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Radicals are ubiquitously involved in numerous chemical and biological processes. They exhibit unique chemical bonding and physical properties applicable to functional materials. With respect to carbon and nitrogen, boron is commonly viewed as a lone-pair electrons acceptor and less often considered as a spin-carrier associated with radicals. In recent years, however, boron-containing radicals have attracted more and more interest on account of their potential applications such as chemical sensors, reagents in organic synthesis, initiators for polymerization and components of extended magnetic systems. This review presents the synthesis, characterization, and reactivity of persistent and stable boron-containing radicals in conjunction with theoretical calculations.

Phosphomolybdic acid (H3PMo12O40) is reported as a green low-cost catalyst for the synthesis of symmetrical / unsymmetrical thiosulfonates via the disproportionate coupling reaction of sulfonyl hydrazide. The attributes of this reported catalytic system include low catalyst loadings (1 mol%), efficient turnover, and high yields (up to 94%). Additionally, this reaction mechanism involves the formation of a thiyl radical and sulfonyl radical via sulfinyl radical disproportionation.

This paper results from combining Electron Spin Resonance (ESR) spectroscopy and atom transfer radical polymerization (ATRP) to study the basic chemistry of radical polymerizations. This combination of analytical and controlled preparation techniques can provide information on chain length of propagating radicals and chain transfer reactions to polymers which has been previously difficult or impossible to study by direct detection of radicals in radical polymerizations by ESR. Two results are obtained by the ESR/ATRP combination technique. The first provides an estimation of the effect of chain length on the structure of methacrylate propagating radicals, especially in the initial stage of radical polymerizations. Model radical precursors of dimeric, trimeric, tetrameric, and pentameric structures of tBMA and MMA were prepared by ATRP. The corresponding model radicals were generated from these precursors and were observed by ESR at various temperatures. The ESR spectra provided direct information on chain length dependent changes of the averaged structures at the chain ends of the model propagating radicals, indicating that distortion in dihedral angles between chain end pπ-orbital and Cβ-H bonds increased with increasing chain length. The second result provides information on the radical migration reactions during the polymerization of acrylates. Radical precursors of ethyl-, n-butyl-, and dodecyl acrylates with various chain lengths (DP = 10, 25, 60, etc.) were prepared by ATRP. Model propagating radicals were generated from these precursors and were observed by ESR at various temperatures. These radicals showed chain length dependent spectroscopic change due to radical migration reactions from model propagating to mid-chain radicals with increasing temperature. Longer chained radicals with the same ester side groups showed easier formation of mid-chain radicals. An examination of side group dependency indicated that, longer alkyl side groups showed easier radical migration reactions. The development of ATRP enabled us to resolve these problems associated with side reactions in radical polymerizations.

The OH-initiated oxidation of p -nitroaniline ( p- NA) was studied both in gas phase and aqueous solution. The calculated results show that addition reactions between p -NA and OH radicals are highly feasible in gas phase. OH addition to the ortho C of the amino group is the most favorable. The major products are p -nitrophenol ( p -NP), 2-amino-5-nitrophenol, 5-amino-2-nitrophenol and p -aminophenol. The H atom of the amino group is the easiest one to be abstracted by OH radical. The undergoing degradation is investigated in the presence of O 2 , which generate oxygen-rich compounds including five-membered heterocycles or six-membered heterocycles and 2-amino-5-nitrophenol or 5-amino-2-nitrophenol. The effect of water solution on the title reactions was considered. The calculated results show that aqueous solution makes the addition reactions between p -NA and OH radical more uniform while H-abstraction reactions are not significant. The total rate constant of OH radicals with p -NA under 298 K, 1 atm is determined to be 6.97 × 10 −11 cm 3 molecule −1 s −1 . With respect to OH radical, the life time of p -NA in atmosphere is estimated to be 4.1 h.

In pulse radiolysis investigations the hydroxyl radical formed in water radiolysis reacts with 2,6-dichloroaniline in radical addition to the ring forming hydroxy-cyclohexadienyl radical and also in hydrogen atom abstraction from the amino group resulting in anilino radical. The hydroxy-cyclohexadienyl radical in the absence of dissolved O 2 partly transforms to anilino radical, when dissolved oxygen is present the radical transforms to peroxy radical. According to chemical oxygen demand measurements the reaction of one OH radical induces the incorporation of 0.6 O 2 into the products. It is a typical value for chlorine atom substituted aromatic molecules, and smaller than found for molecules without chlorine atom (1.0–2.0).

