Free Radical Coupling Research Articles

Synthesis and structural characterization of a novel large type double-layered cyclophanes based on the reaction of bis(N-alkylene benzothiazolium) dibromide and triethylamine

The synthesis of a novel large type double-layered cyclophanes has been successfully accomplished by free radical coupling reaction with the bis(N-alkylene benzothiazolium) dibromide salts as their important synthetic intermediates. The structures of the two-layered cyclophanes and the synthetic intermediates have been elucidated based on the NMR data and X-ray structural analysis, respectively. The two-layered cyclophanes consist of two different geometries, anti-two cyclic lactam amide rings inside and two bridges of disulfide bonds outside, which are unique and novel structures. Their physical properties were investigated by UV–Vis and redox potential, too.

Degradation of Bisphenol A by manganese minerals: The impact of inorganic and organic ligands

Manganese minerals are ubiquitous in aquatic environments, where both inorganic and organic components can form complexes with them, thereby impacting the release of reactive Mn(III) and the fate of pollutants. The available information about these processes remains insufficient, however, impeding the study of the migration and transformation of pollutants in the environment. In this study, pyrophosphate and humic acid were selected as inorganic and organic ligands typical of aquatic environments, and their impacts on the degradation of Bisphenol A by the β- and δ- variants of MnO2 were investigated. The results indicate that 1.0–5.0 mM pyrophosphate promotes the generation of reactive Mn(III) from β-MnO2, resulting in enhanced degradation of Bisphenol A. Pyrophosphate inhibits δ-MnO2-mediated degradation of Bisphenol A due to competition for reactive sites. Humic acids increase the degradation of Bisphenol A mediated by δ-MnO2 by forming complexes with reactive Mn(III) released from the δ-MnO2. The degradation of Bisphenol A mediated by minerals in the presence of ligands occurs primarily through hydroxylation, one-electron transfer, and free radical coupling. These findings provide new insights into the role of ligands in the reductive dissolution of manganese minerals and their synergistic effect on the fate of Bisphenol A in aqueous environments.

Tandem Electrochemical Oxidation of Polycyclic Aromatic Amines Towards Carbazoles and Phenazine-based Aza-helicenes.

Experimental and theoretical study of the regioselectivity and mechanism of polycyclic aromatic amine (PAA) electrochemical oxidation is important for designing nitrogen doped large π-conjugated functional molecules. Herein, we used binary-, ternary-, and quaternary-fused PAAs as electro-oxidative reaction substrates to investigate the yield changes of carbazole and phenazine based aza-helicene other than oligomers, which were obtained through pyrrole and pyrazine annulation pathways. Combined with the restrained electrostatic potential (RESP) and steric hindrance factor analysis of the substrate, the electron spin density distribution of free radical resonance hybrid and the spin population analysis of the atoms in the structure of each free radical tautomer indicate that the degree of delocalized dispersion of N free radical and the resulting change in the spin density distribution of C free radical tautomers determine the reaction regioselectivity. The potential charge of the K-region, Bay-region, and L-region adjacent to the C(α)-C(β1) bond is higher than that of other regions within the molecule, and the charge in these high RESP regions tends to delocalize more strongly toward electron-deficient N free radicals. Thus, the activity of N-C(α)-C(β1) region is increased, which supports the proposed free radical addition and free radical coupling mechanism for the electro-oxidative reaction of PAA.

Facile recovery of lithium as Li2CO3 or Li2O from α-hydroxy-carboxylic acid chelates through pyrolysis and the decomposition mechanism

Chelation of lithium with readily available α-hydroxy carboxylic acids is a sustainable recovery method from spent Li-ion battery wastes as well as a possible lithium extraction method. In this work pyrolytic decomposition of a series of selected lithium chelates of glycolic, lactic, malic and tartaric acid were studied and compared with lithium acetate as a potential lithium recovery method from chelates. In thermogravimetric analysis, the four α-hydroxy carboxylic acid chelates studied showed lower initial decomposition temperatures in the 313–356 °C range, in comparison to the lithium acetate with an initial decomposition temperature of 444 °C. The ease of decomposition of α-hydroxy carboxylic acid chelates may be due to their favorable decomposition pathways through dimerization and tetramerization of α-hydroxy acids via esterification and free radical coupling mechanisms leading to a complex mixtures of products; including alkenes, dienes and enones. The lithium α-hydroxy carboxylic acid chelate pyrolysis residue formed above 300 °C was identified as lithium carbonate and further heating above 700 °C, lithium carbonate decomposes to lithium oxide, allowing the recovery of lithium as Li2CO3 or Li2O.

