Carbon Radicals Research Articles

Conjugation-Induced Spin Delocalization in Helical Chiral Carbon Radicals via Through-Bond and Through-Space Effects.

A class of highly stable hydrocarbon radicals with helical chirality are synthesized, which can be isolated and purified by routine column chromatography on silica gel. These carbon-centered radicals are stabilized by through-bond delocalization and intramolecular through-space conjugation, which is evidenced by Density Functional Theory (DFT) calculation. The high stability enables to directly modify the carbon radical via palladium-catalyzed cross-coupling with the radical being untapped. The structures and optoelectronic properties are investigated with a variety of experimental methods, including Electron Paramagnetic Resonance (EPR), Ultraviolet Visisble Near Infrared (UV-vis-NIR) measurements, Cyclic Voltammetry (CV), Thermogravimetry Analysis (TGA), Circular Dichroism (CD) spectra, High-Performance Liquid Chromatography (HPLC), and X-ray crystallographic analysis. DFT calculations indicated that the 9-anthryl helical radical is more stable than its tail-to-tail σ-dimer over 13.2kJmol-1 , which is consistent with experimental observations.

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.

Divergent Photosensitizer Controlled Reactions of 4‐Hydroxycoumarins and Unactivated Olefins: Hydroarylation and Subsequent [2+2] Cycloaddition

AbstractHerein, we report a photoinduced approach for hydroarylation of unactivated olefins using 4‐hydroxycoumarins as the arylating reagent. Key to the success of this reaction is the conversion of nucleophilic 4‐hydroxycoumarins into electrophilic carbon radicals via photocatalytic arene oxidation, which not only circumvents the polarity‐mismatch issue encountered under ionic conditions but also accommodates a broad substrate scope and inhibits side reactions that were previously observed. Moreover, divergent reactivity was achieved by changing the photocatalyst, enabling a subsequent [2+2] cycloaddition to deliver cyclobutane‐fused pentacyclic products that are otherwise challenging to access in high yields and with high diastereoselectivity. Mechanistic studies have elucidated the mechanism of the reactions and the origin of the divergent reactivity.

Divergent Photosensitizer Controlled Reactions of 4-Hydroxycoumarins and Unactivated Olefins: Hydroarylation and Subsequent [2 + 2] Cycloaddition.

Herein, we report a photoinduced approach for hydroarylation of unactivated olefins using 4-hydroxycoumarins as the arylating reagent. Key to the success of this reaction is the conversion of nucleophilic 4-hydroxycoumarins into electrophilic carbon radicals via photocatalytic arene oxidation, which not only circumvents the polarity-mismatch issue encountered under ionic conditions but also accommodates a broad substrate scope and inhibits side reactions that were previously observed. Moreover, divergent reactivity was achieved by changing the photocatalyst, enabling a subsequent [2+2] cycloaddition to deliver cyclobutane-fused pentacyclic products that are otherwise challenging to access in high yields and with high diastereoselectivity. Mechanistic studies have elucidated the mechanism of the reactions and the origin of the divergent reactivity.

Photoinduced Pyridine N‐oxides Catalyzed Carbon Radical Generation from Alkylboronic Acids

AbstractHerein we report a protocol for the generation of alkyl carbon radicals from alkylboronic acids wherein photoexcited 4‐nitropyridine N‐oxide biradical features a catalyst to promote the nucleo‐homolytic substitution of boronic acids. With a wide range of readily available aliphatic boronic acids, including methyl boronic acid, the developed catalytic system demonstrates broad applicability for alkylation, amination, and cyanation.

