Free Radical Transformations Research Articles

Recent Progress in Free Radical Transformations of Allenamides.

Allenamides are special allenes, and the unique reactivity, selectivity (both stereoselective and regionally selective) and stability of allenamides have been widely studied. In this review, the development of the free radical transformation of allenamides over the last few years will be summarized. This review discusses in detail in three parts: intermolecular radical addition to C- X (X = N, S, O, Se) bonds, metal salt mediated cyclization of allenamides, and photocatalytic cyclization of allenamides. In addition, reasonable details of the mechanisms are provided for the vast majority of these transformations.

Electron Paramagnetic Resonance Analysis of Hydroquinone Polymerization Catalyzed by Small Laccase

AbstractLaccases are multi‐copper oxidases (MCOs) that oxidize a broad range of substrates while reducing molecular oxygen to water. Although much effort has been made to elucidate the catalytic mechanism of laccases, information about reactive catalytic intermediates during catalysis is not yet fully understood. Herein, hydroquinone (HQ) polymerization catalyzed by prokaryotic small laccase (SLAC) was investigated by electron paramagnetic resonance (EPR) spectroscopy to provide more information on the catalytic mechanism. Briefly, free radical intermediates during the catalysis were investigated by real‐time in situ EPR measurement to monitor the formation and transformation of free radicals. A paramagnetic species was identified which was attributed to a copolymer formed by hydroquinone, benzoquinone, and semiquinone radical. In addition, the low‐temperature EPR spectrum of the trinuclear copper center during catalysis not only revealed tentative rotational motion of the histidine imidazole rings coordinating the TNC upon electron uptake but also provided evidence of the formation of oxygen bridge at TNC during the reaction, represented by the observation of a new type 2 copper component featured larger g// values and smaller A// values. Together, EPR spectroscopic insights into the catalytic intermediates during enzymatic hydroquinone polymerization have extended the knowledge of the biophysical characteristics and catalytic mechanism of SLAC.

Microwave-assisted PMS activation Ti3C2/LaFe0.5Cu0.5O3-P catalysts for efficient degradation of CIP: Enhancement of catalytic performance by phosphoric acid etching

The LaFe0.5Cu0.5O3 perovskite synthesised by MOFs template method was compounded with Mxene Ti3C2, and the novel catalyst M-Ti3C2/LFCO-P was subsequently prepared by phosphoric acid etching. This catalyst effectively induced microwave activation of peroxymonosulfate (PMS), and ciprofloxacin (CIP) was efficiently degraded with a degradation rate reaching 95.48 % within 14 min. Characterization results revealed that the MOFs template method and phosphoric acid etching increased specific surface area (from 25.91 m2/g to 142.41 m2/g) of catalyst and improved its pore size distribution. Phosphoric acid etching successfully introduced phosphorus functional groups and promoted the formation of more oxygen vacancies, thereby significantly enhancing the catalytic activity of the catalyst. The conversion of free radicals and the transformation of free radicals to non-free radicals were demonstrated. Additionally, the catalyst exhibited higher electron transfer efficiency. The catalytic reaction was primarily driven by O2– and 1O2, with OH, SO4−, and e- playing auxiliary roles. Furthermore, the degradation pathway and toxicity of CIP were also evaluated. This work provides a new perspective for improving the catalytic degradation activity of antibiotics by perovskite catalysts.

Stereoselective amino acid synthesis by photobiocatalytic oxidative coupling.

Photobiocatalysis-where light is used to expand the reactivity of an enzyme-has recently emerged as a powerful strategy to develop chemistries that are new to nature. These systems have shown potential in asymmetric radical reactions that have long eluded small-molecule catalysts1. So far, unnatural photobiocatalytic reactions are limited to overall reductive and redox-neutral processes2-9. Here we report photobiocatalytic asymmetric sp3-sp3 oxidative cross-coupling between organoboron reagents and amino acids. This reaction requires the cooperative use of engineered pyridoxal biocatalysts, photoredox catalysts and an oxidizing agent. We repurpose a family of pyridoxal-5'-phosphate-dependent enzymes, threonine aldolases10-12, for the α-C-H functionalization of glycine and α-branched amino acid substrates by a radical mechanism, giving rise to a range of α-tri- and tetrasubstituted non-canonical amino acids 13-15 possessing up to two contiguous stereocentres. Directed evolution of pyridoxal radical enzymes allowed primary and secondary radical precursors, including benzyl, allyl and alkylboron reagents, to be coupled in an enantio- and diastereocontrolled fashion. Cooperative photoredox-pyridoxal biocatalysis provides a platform for sp3-sp3 oxidative coupling16, permitting the stereoselective, intermolecular free-radical transformations that are unknown to chemistry or biology.

