Secondary Radicals Research Articles

Laser-Induced Fluorescence Spectroscopy of Large Secondary Alkoxy Radicals: Part I. Spectral Overviews and Vibronic Analysis.

We report vibronically resolved laser-induced fluorescence (LIF) spectra of jet-cooled C5-C10 secondary alkoxy radicals. The LIF spectra demonstrate vibronic structures similar to smaller (C3-C4) secondary alkoxies. For 2-pentoxy and 2-hexoxy, rotationally resolved LIF spectra have also been recorded. Two types of rotational structures have been observed in vibronic bands of each molecule. Extensive quantum chemistry calculations have been performed on 2-pentoxy and 2-hexoxy. The computed results include the relative energies of conformers, their geometries, and the energy separations between the nearly degenerate à and X̃ electronic states (ΔEÃ-X̃). Based on the similarity between the vibronic structures of different secondary alkoxies and calculated molecular parameters, including the relative energies of conformers, the B̃ ← X̃ transition frequencies, and the vibrational frequencies, strong vibronic bands in the LIF spectra are assigned to the origin bands and CO stretch bands of the two lowest-energy conformers of each secondary alkoxy radical. The distinct rotational structures of the two different conformations of 2-pentoxy and 2-hexoxy will be simulated and analyzed in Part II of this series ( J. Phys. Chem. A 2021, DOI: 10.1021/acs.jpca.0c10663).

Decarboxylative C(sp3)-N Cross-Coupling of Diacyl Peroxides with Nitrogen Nucleophiles.

We have disclosed a new radical-mediated decarboxylative C(sp3)-N cross-coupling of diacyl peroxides with nitrogen nucleophiles. The primary and secondary alkyl radicals derived from corresponding diacyl peroxides were generated by copper catalysis or by merging copper catalysis and photoredox catalysis, respectively. Various N-alkyl nitrogen nucleophiles, including indazoles, triazoles, indoles, purine, carbazole, anilines, and sulfonamide, were provided with a broad substrate scope and good functional group tolerance.

Degradation of Congo red by UV photolysis of nitrate: Kinetics and degradation mechanism

The efficiency of Congo red (CR) removal by low-pressure ultraviolet photolysis of nitrate (UV/NO3−) process was investigated in this study. Results showed that CR degradation through direct UV (254 nm) photolysis could be ignored, but CR was obviously removed in UV/NO3− process. Moreover, the contributions of UV, NO2 and OH to the degradation of CR were explored to be roughly 0.63%, 17.46% and 81.90%, respectively. In addition, the observed pseudo-first-order rate constants for the degradation of CR (kobs) were assessed by the effects of different water matrices and operation conditions, including NO3− dosage, CR concentration, solution pH, chloride ion (Cl−), natural organic matter (NOM) and carbonate/bicarbonate (CO32−/HCO3−). The kobs was increased by nearly 12.9 times with the addition of NO3− dosage from 1 mM to 200 mM due to the generation of more active components (e.g., OH and NO2). Higher doses of CR reduced the kobs, which could be attributed to the filter effects of CR on UV irradiation. Although the photolysis of NO3− could be expedited under acidic conditions, the kobs was decelerated because of the protonation of CR (pKa = 4.1, 25 °C). While in alkaline conditions, the kobs was also reduced as the decrease in redox potential of OH. The presence of Cl− (0–7 mM) exhibited no scavenging effect on the kobs, which suggested the secondary radicals (i.e., chlorine radicals (e.g., Cl, Cl2−)) could oxidize CR effectively. NOM restricted the degradation of CR significantly, resulting from the filter effects of NOM (ε = 0.11 LmgC−1cm−1) on UV and the effects of radical scavenging. The kobs was improved obviously with the participation of CO32−/HCO3− (0–7 mM), ascribing to the oxidation of carbonate radical (CO3−) and the increased steady state concentration of NO2. The degradation of CR in UV/NO3− process has the potential for practical applications in real water. Furthermore, eight intermediates were proposed by Liquid chromatography-tandem mass spectrometry (LC-MS/MS). Hydroxylation, nitrification and other aromatic products in the reaction of CR with OH and/or NO2 were readily produced by H-abstraction, addition reaction and electron transfer.

