second bond-dissociation energies is of key importance in controlling the competition between radical dissociations and concerted reductive eliminations.

Copper-Mediated Direct Trifluoromethylation of Trichloromethyl Alkanes

CF3CCl2-containing compounds are of significant synthetic value but are typically synthesized from environmentally harmful hydrochlorofluorocarbons (CFCs). Herein, we report the use of a well-defined Cu(I) complex, [(bpy)Cu(CF3)], as an efficient trifluoromethylating reagent for the direct trifluoromethylation of trichloroalkanes under mild conditions, affording CF3CCl2-containing products with excellent chemoselectivity. This protocol also enabled the gram-scale synthesis of cyhalothric acid ester, which is a key intermediate in the production of pyrethroid pesticides.

New reactivity of late 3d transition metal complexes in catalytic reactions of alkynes

Late 3d metals such as iron, cobalt, nickel, and copper are abundantly present in the Earth's crust and they are produced in huge quantities in the mining industry. Often, these inexpensive metals exhibit unique or special reactivities in catalytic reactions as compared with expensive noble metals such as palladium, iridium, and rhodium. The novel reactivities of 3d metal complexes originate from their unique physical and atomic properties as compared with heavier 4d/5d congeners: smaller ionic and covalent radii, contracted 3d orbitals of smaller sizes and lower energies, lower values of Pauli electronegativity, etc. This review summarizes the recent progress in late 3d transition metal-catalyzed transformations of alkynes. We organize catalytic examples according to each type of novel elementary reactivity exhibited by 3d metal complexes. Each section includes a description of the unique reactivity of the 3d metals, the atomic and theoretical basis of the reactivity and illustrations of catalytic examples: (1) single electron transfer from low-valent metal complexes to alkyl halides, (2) facile reductive elimination from nickel(III), (3) facile reductive elimination from copper(III), (4) cis-to-trans isomerization of alkenyl metal complexes after syn-insertion, (5) ligand-to-ligand hydrogen transfer, (6) hydrogen atom transfer from hydride complexes and (7) protonation of nickel metallacyclopropenes.

Solvent-Dependent C(sp3)-CF3 Reductive Elimination from Neutral Four-Coordinate Cu(III) Complexes

A solvent dependent C(sp3)-CF3 bond-forming reductive elimination from neutral four-coordinate Cu(III) complexes [(L)CuIII(CF3)2(CH2CO2 tBu)] (L=pyridine or its derivatives) is described. Reactions in less polar solvent ClCH2CH2Cl proceed via a concerted bond breaking/bond forming process along with the reorientation of the ligand, while reaction in polar solvent DMF occurs via a rate limiting ligand-dissociation, followed by C(sp3)-CF3 reductive elimination from the resulting three-coordinate intermediate. These mechanistic proposals are supported by kinetic studies that included ligand and temperature effects, as well as DFT calculations.

Catalytic Asymmetric Oxidative Coupling between C(sp3)-H Bonds and Carboxylic Acids

The direct enantioselective functionalization of C(sp3)-H bonds in organic molecules could fundamentally transform the synthesis of chiral molecules. In particular, the enantioselective oxidation of these bonds would dramatically change the production methods of chiral alcohols and esters, which are prevalent in natural products, pharmaceuticals, and fine chemicals. Remarkable advances have been made in the enantioselective construction of carbon-carbon and carbon-nitrogen bonds through the C(sp3)-H bond functionalization. However, the direct enantioselective formation of carbon-oxygen bonds from C(sp3)-H bonds remains a considerable challenge. We herein report a highly enantioselective C(sp3)-H bond oxidative coupling with carboxylic acids. The method applies to allylic and propargylic C-H bonds and employs various carboxylic acids as oxygenating agents. The method successfully synthesized a range of chiral esters directly from readily available alkenes and alkynes, greatly simplifying the synthesis of chiral esters and related alcohols.

BINAP-Accelerated Copper-Mediated Photochemical Late-Stage Trifluoromethylation of Arenes

Site-selective and late-stage C-H trifluoromethylation (including pentafluoroethylation) of arylthianthrenium salts using in situ generated CuCF3 from Ruppert-Prakash reagent under visible light was found to be significantly accelerated by a catalytic amount of rac-BINAP, allowing the reaction to complete within 15 min under very mild conditions without using any additional photoredox catalysts. The process showed a broad substrate scope, high efficiency, and excellent regioselectivity, which holds the promise to accelerate the discovery of fluorinated medicinal compounds.