Aryliodonium Salt-Induced Regioselective Access to meta-Substituted Anilines by Arylation of Azoles.

A highly efficient aryliodonium salt-induced regioselective access to meta-substituted anilines by arylation of azoles has been developed under catalyst-free conditions. This efficient transformation provides a facile and scalable approach to a wide range of biologically active N-arylazoles with moderate to high yields. According to the control experiments, two plausible pathways, including a Michael pathway and a free radical coupling pathway, for the reaction were proposed.

Mn(II) Promoted Divergent-Convergent Domino Reaction Giving Dinuclear Tetrasubstituted Pyrrole Complex.

Domino reaction of benzo[d]thiazole-2-methylamine (S1) has been developed in the presence of MnCl2 ⋅ 4H2O, leading to tetrasubstituted pyrrole coordinated dinuclear Mn(II) complex 1 ([MnClP]2, P-=2,3,4,5-tetrakis(benzo[d]thiazol-2-yl)pyrrol-1-ide). The reaction process has been studied by assigning a series of intermediates based on time-dependent mass spectrometry, control experiments, crystallography, and density functional theory (DFT) theoretical calculation. A plausible mechanism involving an unprecedented divergent-convergent domino sequence has been proposed. Compound S1 could be activated by MnCl2 ⋅ 4H2O via coordination, which divergently produces two intermediates imine II (1-(benzo[d]thiazol-2-yl)-N-(benzo[d]thiazol-2-ylmethyl)methanimine) and alkene C (1,2-bis(benzo[d]thiazol-2-yl)ethene) through oxidative self-condensation and free radical coupling followed by elimination, respectively. They could then react with each other convergently via formal [3+2] cycloaddition to give deprotonated tetrasubstituted pyrrole coordinated intermediate [MnClP] after aromatization. Dimerization of [MnClP] produces the final product 1. Three C-C bonds and one C-N bond are formed through this six-step domino sequence. The corresponding organic skeleton (HP: 2,2',2'',2'''-(1H-pyrrole-2,3,4,5-tetrayl)tetrakis(benzo[d]thiazole)) has been obtained from 1 and shows a higher fluorescent quantum yield (52 %) than the reported 3,4-diphenyl substituted analogue 2,2'-(3,4-diphenyl-1H-pyrrole-2,5-diyl)bis(benzo[d]thiazole) (DPB) (42 %).

Photoredox Synthesis of Silyl Ethers via Radical Coupling of Ketones with Hydrosilanes and [1, 2]‐Brook Rearrangement

AbstractAn expedient synthesis of silyl ethers is reported via photocatalytic free radical coupling of ketones with hydrosilanes and subsequent [1, 2]‐Brook rearrangement. This strategy features the advantages of 100 % atom economy, environmental friendliness, easily available starting materials as well as late‐stage functionalization of complex molecules.

A coupling strategy for excellent mechanical properties and enhanced interfacial performance of polyimide/carbon fiber composite with metal-free catalyzed-curing route

The structure and properties of carbon fiber reinforced composites (CFRC) depend on main factors, especially the unpredictable interface. In this work, a kind of thiocarbonyl disulfide compound is introduced to achieve the catalyzed-cure of phenylethynyl terminated polyimide (PI) and plays the role of coupling medium to constitute covalent-bonded structure at the PI/CF interface. The organocatalyzed cure not only follows the free radical mechanism with an initial curing temperature down to 260.4 °C, but also endows the composite with reduced internal thermal stress and enhanced interface structure. Due to both CF surface-initiated free radical curing reaction and coupling reaction between thiocarbonyl-containing PI matrix and hydroxyl-rich sizing layer, the composite performs an improved interfacial shear strength (IFSS) of 123.9 MPa, being 2.47 times as the thermally cured ones, plus the anti-bacterial reinforcement of the PI matrix. Besides, the interface enhancement brings the CFRC specimen improved electrical properties, performing a 92.6% increase of transverse electrical conductivity to 367.6 S/m. This study gives perspective for improving the interfacial performance of high-temperature processed CFRC with “as-received” fiber reinforcers, plus a clear outlook to future research directions and open challenges to prepare high-performance CFRC.