Direct Oxidation of Aryl Malononitriles Enabling a Copper-Catalyzed Intermolecular Alkene Carbochlorination.

A copper-catalyzed carbochlorination of alkenes with aryl malononitriles and chloride is disclosed. This net oxidative transformation proceeds with activated and unactivated alkenes with moderate to excellent yields. Mechanism experiments suggest addition of the malononitrile radical to form a secondary carbon radical which is intercepted by a chloride source. The resultant products can be transformed into biologically important γ-lactones in one further step.

Ab initio rate coefficients for reactions of 2,5-dimethylhexyl isomers with O2: temperature- and pressure-dependent branching ratios

Chemical kinetics of O2-addition to alkyl radicals (R), termed first O2-addition in the oxidation mechanism of alkanes, are of central importance to next-generation combustion strategies designed for operations in the low- to intermediate-temperature region (<1000 K). In the present work, stationary points on potential energy surfaces (PES), temperature- and pressure-dependent rate coefficients, and branching fractions of product formation from R + O2 reactions initiated by the addition of molecular oxygen (3O2) to the three alkyl radicals of a branched alkane, 2,5-dimethylhexane, are reported. The stationary points were determined utilizing ab initio/DFT methods and the reaction energies were computed using the composite CBS-QB3 method. Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) calculations were employed to compute rate coefficients, from which branching fractions were determined over the pressure range of 10-3-20 atm and the temperature range of 400-900 K on three different surfaces. The quantum chemistry results reveal several distinct features. For the addition of O2 to the tertiary alkyl radical 2,5-dimethylhex-2-yl, the most energetically favorable channel leads to the formation of 2,2,5,5,-tetramethyl-tetrahydrofuran, a cyclic ether intermediate formed coincident with OH in a chain-propagating step from the decomposition of tertiary-tertiary hydroperoxyalkyl (QOOH). On the R + O2 surface of the secondary radical, 2,5-dimethylhex-3-yl, the pathways for the formation of methyl-propanal + iso-butene + OH via concerted C-C and O-O bond scission of tertiary QOOH and that of cyclic ether + OH are the most energetically favorable pathways. The R + O2 surface for the reaction of the primary radical, 2,5-dimethylhex-1-yl, reveals two competitive chain-propagation channels, leading to 2-iso-propyl-4-methyl-tetrahydrofuran + OH and 2,2,5-trimethyltetrahydropyran + OH. Below 100 Torr, the formation of the aforementioned species dominates the respective total R + O2 rate coefficient, while at pressures above 1 atm collisionally stabilized alkylperoxy (ROO) dominates at the temperatures considered here. The results of this study are in very good agreement with the experimentally measured intermediates and products of the 2,5-dimethylhexyl radical + O2 reaction.

Synthesis of gem-Difluoroalkenes via Zn-Mediated Decarboxylative/Defluorinative Cross-Coupling.

An efficient and mild Zn-mediated decarboxylative/defluorinative alkylation of α-trifluoromethyl alkenes using N-hydroxyphthalimide esters as radical precursors was developed. Several α-trifluoromethyl alkenes were readily coupled to a wide range of primary, secondary, and tertiary radicals, affording the desired gem-difluoroethylenes in moderate to excellent yields. This reaction protocol was also successfully applied to the construction of complex molecules such as the bioactive natural dehydroabietic acid and glycosyl groups bearing the gem-difluoroethylene moiety.

Divergent Total Syntheses of (-)-Pseudocopsinine and (-)-Minovincinine.

Herein, the first total syntheses of (-)-pseudocopsinine (1) and (-)-minovincine (3) from a common intermediate 8 are detailed, enlisting late-stage, hydrogen atom transfer (HAT)-mediated free radical bond formations (C20-C2 and C20-OH, respectively) that are unique to their core or structure. The approach to 1 features an Fe-mediated HAT reaction of the intermediate olefin 2, effecting a transannular C20-C2 free radical cyclization of a challenging substrate with formation of a strained [2.2.1] ring system and reaction of a poor acceptor tetrasubstituted alkene with a hindered secondary free radical to form a bond and quaternary center adjacent to another quaternary center. Central to the assemblage of their underlying Aspidosperma skeleton is a powerful [4 + 2]/[3 + 2] cycloaddition cascade of 1,3,4-oxadiazole 9, which affords the stereochemically rich and highly functionalized pentacyclic intermediate 8 as a single diastereomer in one step. The work extends the divergent total synthesis of four to now six different natural product alkaloid classes by distinguishing late stage key strategic bond formations within the underlying Aspidosperma core from the common intermediate 8. Together, the work represents use of strategic bond analysis combined with the strategy of divergent synthesis to access six different natural product classes from a single intermediate.