Cross‐Electrophile C−PIIICoupling of Chlorophosphines with Organic Halides: Photoinduced PIIIand Aminoalkyl Radical Generation Enabled by Pnictogen Bonding

AbstractPnictogen bonding (PnB) has gained recognition as an appealing strategy for constructing novel architectures and unlocking new properties. Within the synthetic community, the development of a straightforward and much simpler protocol for cross‐electrophile C−PIIIcoupling remains an ongoing challenge with organic halides. In this study, we present a simple strategy for photoinduced PnB‐enabled cross‐electrophile C−PIIIcouplings using readily available chlorophosphines and organic halides via merging single electron transfer (SET) and halogen atom transfer (XAT) processes. In this photomediated transformation, the PnB formed between chlorophosphines and alkyl amines facilitates the photogeneration of PIIIradicals andα‐aminoalkyl radicals through SET. Subsequently, the resultingα‐aminoalkyl radicals activate C−X bonds via XAT, leading to the formation of carbon radicals. This methodology offers operational simplicity and compatibility with both aliphatic and aromatic chlorophosphines and organic halides.

Cross-Electrophile C-PIII Coupling of Chlorophosphines with Organic Halides: Photoinduced PIII and Aminoalkyl Radical Generation Enabled by Pnictogen Bonding.

Pnictogen bonding (PnB) has gained recognition as an appealing strategy for constructing novel architectures and unlocking new properties. Within the synthetic community, the development of a straightforward and much simpler protocol for cross-electrophile C-PIII coupling remains an ongoing challenge with organic halides. In this study, we present a simple strategy for photoinduced PnB-enabled cross-electrophile C-PIII couplings using readily available chlorophosphines and organic halides via merging single electron transfer (SET) and halogen atom transfer (XAT) processes. In this photomediated transformation, the PnB formed between chlorophosphines and alkyl amines facilitates the photogeneration of PIII radicals and α-aminoalkyl radicals through SET. Subsequently, the resulting α-aminoalkyl radicals activate C-X bonds via XAT, leading to the formation of carbon radicals. This methodology offers operational simplicity and compatibility with both aliphatic and aromatic chlorophosphines and organic halides.

Photo‐induced Desulfurative Processes for Carbon Radical Generation

AbstractThiols constitute an important family among sulfur‐containing compounds, with well‐established applications in various fields ranging from medicine to material science. For instance, thiol residues are good hydrogen donors which reduce radical species in biological or chemical processes. However, even though the S−H bond activation of thiols for providing access to thiyl radicals has been largely studied, desulfurative processes affording carbon‐based radicals by C−S bond activation have been less explored. In recent years, photoredox catalysis has become the prevalent method for the generation of radicals under soft reaction conditions from readily available starting materials under visible light. In this context, recent studies have been devoted to the development of photocatalytic procedures aiming at the desulfurization of thiol derivatives leading to new C−H, C−C or C‐Het bond formation reactions. This review will cover the synthetic methodologies and strategies for photo‐mediated desulfurization of native thiols, thioethers, sulfonium salts and xanthates to access new organic compounds. This emerging field is especially interesting for new transformations of cysteine and peptide derivatives.

Magnetism in carbon-based fiber materials

Metal-free ferromagnetic materials have gained significant attention as an appealing alternative to traditional inorganic magnets due to their biocompatibility, biodegradability, low production cost, flexibility, solubility in organic solvents, and electrical insulation properties. These features make them highly desirable for biomedical and spintronic applications. The origin of magnetism in these materials is linked to the presence of unpaired electrons, defects, or functional groups that induce local magnetic moments. However, achieving ferromagnetic interaction among persistent carbon radicals within a molecule has been a challenging endeavor. Various strategies have been explored to induce ferromagnetism in carbon-based materials, including defect induction, steric hindrance, and doping with trivalent or pentavalent species like Boron (B) and Nitrogen (N). These approaches have successfully led to the development of organic magnets in different dimensions, ranging from 0D to 3D.Recent advancements in synthesizing organic magnets exhibiting ferromagnetism above room temperature have reignited interest among physicists and chemists in the realm of organic spintronic materials. Furthermore, the integration of magnetic nanoparticles into carbon nanotubes has been a prominent research focus. Understanding the underlying mechanisms of magnetism in carbon-based polymers, such as graphene, carbon nanotubes, and fullerenes, has been the subject of extensive studies over the past few decades. This article presents a critical review of recent research on carbon-based magnetic polymers, including their limitations and applications. The emergence of magnetism in carbon-based 2D materials holds immense potential for spintronics and high-density data storage applications, contributing to the advancement of quantum computing and empowering artificial intelligence.

