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© 2018 American Chemical Society. We examine in a simple organic aerosol the transition between heterogeneous chemistry under well-mixed conditions to chemistry under interfacial confinement. A single reaction mechanism, shown to reproduce observed OH oxidation chemistry for liquid and semisolid C30H62, is used in reaction-diffusion simulations to explore reactivity over a broad viscosity range. The results show that when internal mixing of the aerosol is fast and the particle interface is enriched in C-H groups, ketone and alcohol products, formed via peroxy radical disproportionation, predominate. As viscosity increases the reactions become confined to a shell at the gas-aerosol interface. The confinement is accompanied by emergence of acyloxy reaction pathways that are particularly active when the shell is 1 nm or less. We quantify this trend using a reaction-diffusion index, allowing the parts of the mechanism that control reactivity as viscosity increases to be identified.

The preparation of 9 alkyl 9 borafluorenes and their use as radical precursors in chain reactions were investigated. These organoboranes were found to be excellent precursors of primary and secondary alkyl radicals. Reactions were readily initiated by traces of oxygen and efficient processes involving sulfonyl based radical traps were discovered. Due to the very high reactivity of the intermediate 9 H and 9 alkyl 9 borafluorenes problems of reproducibility were identified. © ARKAT USA Inc.

The effects provoked by ionizing radiation or metabolic disturbances in a living organism result in hyperproduction of reactive species, including O-, C- and N-centered organic and inorganic radicals. Further transformations of such radicals lead to activation of free-radical processes causing damage to biologically relevant molecules. Such processes account for and/or participate in the development of cardiovascular, neurodegenerative, oncological and inflammatory diseases. So, radical-regulatory activity of substances should be mainly associated with their high reactivity towards different types of organic radicals. The subject of this study was investigation of new type tryptophan and its derivatives interactions with O- and C-centered radicals being formed during radiation- and peroxide-induced transformations of organic compounds and with N-centered 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical. Tryptophan and its derivatives displayed the antioxidant properties due to the ability of these compounds to reduce O-centered organic radicals via the reaction of electron transfer from the lone pair of nitrogen atoms. We have found that substances under study can add and oxidize C-centered organic radicals. Serotonin and 5-hydroxytryptophan are characterized by moderate reactivity toward the N-centered DPPH radical as compared with that of classical antioxidants.

International audience; The decomposition of the diazo initiator dimethyl 2,2′‐azobis(isobutyrate) (V‐601), generating the Me2C.(CO2Me) radical, affords essentially the same fraction of disproportionation and combination in media with a large range of viscosity (C6D6, [D6]DMSO, and PEG 200) in the 25–100 °C range. This is in stark contrast to recent results by Yamago et al. on the same radical generated from Me2C(TeMe)(CO2Me) and on other X‐TeR systems (X=polymer chain or unimer model; R=Me, Ph). The discrepancy is rationalized on the basis of an unprecedented RTe.‐catalyzed radical disproportionation, with support from DFT calculations and photochemicaL V‐601 decomposition in the presence of Te2Ph2.

PURPOSE: To demonstrate the effects of weak magnetic fields (> approximately 1 mT) on chemical reactions involving free radicals, in the context of possible effects of environmental electromagnetic radiation on biological systems. MATERIALS AND METHODS: Transient absorption, flash photolysis experiments have been performed to study the kinetics and yields of radical reactions. The triplet state of benzophenone has been used as a convenient source of radical pairs, whose identity is largely immaterial to the investigation of the so-called Low Field Effect. Hydrogen abstraction from surfactant molecules in micelles yields a pair of neutral radicals, one large and one small, in a region of restricted translational and rotational motion. RESULTS: In alkyl sulphate and sulphonate micelles a weak field increases the concentration of free radicals that escape from the micelle to an extent that depends on the structure, dynamics and volume of the space in which the radical pairs are confined. The effect (up to 10%) is typically largest at 1-2 mrT. Smaller effects are found for Brij and TX100 micelles. CONCLUSIONS: Low Field Effects depend strongly on the local environment of the radical pair. Larger effects than observed here might be expected for radicals formed from singlet (rather than triplet) precursors, as would be the case in biological reactions.