The gas-phase reactivity of sulfur-based radical ions of cysteine derivatives and Cys-containing small peptides was studied via the use of ion-molecule reactions inside a quadrupole ion trap mass spectrometer. Both the radical cations M + and the anions (M−2H) − were generated via the gas-phase homolysis of the S NO bond of the S-nitrosylated precursors of the following: Cys, N-Ac-Cys, CysOMe, Gly-Cys, Cys-Gly, γ-Glu-Cys, γ-Glu-Cys-Gly, and Gly-Cys-Arg. The radical ions were allowed to react with the following volatile neutral reagents: allyl iodide, allyl bromide, nitric oxide, 1-propane thiol, 3-mercaptopropionic acid, dimethyl disulfide and dimethyl sulfide. The charge has little effect on the types of products formed, with typical S-based radical reactivity being observed. In several systems (N-Ac-Cys, Cys-Gly, Gly-Cys, γ-Glu-Cys-Gly, and Gly-Cys-Arg) the radical was found to lose its reactivity very rapidly, which is consistent with S-to-α-carbon radical rearrangement via hydrogen atom transfer. The observed radical reactivity serves as a model for studying the chemistry of cysteine-based thiyl radicals in protein systems.

In the course of our attempts to replace the traditional, but harmful, organic solvents in synthetic processes by environmentally favored benign media, we studied the disproportionation of hydrophobic vicinal dihydroarenes in water. This reaction takes place efficiently by either sol–gel-entrapped [(C 8 H 17 ) 3 NCH 3 ][RhCl 4 ] ion pair or by immobilized Pd(OAc) 2 , together with a suitable surfactant to give tetrahydroarenes and fully aromatic products. Since a slow dehydrogenation process accompanies this process the latter products are usually formed in excess. The reactions are also accompanied by transformation of the heterogenized catalysts into sol–gel-stabilized metallic particles, which can be recycled without loss of activity for at least five runs. The rhodium (but not the palladium) promoted disproportionations that follow the pseudo first order rate law. The reactions are influenced by both the electronic and steric structures of the dihydroarenes, as well as by the nature of the surfactant and by the hydrophobicity of the ceramic sol–gel support. Deuterio-labeling indicates slight involvement of the water in the disproportionation process.

PLP catalyzes the 1,2 shifts of amino groups in free radical-intermediates at the active sites of amino acid aminomutases. Free radical forms of the substrates are created upon H atom abstractions carried out by the 5'-deoxyadenosyl radical. In most of these enzymes, the 5'-deoxyadenosyl radical is generated by an iron-sulfur cluster-mediated reductive cleavage of S-adenosyl-(S)-methionine. However, in lysine 5,6-aminomutase and ornithine 4,5-aminomutase, the radical is generated by homolytic cleavage of the cobalt-carbon bond of adenosylcobalamin. The imine linkages in the initial radical forms of the external aldimines undergo radical addition to form azacyclopropylcarbinyl radicals as central intermediates in the catalytic cycles. In the case of lysine 2,3-aminomutase, the multistep catalytic mechanism is corroborated by direct spectroscopic observation and characterization of a product radical trapped during steady-state turnover. Analogues of the substrate-related radical having substituents adjacent to the radical center to stabilize the unpaired electron are also observed and characterized spectroscopically. A functional allylic analogue of the 5'-deoxyadenosyl radical is observed spectroscopically. A high-resolution crystal structure fully supports the mechanistic proposals. Evidence for a similar free radical mediated amino group transfer in the adenosylcobalamin-dependent lysine 5,6-aminomutase is provided by spectroscopic detection and characterization of radicals generated from the 4-thia analogues of the lysine substrates. This article is part of a Special Issue entitled: Pyridoxal Phospate Enzymology.

2,2'-Azobis(2-amidinopropane) dihydrochloride (AAPH), a free radical-generating azo compound, is gaining prominence as a model oxidant in small molecule and protein therapeutics, namely for its ability to initiate oxidation reactions via both nucleophilic and free radical mechanisms. To better understand its degradation pathways, AAPH was degraded at 40°C in aqueous solutions over a wide pH range. Samples were analyzed via liquid chromatography-ultraviolet spectroscopy and liquid chromatography-tandem mass spectrometry (LC-MS/MS). The thermal decomposition rate of AAPH to form radical species averaged 2.1 × 10(-6) s(-1) and did not vary significantly with pH. The hydrolysis rate increased exponentially with pH, showing hydroxide ion dependence. A mechanism for AAPH hydrolysis is proposed. The LC-MS/MS results provided evidence that the alkoxyl radical is a major radical species in solution. The LC-MS/MS results also showed a radical disproportionation reaction and enabled the generation of an overall reaction scheme showing the various side and termination products of AAPH degradation.