Shear Stress Triggers Ultrathin-Nanosheet Carbon Nitride Assembly for Photocatalytic H2O2 Production Coupled with Selective Alcohol Oxidation.

Coupled photocatalysis without cocatalysts can maximize the utilization of photons and atoms, which puts forward higher demands on photocatalysts. Polymeric carbon nitride (CN) has become the most promising photocatalyst, but still suffers from major drawbacks of insufficient catalytic sites and low quantum efficiency. Herein, we report a fluid shear stress-assisted molecular assembly to prepare ultrathin-nanosheet-assembled acanthosphere-like CN (ASCN) with nitrogen vacancy (Nv) and carbonyl modification. Shear stress breaks the stacking interactions between layers and cuts the stacked structure into ultrathin layers, which are further reassembled into acanthosphere bundles driven by "centrifugal force". Benefitted greatly from the ultrathin nature that provides more exposed active sites and improves charge carrier separation, ASCN-3 exhibits a 20-fold higher activity than the bulk counterpart toward oxygen reduction to H2O2 coupled with 4-methoxybenzyl alcohol (4-MBA) oxidation to anisaldehyde (AA), with significantly increased turnover frequency (TOF) values (TOF: 1.69 h-1 for H2O2 and 1.02 h-1 for AA). Significantly, ASCN-3 exhibits 95.8% conversion for 4-MBA oxidation with nearly 100% selectivity. High apparent quantum yields of 11.7% and 9.3% at 420 nm are achieved for H2O2 photosynthesis and 4-MBA oxidation. Mechanism studies suggest that carbonyl induces holes concentrated at the neighboring melem unit to directly oxidize the Cα-H bond of 4-MBA to produce carbon radicals, and Nv as oxygen adsorption active site traps electrons to form a superoxide radical that further combines with the shed protons into H2O2. This work presents a simple physical method to break the layered stack of CN for creating hierarchical assembly for coupled photocatalysis.

Study on the microcosmic mechanism of inhibition of coal spontaneous combustion by antioxidant diphenylamine

Diphenylamine has been applied to suppress coal spontaneous combustion with good results, but its microscopic reaction mechanism is not clear. In this paper, the free radical reaction process of coal spontaneous combustion is calculated more systematically, and the quantum chemical method is applied to analyze in detail the process of diphenylamine depletion of reactive free radicals in coal from three perspectives: hydrogen atom transfer, direct radical capture, and regeneration cycle chain. By destroying the molecular structure of coal to generate free radicals as the initial condition of coal spontaneous combustion, the oxidation process of the free radical chain reaction was analyzed, the thermodynamic data of all reactions were calculated, the microscopic mechanism of free radical capture by diphenylamine was revealed, and the transition state structure model of the reaction process was given. The active sites of each reactant were determined by quantitative molecular surface analysis, the spontaneity of the reaction was measured by the free energy change, and the interaction relationship between different conformations was graphically represented by applying the IRI (interaction region indicator) method. The results show that most of the free radical reactions during the spontaneous combustion of coal are exothermic, and they provide heat for the spontaneous combustion of coal, which can react directly with oxygen when accumulated to a certain temperature. Diphenylamine can break the N-H bond, allowing hydroxyl radicals and carbon radicals to obtain hydrogen atoms to form stable compounds, and the generated diphenylamine radicals can reduce the content of free radicals in coal by direct capture. In addition, the process of diphenylamine inhibition of coal spontaneous combustion has a circular regeneration chain, which can indeed effectively reduce the content of reactive radicals and inhibit the spontaneous combustion of coal, which is reflected in the macroscopic level of reducing the generation of coal spontaneous combustion indicator gas, which is consistent with the experimental results.

