Benzylic C-H Esterification with Limiting C-H Substrate Enabled by Photochemical Redox Buffering of the Cu Catalyst

Copper-catalyzed radical-relay reactions provide a versatile strategy for selective C-H functionalization; however, reactions with peroxide-based oxidants often require excess C-H substrate. Here, we report a photochemical strategy to overcome this limitation by using a Cu/2,2'-biquinoline catalyst that supports benzylic C-H esterification with limiting C-H substrate. Mechanistic studies indicate that blue-light irradiation promotes carboxylate-to-copper charge transfer, reducing resting-state CuII to CuI, which activates the peroxide to generate an alkoxyl radical hydrogen-atom-transfer species. This "photochemical redox buffering" introduces a unique strategy to sustain the activity of Cu catalysts in radical-relay reactions.

Access to acridones by tandem copper(I)-catalyzed electrophilic amination/Ag(I)-mediated oxidative annulation of anthranils with arylboronic acids

An efficient and practical approach for the synthesis of medicinally important acridones was developed from anthranils and commercially available arylboronic acids by a tandem copper(I)-catalyzed electrophilic amination/Ag(I)-mediated oxidative annulation strategy. This new and straightforward protocol displayed a broad substrate scope (25 examples) and high functional group tolerance. What's more, a possible mechanistic proposal was also presented.

Photoinduced Copper-Catalyzed Asymmetric C-O Cross-Coupling

The construction of carbon-heteroatom bonds is one of the most active areas of research in organic chemistry because the function of organic molecules is often derived from the presence of heteroatoms. Although considerable advances have recently been achieved in radical-involved catalytic asymmetric C-N bond formation, there has been little progress in the corresponding C-O bond-forming processes. Here, we describe a photoinduced copper-catalyzed cross-coupling of readily available oxime esters and 1,3-dienes to generate diversely substituted allylic esters with high regio- and enantioselectivity (>75 examples; up to 95% ee). The reaction proceeds at room temperature under excitation by purple light-emitting diodes (LEDs) and features the use of a single, earth-abundant copper-based chiral catalyst as both the photoredox catalyst for radical generation and the source of asymmetric induction in C-O coupling. Combined experimental and density functional theory (DFT) computational studies suggest the formation of π-allylcopper complexes from redox-active oxime esters as bifunctional reagents and 1,3-dienes through a radical-polar crossover process.

Copper-Catalyzed Electrophilic Amination of Arylboronic Acids with Anthranils: An Access to N-Aryl-2-aminophenones

An efficient copper-catalyzed electrophilic amination strategy has been established for the rapid synthesis of N-aryl-2-aminophenones from readily available arylboronic acids/esters and anthranils. This protocol features good functional group tolerance, broad substrate scope, and operational simplicity. Moreover, a tandem C-H borylation and C-N coupling protocol has also been developed to transform simple arenes to the valuable N-aryl-2-aminophenones in one pot. Additionally, the synthetic potential of this methodology is further demonstrated by the synthesis of various useful N-heterocycles and derivatives.

His18 promotes reactive oxidative stress production in copper-ion mediated human islet amyloid polypeptide aggregation

Copper ions play a critical role in human islet amyloid polypeptide (hIAPP) aggregation, which has been found in more than 90% of patients with type-2 diabetes (T2D). The role of Cu(ii) in the cell cytotoxicity with hIAPP has been explored in two aspects: inhibiting the formation of fibrillar structures and stimulating the generation of reactive oxygen species (ROS). In this work, we carried out spectroscopic studies of Cu(ii) interacting with several hIAPP fragments and their variants as well. Electron paramagnetic resonance (EPR) measurements and Amplex Red analysis showed that the amount of H₂O₂ generated in hIAPP(11-28) solution co-incubated with Cu(ii) was remarkably more than hIAPP(1-11) and hIAPP(28-37). Furthermore, the H₂O₂ level was seriously reduced when His18 of hIAPP(11-28) was replaced by Arg(R) or Ser(S), indicating that His18 is the key residue of Cu(ii) binding to hIAPP(11-28) to promote H₂O₂ generation. This is likely because the donation of electrons from the peptide to Cu(ii) ions would result in the formation of the redox-active complexes, which could stimulate the formation of H₂O₂. Overall, this study provides further insight into the molecular mechanism of Cu(ii) induced ROS generation.

His18 promotes H₂O₂ production in copper-ion mediated hIAPP aggregation.

Ultrasound-Mediated Atom Transfer Radical Polymerization (ATRP)

Ultrasonic agitation is an external stimulus, rapidly developed in recent years in the atom transfer radical polymerization (ATRP) approach. This review presents the current state-of-the-art in the application of ultrasound in ATRP, including an initially-developed, mechanically-initiated solution with the use of piezoelectric nanoparticles, that next goes to the ultrasonication-mediated method utilizing ultrasound as a factor for producing radicals through the homolytic cleavage of polymer chains, or the sonolysis of solvent or other small molecules. Future perspectives in the field of ultrasound in ATRP are presented, focusing on the preparation of more complex architectures with highly predictable molecular weights and versatile properties. The challenges also include biohybrid materials. Recent advances in the ultrasound-mediated ATRP point out this approach as an excellent tool for the synthesis of advanced materials with a wide range of potential industrial applications.