Constructing Nano-Heterostructure with Dual-Site to Boost H2O2 Activation and Regulate the Transformation of Free Radicals.

A major issue with Fenton-like reaction is the excessive consumption of H2O2 caused by the sluggish regeneration rate of low-valent metal, and how to improve the activation efficiency of H2O2 has become a key in current research. Herein, a nano-heterostructure catalyst (1.0-MnCu/C) based on nano-interface engineering is constructed by supporting Cu and MnO on carbon skeleton, and its kinetic rate for the degradation of tetracycline hydrochloride is 0.0436 min-1, which is 2.9 times higher than that of Cu/C system (0.0151 min-1). The enhancement of removal rate results from the introduced Mn species can aggregate and transfer electrons to Cu sites through the electron bridge Mn-N/O-Cu, thus preventing Cu2+ from oxidizing H2O2 to form O2 •-, and facilitating the reduction of Cu2+ and generating more reactive oxygen species (1O2 and ·OH) with stronger oxidation ability, resulting in H2O2 utilization efficiency is 1.9 times as much as that of Cu/C. Additionally, the good and stable practical application capacity in different bodies demonstrates that it has great potential for practical environmental remediation.

Activation of peroxymonosulfate with natural pyrite-biochar composite for sulfamethoxazole degradation in soil: Organic matter effects and free radical conversion

Peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs) represent an effective method for the remediation of antibiotic-contaminated soils. In this study, a natural pyrite-biochar composite material (FBCx) was developed, demonstrating superior activation performance and achieving a 76% removal rate of SMX from soil within 120 min. There existed different degradation mechanisms for SMX in aqueous and soil solutions, respectively. The production of 1O2 and inherent active species produced by soil slurry played an important role in the degradation process. The combination of electron paramagnetic resonance (EPR) and free radical probe experiments confirmed the presence of free radical transformation processes in soil. Wherein, the·OH and SO4·− generated in soil slurry did not directly involve in the degradation process, but rather preferentially reacted with soil organic matter (SOM) to form alkyl-like radicals (R·), thereby maintaining a high concentration of reactive species in the system. Furthermore, germination and growth promotion of mung bean seeds observed in the toxicity test indicated the environmental compatibility of this remediation method. This study revealed the influence mechanism of SOM in the remediation process of contaminated soil comprehensively, which possessed enormous potential for application in practical environments.

Using Mechanochemistry to Activate Commodity Plastics as Initiators for Radical Chain Reactions of Small Organic Molecules.

Radical initiators such as azo compounds and organic peroxides have been widely used to facilitate numerous transformations of free radicals, which enable the efficient synthesis of structurally complex molecules, natural products, polymers, and functional materials. However, these high-energy reagents are potentially explosive and thus often require special precautions or delicate operating conditions. We postulated that a more convenient and safer alternative for radical chain initiation could be developed by mechanical activation of thermodynamically stable covalent bonds. Here, we show that commodity plastics such as polyethylene and poly(vinyl acetate) are capable of acting as efficient initiators for radical chain reactions under solvent-free mechanochemical conditions. In this approach, polymeric mechanoradicals, which are generated by homolytic cleavage of the polymer chains in response to the applied mechanical energy provided by ball milling, react with tris(trimethylsilyl)silane to initiate radical chain dehalogenation of organic halides. Preliminary calculations support our proposed force-induced radical chain mechanism.

