Radical Chain Mechanism Research Articles

Synthesis, catalytic activity and the structural transformation of dimeric mono-Fe (Ⅲ)-substituted Keggin-type polyoxotungstates in the oxidation of cyclohexanol with H2O2

Three dimeric mono-Fe(III)-substituted polyoxotungstates [C16H33(CH3)3N]10[(PW11FeO39)2O]·3.5H2O (1), [(C2H5)4N]10[(PW11FeO39)2O]·3H2O (2) and [(CH3)4N]10[(PW11FeO39)2O]·4H2O (3) were synthesized and characterized by elemental analysis, TGA-DSC, FT-IR, UV–vis and SEM. The catalytic activities of them were detected in the oxidation of cyclohexanol with H2O2 as oxidant. 1 with the longest carbon chain of organic countercations showed the best catalytic activity and exhibited a reaction-controlled phase transfer behavior. Many kinds of alcohols can be also efficiently oxidized into corresponding aldehydes or ketones based on 1. The fresh catalyst, the recovered one and the catalyst 1 treated with H2O2 were characterized by IR, UV and MALDI-TOF mass spectroscopy. The results manifested that the dimeric 1 decomposed into monomeric [C16H33(CH3)3N]4PW11Fe(H2O)O39·H2O (4) after the reaction. The temperature and H2O2 had an influence on the dimer-monomer transformation process. With the addition of H2O2, the solubility of 1 in the reaction solution was enhanced and 1 gradually decomposed into 4 with the temperature increasing, until decomposed completely at high temperature (70, 80 and 90 °C), eg., the dimer-monomer transformation completed at the first 3 h of the reaction at 70 °C. The mechanism of 1-catalyzed oxidation of cyclohexanol system with H2O2 was mainly a radical-chain mechanism. It was found that a faster reaction rate was observed during the transformation process based on 1 compared to 4 at the same amount of Fe in the catalyst. This suggested that there should exist the equilibrium of [(PW11FeO39)2O]10−, [PW11Fe(OH)O39]5- and [PW11Fe(H2O)O39]4- anions during the transformation process based on the radical decomposition of H2O2, which all played a catalytic role in the oxidation of cyclohexanol. But after the transformation, the [(PW11FeO39)2O]10- anion entirely decomposed into [PW11Fe(H2O)O39]4- anion until the reaction ended.

Red-Light-Mediated Barton–McCombie Reaction

Abstract A red-light-mediated Barton–McCombie reaction is described, in which chlorophyll a is used as a photocatalyst and tris(trimethylsilyl)silane or Hantzsch ester is used as the hydrogen source. The reaction can be performed with a set of easily available equipment and reagents, and a variety of linear and cyclic xanthates could be applied. In contrast to the traditional conditions, the reaction does not involve toxic organotin or an explosive radical initiator. The reaction mechanism was analyzed both by experiments and computation, and it was suggested that the radical chain mechanism initiated by excitation of complex followed by charge transfer is likely to be operative.

Direct α‐Arylation of Alcohols with Aryl Halides through a Radical Chain Mechanism

AbstractAlcohols were found to be arylated directly at their α‐C−H bond with aryl halides in the presence of a base and a substoichiometric amount of t‐BuOOt‐Bu through a homolytic aromatic substitution mechanism.magnified image

Photocatalytic Deoxygenation of Sulfoxides Using Visible Light: Mechanistic Investigations and Synthetic Applications.

The photocatalytic deoxygenation of sulfoxides to generate sulfides facilitated by either Ir[(dF(CF3)ppy)2(dtbbpy)]PF6 or fac-Ir(ppy)3 is reported. Mechanistic studies indicate that a radical chain mechanism operates, which proceeds via a phosphoranyl radical generated from a radical/polar crossover process. Initiation of the radical chain was found to proceed via two opposing photocatalytic quenching mechanisms, offering complementary reactivity. The mild nature of the radical deoxygenation process enables the reduction of a wide range of functionalized sulfoxides, including those containing acid-sensitive groups, in typically high isolated yields.

Site-Selective Thiolation of (Multi)halogenated Heteroarenes.

A general and simple strategy for the site-selective thiolation of various pharmaceutically relevant electron-rich heteroarenes with thiols is reported. This mild and reliable photocatalytic protocol enables C-S coupling at the most electron-rich position of the (multi)halogenated substrates, complementing established methodologies. Experimental and computational studies suggest a radical chain mechanism with the key step being a homolytic aromatic substitution of the heteroaryl halide by an electrophilic thiyl radical, highlighting an underdeveloped reactivity mode.