Three Distinct Oxidation States (II/II, II/III, and III/III) of Diorganocopper Complexes

In this report, we present a structurally and spectroscopically characterized diorganocopper system in three distinct oxidation states: [CuIICuII] (1), [CuIICuIII] (2), and [CuIIICuIII] (3). These states are stabilized by a macrocyclic ligand scaffold featuring two square-planar coordination {C2 NHCN2 pyrazole}. We have analyzed the geometric and electronic structures using X-ray diffraction (XRD) and multiple spectroscopic methods including nuclear magnetic resonance (NMR), UV-vis, and electron paramagnetic resonance (EPR) spectroscopies, in combination with density functional theory (DFT) calculations. Remarkably, this study provides a structural determination of mixed-valence diorganocopper(II,III) complex 2, which is at the borderline between valence-trapped or charge-localized class I systems and charge moderately delocalized class II systems in Robin and Day classification. These findings enhance our understanding of the systematic structural and electronic changes that occur in diorganocopper complexes in response to redox transformations.

Mechanistic insights into copper-mediated benzylic C(sp3)-H bond trifluoromethylation

The mechanisms underlying copper-mediated trifluoromethylation of benzylic C(sp3)-H bonds were investigated using density functional theory (DFT) calculations. Two distinct pathways were identified: radical recombination/reductive elimination and single-electron transfer (SET). In the radical recombination/reductive elimination pathway, the CuII species recombines with benzyl radicals to generate a CuIII intermediate, which subsequently undergoes reductive elimination. Conversely, the SET pathway involves single-electron transfer from benzyl radicals to CuII species, forming a cationic benzylic intermediate and CuI species, followed by coupling with a CF3 group coordinated to Cu. DFT calculations revealed that the radical recombination/reductive elimination pathway is favoured for trifluoromethylation of primary and secondary benzylic C(sp3)-H bonds, with the reductive elimination step being rate-determining. In contrast, the SET pathway exhibits preference for trifluoromethylation of tertiary benzylic C(sp3)-H bonds. These mechanistic insights have significant implications for enhancing the selectivity of copper-mediated trifluoromethylation reactions.

Well-Defined Highly-Coordinated Copper(III) Iodide and Pincer Tris(trifluoromethyl)copper Complexes

Copper(III) iodide and bromide complexes representing a unique combination of highly-coordinated metal and soft polarizable anions were synthesized and fully characterized, including X-ray crystallography. Ligand substitution in well-defined highly-coordinated copper complex PyCu(CF3)3 with pincer ligands was achieved to give formally octahedral copper(III) complexes.

Oxidative Substitution of Organocopper(II) by a Carbon-Centered Radical

Copper-catalyzed coupling reactions of alkyl halides are believed to prominently involve copper(II) species and alkyl radicals as pivotal intermediates, with their exact interaction mechanism being the subject of considerable debate. In this study, a visible light-responsive fluoroalkylcopper(III) complex, [(terpy)Cu(CF3)2(CH2CO2tBu)] Trans-1, was designed to explore the mechanism. Upon exposure to blue LED irradiation, Trans-1 undergoes copper-carbon bond homolysis, generating Cu(II) species and carbon-centered radicals, where the carbon-centered radical then recombines with the Cu(II) intermediate, resulting in the formation of Cis-1, the Cis isomer of Trans-1. Beyond this, a well-defined fluoroalkylcopper(II) intermediate ligated with a sterically hindered ligand was isolated and underwent full characterization and electronic structure studies. The collective experimental, computational, and spectroscopic findings in this work strongly suggest that organocopper(II) engages with carbon-centered radicals via an "oxidative substitution" mechanism, which is likely the operational pathway for copper-catalyzed C-H bond trifluoromethylation reactions.