Visible‐Light‐Induced C2 Alkylation of Heterocyclic N‐Oxides with N‐Hydroxyphthalimide Esters under Metal‐Free Conditions

AbstractA visible‐light‐induced site selective C−H alkylation of heterocyclic N‐oxides with N‐hydroxyphthalimide esters was developed using Eosin Y as the photocatalyst in the presence of Cs2CO3 under redox‐neutral and mild conditions. Using N‐hydroxyphthalimide esters as the radical precursors, quinoline and pyridine N‐oxides were readily coupled with a wide range of primary, secondary, and tertiary radicals to afford the desired alkylated heterocyclic N‐oxides in moderated to excellent yields. Importantly, this reaction protocol also successfully demonstrated its applications for the construction of glycosyl or bioactive natural dehydroabietic acid containing heterocyclic N‐oxides, as well as the pharmaceutical and agrochemical active alkylated quinine‐based functional molecules (potential antimalarial drug).magnified image

The coupling of alkylboronic acids with α-(trifluoromethyl)styrenes by Lewis base/photoredox dual catalysis

Under Lewis base/photoredox dual catalysis, the defluorinative alkylation of alkylboronic acids with α-(trifluoromethyl)styrenes has been reported. In this system, both primary and secondary alkyl radicals produced by alkylboronic acids can react with α-(trifluoromethyl)styrenes to generate alkylated gem-difluoroalkenes. In general, the method has mild conditions, a wide substrate scope, and good functional group tolerance, which provides a new route for the synthesis of gem-difluoroalkenes.

Hydroxylamines inhibit tyrosine oxidation and nitration: The role of their respective nitroxide radicals

In vivo, nitroxide antioxidants distribute within minutes throughout all tissues, but are reduced to their respective hydroxylamines due to the cellular reducing environment, which apparently limits their application. To distinguish their antioxidative activity from that of their respective nitroxides, the kinetics and mechanism of their inhibitory effect on the enzymatic oxidation and nitration of tyrosine have been studied. The inhibitory effect of the hydroxylamines on the oxidation and nitration of tyrosine induced by HRP/H2O2 and HRP/H2O2/nitrite was investigated by following the kinetics of the formation of their respective nitroxides, H2O2 decomposition, release of O2 and accumulation of tyrosine oxidation and nitration products. The distinction between the antioxidative activities of nitroxides and of their respective hydroxylamines is hindered due to oxidation of hydroxylamines to nitroxides, which catalytically inhibit tyrosine oxidation and nitration. The results demonstrate that (i) hydroxylamines inhibit tyrosine oxidation and nitration and their inhibitory effect increases as the reduction potential of their respective nitroxides decreases; (ii) the 6-membered ring hydroxylamines are more effective antioxidants than the 5-membered hydroxylamine derived from 3-carbamoyl proxyl and (iii) the 6-membered ring hydroxylamines are as effective antioxidants as their respective nitroxides, whereas the 3-carbamoyl proxyl is even a weaker antioxidant than its respective hydroxylamine. In general, cyclic hydroxylamines are more effective antioxidants than common antioxidants such as ascorbic and uric acids, which are depleted giving rise to secondary radicals that, might be toxic. In the case of hydroxylamines, the secondary radicals are their respective nitroxides, which are efficient catalytic antioxidants.