Three modes of amine degradation have been previously identified—direct oxidation by metal ions, thermal disproportionation and autoxidation. Results are presented in which products of disproportionation reactions of quaternary compounds are compared to those seen in the thermal degradation of methyldiethanolamine, MDEA. This strongly suggests a role of a quaternary or a protonated amine intermediate in this reaction. Products of MDEA autoxidation at 140 °C also show characteristics of disproportionation reactions. Potential mechanisms are presented to account for the products seen.

Intramolecular 1-n H-shift ( n = 2, 3… 7) reactions in alkoxy, alkyl and peroxy radicals were studied by density functional theory (DFT) at the B3LYP/6-311+G∗∗ level and compared with respective intermolecular H-transfers. It was found that starting from 1 to 3 H-shift the barrier heights stepwise decrease with increasing n reaching minimum for 1–5 and 1–6 H-shifts. This dependence can be ascribed to the decrease of the strain with increasing transition state (TS) ring size, which is minimal in six- and seven-member rings. The barrier heights of H-shifts in alkyl radicals are systematically larger than those in alkoxy radicals: the respective activation energies (E a ) of 1–5 and 1–6 H-shifts are about 59–67 kJ/mol for alkyl radical and 21–34 kJ/mol for alkoxy radicals. Further increase of the TS ring size in 1–7 H-shifts leads to the increase of the barrier to 44 kJ/mol in the hexyloxy radical and 84 kJ/mol for n-heptyl radical. We have also found that intermolecular H-transfer reactions in all three types of free radicals have smaller barriers than respective intramolecular 1–5 or 1–6 H-shifts by 4–25 kJ/mol. The mentioned difference can be explained in terms of enhanced nonbonding repulsion interaction in the cyclic TS structures compared to respective intermolecular TS. B3LYP/6–311+G∗∗ geometric parameters and imaginary frequencies for 1-n H-shifts TS are consistent with respective calculated barrier heights. Reactivity of some other radicals compared to alkoxy, peroxy and alkyl radicals as well as other factors influencing their reactivity (π-conjugation, steric effect and ring strain in cyclic TS, etc.) are also briefly discussed in relation to free radical reactions in polymer systems.

A method for the study of reactions of open-shell neutrals (radicals) and radical cations is described. Pyrolysis (25-1500 degrees C) of thermally labile compounds, such as, 1,5-hexadiene via a Chen nozzle yields a seeded beam of reactive species in helium. The pyrolysis products are then analyzed by electron ionization (EI) or reacted with stored ions. Electron ionization of the pyrolysis products of 1,5-hexadiene shows that both the allyl radical and allene are generated. Reactions of benzene radical cations and the pyrolysis products of 1,5-hexadiene result in carbon-carbon bond formation. Those reactions of allyl radical with the benzene radical cation yield the C(7)H(7) (+) ion of m/z 91, permitting an unusual entry into arenium ions. The reaction of allene with benzene radical cation in contrast yields C(9)H(10) (+). and C(9)H(9) (+).

The metal salts mediated oxidative free radical reaction of 2-amino-1,4-benzoquinones is described. Imine radicals can be generated by the oxidation of 2-amino-1,4-benzoquinones with Mn(III) and Ag(II). The dimeric products 4 and 14 were formed via the intermolecular radical coupling reaction of the corresponding radical intermediates 5 and 15. In the presence of styrene, twistane 17 was afforded from 2-phenylamino-1,4-benzoquinone 1 via a radical annulation reaction of imine radical 5.

EPR spectroscopy has been used for the study of the free radicals formed in the single crystals of 2-thiothymine (5-methyl-2-thiouracil) γ-irradiated at room temperature (300 K). The radicals formed by a net hydrogen-atom addition to C(6) (5-thymyl or 5-yl radicals) and by a net hydrogen abstraction from the methyl group (7-thymyl or 7-yl radicals) has been studied in detail. The hyperfine tensors of proton couplings in these radicals are given together with their g-tensors. It has been shown that the calculated hyperfine proton couplings and relative orientation of the coupling tensors along the pyrimidine ring are similar to those observed earlier in 5-yl and 7-yl radicals in the single crystals of thymine and its derivatives. Although no large differences in hyperfine couplings were found, calculated values of the g-tensor elements of the 5-yl radical are appreciable larger, what is expected due to the presence of sulfur at C(2) in 2-thiothymine. Also, the observed superhyperfine 1:2:2:1 splitting of the resonance lines of the 5-yl radical indicated that the presence of sulfur in 2-thiothymine slightly affects the spin distribution in this radical insomuch that appreciable amount of spin is located on one of N–H fragments of 2-thiothymine.