The reactions of hydroxyl radical ( OH) with guanidine, 1,1-dimethylguanidine, and 1-ethylguanidine have been investigated by the method of pulse radiolysis. The characteristic absorption observed in the OH reaction with guanidine under basic conditions was assigned to a nitrogen-centered radical. On the other hand, reducing carbon-centered radicals were generated in the OH reaction with 1,1-dimethylguanidine and 1-ethylguanidine over a pH range of 7–13, which may suggest that the primarily formed nitrogen-centered radicals at a high pH are converted into carbon-centered radicals by hydrogen atom shift. Similar transient species were also observed in the reaction of sulfate radical anion with the guanidine derivatives.

The authors thank EaStCHEM for funding. The elusive neutral bicarbonate radical and the carbonate radical anion form an acid/conjugate base pair. We now report experimental studies for a model of bicarbonate radical, namely methyl carbonate (methoxycarbonyloxyl) radical, complemented by DFT computations at the CAM-B3LYP level applied to the bicarbonate radical itself. Methyl carbonate radicals were generated by UV irradiation of oxime carbonate precursors. Kinetic EPR was employed to measure rate constants and Arrhenius parameters for their dissociation to CO2 and methoxyl radicals. With oleate and cholesterol lipid components methyl carbonate radicals preferentially added to their double bonds; with linoleate and linolenate substrates abstraction of the bis-allylic H-atoms competed with addition. This contrasts with the behavior of ROS such as hydroxyl radicals that selectively abstract allylic and/or bis-allylic H atoms. The thermodynamic and activation parameters for bicarbonate radical dissociation, obtained from DFT computations, predicted it would indeed have substantial lifetime in gas and nonpolar solvents. The acidity of bicarbonate radicals was also examined by DFT methods. A noteworthy linear relationship was discovered between the known pKa's of strong acids and the computed numbers of microsolvating water molecules needed to bring about their ionization. DFT computations with bicarbonate radicals, solvated with up to eight water molecules, predicted that only 5 water molecules were needed to bring about its complete ionization. On comparing with the correlation, this indicated a pKa of about -2 units. This marks the bicarbonate radical as the strongest known carboxylic acid. Postprint

Radical intermediates can undergo specific reactions, such as intramolecular rearrangements, i.e., the transpositions of radical centers, which are not known in classical ionic organic reactions. 1,5-Transposition of a radical center to a non-activated carbon atom are of great synthetic importance. It can be successfully applied for the introduction of different functional groups (oxygen, nitrogen, sulfur, halogens) onto a carbon atom remote from the present functional group. In addition to functionalization of a remote non-activated carbon atom, the formation of new C-C bonds on the ?-carbon atom have also been achieved. 1,5-Transposition of the radical centers takes place from alkoxyl, aminyl and carbon radicals to a remote carbon atom. Relocation of the radical centers preferentially involves 1,5-transfer of a hydrogen atom, although migrations of some other groups are known. The reactions of the carbon radical generated by 1,5-relocation of the radical center are presented and their synthetic applications are reviewed.

International audience; The synthesis and physical characterization of a new class of N-heterocycle-boryl radicals is presented, based on five membered ring ligands with a N(sp(2)) complexation site. These pyrazole-boranes and pyrazaboles exhibit a low bond dissociation energy (BDE; BH) and accordingly excellent hydrogen transfer properties. Most importantly, a high modulation of the BDE(BH) by the fine tuning of the N-heterocyclic ligand was obtained in this series and could be correlated with the spin density on the boron atom of the corresponding radical. The reactivity of the latter for small molecule chemistry has been studied through the determination of several reaction rate constants corresponding to addition to alkenes and alkynes, addition to O-2, oxidation by iodonium salts and halogen abstraction from alkyl halides. Two selected applications of N-heterocycle-boryl radicals are also proposed herein, for radical polymerization and for radical dehalogenation reactions.

Using a redox titration method in the pulse radiolysis of N2O-saturated phosphate buffer solution, the redox properties of the intermediate radicals derived from OH radical reactions of 5-substituted uracils 1c−h (R = F, Cl, Br, NO2, NH2, OH) have been characterized. While the primary intermediates were oxidizing C(5) radicals 2c−h and reducing C(6) radicals 3c−h of the respective OH adducts, a common allyl-type radical of “redox ambivalence” with limiting mesomeric forms of reducing carbon-centered 5-oxo-5,6-dihydrouracil-6-yl radical (4) and oxidizing oxygen-centered uracil-5-oxyl radical (5) was secondarily formed via elimination of 5-substituents from 3c−h. The rate of 5-substituent elimination decreased with varying structures of the 5-substituents in the order halo > amino, nitro > hydroxy groups. The common radical 4/5 showed weaker reducing/oxidizing abilities than the typical pyrimidine C(6) and C(5) radicals. The product study demonstrated that the one-electron reduction of 5-oxyl radical 5 by N...