Radiation-Chemical Transformations of Ethylene Glycol and Its Deuterated Derivative in Deaerated Aqueous Solutions

This study is devoted to the effect of complete deuteration of the ethylene glycol carbon skeleton on the formation of its radiation-induced free-radical transformations products in deaerated aqueous solutions at pH 7.00 ± 0.10 and a dose rate of 0.064 ± 0.002 Gy/s. It has been established that the chain process of fragmentation of carbon-centered ethylene glycol radicals (∙CH(OH)CH2OH) plays an important role in the formation of acetaldehyde. The replacement of protium atoms located at carbon atoms by deuterium decreased the radiation-chemical yield of acetaldehyde in aqueous 1 M solutions of diols by a factor of 8. The results of this study indicate that the use of deuterium exchange at the carbon skeleton of organic molecules is a promising way for increasing their resistance to radiation-induced free radical fragmentation processes.

Atomic-level analysis of migration and transformation of organic sodium in high-alkali coal pyrolysis using reactive molecular dynamics simulations

Monitoring the migration and transformation of free radicals and alkali metals (such as tar- and organic gas-bonded sodium) in high-alkali coal pyrolysis through experimental methods alone is a challenging task. To address this challenge, in this work we employed reactive force field (ReaxFF) molecular dynamics simulations to model the transformation behavior of sodium at the atomic level. The different forms of sodium [sodium atoms (Na(g)), sodium hydroxide (NaOH(g)), tar-bonded sodium, and organic gas-bonded sodium] were carefully analyzed to gain insights into their migration during the pyrolysis process. The results show that inherent organic sodium forms binary or multiple coordination structures with oxygen atoms in the coal matrix during pyrolysis. During pyrolysis, inherent organic sodium was transformed into three main sodium species: Na atoms, NaOH, and Na·H2O. The repeated reactions between these sodium-containing intermediates and the coal matrix strengthen the three-dimensional network structure of the coal matrix and hinder its graphitization. The preponderance of Na(g) predominantly stems from organic sodium C1–40+Na, while NaOH and Na·H2O contribute to a lesser extent. The formation of NaOH and Na·H2O can be primarily attributed to the presence of Na(g), with merely a limited portion arising from organic sodium (C1–40+Na). The results also show that organic sodium inhibits char and tar formation at high temperatures, whereas at low temperatures it promotes char formation and inhibits tar production.

Biochar–goethite composites inhibited/enhanced degradation of triphenyl phosphate by activating persulfate: Insights on the mechanism

In this study, the biochar–goethite composites (MBC@FH) were synthesized through co–ball milling and the degradation of triphenyl phosphate (TPhP) was compared in persulfate (PDS) alone system and MBC@FH&PDS systems. The results showed that TPhP can be effectively degraded in PDS alone system and degradation efficiency reached up to 90 % within reaction of 8 h, at a PDS concentration of 10 mM, a reaction temperature of 30 °C and a system pH of 6.12. The obvious degradation can be ascribed to the reactive oxygen species (ROS) generated by self–decompose of PDS, among which 1O2, ∙OH and O2∙− play a major role in the degradation process. Although 350 °C biochar–goethite composites (MBC35@FH) and 800 °C biochar–goethite composites (MBC80@FH) facilitated PDS activation to produce more ROS, the catalytic degradation of TPhP was different in their systems. The degradation of TPhP was inhibited by MBC35@FH due to its stronger adsorption for TPhP, while MBC80@FH promoted TPhP degradation and degradation efficiency was up to 100 % within 6 h. 1O2 and SO4∙− played a stronger degradation role than ∙OH and O2∙− in above systems. The transformation of Fe species, functional groups (oxygen–containing functional groups, pyrrolic nitrogen) and persistent free radicals (PFRs) on the MBC@FH were involved in the PDS activation to produce ROS. Furthermore, MBC80@FH was more capable of activating PDS than MBC35@FH due to its abundant defect sites, larger specific surface area, more PFRs, higher Fe content and stronger electron transfer capability. In addition, seven possible TPhP intermediates were identified and possible degradation pathways of TPhP were proposed accordingly. This study illustrated that not all metallic carbon catalysts are necessarily beneficial for organic contaminants degradation.