The impact of nitroxy-substituents, –ONO 2 , on the dominant chemical reactions in the atmosphere is still not fully understood. A theoretical DFT and coupled cluster theory analysis of alkyl radicals with nitroxy-substituents on the radical carbon revealed that such radicals are unstable, dissociating by O–N bond scission and leading to a carbonyl compound and NO 2 . This dissociation occurs sequentially after completion of the reaction forming the α-nitroxy-substituted alkyl radical, such that the formation process is independent of the subsequent O–N bond scission. The presence of a β-nitroxy substituent in alkoxy radicals decreases the barrier to decomposition by 12 kJ mol −1 .

Alkoxy radicals are key intermediates in the atmospheric degradations of volatile organic compounds, and can typically undergo reaction with O2, unimolecular decomposition or unimolecular isomerization. Previous structure–reactivity relationships for the estimation of rate constants for these processes for alkoxy radicals [Atkinson, R., 1997. Atmospheric reactions of alkoxy and β-hydroxyalkoxy radicals. International Journal of Chemical Kinetics, 29, 99–111; Aschmann, S.M., Atkinson, R., 1999. Products of the gas-phase reactions of the OH radical with n-butyl methyl ether and 2-isopropoxyethanol: reactions of ROC(O )

The continuous development of more active and stable catalysts in atom transfer radical polymerization (ATRP) has increasingly required a thorough knowledge of concurrent electron transfer reactions that can affect catalyst performance. Special attention is provided in this short review to such processes, including disproportionation, most pronounced in Cu-mediated ATRP, the reduction of radicals to carbanions or oxidation to carbocations, and radical coordination to the metal catalyst resulting in the interplay of controlled radical polymerization mechanisms.

The structures and energies of forming C1′, C2′, C3′ and C4′ radicals in sugar, and C5′ radical in various nucleotides have been studied by ab initio and DFT methods. Among these radicals, the radical at C1 position of sugar forms at lower energy level, and the efficiency of generating this radical will be more than other radicals. The puckering of sugar in 2-deoxy adenosine 5-monophosphate and 2-deoxy thymidine 5-monophosphate after radical formation at C1 is significant, herein C2 endo is observed in C1′ sugar radical compared to the C3 endo structure in its non-radical counterpart. In the case of 2-deoxy guanosine 5-monophosphate and 2-deoxy cystidine 5-monophosphate, C2 endo structures in sugar are found and distinct flattening of C3 center occurs after radical formation. There are intramolecular hydrogen bonds in the non-radical and radical structures of 2-deoxy guanosine 5-monophosphate and 2-deoxy adenosine 5-monophosphate nucleotides. In addition, significant variation of puckering amplitudes and phase angles between non-radical and C1′ sugar radical are also found.

The stable radicals derived from different compounds were detected in process of styrene autopolymerization. The nitroxide radicals are produced from nitrosocompound, hindered hydroxylamine, nitrophenols and nitroanisoles. The phenoxyl radicals are formed from quinine methides, and naphtoxyl radicals are generated from 2-nitro-1-naphtol. The radicals are identified, the kinetics of their formation and follow-up evolution are studied. These radicals can participate in process of living radical polymerization as the mediators and can effect significantly on kinetics of polymerization and structure of the resulting polymer.

Effects of α-tocopherol, PMC, and a number of the respective sulfur-containing analogues on reactions involving various organic radicals were studied. The test compounds were found to interact with alkyl radicals more effectively than with peroxyl radicals. The presence of a sulfur atom in structures of the respective analogues did not produce significant effects on reactivity. Derivatives of 5-hydroxy-1,3-benzoxathiol-2-one and 6-hydroxy-1,4-benzoxathiin-2(3H)-one displayed a high reactivity toward α-hydroxyalkyl radicals.

The body constantly reacts with oxygen as part of the energy producing processes of cells. Oxidative stress is a dysbalance between the production of free radicals as products of these reactions and antioxidant properties of cells. The factors influencing the production of free radicals are physical agents, chemical agents and biological agents. Free radicals are paramagnetic molecules with short time-period for their detection by electron spin resonance (ESR) spectroscopy. The free radical stabilization can be gained by freezing a solution of an organic radical or bonding to spin trapping agents. The spin trapping agents are diamagnetic compounds which rapidly scavenge transient radicals to form stable paramagnetic spin adducts radicals. Because this secondary radical retains an unpaired electron, it can often be detected by electron spin resonance. From ESR spectra can be obtained structural information and kinetic information, information about the formation and decay of the radicals. To study the process of free radical generation is an important step towards reducing the deteriorating effects of oxidative stress.