A DFT study of radiation induced alkoxy radical formation in crystalline α-L-rhamnose has been performed to better understand the processes leading to selective radical formation in carbohydrates upon exposure to ionizing radiation at low temperatures. The apparent specificity of radiation damage to carbohydrates is of great interest for understanding radiation damage processes in the ribose backbone of the DNA molecule. Alkoxy radicals are formed by deprotonation from hydroxyl groups in oxidized sugar molecules. In α-L-rhamnose only one alkoxy radical is observed experimentally even though there are four possible sites for alkoxy radical formation. In the present work, the origin of this apparently specific action of radiation damage is investigated by computationally examining all four possible deprotonation reactions from oxygen in the oxidized molecule. All calculations are performed in a periodic approach and include estimates of the energy barriers for the deprotonation reactions using the Nudged Elastic Band (NEB) method. One of the four possible radical sites is ruled out due to the lack of a suitable proton acceptor. For the other three possible sites, the reaction paths and energy profiles from primary cationic radicals to stable, neutral alkoxy radicals are compared. It is found that deprotonation from one site (corresponding to the experimentally observed radical) differs from the others in that the reaction path is less energy demanding. Hence, it is suggested that the alkoxy radical formation is not necessarily site specific, but that the observed radical is formed in much greater abundance than the others due to the different energetics of the processes and reaction products.

The metal-insulator phase transition accompanying charge disproportionation was studied for two charge-transfer salts, θ-(BDT-TTP) 2 Cυ(NCS) 2 and (TTM-TTP)I 3 . In the former compound, the localized generates a charge disproportionation such as (+0.1, +0.9). On the other hand, the charge disproportionation was not found in (TTM-TTP)I 3 . Alternatively, the intra-molecular charge distribution becomes asymmetric below the phase transition temperature Both phenomena are derived from the localization of charge with a symmetry breaking.

This article covers radical rearrangements that do not involve the redistribution of the carbon skeleton of the substrate. The focus is on the rearrangement of ester-substituted radicals and on hydrogen atom abstractions, and the emphasis is on the radical rearrangements themselves rather than on their application in synthesis, which is left to the imagination and creativity of the practitioner. Keywords: alkoxy radicals; aminium radical cations; aryl radicals; vinyl radicals; acyloxyalkyl radicals; phosphatoxyalkyl radicals; alkene radical cations; radical ion pairs; hydrogen atom abstraction

This account is focused on carbon-carbon, and carbon-sulphur bond-forming reactions through the use of carbon- and sulphur-centred radicals in water, respectively. It intends to show the scope and applications on these types of radicals with a synthetic goal, thus helping the Radical Synthetic Chemist to grab the fundamental aspects devoted to the synthesis of a wide array of organic compounds through radical methods in environmentally friendly media. This account excludes the syntheses of organic substrates through the aid of organometallic / metal-centred radicals such as other Group IV-radicals and transition-metal radicals, and the important class of radical reductions currently accomplished in water through different metallic reducing species. The array of substrates synthesized through the present method encompasses intermolecular and intramolecular radical carbon-carbon bond formation reactions in water, and radical carbon-sulphur bond formation reactions. Among the radical carbon-carbon bond formation reactions, special attention is devoted to atom-transfer radical reactions, radical addition reactions to olefins, radical addition to carbon-nitrogen double bonds, and carboaminohydroxylation of alkenes, and the analogues of Knovonoegel reactions, synthesis of functionalized enol ethers, carboazidation and azidation, and ynol formation, and radical cyclization reactions in water. Fil: Postigo, Jose Alberto. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay; Argentina