Renewable ultrathin carbon nitride nanosheets and its practical utilization for photocatalytic decarboxylation free radical coupling reaction

It is attractive to develop a durable and renewable carbon nitride-based photocatalytic system for compelling photocatalytic organic synthesis, but remains a huge challenge. In this study, cost-effective ultrathin carbon nitride nanosheets (denoted as CNS) with abundant active sites and visible light harvesting capacity were designed and prepared, with the aim of realizing the decarboxylation free radical coupling reaction between N-arylglycine and aldehydes. The as-prepared catalyst enabled good to high yields (up to 97%) of 1,2-amino alcohols with a wide substrate range and desired functional group tolerance. Furthermore, CNS provided a long-term durability, and could be straightforwardly recovered through simple centrifugation of reaction solutions. In addition, this practical strategy is also applicable to gram-scale photosynthesis as well as the synthesis of some other complex molecules, providing an alternative to homogeneous organic photocatalysts for the synthesis of valuable pharmaceutical intermediates for future applications.

Alcohol imination catalyzed by carbon nanostructures synthesized by C(sp2)-C(sp3) free radical coupling

Imines are important intermediates for synthesizing various fine chemicals, with the disadvantage of requiring the use of expensive metal-containing catalysts. We report that the dehydrogenative cross-coupling of phenylmethanol and benzylamine (or aniline) directly forms the corresponding imine with a yield of up to 98%, and water as the sole by-product, in the presence of a stoichiometric base, using carbon nanostructures as the "green" metal-free carbon catalysts with high spin concentrations, which is synthesized by C(sp2)-C(sp3) free radical coupling reactions. The catalytic mechanism is attributed to the unpaired electrons of carbon catalysts to reduce O2 to O2·-, which triggers the oxidative coupling reaction to form imines, whereas the holes in the carbon catalysts receive electrons from the amine to restore the spin states. This is supported by density functional theory calculations. This work will open up an avenue for synthesizing carbon catalysts and offer great potential for industrial applications.

Visible-Light-Driven Iodine-Catalyzed Synthesis of Unsymmetrical Disulfides via Oxidative Coupling

AbstractHerein, a facile and environmentally friendly oxidative coupling reaction to obtain unsymmetrical disulfide compounds, without using metal or organic dyes as photocatalyst and only using a catalytic amount of iodine, was developed. Oxygen is used as oxidant in the reaction and the substrate scope is wide. A gram-scale synthesis also highlights the synthetic practicality of this photochemical strategy. Moreover, our mechanistic studies support the formation of disulfide products through free-radical coupling.

Visible-Light-Induced Acylative Coupling of Benzoic Acid Derivatives with Alkenes to Dihydrochalcones.

A strategy was developed for the visible-light-induced photocatalytic synthesis of dihydrochalcone via the deoxygenation and coupling of benzoic acid derivatives with alkenes using diphenyl sulfide as the O-transfer reagent. Under mild photoredox conditions, a series of dihydrochalcone derivatives were produced in moderate to good yields. A mechanism for the visible-light-induced free-radical coupling was proposed on the basis of the control experiments. The protocol provides a new strategy the generation of acyl radicals from carboxylic acids and the synthesis of dihydrochalcones.

Electrocatalytic Activity of Polyaniline in Magnesium-Sulfur Batteries.

Rechargeable magnesium-sulfur (Mg-S) batteries offer the potential for inexpensive energy storage alternatives to other metal-ion batteries for the grid scale and household applications. Despite all economic and resource advantages, Mg-S battery chemistry suffers from a complicated reaction mechanism and extremely slow reaction kinetics. To improve the kinetics, we improvise a new electrode architecture where a conductive polymer is used along with a carbon network. This report will bring an important insight of electrocatalytic activity of polyaniline, on the basis of free-radical coupling and is a completely new concept in Mg-S battery chemistry. By the combined electron spin resonance spectroscopy, X-ray photoelectron spectroscopy, and fluorescence lifetime measurements, we perceived that the polyaniline anchors the S3•- species from the electrolyte/catholyte through a free-radical-coupling process and thus promotes the reduction to end-discharged products, via a chemical adduct. The concept of free-radical catalysis in Mg/S batteries will open a new knowledge to enhance the active material utilization in the Mg-S batteries.