The views from solution-type chemistry cannot be transposed to polymer melts. The bimolecular reaction between hydroperoxide and alcohol groups is a typical example in this respect. It is the dominant hydroperoxide decomposition reaction in the advanced stages of polyethylene processing in the high temperature range (170–200 °C). In part, the alkoxy/alkoxy radical pair resulting from the decomposition reacts directly in the cage by disproportionation and combination. In part, it is transformed into a caged alkoxy/alkyl radical pair following hydrogen abstraction by one of the alkoxy radicals. The alkoxy/alkyl radical pair reacts by combination or disproportionation in the cage or is transformed into an alkyl/alkyl radical pair on hydrogen abstraction by the remaining alkoxy radical. Combination or disproportionation of the alkyl/alkyl radical pair is considerably faster than oxygen addition and/or cage disruption. The bimolecular reactions of the three caged radical pairs remove the free radicals from the polymer melt. There is no chain reaction initiated by those radicals. This explains previous findings concerning the absence of initiation by the hydroperoxides formed and decomposed in polyethylene melts. The relative quantities of oxidation products calculated from the model are compatible with the experimental results. The comparison yields quantitative values for the probabilities of the different cage reactions. The rate constants for the reactions between the alkoxy/alkoxy and alkoxy/alkyl radical pairs can be deduced from the probabilities, taking intramolecular hydrogen abstraction by the alkoxy radical as a reference. They are comparable to the corresponding rate constants for low molecular mass compounds in solution. The knowledge of these values allows calculation of the oxidation product yields at any temperature.

The equilibrium constants for disproportionation of anthracene and perylene radical anions into neutral and dianion both in the absence and presence of solvent and for ion pairing of the dianions to tetraethylammonium ion have been computed by density functional B3LYP/6-311+G* theory. The disproportionation equilibrium is shown to be highly influenced by differential solvation effects and only modestly by ion pairing. Keywords: Radical anions, Dianions, Disproportionation, Ion pairing, Solvation, Computations

Synthesis of novel heterocycles using radical reactions have been described. Reaction mechanism of their formation is also presented.

Sylvatesmin (SYL) and lantbeside (LAN) are two lignans, isolated from a Chinese folk medicinal herb, Lancea tibetica . Their abilities of scavenging oxidizing free radical models, OH, SO 4 − and N 3 , were investigated in aqueous solution by pulse radiolysis techniques. The OH-adduct radicals with small amount of oxidized products were formed by the reaction of SYL or LAN with OH radical. SYL undergoes one-electron oxidation either by SO 4 − or N 3 radicals, LAN can only be detected to react with stronger oxidant SO 4 − radical anion. No reaction between LAN and N 3 radical was detectable. The relationship of structure with the abilities of scavenging free radicals was discussed. The reaction rate constants were determined by analysis of the build-up trace of the radical products.

In the radiolysis of N2O-saturated aqueous solutions the OH radicals in their reactions with polymers give rise to polymer-derived radicals. The kinetics of the formation and decay of these radicals are reviewed. Once a larger number of radicals are formed on one polymer molecule, e.g. upon pulse radiolysis, close-by radicals recombine more rapidly while the more distant ones survive for much longer times. Intermolecular cross-linking (between two polymer chains, increase in molecular weight) and intramolecular cross-linking (formation of small loops, no increase in polymer weight) are competing processes, and their relative yields thus depend on the dose rate and polymer concentration. Hydrogen-transfer reactions within the polymer, e.g. transformation of a secondary radical into a tertiary one, are common and facilitated by the high local density of reactive sites. Due to repulsive forces, the lifetime of radicals of charged polymers is substantially increased. This enables even relatively slow β-fragmentation reactions to become of importance. In the case of poly(methacrylic acid) this even leads to an unzipping, and the situation of equilibrium polymerisation is approached. Heterolytic β-fragmentation is possible when adequate leaving groups are available, e.g. in polynucleotides and DNA. In the presence of O2, chain scission occurs via oxyl radicals as intermediates.

Reactions of the adduct radical obtained by addition of the initiator-derived 2-carbomethoxy-2-propyl radical to methyl α-[(2-methyl-2-carbomethoxy)propyl]acrylate (the unsaturated dimer of methyl methacrylate (MMAD)) have been monitored by electron paramagnetic resonance spectroscopy. Rate constants for addition of the 2-carbomethoxy-2-propyl radical to MMAD, mutual reactions between the adduct radicals, and reactions of the adduct radical and the 2-carbomethoxy-2-propyl radical, and β-fragmentation of the adduct radical were evaluated by steady-state and non-steady-state kinetic analysis in tandem with computer modeling and simulations involving parameter optimization procedures. The resulting rate constants showed that addition and radical–radical reactions between low molecular weight radicals are suppressed due to the marked steric congestion of the carbon-carbon double bond and radical center, respectively, and that β-fragmentation can compete with radical–radical reactions of the adduct radicals depending on the radical concentration.