Site‐Selective Direct Intermolecular C(sp3)−H Alkylation of Saccharides and Switching of Reaction Sites by Changing Photocatalysts

AbstractWe developed a site‐selective intermolecular C(sp3)−H alkylation of saccharides with electron‐deficient alkenes using photocatalysis by anthraquinone and tetrabutylammonium decatungstate (TBADT). The main reaction site of anthraquinone‐catalyzed C(sp3)−H alkylation is determined by the weakness of the C−H bond. The reaction site can be switched by changing the photocatalyst to TBADT, and the site‐selectivity is controlled by the steric hindrance between the bulkier TBADT photocatalyst and the substituents of the saccharides. The reaction is compatible with several electron‐deficient alkenes and saccharides, and provides C‐saccharides in good yields. Furthermore, the monoalkylated product was obtained in excellent yield, even on the gram scale, under TBADT photocatalysis. Applicability was demonstrated by site‐selective C(sp3)−H alkylation of glycosyl derivatives. The reactions proceed via a site‐selective hydrogen atom transfer between the photocatalyst and saccharide, and the generated carbon radical reacts with an electron‐deficient alkene to give alkylated products.

Regio- and Enantioselective Catalytic δ-C-H Amidation of Dioxazolones Enabled by Open-Shell Copper-Nitrenoid Transfer.

Controlling regio- and enantioselectivity in C-H functionalization reactions is of paramount importance due to their versatile synthetic utilities. Herein, we describe a new approach for the asymmetric δ-C(sp3)-H amidation catalysis of dioxazolones using a Cu(I) precursor with a chiral bisoxazoline ligand to access six-membered lactams with high to excellent regio- and enantioselectivity (up to >19:1 rr and >99:1 er). Combined experimental and computational mechanistic studies unveiled that the open-shell character of the postulated Cu-nitrenoids enables the regioselective hydrogen atom abstraction and subsequent enantio-determining radical rebound of the resulting carbon radical intermediates. The synthetic utility of this asymmetric cyclization was demonstrated in the diastereoselective introduction of additional functional groups into the chiral δ-lactam skeleton as well as in the rapid access to biorelevant azacyclic compounds.

In situ selective deuterated (hydrogenated) aryl alkynes to alkenes on Pd/Ni(OH)2-nickel foam

The preparation of deuterated alkenes via electrochemistry from a readily available deuterated source has the advantages of mild reaction conditions, sustainability, and cost-effectiveness. Hydrogen isotopic exchange is a simple method to introduce deuterium atoms into molecules without prior functionalization. However, this process typically requires complex ligand catalysts, costly sources of deuterium, and strong bases and acids to activate the robust C-H bonds. The deuterated incorporation sites and their corresponding numbers are difficult to control. In this study, an in-situ electrochemical deuteration system is reported that is based on a Pd/Ni(OH)2 nanoflake catalyst grown on a commercial nickel foam conductive substrate was used as the cathodic working electrode. A room-temperature, one-pot, two-step transformation of 4-ethynylaniline to deuterated 4-vinylanilin was achieved with a conversion rate of up to 96%, selectivity of 98%, and a deuterium ratio of 99% using D2O as the deuterium source. The interaction between Pd and Ni(OH)2-NF promoted the rapid cleavage of D2O during the semi-deuteration of aryl alkynes. The key deuterium and carbon radical intermediates were confirmed using electron paramagnetic resonance spectroscopy. A wide range of substrates and easy access to hydrogenated alkenes highlight the promising applications.

Ketyl Radical Anion Mediated Radical Polymerization and Anionic Ring-Opening Polymerization to Give Polymers with Low Molecular Weight Distribution.