Determination of Curcumin in Food with Homogenous Liquid-Phase Microextraction Preconcentration and Spectrophotometric Determination

Antioxidants play a role in the transformation of free radicals into more stable chemical structures. Work on phenolic antioxidants has accelerated in recent years. Curcumin is a natural phenolic with antioxidant properties. In this study, a salt-supported homogenous liquid-phase microextraction method was used for preconcentration and spectrophotometric determination of curcumin for the first time. The curcumin was transferred into the organic phase using cyclohexylamine as the extraction solvent at pH values between 2.0 and 7.0 and determined by spectrophotometry. The results showed that the performance of the method for curcumin was unaffected by pH. The pH of the sample solution, the mass of salt, and the volume of the solvent were optimized, and the influence of the matrix types was investigated. The limits of detection and quantification were 1.44 and 4.80 µg L−1 for curcumin. The developed method was employed for the analysis of food samples.

Dynamic room-temperature phosphorescence by reversible transformation of photo-induced free radicals

Dynamic room-temperature phosphorescence by reversible transformation of photo-induced free radicals

Origins of selective differential oxidation of β-lactam antibiotics with different structure in an efficient visible-light driving mesoporous g-C3N4 activated persulfate synergistic mechanism

In the study, an efficient ‘1 + 1 > 2′ synergistic coupling system driven by visible light consisting of mesoporous g-C3N4 (MCN) and persulfate (PS)was constructed. The free radical transformation, electron transfer and non-covalent interaction between the MCN layer and PS in the system were explored via experiments and DFT calculations. The similarity for the fate of the seven β-lactam antibiotics with typical structures in the oxidation system was studied systematically in depth. First, the consistencies and differences of the seven antibiotics were summarized from three aspects: three-dimensional structures, electron cloud distributions, and the vulnerable sites. Notably, the selective differential degradation of β-lactam antibiotics in the MCN/PS system was speculated to be related with the molecular ionization potential (MIP), as a key index to describe the difficulty of oxidation. The distribution relationship between MIP and the oxidation kinetic constant (K) was explored and showed the following trend: a higher MIP indicates a weaker ability to provide electrons, and this leads to a greater resistance to oxidative degradation. In total, four main oxidation pathways of β-lactam antibiotics were systematically summarized combining HPLC-QTOF-MS and the simplified Fukui function calculation. The toxicity assessment of intermediate products provided by the T.E.S.T software of USEPA also shows a decreasing trend in the oxidation process. In the end, the superior practicability and stability of the MCN/PS system was verified by complex environment simulation and cyclic test. This research clarified the selective differential degradation mechanism of β-lactam antibiotics and provided a possible idea for the effective removal of refractory organic pollutants in water.

Two configurations of a dielectric barrier discharge enhanced nanoparticle (CexTi1-x)O2 catalyst for the removal of low-concentration SO2

• Reported on NTP enhanced nanoparticle (Ce x Ti 1-x )O 2 catalyst to remove low-concentration SO 2 . • The desulfurization efficiency under the ppc configuration is 30–400% higher than that of the catalyst alone, and the IPC configuration is 100% desulfurization at room temperature and pressure. • NTP increases the specific surface area of the catalyst and generates more unsaturated adsorbed oxygen and oxygen vacancies. • CCD spectrum is used to detect the discharge intermediate, and COMSOL is used to simulate the dynamic change of the intermediate in the discharge area. Removing low-concentration SO 2 present in flue gas is a worldwide problem. Herein, we introduce different technologies for coupling nonthermal plasma (NTP) with mixed CeO 2 -TiO 2 catalysts in a dielectric barrier discharge (DBD) reactor for the degradation of low-concentration SO 2 , including an in-plasma catalysis configuration (IPC) and a post-plasma catalysis configuration (PPC). Thermal catalysis at various Ce:Ti molar ratios and desulfurization efficiencies using different NTP-enhanced catalytic coupling processes were compared. We show that the SO 2 -removal efficiency in the PPC is enhanced by 20–30% by NTP, while 100% SO 2 -removal efficiency in the IPC was recorded at room temperature and atmospheric pressure, with (Ce 0.7 Ti 0.3 )O 2 exhibiting the best catalytic activity. To understand the influence of NTP on the activity of the catalytic reaction, catalytic behavior during (Ce 0.7 Ti 0.3 )O 2 thermal catalysis and during the nonthermal plasma reaction was analyzed by X-ray diffractometry, scanning electron microscopy, X-ray photoelectron spectroscopy, and N 2 physisorption experiments. The results suggest that plasma engraving increases the specific surface area of the catalyst and generates more oxygen vacancies or unsaturated active oxygen sites that promote the catalytic desulfurization reaction. Charged-coupled-device spectroscopy and COMSOL numerical simulations were used to reveal the types and transformations of reactive intermediate free radicals in the dielectric barrier discharge space. The results show that a large number of active N 2 /O 2 free radicals participate in transferring energy in the reactor, and that the first metastable state N 2 A 3 ∑ u + of N 2 is the most important excited state, as it promotes the formation and transformation of other active intermediates.