Role of antioxidants in food

Antioxidant is any substance that delays, prevents or removes oxidative damage to a target molecule. It can be found in many foods, including fruits and vegetables. The role of antioxidants in foods is to retard or control oxidation. The process of autoxidation and development of rancidity in foods involves a free radical chain mechanism via initiation, propagation and termination steps. While radicals are produced in the ‘initiation’ step, they react with unsaturated fatty acids by abstracting a hydrogen atom from a site which requires the least energy that is the allylic or diallylic position in the ‘propagation’ steps. The reactions in the propagation step make up a chain reaction until a ‘termination’ reaction occurs. Due to high stability and low volatility, it helps to maintain the level of nutrients, the texture, colour, taste, freshness, functionality, aroma, and appeal to consumers such as the older person. Various antioxidants found in food viz. natural antioxidants, synthetic antioxidants, dietary antioxidant, endogenous antioxidant play an important role in preservation of food. Vitamin C, Vitamin E, α-carotene, Lycopene, Polyphenol etc. is main sources of antioxidants. They may be present in foods as endogenous factors or may be added to preserve their lipid components from quality deterioration. The most widely used antioxidants in foods include butylated hydroxyl anisole (BHA), butylated hydroxyl toluene (BHT), propyl gallate (PG) and tertiary butyl hydroquinone (TBHQ). These antioxidants may be used at 200 ppm in bulk oils and at 200 ppm, based on the lipid content of other foods. The benefits of antioxidants include whole foods and beverages (e.g., acai berry, gogi berry, green tea) as well as isolated substances sold primarily as dietary supplements (e.g., vitamin C, lycopene, selenium) or added to foods (e.g., vitamin E). It reduces the risk of developing certain diseases such as; cancer, heart disease, stroke, and arthritis etc.

Open-Shell Fluorination of Alkyl Bromides: Unexpected Selectivity in a Silyl Radical-Mediated Chain Process.

We disclose a novel radical strategy for the fluorination of alkyl bromides via the merger of silyl radical-mediated halogen-atom abstraction and benzophenone photosensitization. Selectivity for halogen-atom abstraction from alkyl bromides is observed in the presence of an electrophilic fluorinating reagent containing a weak N-F bond despite the predicted favorability for Si-F bond formation. To probe this surprising selectivity, preliminary mechanistic and computational studies were conducted, revealing that a radical chain mechanism is operative in which kinetic selectivity for Si-Br abstraction dominates due to a combination of polar effects and halogen-atom polarizability in the transition state. This transition-metal-free fluorination protocol tolerates a broad range of functional groups, including alcohols, ketones, and aldehydes, which demonstrates the complementary nature of this strategy to existing fluorination technologies. This system has been extended to the generation of gem-difluorinated motifs which are commonly found in medicinal agents and agrochemicals.

The role of organic electron donors in the initiation of BHAS base-induced coupling reactions between haloarenes and arenes

Coupling reactions between haloarenes and arenes (including heteroarenes) that are conducted without added transition metals but in the presence of KOtBu or NaOtBu, have been a topic of great interest since their discovery in 2008. Diverse organic structures act as additives that assist these reactions. These additives are converted into organic electron donors by the butoxide base and this leads to initiation of the coupling reactions, which proceed by radical chain mechanisms. This review provides an overview of the initiation stages of these reactions.

Experimental studies of radical reactions of hydrocarbons conversion

Radical reactions are traditional objects of research in various fields of chemical kinetics for many decades; this is evidenced by works of academic A. L. Buchachenko [1–4]. On the example of these reactions the general regularities of chemical compounds transformation are found and explained. At the same time, the transformations of dispersed organic matter and hydrocarbon are the basis of oil and gas generation processes and are key in industrial oil and gas processing. These reasons make it very important to establish the mechanisms of transformation of organic matter and its transformation products in the development of processes in various external conditions. As a result of innovative researches by I. I. Nesterov [5–9], it can be considered reliably established that the transformation processes of dispersed organic matter proceed by a discrete radical-chain mechanism. The fragmentation of molecules with long chains of carbon atoms and ring molecules into smaller ones (cracking of hydrocarbons) is the basis of the conversion process of dispersed organic matter into hydrocarbons [10]. Over decades of research, the basic laws of these processes have been revealed, but some aspects of their mechanism and kinetics remain controversial. In this context, the aim of our study is to investigate the influence of wave fields to change the structure and composition of the hydrocarbons and oil NSOcompounds. Research methods are irradiation of crude oils and their fractions by a wave field with a constant frequency of 50 Hz and fixation by chromatographic methods of changing the fractional composition depending on the time of irradiation.