Reductive dehalogenation of 5-bromouracil by aliphatic organic radicals CO 2 − , CH 2 OH, CH(CH 3 )OH, and CH(CH 3 )O − have been studied in oxygen free aqueous solutions in the presence of organic additives: formate, methanol or ethanol. For radicals production 60 Co γ-radiolysis was employed and the yield of bromide was measured by means of ion chromatography. Both radical anions have reducing potential negative enough to transfer an electron to BrU producing bromide ion and U radical. High yields of bromide have been measured increasing proportional to the concentration of the corresponding organic additives at a constant dose rate. This is characteristic for a chain process where regeneration of radical ions occurs by H-atom abstraction by U radical from formate or ethanol. Results with the neutral radicals conformed earlier proposition that the reduction reaction of α-hydroxyalkyl radicals proceeds by the proton-coupled electron transfer mechanism ( Matasovic and Bonifacic, 2007 ). Thus, while both CH 2 OH and CH(CH 3 )OH did not react with BrU in water/alcohol solutions, addition of bicarbonate and acetate in mmol dm –3 concentrations, pH 7, brought about chain debromination to occur in the case of CH(CH 3 )OH radical as reactant. Under the same conditions phosphate buffer, a base with higher bulk proton affinity, failed to have any influence. The results are taken as additional proofs for the specific complex formation of α-hydroxyalkyl radicals with suitable bases which enhances radicals' reduction potential in comparison with only water molecules as proton acceptors. Rate constants for the H-atom abstraction from ethanol and formate by U radicals have been estimated to amount to about ≥85 and 1200 dm 3 mol –1 s –1 , respectively.

Toluene pyrolysis was studied between 350 °C and 400 °C under 700 bar pressure using confined pyrolysis (gold cells). A detailed kinetic model consisting of 30 free-radical reactions was developed to describe the thermal cracking of toluene at low conversion. The mechanism considered is consistent with the experimental results concerning conversion as well as formation of the main products (benzene, methane, biaromatic compounds). This study shows that high pressure and low temperature are favorable to bimolecular reactions. Thus, additions of benzyl radicals to aromatic rings lead to the formation of biaromatic compounds. Yet, the most important conclusions concern the initiation reactions for which a bimolecular mechanism (or RRD: reverse radical disproportionation) needed to be considered. The relative importance of the monomolecular vs. bimolecular initiation mechanism changes as a function of temperature and pressure. The extension of the published mechanisms by our results allows to significantly increase the temperature–pressure domain for the modeling of toluene pyrolysis.

Over the last thirty years organic azides have drawn a great deal of attention as radical traps for carbon- and heteroatom-centered radicals, both in intra- and in intermolecular processes. The resulting intermediates (nitrogen-centered radicals such as triazenyls, aminyls, or even iminyls) can be conveniently employed in the synthesis of a variety of cyclic and acyclic nitrogen-containing compounds.

Boron derivatives are becoming key reagents in radical chemistry. Here, we describe reactions where an organoboron derivative is used as a radical initiator, a chain-transfer reagent, and a radical precursor. For instance,B-alkylcatecholboranes, easily prepared by hydroboration of alkenes, represent a very efficient source of primary, secondary, and tertiary alkyl radicals. Their very high sensitivity toward oxygen- and heteroatom-centered radicals makes them particularly attractive for the development of radical chain processes such as conjugate addition, allylation, alkenylation, and alkynylation. Boron derivatives have also been used to develop an attractive new procedure for the reduction of radicals with alcohols and water. The selected examples presented here demonstrate that boron-containing reagents can efficiently replace tin derivatives in a wide range of radical reactions.

The application of acyl radicals in radical addition reactions in the absence of a CO atmosphere is generally limited to aryl or alpha-unsubstituted alkyl acyl radicals due to competing decarbonylations where the rate constant for this degradation process surpasses 104 s-1. In this work, a potential solution to avoid the problem of decarbonylations is presented employing N-acyl oxazolidinones which are reduced to acyl radical equivalents in the presence of samarium diiodide and water. In the company of an acrylamide, acrylate, or acrylonitrile, the product from a formal acyl radical addition is obtained in yields up to 87%. Examples are given where the decarbonylation rate constants even exceed 108 s-1. It is proposed that the reaction proceeds via a ketyl-like intermediate.

Alkyl radicals produced in the indirect reduction of alkyl halides or alkyldimethylsulfonium salts by electrochemically generated aromatic radical anions couple fast with the latter and alkylated or dialkylated dihydro compounds are formed. Rate constants measured for the coupling reaction between on one hand methyl, primary, secondary and tertiary alkyl radicals as well as benzyl and cumyl radicals and on the other hand a wide spectrum of electrochemically generated aromatic radical anions are found to be about 1×10 9 M −1 s −1 . Previous measurements of coupling rate constants for primary alkyl radicals have been re-evaluated since they were affected by the presence of an S N 2 reaction occurring between the alkyl halides used as radical precursors and the aromatic radical anions. New experiments are also included using alkyldimethylsulfonium salts as precursors in order to prevent such S N 2 artefacts. It is concluded that sterical hindrance does not play a significant role for the radical–radical anion coupling reactions. In general the rate constants for the coupling reactions are all close to 10 9 M −1 s −1 .