A series of 1,5-dialkyl-6-oxoverdazyl radicals with differing N1 and N5 substituents were synthesized as a means to observe the effects of different alkyl groups on the reactivity of verdazyl radicals towards disproportionation with a methyl group as the benchmark. Using previously described cycloaddition chemistry with verdazyl radical-derived azomethine imines via disproportionation, methyl acrylate was used to trap the disproportionation products in situ in order to observe the N1 versus N5 site-selectivity of this process. Surprisingly, in most cases the larger alkyl groups proved to be the more reactive as compared to methyl explained by the weakening of the C–H bond involved in a hydrogen atom abstraction as part of the disproportionation. These results contrast with the idea that greater steric bulk generally makes radicals less reactive.

Kinetics of reactions of model oligosiloxanols, 1,1,3,3,3-pentamethyldisiloxane-1-ol (MDH) and 1,1,3,3,5,5,5-heptamethyltrisiloxane-1-ol (MD2H), which occur in the presence of phosphazenium superbase, hexapyrrolidine-diphosphazenium hydroxide, in an acid–base inert solvent, toluene, was studied using sampling and gas chromatographic analysis method. In addition, kinetics of reactions of MDH and MD2H with trimethylsilanol (MH) was studied. In the MDH and MD2H systems the oligosiloxanol condensation competes with the oligosiloxanol disproportionation, the latter being the dominating process. The disproportionation products, i.e. MDn+1H and MDn−1H, n=1, 2, … undergo analogous consecutive disproportionation and condensation reactions. The kinetic law was derived and rate parameters determined from initial rates and by computer simulation to the best agreement with experimental data. Both competing reactions, the disproportionation and the condensation, conform to the same general kinetic law being first internal order in substrate and first order in catalyst. Activation parameters of these reactions were determined. The results were interpreted in terms of a bimolecular mechanism in which nucleophilic attack of the silanolate anion directed to silicon of the silanol group causes the cleavage of one of its geminal bonds to oxygen, either the one to hydroxyl leading to condensation or the one to siloxane which leads to disproportionation. The latter is faster as the silanolate is a better leaving group compared with OH−. Moreover, in the pentacoordinate silicon transition state (or intermediate) the siloxane substituent preferentially enters the apical position, thus driving the OH substituent into the unreactive equatorial position.

The two carbon-centered radicals 4-morpholinomethyl ( 4 ) and 4-methylmorpholin-3-yl ( 5 ), generated from the primary cation radical intermediate 3 by β-deprotonation, are the major radical species in reaction mixtures of N -methylmorpholine- N -oxide ( 1 ) as demonstrated by trapping reactions with γ-tocopherol. Carbon–carbon coupling products originating from the recombination of 4 and the reaction between 4 and 5 have been identified. Recombination of 4 with the primary cation radical 3 was shown to be the key step in the ‘disproportionation’ of 3 into N -methylmorpholine, morpholine and formaldehyde.

The presence of nitroxyl radicals affects mainly trans-vinylene and alcohol group formation. The influence of the nitroxyl radicals on the yield of carbonyl and γ-lactone formation is negligible. The effect on hydroperoxide decomposition is best explained by elimination of the chain reaction of induced hydroperoxide decomposition. This is a consequence of the reactions of nitroxyl radicals with alkyl radicals. These reactions involve geminate alkyl radicals as well as single alkyl radicals resulting from prior destruction of a radical pair by reaction with a nitroxyl radical. This is a major aspect of the kinetics of the reactions. The nitroxyl radicals do not disrupt rapidly enough the geminate radicals formed on hydroperoxide thermolysis to yield typical homogeneous, solution-type kinetics. They terminate chains on reaction with geminate and with single radicals. The increase of the rate of formation as well as of the maximum amount of trans-vinylene groups in the presence of nitroxyl radicals can be easily understood. It is caused by the disproportionation of macroalkyl radicals with nitroxyl radicals. It is more difficult to explain the significant decrease of the rate of formation and maximum amount alcohol groups. The only explanation in agreement with mechanisms and kinetics involves unchanged formation of alcohol groups in the presence of nitroxyl radicals. However, the alcohol groups formed by intramolecular hydrogen abstraction by alkoxy radicals are destroyed rapidly after formation of hydroxylamine ethers in the 4-position.