The role of sodium metabisulphite in depressing pyrite in chalcopyrite flotation using saline water

In response to the high pyrite recovery in copper flotation using seawater, this study investigated the role of sodium metabisulphite (MBS), a widely used pyrite depressant, in depressing the flotation of copper-activated pyrite to identify a solution to maximising pyrite rejection. It started with chalcopyrite flotation against pyrite with MBS added in the mill before grinding and in the conditioning cell before flotation. The surface properties of pyrite exposed to different reagent schemes were investigated by X-ray photoelectron spectroscopy (XPS) analysis. The reactions on pyrite surface responsible for different flotation results were then investigated by cyclic voltammogram (CV) measurements and discussed in terms of a radical chain mechanism involving the redox cycling of Cu(I/II) induced by sulphite species. It was found that the addition of MBS during the conditioning stage was more efficient in depressing the flotation of copper-activated pyrite. This is because the combination of sulphite ions and oxygen may form a strongly oxidising radical SO5·- which can oxidise Cu(I)S, the copper activation product on pyrite surface, to form hydrophilic Cu(OH)2/CuSO4. The addition of MBS at low dosages in the mill depressed pyrite flotation to a less extent due to the less oxygen available, while its depression effect was even weakened at high dosages due to the consumption of dissolved oxygen and a strongly reducing environment which favoured copper activation on pyrite surface.

Kinetics of iron(III)-catalyzed oxidation of arsenic(III) in acidic solutions with SO2/O2 gas mixture using different iron sources

Oxidation of arsenic(III) to arsenic(V) is very important in arsenic removal and immobilization operations. Limited studies on arsenic(III) oxidation at pH below 1.5 as is the case of acid plant and leaching process effluents have been reported. In this work, the kinetics of As(III) oxidation with SO2/O2 gas mixture at low pH (0.3–1.0) sulfuric acid solutions was investigated in the presence of different iron sources as catalyst: (i) Fe(III); (ii) Fe(II); and (iii) FeOOH residue. In the absence of Fe(III) the initial rate of As(III) oxidation at 90 °C is 0.6 mmol/L·min increasing to 1.1 mmol/L·min with Fe(III)/As(III) molar ratio = 0.1. Higher additions of Fe(III) bring only modest increase in rate. In the presence of Fe(II) the oxidation of As(III) is delayed initially due to competing oxidation of Fe(II) subsequently accelerating with the build-up of Fe(III), which acts as catalyst. Use of FeOOH (industrial goethite residue) provides a cost-effective source of Fe(III) catalytic species while simultaneously acts as neutralizing agent. A complex kinetic behavior is followed. Oxidation is reaction-controlled over the temperature range 25–50 °C shifting to mixed regime control from 70 to 80 °C becoming finally fully mass-transfer controlled at 90 °C. The catalytic role of Fe(III) is rationalized on the basis of a radical chain mechanism.

Facile Synthesis of α-N-Heterocyclic Carbene-Boryl Ketones from N-Heterocyclic Carbene-Boranes and Alkenyl Triflates.

Reactions of readily available alkenyl triflates with N-heterocyclic carbene (NHC)-boranes in the presence of diisopropyl ethyl amine provided about three dozen stable α-NHC-boryl ketones. Isolated yields were typically 40-56% for B-unsubstituted NHC-boranes (NHC-BH3), and somewhat lower for NHC-boranes with B-substituents (NHC-BH2R). The requisite alkenyl triflates can be made separately or prepared in situ from either ketones or alkynes. The experimental evidence supports a radical chain mechanism that involves the following: (1) addition of an NHC-boryl radical to the alkenyl triflate, (2) fragmentation to give the α-NHC-boryl ketone, SO2, and trifluoromethyl radical, and (3) hydrogen abstraction by trifluoromethyl radical from the starting NHC-borane to return the NHC-boryl radical along with trifluoromethane. Reactions 1 and 3 are both new and evidently rather fast.