Catalytic Ozonation Mechanism of Norfloxacin Wastewater Using Cu-Cufe2o4: Oxygen Vacancies Initiate Free Radical and Non-Free Radical Coupling Reactions

Catalytic Ozonation Mechanism of Norfloxacin Wastewater Using Cu-Cufe2o4: Oxygen Vacancies Initiate Free Radical and Non-Free Radical Coupling Reactions

Tris(4-bromophenyl)aminium Hexachloroantimonate-Mediated Intermolecular C(sp2)-C(sp3) Free Radical Coupling of Vindoline with β-Ketoesters and Related Compounds.

A powerful tris(4-bromophenyl)aminium hexachloroantimonate (BAHA) mediated regioselective intermolecular coupling reaction of vindoline with a wide range of substrates that include β-ketoesters, β-diketones, β-ketoaldehydes, β-ketonitriles, β-ketolactones, β-ketolactams, β-cyanoesters, and malononitriles is detailed. The BAHA-promoted intermolecular sp3/sp2 coupling, representing a special class of selective C-H functionalization reactions with direct carbon-carbon bond formation, proceeds with generation of a quaternary center bound to the aryl C15 center of vindoline capable of accommodating of the vinblastine C16' methyl ester and functionalized for subsequent divergent heterocycle introduction. A comprehensive examination of the reaction scope, optimization of subtle reaction parameters, and key insights into the reaction mechanism are described. Contrary to what might be prevailing expectations, studies suggest the plausible mechanism entails initial single-electron oxidation of the substrate enolate, not vindoline, and subsequent regiospecific addition of the resulting electrophilic radical to vindoline. As such and beyond the new arylation reaction with vindoline, the studies define a host of new, previously unrecognized, applications of BAHA and related triarylaminium radical cations that arises from their ability to generate stabilized electrophilic radicals from β-ketoesters and related substrates under nonreducing and metal-free conditions. Those exemplified herein include mediating stabilized enolate free radical arylation, dimerization, allylation, alkene addition, and α-oxidation reactions.

Sub-zero and room-temperature sodium–sulfur battery cell operations: A rational current collector, catalyst and sulphur-host design and study

Minimizing polysulfide-shuttling while using a high-sulfur loaded cathode is vital in the effort to realize practical room-temperature sodium-sulfur (RT Na–S) batteries. Because of their inexpensive, highly abundant materials, these batteries have great potential as future, low-cost, stationary energy storage devices. The sluggish kinetics associated with the electrochemical conversion of electrolyte-soluble Na-polysulfides into insoluble final products causes a rapid accumulation of the intermediates in the electrolyte that escalates the polysulfide-shuttling. We report indium tin oxide nanoparticles decorated onto activated carbon cloth (ITO@ACC) as an electrocatalytic substrate that immobilizes the higher-order Na-polysulfides and also promotes their conversion into the insoluble end-discharge products and thus suppresses the accumulation of intermediate products in the electrolyte. The conversion into the insoluble end-discharge product is confirmed by electron paramagnetic resonance (EPR) spectroscopy to be via a free-radical coupling mechanism. At 20 °C, the ITO@ACC-catalyzed sodium-sulfur batteries (6.8 mg(S) cm–2) gave an early cycle capacity of 684 mA h g(S)–1 and after 1000 cycles the capacity was maintained at 445 mA h g(S)–1 at a 0.5C rate. Moreover, a subzero-temperature sodium-sulfur battery is also realized for the first time; operating at –10 °C, the initial specific capacity of the battery operating at 0.1C at this temperature was 342 mA h g(S)–1 and after 100 cycles, the capacity was maintained at 310 mA h g(S)–1.