Alkoxides, alkyl compounds, amides and hydrides of alkali metals (M) and barium, such as MOR, Ba(OR)2, n-BuM, PhM, MN(SiMe3)2 and MAlH4 showed high catalytic activities versus the disproportionation reactions of PhSiH3 to produce SiH4, Ph2SiH2 and Ph3SiH. A good correlation between the catalyst basicities and the catalytic activities was observed, and a reaction mechanism involving the metal hydride and alkyl metal was proposed. A considerable amount of SiH4 was produced in the reduction of PhSiCl3 with LiAlH4 when over three moles of LiAlH4 was used.

Photoinduced isomerization of secondary alkyl radicals to primary alkyl radicals in glassy solids was studied by ESR spectroscopy. The efficiencies for 2-, 3- and 4-heptyl radicals in 77 K perdeuterated methylcyclohexane decrease in this order. Molecular orbital calculations indicate that the isomerization occurs through a hydride ion transfer from the methyl groups to the radical site in the lowest Rydberg state of secondary alkyl radicals. The dependence of its efficiency on the position of the radical site can be explained by variation of the negative charge at the methyl groups with the position of the radical site.

The potential-determining reaction of cation radicals formed upon electrochemical oxidation of diphenylselenide is an orbital-controlled disproportionation both in the presence and in the absence of specially added nucleophiles (water, benzene, toluene, pyridine). The active species of ensuing bulk reactions leading to stable products was shown to be a dication of diphenylselenide which reacts with the starting molecule or with an appropriate external nucleophile.

The reactions of hydroxyl radicals with a number of stable alkenes have been studied in low-temperature matrices. The reactions were initiated by broad band UV–visible irradiation of matrices containing H2O2 and the alkene under investigation. The hydroxyalkyl radical products were identified principally by comparison of their spectra with the spectra of corresponding stable alcohols. Accordingly, IR spectra were recorded for the following series of alcohols isolated in argon matrices — methanol, ethanol, ethanol-d6, propan-1-ol, propan-2-ol, butan-2-ol, 2-methylpropan-1-ol (iso-butyl alcohol), 2-methylpropan-2-ol (tert-butyl alcohol), 2-methylbutan-2-ol (tert-amyl alcohol), 3-methylbutan-2-ol and 2,3-dimethylbutan-2-ol. The hydroxyalkyl radicals, which appear to be formed from the alkenes studied were as follows — from ethene, 2-hydroxyethyl radical; from cis- or trans-but-2-ene, 1-methyl-2-hydroxypropyl radical; from propene, 1-methyl-2-hydroxyethyl and 2-hydroxypropyl radicals; from but-1-ene, 1-hydroxymethylpropyl and 2-hydroxybutyl radicals; from 2-methylpropene (iso-butene), 1,1-dimethyl-2-hydroxyethyl and 2-methyl-2-hydroxypropyl radicals; the radical products from buta-1,3-diene and isoprene could not be identified. In the cases, where two radical products were possible, i.e. when propene, but-1-ene or 2-methylpropene were the substrates, it was found that the concentration of the secondary or tertiary radical always exceeded that of the primary radical. However, the relative concentration of these radicals appears to be determined by subsequent photolysis to give carbonyl compounds. There seems, therefore, to be little preference for the secondary and tertiary radicals over the primary radicals in the primary addition process. Comments on the mechanism of the transformation from radical to carbonyl compound based upon identification of intermediates within the matrix and isotopic substitution experiments are made. The characterisation of the 2-hydroxyethyl radical has been backed up by experiments utilising isotopic substitution with 13C and D (2H). The other radicals have been identified with varying degrees of certainty. Those radicals, which are observed at the highest concentration and which are, therefore, characterised more certainly are — 2-hydroxyethyl (1), from ethene; 1-methyl-2-hydroxypropyl (2), from cis- and trans-but-2-ene; 1-methyl-2-hydroxyethyl (3), from propene; 1-hydroxymethylpropyl (5), from but-1-ene; and 1,1- dimethyl-2-hydroxyethyl (8), from 2-methylpropene.