The development of novel polymerization capable of yielding polymers with low molecular weight distribution (Đ) is essential and significant in polymer chemistry, where monofunctional initiator contains only one initiation site in these polymerizations generally. Here, ketyl radical anion species is introduced to develop a novel Ketyl Mediated Polymerization (KMP), which enables radical polymerization at carbon radical site and anionic ring-opening polymerization at oxygen anion site, respectively. Meanwhile, polymerization and corresponding organic synthesis generally couldn't be performed simultaneously in one pot. Through KMP, organic synthesis and polymerization are achieved in one pot, where small molecules (cyclopentane derivates) and polymers with low Đ are successfully prepared under mild condition simultaneously. At the initiation step, both organic synthesis and polymerization are initiated by single electron transfer reaction with ketyl radical anion formation. Cyclopentane derivates are synthesized through 3-3 coupling reaction and cyclization. Polystyrene and polycaprolactone with low Đ and a full monomer conversion are prepared by KMP via radical polymerization and anionic ring-opening polymerization, respectively. This work therefore enables both organic synthesis and two different polymerizations from same initiation system, which saves time, labour, resource and energy and expands the reaction mode and method libraries of organic chemistry and polymer chemistry.

Ketyl Radical Anion Mediated Radical Polymerization and Anionic Ring‐Opening Polymerization to Give Polymers with Low Molecular Weight Distribution

AbstractThe development of novel polymerization capable of yielding polymers with low molecular weight distribution (Đ) is essential and significant in polymer chemistry, where monofunctional initiator contains only one initiation site in these polymerizations generally. Here, ketyl radical anion species is introduced to develop a novel Ketyl Mediated Polymerization (KMP), which enables radical polymerization at carbon radical site and anionic ring‐opening polymerization at oxygen anion site, respectively. Meanwhile, polymerization and corresponding organic synthesis generally couldn't be performed simultaneously in one pot. Through KMP, organic synthesis and polymerization are achieved in one pot, where small molecules (cyclopentane derivates) and polymers with low Đ are successfully prepared under mild condition simultaneously. At the initiation step, both organic synthesis and polymerization are initiated by single electron transfer reaction with ketyl radical anion formation. Cyclopentane derivates are synthesized through 3–3 coupling reaction and cyclization. Polystyrene and polycaprolactone with low Đ and a full monomer conversion are prepared by KMP via radical polymerization and anionic ring‐opening polymerization, respectively. This work therefore enables both organic synthesis and two different polymerizations from same initiation system, which saves time, labour, resource and energy and expands the reaction mode and method libraries of organic chemistry and polymer chemistry.

Fe-Catalyzed Regioselective C(sp3)–H-Abstraction by Tertiary Cyclopropyl Radicals

Tertiary carbon radicals are usually more stable than secondary carbon radicals, and the hydrogen atom transfer (HAT) from secondary C(sp3)–H bonds to tertiary carbon radicals is normally considered to be a thermodynamically disfavored pathway. Herein, we disclose an unusual protocol involving a C(sp3)–H functionalization enabled by a tertiary cyclopropyl radical and proceeding through hydrogen abstraction from a variety of secondary and tertiary aliphatic C(sp3)–H bonds. The reaction can furnish valuable, densely functionalized cyclopropane derivatives. Sufficient mechanistic experiments and density functional theory (DFT) calculations have provided theoretical insights into the chemo- and regioselectivity in this underexplored HAT protocol. The method features broad functional group compatibility and high product diversity and offers a new prospect for radical C(sp3)–H functionalization.