One-Pot Free Radical Polymerization/Hydroxyl-Isocyanate Reaction: A Facile Strategy to Synthesize Hyperbranched Glycopoly(MaM/IM) with Tunable Structures

A facile strategy for the free radical polymerization and coupling of −OH groups and −NCO groups in one pot was carried out to synthesize hyperbranched poly(6-O-methacryloyl mannose/2-isocyanatoeth...

Degradation kinetics and mechanism of bis(2-chloroethyl) ether by electromagnetic induction electrodeless lamp activated persulfate

Bis(2-chloroethyl) ether (BCEE) has become one of the most frequently detected chlorinated ether contaminants in the US and Europe, and is classified as a B2 carcinogen. In this study, the degradation rate of BCEE by mercury lamps, xenon lamp and electromagnetic induction electrodeless lamp (EIEL) activated persulfate were compared, and EIEL activated persulfate was confirmed to have higher degradation capability and lower energy consumption. In this sense, the degradation kinetics and mechanism in EIEL system were further investigated. The degradation reaction followed pseudo first-order, and the removal rate of BCEE exceeded 95% in 60 min when the initial pH, the concentration of BCEE and Na2S2O8 were 3, 4 mg L−1 and 15 mM, respectively. Presence of inorganic anions and humic acids would reduce the degradation rate constant. In accordance with the results of electron paramagnetic resonance and quenching experiments, SO4−· was dominant in the acidic regime and OH· was dominant in the alkaline regime. Meanwhile, OH· had higher degradation rate with BCEE when initial pH was 7. Seven degradation products were identified and the reaction pathways included OH· substitution and free radical coupling. Although the total organic carbon was eliminated slowly during the degradation of BCEE, the predicted toxicity of most degradation products to Fathead minnow, Daphnia magna and oral rat were lower than BCEE.

Surface Engineering of Organic Polymers by Photo‐induced Free Radical Coupling with p‐Dimethylaminophenyl Group as A Synthesis Block

AbstractThis contribution presents a versatile strategy to transfer unactivated surface C–H bonds of polymeric substrates to C–R–F groups (R stands for a spacer and F stands for functional group) by UV light‐assisted surface free radical coupling. With p‐dimethylaminophenyl radical as a building block, various reactive groups, such as –CHO, –SH, –B(O)OH, –CN and –SO3−, are successfully grafted onto typical commercial polymer substrates including biaxially oriented polypropylene, polyethylene phthalate, ethylene tetrafluoroethylene copolymer and dimethyl silicone rubber. Fluorencence microscope images, X‐ray photoelectron spectroscopy, UV‐vis and fluorescence spectra support that the above functional groups are covalently bonded to the polymer substrates. The concentration of the reactive groups immobilized on the different substrates is in the order of 10−8 to 10−7 mmol mm−2 determined by UV‐vis spectrometry. In addition, as a proof‐of‐concept, the introduced functional groups are demonstrated by their special reactions, for example, –CHO reacting with –NH2 to synthesize Schiff base, –SH to detect silver ions, and the complexes of –C=N and Zn2+ to degrade Rhodamine B. The results indicate that one can fabricate an integrated molecular library on the surface of polymeric substrates at the molecular level by selecting the building blocks.

Graft modification of lignin-based cellulose via enzyme-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization and free-radical coupling

In this study, a green approach combining enzyme-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization and free-radical coupling was developed for the modification of jute fiber, which is a typical lignin-based cellulose. Jute fiber surface was covered by rich amount of lignin, which offered great opportunities for further functional modification. The controlled polymerization of vinyl monomers, acrylamide (AM) or butyl acrylate (BA), was carried out by horseradish peroxidase (HRP)-initiated RAFT to form well-defined polymers with well-controlled molecular weights and structures. Enzymatic grafting by HRP occurred between the free radicals of well-defined polymers and free radicals of lignin on jute. Gel permeation chromatography (GPC) analysis indicated the alkyl chain length of polymers prepared via HRP-initiated RAFT polymerization was well-controlled. Other results of flourier transformed infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) revealed that well-controlled alkyl chains prepared via enzymatic catalysis were grafted on the exposed lignin of jute. The study explores a new and eco-friendly modification method for lignin-based materials with the controlled graft chain structure via two different catalysis with HRP.