Comparison of radical processes in non-aged and radiation-aged polyethylene unprotected or protected by antioxidants

Silane cross-linked polyethylene was examined by electron paramagnetic resonance (EPR) spectroscopy before and after gamma-irradiation at a dose rate of 77.7 Gy/h to a dose of 374 kGy, which corresponds to approximately 40 years of radiation aging in nuclear power plants (NPP). Radicals generated after post-aging by gamma radiation at 77 K (10 kGy, 2.2 kGy/h) were identified in a polymer unprotected, doped with phenolic antioxidant, thioether antioxidant or both. The radical processes in non-aged and long-term aged polyethylene differed significantly. In the second case, peroxyl radicals dominated at lowest temperatures, while the content of secondary alkyl radicals increased gradually during thermal annealing to 220 K, whereas above ambient temperature tertiary or primary alkyl radicals appeared. The stability of alkyl radicals was related to the rapid consumption of oxygen and, consequently, its depletion in earlier stages of radical processes. Phenolic antioxidant suppressed the above effects and protected the cross-linked structure of the polymer matrix.

Energy-efficient erythromycin degradation using UV-LED (275 nm)/chlorine process: Radical contribution, transformation products, and toxicity evaluation

In this study, we investigated the degradation mechanism of erythromycin (ERY) during UV-LED/chlorine treatment using a 275-nm ultraviolet light-emitting diode (UV-LED). This wavelength is known to generate fewer disinfection byproducts (DBPs), and to have higher energy and photon yield efficiency compared to low pressure mercury (LP-UV) lamp which emits 254 nm of UV radiation. The degradation of ERY during the UV-LED/chlorine reaction followed pseudo-first-order kinetics. While Cl• and ClO• radicals along with other secondary radicals played key roles in the degradation of ERY at alkaline pH conditions, •OH radical was the main contributor at acidic pH conditions. Using ultra-high performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QToF-MS), we tentatively identified six byproducts. Trace amounts of DBPs, such as chloroform (CHCl3) and chlorate (ClO3−) ions, were also detected at less than 0.3 mg/L. There was no residual antibiotic effect at the end of the UV-LED/chlorine reaction due to the complete degradation of important moieties, such as macrolide, in ERY. Toxicity decreased by 20% after 20 min during the UV-LED/chlorine process of ERY (1.0 mg/L) degradation. Finally, we confirmed the inactivation of ARB and ARG during the UV-LED/chlorine process.

Kinetics and pathways of the degradation of PPCPs by carbonate radicals in advanced oxidation processes

The carbonate radical (CO3•−) is a typical secondary radical observed in engineering and natural aquatic systems. This study investigated the degradation kinetics of 20 pharmaceuticals and personal care products (PPCPs) by CO3•− and the transformation pathways of a typical PPCP (naproxen) that is susceptible to CO3•−. CO3•− is highly selective for compounds containing aniline and phenolic hydroxyl groups as well as naphthalene rings, such as sulfamethoxazole, sulfamethazine, salbutamol, propranolol, naproxen, and macrolide antibiotics such as azithromycin, for which the second-order rate constants range from 5.6 × 107 M−1s−1 to 2.96 × 108 M−1s−1. A good linear relationship is observed between the natural logarithms of kCO3•− and the negative values of the Hammett Σσp+ constant for aromatic PPCPs, indicating that electron-donating groups promote the attack of benzene derivatives by CO3•−. The contribution of CO3•− to naproxen degradation is significant in different processes such as UV/H2O2, UV/persulfate, UV/chlorine, and UV/monochloramine, in the presence of HCO3−, which compensates for the decreased contributions of primary radicals. In particular, the formation of CO3•− increases the first-order rate constant of naproxen by 127% in UV/monochloramine in the presence of 50 mM HCO3− compared to that without HCO3−. Natural organic matter (NOM) exerts a slight scavenging effect on CO3•−, decreasing the inhibition effect of NOM on the degradation of naproxen by UV/H2O2 in the presence of HCO3−. The pathways involved in the transformation of naproxen by CO3•− include decarboxylation, hydroxylation, ketonization, demethylation and aldolization. In addition, the alteration of the genotoxicity during naproxen degradation by CO3•− was negligible.