Catalytically Relevant Organocopper(III) Complexes Formed through Aryl-Radical-Enabled Oxidative Addition

Stepwise oxidative addition of copper(I) complexes to form copper(III) species via single electron transfer (SET) events has been widely proposed in copper catalysis. However, direct observation and detailed investigation of these fundamental steps remain elusive owing largely to the typically slow oxidative addition rate of copper(I) complexes and the instability of the copper(III) species. We report herein a novel aryl-radical-enabled stepwise oxidative addition pathway that allows for the formation of well-defined alkyl-CuIII species from CuI complexes. The process is enabled by the SET from a CuI species to an aryl diazonium salt to form a CuII species and an aryl radical. Subsequent iodine abstraction from an alkyl iodide by the aryl radical affords an alkyl radical, which then reacts with the CuII species to form the alkyl-CuIII complex. The structure of resultant [(bpy)CuIII(CF3)2(alkyl)] complexes has been characterized by NMR spectroscopy and X-ray crystallography. Competition experiments have revealed that the rate at which different alkyl iodides undergo oxidative addition is consistent with the rate of iodine abstraction by carbon-centered radicals. The CuII intermediate formed during the SET process has been identified as a four-coordinate complex, [CuII(CH3CN)2(CF3)2], through electronic paramagnetic resonance (EPR) studies. The catalytic relevance of the high-valent organo-CuIII has been demonstrated by the C-C bond-forming reductive elimination reactivity. Finally, localized orbital bonding analysis of these formal CuIII complexes indicates inverted ligand fields in σ(Cu-CH2) bonds. These results demonstrate the stepwise oxidative addition in copper catalysis and provide a general strategy to investigate the elusive formal CuIII complexes.

High-Valent Copper Catalysis Enables Regioselective Fluoroarylation of Gem-Difluorinated Cyclopropanes

Transition-metal (TM) catalyzed reaction of gem-difluorinated cyclopropanes (gem-DFCPs) has drawn much attention recently. The reaction generally occurs via the activation of the distal C─C bond in gem-DFCPs by a low-valent TM through oxidative addition, eventually producing mono-fluoro olefins as the coupling products. However, achieving regioselective activation of the proximal C─C bond in gem-DFCPs that overcomes the intrinsic reactivity via TM catalysis remains elusive. Here, a new reaction mode of gem-DFCPs enabled by high-valent copper catalysis, which allows exclusive activation of the congested proximal C─C bond is presented. The reaction that achieves fluoroarylation of gem-DFCPs uses NFSI (N-fluorobenzenesulfonimide) as electrophilic fluoro reagent and arenes as the C─H nucleophiles, enabling the synthesis of diverse CF3-containing scaffolds. It is proposed that a high-valent copper species plays an important role in the regioselective activation of the proximal C─C bond possibly via a σ-bond metathesis.

An Organocopper(III) Fluoride Triggering C-CF3 Bond Formation

Copper(III) fluorides are catalytically competent, yet elusive, intermediates in cross-coupling. The synthesis of [PPh4 ][CuIII (CF3 )3 F] (2), the first stable (isolable) CuIII -F, was accomplished via chloride addition to [CuIII (CF3 )3 (py)] (1) yielding [PPh4 ][CuIII (CF3 )3 Cl(py)] (1⋅Cl), followed by treatment with AgF. The CuIII halides 1⋅Cl and 2 were fully characterized using nuclear magnetic resonance (NMR) spectroscopy, single crystal X-ray diffraction (Sc-XRD) and elemental analysis (EA). Complex 2 proved capable of forging C-CF3 bonds from silyl-capped alkynes. In-depth mechanistic studies combining probes, theoretical calculations, trapping of intermediate 4a ([PPh4 ][CuIII (CF3 )3 (C≡CPh)]) and radical tests unveil the key role of the CuIII acetylides that undergo facile 2e- reductive elimination furnishing the trifluoromethylated alkynes (RC≡CCF3 ), which are industrially relevant synthons in drug discovery, pharma and agrochemistry.

Site-Specific Deaminative Trifluoromethylation of Aliphatic Primary Amines

The introduction of trifluoromethyl groups into organic molecules is of paramount importance in modern synthetic chemistry and medicinal chemistry. While methods for constructing C(sp2 )-CF3 bonds have been well established, the advancement of practical and comprehensive approaches for forming C(sp3 )-CF3 bonds remains considerably restricted. In this work, we describe an efficient and site-specific deaminative trifluoromethylation reaction of aliphatic primary amines to afford the corresponding alkyl trifluoromethyl compounds. The reaction proceeds at room temperature with readily accessible N-anomeric amide (Levin's reagent) and bench-stable bpyCu(CF3 )3 (Grushin's reagent, bpy=2,2'-bipyridine) under blue light. The protocol features mild reaction conditions, good functional group tolerance, and moderate to good yields. Remarkably, the method can be applied to the direct, late-stage trifluoromethylation of natural products and bioactive molecules. Experimental mechanistic studies were conducted, and a radical mechanism is proposed, wherein the dual roles of Grushin's reagent have been elucidated.