The Role of Iodanyl Radicals as Critical Chain Carriers in Aerobic Hypervalent Iodine Chemistry

Summary Selective O 2 utilization remains a substantial challenge in synthetic chemistry. Biological small-molecule oxidation reactions often utilize aerobically generated high-valent catalyst intermediates to effect substrate oxidation. Available synthetic methods for aerobic oxidation catalysis are largely limited to substrate functionalization chemistry by low-valent catalyst intermediates (i.e., aerobically generated Pd(II) intermediates). Motivated by the need for new chemical platforms for aerobic oxidation catalysis, we recently developed aerobic hypervalent iodine chemistry. Here, we report that in contrast to the canonical two-electron oxidation mechanisms for the oxidation of organoiodides, the developed aerobic hypervalent iodine chemistry proceeds via a radical chain mechanism initiated by the addition of aerobically generated acetoxy radicals to aryl iodides. Despite the radical chain mechanism, aerobic hypervalent iodine chemistry displays substrate tolerance similar to that observed with traditional terminal oxidants, such as peracids. We anticipate that these insights will enable new sustainable oxidation chemistry via hypervalent iodine intermediates.

Quantum chemical evaluation of the role of $${{{\text{HO}}_{2}}^{ \cdot }}$$ radicals in the kinetics of the methyl linoleate oxidation in micelles

The process of lipid peroxidation (LPO) plays an extremely important role in the human body due to the fact that its uncontrolled development can lead to oxidative stress and a number of serious diseases. The traditional approach to the analysis of the mechanism and kinetics of LPO is based on the well-known ideas that emerged from the study of radical-chain oxidation of hydrocarbons in a homogeneous medium. However, the distinctive feature of LPO is that this process is heterogeneous and the processes of diffusion of active intermediates between the aqueous and hydrocarbon phases should play a significant role in it. In the present work, an attempt has been made to theoretically estimate the contribution of these processes to the oxidation kinetics of model substances used in practice in the study of LPO. In the course of the calculations, a quantum chemical and kinetic analysis of the role of hydroperoxyl radical in the radical-chain mechanism of oxidation of methyl linoleate in micelles was carried out. The molecular dynamics method shows the important role of changing the dynamic rigidity of a hydrocarbon fragment of a chain during the formation of a peroxyl radical and hydroperoxyl group during the oxidation of a substrate in a heterogeneous medium. Quantum chemical calculations of the thermodynamics of reactions involving $${{{\text{HO}}_{2}}^{ \cdot }}$$ radicals and the effects of their solvation made it possible to estimate the kinetic constants of the reaction rates. Using kinetic modeling, their relative contribution to the oxidation kinetics was revealed. In this case, it is possible to explain a number of anomalies associated with the oxidation of polyunsaturated fatty acids and their esters in micelles compared with their oxidation in a homogeneous hydrocarbon medium.

An Olefinic 1,2‐Boryl‐Migration Enabled by Radical Addition: Construction ofgem‐Bis(boryl)alkanes

A series of in situ formed alkenyl diboronate complexes from alkenyl Grignard reagents (commercially available or prepared from alkenyl bromides and Mg) with B2 Pin2 (bis(pinacolato)diboron) react with diverse alkyl halides by a Ru photocatalyst to give various gem-bis(boryl)alkanes. Alkyl radicals add efficiently to the alkenyl diboronate complexes, and the adduct radical anions undergo radical-polar crossover, specifically, a 1,2-boryl-anion shift from boron to the α-carbon sp2 center. This transformation shows good functional-group compatibility and can serve as a powerful synthetic tool for late-stage functionalization in complex compounds. Measurements of the quantum yield reveal that a radical-chain mechanism is operative in which the alkenyl diboronates acts as reductive quencher for the excited state of the photocatalyst.

Auto-ignition control using an additive with adaptable chemical structure. Part I: Development of a kinetic model for 1,3-cyclohexadiene and 1,3,5-hexatriene combustion

1,3-cyclohexadiene (1,3-CHD) can be transformed into cis-1,3,5-hexatriene (1,3,5-HT) upon light irradiation, which makes it a potential additive able to change the reactivity of a conventional fuel. This paper presents the development of a detailed chemical kinetic model for the low-temperature (500–1200 K) combustion of 1,3-cyclohexadiene and 1,3,5-hexatriene. Theoretical calculations were performed to compute the thermochemistry of a large number of intermediates involved in the reaction mechanism, and for several kinetic parameters. In particular, the pericyclic reactions of 1,3-cyclohexadiene, linking it to cis-1,3,5-hexatriene, were studied theoretically. It was shown that 1,3,5-HT is inherently a secondary molecule of the 1,3-CHD mechanism and a comprehensive set of its oxidation reactions were included. Simulation of literature data (ignition delays and products speciation) measured in rapid compression machines for 1,3-CHD were performed using the newly developed kinetic model. A good agreement with experiments was found, and kinetic analyses highlighted the decomposition mechanism of 1,3-CHD and the most sensitive reactions affecting the auto-ignition delay times. Simulations of cis-1,3,5-HT auto-ignition in an RCM were also performed and compared to the ignition behaviour of 1,3-CHD. The simulation results showed that 1,3,5-HT combustion involves an induction period characterized by the predominant formation of 1,3-CHD, whose decomposition starts the radical chain mechanism.