Mechanism for sonochemical reduction of Au(III) in aqueous butanol solution under Ar based on the analysis of gaseous and water-soluble products

When an aqueous Au(III) solution containing 1-butanol was sonicated under Ar, Au(III) was reduced to Au(0) to form Au particles. This is because various reducing species are formed during sonication, but the reactivity of these species has not yet been evaluated in detail. Therefore, in this study, we analyzed the effects of Au(III) on the rates of the formation of gaseous and water-soluble compounds (CH4, C2H6, C2H4, C2H2, CO, CO2, H2, H2O2, and aldehydes), and the rate of Au(III) reduction as a function of 1-butanol concentration. The following facts were recognized: 1) for Au(III) reduction, the contribution of the radicals formed by the pyrolysis of 1-butanol was higher than that of the secondary radicals formed by the abstraction reactions of 1-butanol with ·OH, 2) ·CH3 and CO acted as reductants, 3) the contribution of ·H to Au(III) reduction was small in the presence of 1-butanol, 4) aldehydes and H2 did not act as reductants, and 5) the types of species that reduced Au(III) changed with 1-butanol concentration.

Effect of Additional Pin Electrode Placing at the Bottom of the Plasma Jet and Flow Behavior on Radical Generation and Propagation

In this article, the effect of an additional pin electrode at the bottom of the plasma jet on a double electrode atmospheric pressure plasma (APP) jet in radical generation and propagation along the jet were investigated. An additional pin electrode at the bottom of the plasma jet connecting to the ground voltage resulted in triggered glow discharge near the additional pin electrode is similar to the streamer-triggered glow discharge at the ground electrode for double electrode configuration. This leads to an increase in the intensity of excited radicals along the plasma jet with the secondary radical generation at the bottom of the plasma jet. The effect of changes in flow behavior from laminar to turbulent flow on the generation and propagation of radicals along the jet were also investigated for the double electrode configuration. Experimental observations confirm that changes in flow regimes affecting the radical generation and propagation. Increase of flow in the laminar regime increases the radical intensity and decreases the intensity as the flow becomes turbulent.

Infiltration of the Mascagni Node is a Strong Prognostic Factor for Gall Bladder Cancer Discovered in Resected specimen.

Introduction: The infiltrative node is a strong prognostic indicator of survival in gallbladder cancer. The profile of the Mascagni ganglion (Ganglion of the neck of the gallbladder) has never been studied and taken into account for the form of histological discovery. The purpose of this retrospective study is to assess the impact of the Mascagni lymph node profile (GGM) and the prediction of the presence of residual tumor tissue after a histological discovery of gallbladder cancer. Material and method: Forty-one patients were collected. Group A (No infiltrated Mascagni node) is made up of 15 patients while group B (infiltrated Mascagni node) is made up of 26 patients. Complementary secondary surgery was retained in 27 patients (65.8%) who underwent complimentary radical surgery in 16 (39%) of them and palliative surgery for 10 others (24,4%). Sixteen patients (39%) did not have secondary surgery. The mean time to reopen was 93.9 (48-166) in group A and 85.6 (21-230) days (p=0.5). Only 2 patients (13.3%) of group A had a residual tumor (carcinomatosis) and 100% of patients of group B presented this residual tumor with or without second surgery (p&lt;0.001). Result: No patient of group A had infiltrated lymph node even after second surgery while 92.3% (24/26) of group B had finally infiltrated lymph node (results of two interventions) (p&lt;0.001). Survival is significantly higher in-group A than in-group B. with global five-year survival reaching 46.6% vs only 03.8%. Free survival is significantly higher for group A than group B (73.3% (11/15) vs 03.8% (3/26). The survival is also significantly higher for both cholecystectomy alone and radical secondary surgery with 62.5% (05/08) vs 00% (00/16) (p &lt; 0.05) and 85.60% (06/07) vs 30% (03/10) respectively (p&lt;0.01). Conclusion: GGM is a strong prognostic factor for gall bladder cancer discovered on the resected specimens. It must be noted systematically on the histological reports and be taken into account before the secondary radical surgery.