Several reactions in heterogeneous catalysis take place via radical intermediates. The present article illustrates the typical aspects of the radical chemistry in heterogeneous catalysis that very often differ from those in the radical chemistry in fluid homogeneous phases because of the role of the surface in the generation, stabilization, and transformation of radical species. The various routes for the formation of radicals on a surface will be taken into consideration including electron transfer, atom transfer, and molecular dissociation. A particular emphasis is given to the electron magnetic resonance techniques, which have the paramount role in the detection and also in the description of the radical species because of their high sensitivity that allows the detection of low abundance species. In the final part of the article some selected examples of heterogeneous catalytic processes involving radical steps will be discussed with particular emphasis on catalytic oxidation on metal oxides. Keywords: radicals; heterogeneous catalysis; surface; surface stabilized radicals; electron magnetic resonance; EPR; ESR; metal oxides; catalytic oxidation; surface radical intermediates

Proteins damaged by free-radical-generating systems in the presence of oxygen yield relatively long-lived protein hydroperoxides. These hydroperoxides have been shown by e.p.r. spectroscopy to be readily degraded to reactive free radicals on reaction with iron(II) complexes. Comparison of the observed spectra with those obtained with free amino acid hydroperoxides had allowed identification of some of the protein-derived radical species (including a number of carbon-centred radicals, alkoxyl radicals and a species believed to be the CO2 radical anion) and the elucidation of novel fragmentation and rearrangement processes involving amino acid side chains. In particular, degradation of hydroperoxide functions on the side chain of glutamic acid is shown to result in decarboxylation at the side-chain carboxy group via the formation of the CO2 radical anion; the generation of an identical radical from hydroperoxide groups on proteins suggests that a similar process occurs with these molecules. In a number of cases these fragmentation and rearrangement reactions give rise to further reactive free radicals (R., O2-./HO2., CO2-.) which may act as chain-carrying species in protein oxidations. These studies suggest that protein hydroperoxides are capable of initiating further radical chain reactions both intra- and inter-molecularly, and provide information on some of the fundamental mechanisms of protein alteration and side-chain fragmentation.

Free radicals centred on tin and silicon react with 1,6, 6aλ4-trithiapentalenes (1) at the 1(6)-position to give substituted pentadienyl radicals (3) which have been studied by e.s.r. spectroscopy.

The radicals formed during photolysis of di-t-butyl peroxide and a number of alkenyldimethylsilanes have been examined by e.s.r. spectroscopy. Only carbon-centred radicals were observed. These were either secondary alkyl radicals formed by the addition of the initially formed silyl radical to a double bond and/or allyl radicals formed by hydrogen-atom abstraction from the alkenyl group. In most cases addition to the double bond was an intermolecular process. However, pent-4-enylsilyl radicals undergo intramolecular addition with endo-cyclization to form the six-membered ring being strongly favoured over exo-cyclization to form the five-membered ring. This preference was confirmed by product studies on 3,3-dimethylpent-4-enyldimethylsilane. The factors that control homolytic exo : endo cyclization rates for species in which the radical centre is located on a second row element are adumbrated.

The ionization potentials of methyl, allyl, and benzyl radicals have been measured directly by electron impact. The values found were methyl 9.95±0.03 ev, allyl 8.16±0.03 ev, and benzyl 7.73±0.08 ev. The radicals were produced by thermal decomposition of suitable compounds in a mass spectrometer designed for the study of free radicals. Reactions of the radicals after formation led to the expected dimers, but the disproportionation reactions were found to be negligibly slow.

The ratios of rate constants for disproportionation to combination have been measured for ethyl radicals and for i-propyl radicals in liquid methane between −181 and −94 °C. The radicals were generated by γ-radiolysis of dilute methane solutions of ethylene-d4 or propylene-d6. The activation energy for combination was found to exceed that for disproportionation by 290 ± 30 cal mol−1 for ethyl radicals and by 255 ± 25 cal mol−1 for i-propyl radicals. In both cases the disproportionation—combination ratio in the liquid, extrapolated to room temperature, is greater than that in the gas phase by a factor of about 2.5. These results are interpreted as indicating that disproportionation and combination reactions proceed by way of different transition states.