Selective One‐Step Aerobic Oxidation of Cyclohexane to ϵ‐Caprolactone Mediated by N‐Hydroxyphthalimide (NHPI)

AbstractThe selective one‐step aerobic oxidation of cyclohexane to ϵ‐caprolactone was achieved in the presence of N‐hydroxyphthalimide (NHPI) and aldehyde under mild conditions. Remarkably, 12 % of cyclohexane was converted with a selectivity of 77 % of ϵ‐caprolactone and 15 % of KA oil. Control experiments indicated that NHPI accelerated the oxidation of aldehydes and peroxy radicals generated from aldehydes in situ were the key intermediates in the period of CH bond activation. 2,2,6,6‐Tetramethylpiperidine 1‐oxyl (TEMPO) addition and a series of m‐chloroperoxybenzoic acid (m‐CPBA) oxidation experiments showed that the oxidation proceeded via a complex radical chain mechanism.

Density functional theory study of selective aerobic oxidation of cyclohexane: the roles of acetic acid and cobalt ion.

A computational study of cyclohexane autoxidation and catalytic oxidation to a cyclohexyl hydroperoxide intermediate (CyOOH), cyclohexanol, and cyclohexanone has been conducted using a hybrid density functional theory method. The activation of cyclohexane and O2 is the rate-determining step in the formation of CyOOH due to its relatively high energy barrier of 41.2kcal/mol, and the subsequent reaction behavior of CyOOH controls whether the production of cyclohexanol or cyclohexanone is favored. Using CH3COOH or (CH3COO)2Co as a catalyst reduces the energy barriers required to activate cyclohexane and O2 by 4.1 or 7.9kcal/mol, respectively. Employing CH3COOH improves the CyOOH intramolecular dehydration process, which favors the formation of cyclohexanone. The energy barrier to the decomposition of CyOOH to CyO·, an important precursor of cyclohexanol, decreases from 35.5kcal/mol for autoxidation to 25.9kcal/mol for (CH3COO)2Co catalysis. (CH3COO)2Co promotes the autoxidation process via a radical chain mechanism. The computational results agree with experimental observations quite well, revealing the underlying role of CH3COOH and Co ion in cyclohexane oxidation. Graphical abstract Through DFT analysis of cyclohexane autoxidation and catalytic oxidation, we reveal the mechanism of the effects of CH3COOH and Co2+ on the reaction routes.

Halogen-Dependent Surface Confinement Governs Selective Alkane Functionalization to Olefins.

The product distribution in direct alkane functionalization by oxyhalogenation strongly depends on the halogen of choice. We demonstrate that the superior selectivity to olefins over an iron phosphate catalyst in oxychlorination is the consequence of a surface-confined reaction. By contrast, in oxybromination alkane activation follows a gas-phase radical-chain mechanism and yields a mixture of alkyl bromide, cracking, and combustion products. Surface-coverage analysis of the catalyst and identification of gas-phase radicals in operando mode are correlated to the catalytic performance by a multi-technique approach, which combines kinetic studies with advanced characterization techniques such as prompt-gamma activation analysis and photoelectron photoion coincidence spectroscopy. Rationalization of gas-phase and surface contributions by density functional theory reveals that the molecular level effects of chlorine are pivotal in determining the stark selectivity differences. These results provide strategies for unraveling detailed mechanisms within complex reaction networks.

Degenerative xanthate transfer to olefins under visible-light photocatalysis.

The degenerative transfer of xanthates to olefins is enabled by the iridium-based photocatalyst [Ir{dF(CF3)ppy}2(dtbbpy)](PF6) under blue LED light irradiation. Detailed mechanistic investigations through kinetics and photophysical studies revealed that the process operates under a radical chain mechanism, which is initiated through triplet-sensitization of xanthates by the long-lived triplet state of the iridium-based photocatalyst.