The role of carbonate in sulfamethoxazole degradation by peroxymonosulfate without catalyst and the generation of carbonate racial

Peroxymonosulfate (PMS) can be activated by various catalysts to degrade organic contaminants in wastewater treatment processes. In this research, the co-activation of PMS by sulfamethoxazole (SMX) and carbonate is investigated. The results show that SMX can be degraded in situ, and the main reactive oxygen species are singlet oxygen and carbonate radicals. Only singlet oxygen is detected when SMX is degraded by PMS without carbonate. However, both carbonate radicals and hydroxyl radicals are measured in the presence of carbonate. Among which, hydroxyl radicals are identified by electron paramagnetic resonance spectroscopy method, and carbonate radicals are confirmed by radical quenching experiments, as well as the variation of SMX degradation kinetic and the appreance of carbonyldioxy derivatives by-products in the presence of carbonate anions. Based on this phenomenon, it is proposed that carbonate can enhance the decomposition of PMS and the generation of secondary free radicals (carbonate radicals). Thus carbonate radicals can enhance the oxidizability for sulfonamide antibiotics in the PMS-based advanced oxidation systems. These results suggest that the addition of carbonate is an important enhancement method for the treatment of sulfonamide antibiotics in water.

Generation of Alkyl Radical through Direct Excitation of Boracene-Based Alkylborate.

The generation of tertiary, secondary, and primary alkyl radicals has been achieved by the direct visible-light excitation of a boracene-based alkylborate. This system is based on the photophysical properties of the organoboron molecule. The protocol is applicable to decyanoalkylation, Giese addition, and nickel-catalyzed carbon-carbon bond formations such as alkyl-aryl cross-coupling or vicinal alkylarylation of alkenes, enabling the introduction of various C(sp3) fragments to organic molecules.

Synthetic Melanin Coated Carbon Fiber Micro-Electrodes for Selective Detection of Peroxynitrite

Melanin is a natural pigment produced by melanocytes and has many physiological functions. Recent studies have suggested that melanin can behave like a free radical scavenging antioxidant. In biological systems, free radicals can damage a wide array of the essential components in the human body, such as cells/tissues, DNA, proteins, and lipids. Peroxynitrite (PON) has emerged as a combination of two free radicals and it can yield secondary free radicals per homolytic decomposition. PON is a biological oxidant that has been linked with oxidative damage and disruption of redox control systems. The short lifespan and fast reactivity of PON add more challenge to measure its concentration under physiologic conditions.Carbon fiber electrodes (CFEs) have the advantage of providing a minimally invasive and a non-toxic platform when compared to other metal-based electrodes. In this research study, synthetic melanin-coated CFEs were prepared using electro-deposition of 5,6-dihydroxyindole monomer. Surface characterizations including scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were utilized to confirm the melanin deposition and to examine the morphological differences between the modified CFEs before and after exposure to PON. Cyclic voltammetry, differential pulse voltammetry, and amperometry were used to test the performance of CFEs modified with synthetic melanin in terms of detection and quantification of PON. Our preliminary findings have shown that synthetic melanin modified CFEs have greater sensitivity when compared to glassy carbon electrodes and other macro-electrodes. In addition, melanin-coated CFEs exhibit high selectivity when they tested for PON detection in the presence of biological interferents. The present results indicate the possibility of using melanin-coated CFEs as a sensitive and selective sensing platforms for peroxynitrite detection in the nanomolar range at physiologic conditions.

Theoretical calculation of low-temperature oxidation of heptyl radicals and O2

n-Heptane, as an alternative gasoline fuel, was used for research on low-temperature oxidation in this study, where the reactions of alkyl radicals (C7H15) and O2 play an important role. The related chemical reaction kinetics on four C7H15 radicals and O2 were investigated. The potential energy surfaces (PESs) of C7H15O2 were obtained by the quantum chemical calculation method of CBS-QB3. The pressure- and temperature- dependence of the rate constants was also calculated by solving master equations based on Rice–Ramsperger–Kassel–Marcus theory. In this work, all possible reaction pathways in the oxidation process were considered and calculated. Formation, concerted elimination, and intramolecular H atom transfer reactions of initial product RO2 are very critical to the low-temperature combustion. For four C7H15 radicals, the energy barriers that form RO2 from the HR + O2 association are typically zero, and barriers to subsequent reactions are different by up to 30 kcal/mol. It is most advantageous to form QOOH from RO2 through a six- or seven-member ring transition state. In terms of kinetic aspect, compared with the reaction of O2 plus secondary radicals (HR2, HR3, HR4), the reaction of adding O2 to primary radical (HR1) is less competitive at low temperatures. Moreover, the kinetic data obtained in this study will play an important role in the kinetic model of low-temperature combustion for n-heptane.