Synthesis of 1-Trifluorometylindanes and Close Structures: A Mini Review

This review describes methods for the synthesis of 1-trifluomethylindanes and close structures, which are still quite rare and scarcely available compounds. There are two main approaches to obtain 1-CF3-indanes. The first one is the construction of an indane system from CF3 precursors; the main methods are acid-mediated Friedel–Crafts cyclization, transition metal-catalyzed [3+2] annulation, and free-radical transformations. The second approach is the trifluoromethylation of a ready-made indane core by various CF3 sources, such as Ruppert–Prakash or Togni reagents. Many of these synthetic procedures possess high regio- and stereo-selectivity, allowing the preparation of unique 1-CF3-indane structures. In recent years, great attention has been paid to the synthesis of 1-CF3-indanes, due to the discovery of important biologically active properties for these compounds.

The mechanism of formation of boundary lubricating films during friction in a medium of di(2-ethylhexyl) sebacate

In this work, the mechanism of formation of boundary lubricating films during friction in synthetic oil di(2-ethylhexyl) sebacate (DEHS) was studied. The study of the kinetics of the interaction of DEHS with the metal surface showed that the formation of the boundary film proceeds in the first 40 s of friction. The unoxidized iron detected by the XPS method on the friction track accelerates the chemical transformations of lubricant molecules. Removing the oxide film from the friction surface is a mechanically activated process. When DEHS interacts with the iron surface, the reactions of decarboxylation of molecules proceed, as a result free radicals are formed. The sequence of transformations of free radicals and compounds with CC bonds leads to the formation of surface films upon friction. The retention of the film on the surface occurs due to the forces of interaction of the metal with free radicals or unsaturated bonds of organic molecules.

Endorsing Organic Porous Polymers in Regioselective and Unusual Oxidative C═C Bond Cleavage of Styrenes into Aldehydes and Anaerobic Benzyl Alcohol Oxidation via Hydride Elimination.

Oxidative cleavage of styrene C═C double bond is accomplished by employing a nitrogen-rich triazine-based microporous organic polymer as an organocatalyst. We report this regioselective reaction as first of its kind with no metal add-ons to afford benzaldehydes up to 92% selectivity via an unusual Wacker-type C═C bond cleavage. Such a reaction pathway is generally observed in the presence of a metal catalyst. This polymer further shows high catalytic efficiency in an anaerobic oxidation reaction of benzyl alcohols into benzaldehydes. The reaction is mediated by a base via the in situ generation of hydride ions. This study is supported by experiments and computational analyses for a free-radical transformation reaction of oxidative C═C bond cleavage of styrenes and a hydride elimination mechanism for the anaerobic oxidation reaction. Essentially, the study unveils protruding applications of metal-free nitrogen-rich porous polymers in organic transformation reactions.