A high-performance self-healing polymer binder for Si anodes based on dynamic carbon radicals in cross-linked poly(acrylic acid)

Silicon (Si), which exhibits high theoretical capacity, is a promising material for high energy density anode material in Lithium Ion Batteries (LIBs) because it is abundant in nature, inexpensive, and eco-friendly. However, Si has excessive volume expansion with charge and discharge cycles, resulting in reduced capacity reversibility and collapsing electrode structures. To overcome this limitation, polymer binders equipped with various functions have been used to improve the electrochemical performance of Si. The present study developed a self-healing cross-linked poly(acrylic acid) (PAA) binder for Si anodes functionalized with diarylbibenzofuranone (DABBF), which exhibits self-healing from reversible C-C bonding and the debonding of dynamic carbon radicals. When applied to Si anodes, the PAA-based binder functionalized with DABBF formed a three-dimensional network, stabilizing the Si structure and achieving improved adhesion and mechanical properties. Its self-healing performance enabled the Si anodes to exhibit stable capacity retention. In addition, the adhesion performance and mechanical properties of Si anodes were higher when DABBF-functionalized PAA was used as a binder. At a current density of 0.5 C, a high initial coulombic efficiency (ICE) of 77.89% was achieved, and after 500 cycles, the capacity remained high, 1774.45 mAh g−1. A novel cross-linked polymer binder with self-healing ability was successfully developed for the first time using dynamic carbon radicals, and its excellent electrochemical performance significantly improved the stability of Si anodes.

Solid-state mechanochemistry advancing two dimensional materials for lithium-ion storage applications: A mini review

The vigorous development of two-dimensional (2D) materials brings about numerous opportunities for lithium-ion batteries (LIBs) due to their unique 2D layered structure, large specific surface area, outstanding mechanical and flexibility properties, etc. Modern technologies for production of 2D materials include but are not limited to mechanochemical (solid-state/liquid-phase) exfoliation, the solvothermal method and chemical vapor deposition. In this review, strategies leading to the production of 2D materials via solid-state mechanochemistry featuring traditional high energy ball-milling and Sichuan University patented pan-milling are highlighted. The mechanism involving exfoliation, edge selective carbon radical generation of the 2D materials is delineated and this is followed by detailed discussion on representative mechanochemical techniques for tailored and improved lithium-ion storage performance. In the light of the advantages of the solid-state mechanochemical method, there is great promise for the commercialization of 2D materials for the next-generation high performance LIBs.

The Glutathione System: A Journey from Cyanobacteria to Higher Eukaryotes.

From bacteria to plants and humans, the glutathione system plays a pleiotropic role in cell defense against metabolic, oxidative and metal stresses. Glutathione (GSH), the γ-L-glutamyl-L-cysteinyl-glycine nucleophile tri-peptide, is the central player of this system that acts in redox homeostasis, detoxification and iron metabolism in most living organisms. GSH directly scavenges diverse reactive oxygen species (ROS), such as singlet oxygen, superoxide anion, hydrogen peroxide, hydroxyl radical, nitric oxide and carbon radicals. It also serves as a cofactor for various enzymes, such as glutaredoxins (Grxs), glutathione peroxidases (Gpxs), glutathione reductase (GR) and glutathione-S-transferases (GSTs), which play crucial roles in cell detoxication. This review summarizes what is known concerning the GSH-system (GSH, GSH-derived metabolites and GSH-dependent enzymes) in selected model organisms (Escherichia coli, Saccharomyces cerevisiae, Arabidopsis thaliana and human), emphasizing cyanobacteria for the following reasons. Cyanobacteria are environmentally crucial and biotechnologically important organisms that are regarded as having evolved photosynthesis and the GSH system to protect themselves against the ROS produced by their active photoautotrophic metabolism. Furthermore, cyanobacteria synthesize the GSH-derived metabolites, ergothioneine and phytochelatin, that play crucial roles in cell detoxication in humans and plants, respectively. Cyanobacteria also synthesize the thiol-less GSH homologs ophthalmate and norophthalmate that serve as biomarkers of various diseases in humans. Hence, cyanobacteria are well-suited to thoroughly analyze the role/specificity/redundancy of the players of the GSH-system using a genetic approach (deletion/overproduction) that is hardly feasible with other model organisms (E. coli and S. cerevisiae do not synthesize ergothioneine, while plants and humans acquire it from their soil and their diet, respectively).