Cu(i)-O2 oxidation reactions in a fluorinated all-O-donor ligand environment

Investigation of Cu-O2 oxidation reactivity is important in biological and anthropogenic chemistry. Zeolites are one of the most promising Cu/O based oxidation catalysts for development of industrial-scale CH4 to CH3OH conversion. Their oxidation mechanisms are not well understood, however, highlighting the importance of the investigation of molecular Cu(i)-O2 reactivity with O-donor complexes. Herein, we give an overview of the synthesis, structural properties, and O2 reactivity of three different series of O-donor fluorinated Cu(i) alkoxides: K[Cu(OR)2], [(Ph3P)Cu(μ-OR)2Cu(PPh3)], and K[(R3P)Cu(pinF)], in which OR = fluorinated monodentate alkoxide ligands and pinF = perfluoropinacolate. This breadth allowed for the exploration of the influence of the denticity of the ligand, coordination number, the presence of phosphine, and KF/O interactions on their O2 reactivity. KF/O interactions were required to activate O2 in the monodentate-ligand-only family, whereas these connections did not affect O2 activation in the bidentate complexes, potentially due to the presence of phosphine. Both families formed trisanionic, trinuclear cores of the form {Cu3(μ3-O)2}3-. Intramolecular and intermolecular substrate oxidation were also explored and found to be influenced by the fluorinated ligand. Namely, {Cu3(μ3-O)2}3- from K[Cu(OR)2] could perform both monooxygenase reactivity and oxidase catalysis, whereas those from K[(R3P)Cu(pinF)] could only perform oxidase catalysis.

Atom Transfer Radical Polymerization Enabled by Sonochemically Labile Cu-carbonate Species

Atom transfer radical polymerization (ATRP) has been previously mediated by ultrasound using a low concentration of copper complex in water (sono-ATRP) or by addition of piezoelectric materials in organic solvents (mechano-ATRP). However, these procedures proceeded slowly and yielded polymers contaminated by new chains initiated by hydroxyl radicals or by residual piezoelectrics. Unexpectedly, in the presence of sodium carbonate, rapid sono-ATRP of methyl acrylate in DMSO was achieved (80% conversion in <2 h) with excellent control of molecular weights and low dispersities (M_w/M_n < 1.2). The in situ formed Cu^II/L-CO₃ complex in the the presence of ultrasound generated Cu^I/L species as activators for ATRP and carbonate radical anions. The latter were scavenged by DMSO that was oxidized to dimethyl sulfone. This simple and robust process employs low-intensity ultrasound, air-stable Cu^II/L catalysts, and carbonate or bicarbonate salts (washing soda or baking soda) to prepare well-defined polyacrylates.

Tuning radical reactivity for selective radical/radical cross-coupling

Radical/radical cross-coupling represents a straightforward way for the construction of new chemical bonds in theory and has received more and more attention over the past several years. Until now, it is challenging to achieve the selective bond formation between two transient radicals since selective radical cross-coupling usually happens between persistent radical and transient radical. However, the number of persistent radicals is much less than that of transient radicals. These issues limit the application of radical/radical cross-coupling in the construction of new chemical bonds. To solve this, we proposed a novel strategy "tuning radical reactivity" that could tune transient radical into persistent radical. This paper will introduce the concept and recent developments of "tuning radical reactivity".

Oxidation-induced C-H amination leads to a new avenue to build C-N bonds

In this work, we develop an oxidation-induced C-H functionalization strategy, which not only leads to a new avenue to build C-N bonds, but also leads to different site-selectivity compared with "classic directing-groups". The high selectivity of our new catalytic system originates from the heterogeneous electron-density distribution of the radical cation species which are induced by single electron transfer between the aromatics and oxidant-Cu(ii) species.

Copper-catalyzed selective radical–radical cross-coupling for C–S bond formation: an access to α-alkylthionitriles

Oxidative radical–radical cross-coupling: a new protocol for C–S bond formation was developed by selective cross-coupling between a thiyl radical and an isobutyronitrile radical.

Synthesis of a heterogeneous Cu(OAc)2-anchored SBA-15 catalyst and its application in the CuAAC reaction

A dinuclear Cu(OAc)₂-anchored SBA-15 silica was synthesized, confirmed and showed excellent catalytic activity for the azide–alkyne cycloaddition reaction in water.

Elemental sulfur as a sulfuration agent in the copper-catalyzed C-H bond thiolation of electron-deficient arenes

By utilizing elemental sulfur as the thiolation agent and oxidant, a copper-catalyzed direct C-H bond thiolation of electron-deficient arenes was demonstrated. Various electron-deficient arenes were proved to be suitable for this transformation. Preliminary mechanistic studies indicated that this reaction underwent a radical pathway, in which the trisulfur radical anion (S₃˙^-) might play a vital role. Meanwhile, KIE experiments suggested that C-H bond cleavage was not involved in the rate-determining step.

Coordination strategy-induced selective C–H amination of 8-aminoquinolines

Using coordination-strategy, we realized a selective C–H amination of 8-aminoquinolins on the C5 position employing azoles as amines sources.

Cu(ii)-Mediated keto C(sp3)–H bond α-acyloxylation of N,N-dialkylamides with aromatic carboxylic acids

The first example of Cu(ii)-mediated oxidative coupling of aromatic carboxylic acids with the C(sp³)–H bond adjacent to the keto group of N,N-dialkylamides has been developed.

Aromatic C–H bond cleavage by using a Cu(i) ate-complex

Ate-complex [Cu(O^tBu)₂]Na was found to be able to cleave C–H bond of benzothiazole (ArH) producing [ArCuI(O^tBu)]Na with a rate constant of 3.2 × 10⁻² mol⁻¹ L s⁻¹ at −50 °C and with an activation enthalpy of 0.73 kcal mol⁻¹ at room temperature.