The influence of H/D kinetic isotope effect on radiation-induced transformations of hydroxyl-containing compounds in aqueous solutions

Vicinal diols and its derivatives can be exploited as model compounds for the investigation of radiation-induced free-radical transformations of hydroxyl-containing biomolecules such as carbohydrates, phospholipids, ribonucleotides, amino acids, and peptides. In this paper, for the first time, the prospects of isotope reinforcement approach in inhibiting free-radical transformations of hydroxyl-containing compounds in aqueous solutions are investigated on the example of radiolysis of 1,2-propanediol and 1,2-propanediol-2-d1 aqueous solutions. At an absorbed dose rate of 0.110 ± 0.003 Gy·s−1 a profound kinetic isotope effect (KIE) is observed for the non-branched chain formation of acetone, which is a final dehydration product of predominant carbon-centred radicals CH3·C(OH)CH2OH. In 0.1 and 1 M deaerated solutions at pH 7.00 ± 0.01, the values of KIE are 8.9 ± 1.7 and 15.3 ± 3.1, respectively. A rationale for the fact that a strong KIE takes place only in the case of chain processes, which may occur during free-radical transformations of vicinal diols, is also provided herein based on the results of 2-propanol and 2-propanol-2-d1 indirect radiolysis. Lastly, the lack of KIE is shown in the case of 2-butanone formation from 2,3-butanediol or 2,3-butanediol-2,3-d2. This indicates that the type (primary, secondary) of the β-carbonyl radicals formed as a result of CH3·C(OH)CH(OH)R (R = H, CH3) dehydration determines the manifestation of the effect.

Insights into heterogeneous catalytic activation of peroxymonosulfate by natural chalcopyrite: pH-dependent radical generation, degradation pathway and mechanism

In this study, natural chalcopyrite (NCP) was employed in the activation of peroxymonosulfate (PMS) for bisphenol S (BPS) degradation. Firstly, the NCP catalyst was characterized via X-ray diffraction (XRD), scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) techniques. Then, several key parameters such as catalyst dosage, PMS dosage and initial pH were investigated in NCP/PMS system. Furthermore, the transformation of various free radicals (SO4•−, •OH and O2•−) with the changes of initial pH were investigated by quenching experiments and electron spin resonance (ESR) study. Also, sulfur species cycling of copper and iron species were investigated via exogenous Cu2+ and Fe3+ addition experiments and X-ray photoelectron spectroscopy (XPS) analysis, the result indicated that sulfur species promoted Fe3+/Fe2+ and Cu2+/Cu+ cycles on the NCP surface. Furthermore, thirteen major degradation intermediates of BPS were detected by UPLC-QTOF-MS/MS and density functional theory (DFT) method was used to illustrate possible reaction pathways of BPS. Finally, a reasonable reaction mechanism of NCP/PMS system for BPS degradation was proposed on the basis of the comprehensive analysis. In brief, this work helps to provide useful information for the application of natural metallic sulfide minerals in treatment of contaminated waters.

Photoreactivity of humic-like polyphenol material under irradiation with different wavelengths explored by FTICR MS and deuteromethylation.

The goal of this study was to establish reactivity of lignin-derived synthetic polyphenolic material under irradiation by ultraviolet (254 nm) and visible (460 and 525 nm) light in order to deeper examine relationships between the optical properties of this complex mixture and its individual constituents. In all photoirradiation experiments, blue shift of the fluorescence spectrum was observed. We aimed at understanding whether these changes could be explained on the basis of the chromophore interactions hypothesis, which implies destruction of electron-acceptor pairs via free radical transformations to be responsible for the alteration of optical properties. For this, changes in molecular composition were explored by Fourier transform ion cyclotron resonance mass spectrometry. Irradiation with UV resulted in a pronounced oxidation of polyphenols, which was manifested in the van Krevelen diagram by the formation of components with higher O/C ratio. At the same time, irradiation by visible light had led to the appearance of more condensed molecules depleted of oxygen. Consideration of changes in relative contribution of 500 most abundant components in polyphenol materials revealed higher transformation yields under UV light as compared to the visible light. Further studies using deuteromethylation followed by Fourier transform ion cyclotron resonance mass spectrometry enabled to enumerate the number of carboxylic groups in individual components of the parent polyphenol material. It was shown that at all wavelengths irradiation mainly impacted carboxylic-rich unsaturated and aromatic compounds, which can be considered as strong electron-acceptors. We suggest that their transformation is responsible for the blue shift of fluorescence spectrum, thus emphasizing the role of chromophore interaction mechanism of the optical properties formation.