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

Overcoming Copper Reduction Limitation in Asymmetric Substitution: Aryl-Radical-Enabled Enantioconvergent Cyanation of Alkyl Iodides

Cu-catalyzed enantioconvergent cross-coupling of alkyl halides has emerged as a powerful strategy for synthesizing enantioenriched molecules. However, this approach is intrinsically limited by the weak reducing power of copper(I) species, which restricts the scope of compatible nucleophiles and necessitates extensive ligand optimization or the use of complex chiral scaffolds. To overcome these challenges, we introduce an aryl-radical-enabled strategy that decouples the alkyl halide activation step from the chiral Cu center. We demonstrate that merging aryl-radical-enabled iodine abstraction with Cu-catalyzed asymmetric radical functionalization enables the conversion of racemic α-iodoamides to enantioenriched alkyl nitrile products with good yield and enantioselectivity. The rational design of chiral ligands identified a new class of carboxamide-containing BOX ligands. Mechanistic studies support an aryl-radical-enabled pathway and the unique hydrogen-bonding ability in the newly designed BOX ligands. This aryl-radical-enabled asymmetric substitution reaction has the potential to significantly expand the scope of Cu-catalyzed enantioconvergent cross-coupling reactions.

Speciation and Reactivity of Mono- and Binuclear Ni Intermediates in Aminoquinoline-Directed C-H Arylation and Benzylation

This paper describes detailed organometallic studies of the aminoquinoline-directed Ni-catalyzed C-H functionalization of 2,3,4,5-tetrafluoro-N-(quinolin-8-yl)benzamide with diaryliodonium reagents. A combination of 19F NMR spectroscopy and X-ray crystallography is used to track and characterize diamagnetic and paramagnetic intermediates throughout this transformation. These provide key insights into both the cyclometalation and oxidative functionalization steps of the catalytic cycle. The reaction conditions (solvent, ligands, base, and stoichiometry) play a central role in the observation of a NiII precyclometalation intermediate as well as in the speciation of the NiII products of C-H activation. Both mono- and binuclear cyclometalated NiII species are observed and interconvert, depending on the reaction conditions. Cyclic voltammetry reveals that the NiII/III redox potentials for the cyclometalated intermediates vary by more than 700 mV depending on their coordination environments, and these differences are reflected in their relative reactivity with diaryliodonium oxidants. The oxidative functionalization reaction affords a mixture of arylated and solvent functionalization organic products, depending on the conditions and solvent. For example, conducting oxidation in toluene leads to the preferential formation of the benzylated product. A series of experiments implicate a NiII/III/IV pathway for this transformation.

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.

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.

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

Despite the recent advancements of Cu catalysis for the cross-coupling of alkyl electrophiles and the frequently proposed involvement of alkyl-Cu(III) complexes in such reactions, little is known about the reactivity of these high-valent complexes. Specifically, although the reversible interconversion between an alkyl-CuIII complex and an alkyl radical/CuII pair has been frequently proposed in Cu catalysis, direct observation of such steps in well-defined CuIII complexes remains elusive. In this study, we report the synthesis and investigation of alkyl-CuIII complexes, which exclusively undergo a Cu-C homolysis pathway to generate alkyl radicals and CuII species. Kinetic studies suggest a bond dissociation energy of 28.6 kcal/mol for the CuIII-C bonds. Moreover, these four-coordinate complexes could be converted to a solvated alkyl-CuIII-(CF3)2, which undergoes highly efficient C-CF3 bond-forming reductive elimination even at low temperatures (-4 °C). These results provide strong support for the reversible recombination of alkyl radicals with CuII to form alkyl-CuIII species, an elusive step that has been proposed in Cu-catalyzed mechanisms. Furthermore, our work has demonstrated that the reactivity of CuIII complexes could be significantly influenced by subtle changes in the coordination environment. Lastly, the observation of the highly reactive neutral alkyl-CuIII-(CF3)2 species (or with weakly bound solvent molecules) suggests they might be the true intermediates in many Cu-catalyzed trifluoromethylation reactions.