The aim of this series of two publications is to characterise important radical reactions of organic molecules in atmospheric water phases, such as cloud, fog, and rain droplets, or on wetted aerosols. In this first study the reactions occurring in a mixed peroxyl radical system generated from the OH radical attack on a model compound, 2-butoxyethanol, in oxygenated aqueous solution are investigated in the absence of trace compounds typically found in such natural waters. Using gamma-radiation emitted from a Co-60 source, We have produced steady-state concentrations of O-2(-)/HO2- and OH radicals comparable to those predicted for daytime conditions in cloud or fog droplets. The reactions of the peroxyl radicals derived from 2-butoxyethanol (C4H9OCH2CH2OH) in oxygenated aqueous solution have been investigated by measuring detailed product distributions resulting from the oxidation process. The observed and quantified products are mainly esters, ketones, aldehydes and hydroperoxides and accounted for 83% of the removal of 2-butoxyethanol (expressed as a carbon balance). A mechanism for the formation of the reaction products is proposed. It includes the OH attack at four carbon centers of the molecule producing carbon centered radicals, the formation of peroxyl radicals by addition of oxygen to these radicals, the decay of the peroxyl radicals in a reaction system of six different peroxyl radicals and superoxide via bimolecular and unimolecular reactions and the reactions of intermediary produced oxyl radicals. (C) 2000 Elsevier Science Ltd. All rights reserved.

The ESR spectrum of the propagating radical of cyclohexyl acrylate (CHA) overlapping with a three-line spectrum of a mid-chain radical was recorded in benzene as a non-polar solvent. The hyperfine coupling constants for the α- and β-hydrogens of the propagating radical of CHA were assigned based on the spectrum of the polymer radical of CHA-α-d. The polymerization of CHA was found to proceed in the presence of two types of radical species with different reactivities, the propagating radical and the mid-chain radical. The content of the mid-chain radical produced by abstraction of the α-hydrogen of the monomeric unit of the polymer was found to increase up to 25% with increasing conversion. The content of branching in the polymers arising from the hydrogen abstraction, 1.2–3.5%, determined by 13C NMR spectroscopy was also found to increase with progress of the polymerization. The rate of the intramolecular formation of the mid-chain radical relative to propagation was estimated from the branching content at an infinitely low conversion, which was obtained by extrapolation, to be 0.65% at 1 mol/l of CHA. The absolute rate coefficients for the propagation (kp) and termination (kt) of CHA were determined using kinetic data obtained by FT-near infrared spectroscopy and the concentration of the propagating radical extrapolated to 0% conversion to minimize the influence of the mid-chain radicals: k p =1224 l/mol s and k t =5.7×10 5 l/mol s at 60°C. These seemed to be apparent values because the propagating radical is repeatedly converted to the less reactive mid-chain radical by the intramolecular hydrogen abstraction during the lifetime of each radical. Slow initiation by the mid chain radical would result in the propagating radical which propagates further and accompanies the conversion to the mid-chain radical.

A simple water soluble diselenide derivative 1 shows radical scavenger properties towards alkyl and hydroxyl radicals (k3 (0°C)=6.8×108 M−1 s−1) in Fenton-type chemistry. The reaction rate between produceded alkyl radicals 2 and the diselenide overwhelms self-termination and halogen transfer reactions.

Products derived from the homolysis of the aromatic CI bond of ortho -iodobenzophenones by photolysis or tributyltin radicals indicate 1,5-hydrogen migration. The reactions include iodine migration, hydrogen abstraction and arylation.

In contrast to similar oxiranylcarbinyl radical systems, 3-arylaziridinylcarbinyl radicals have been shown to give only products of CN bond cleavage.

The reaction of apparently long-lived, oxygen-centered gas-phase free radicals, which may cause damage to a living body, from combustion smoke of poly(methyl methacrylate) with typical radical scavengers, diphenylamine and 2,6-di-t-butyl-4-methylphenol has been studied. ESR spectra of the spin adducts of gas-phase radicals were observed in the presence of both radical scavengers. Results show that gas-phase radicals in combustion smoke do not react with these radical scavengers directly, but undergo complicated reactions like induced decomposition. In the case of diphenylamine, it was observed that radical concentrations increased with exposure. It is suggested that this increase may be caused by the reaction of organic peroxides or hydroperoxides in the combustion smoke with diphenylamine.

High to moderate levels of stereoselectivity are observed in chiral Lewis acid promoted allyl transfer reactions of radicals substituted α to oxazolidinones. Similar selectivity patterns are observed for reactions in which such radicals are generated by addition to an acrylimide or by reactions in which the same radical is generated from the corresponding bromide. A good correlation is obtained for selectivity vs. the Taft steric parameter for the alkyl group attached to the radical center.

Hydroxyl radicals are observed to react with 1-bromo-2-fluorobenzene to form C 6 H 4 BrF·OH and (C 6 H 4 BrF) +. in neutral and acidic solutions respectively. The nature and reactivity of the solute radical cation, (C 6 H 4 BrF) +. , formed under different acidic conditions are discussed.

Laser flash photolysis of [CpM(CO) 3 $#x0005D; 2 (M = W, Mo, and Cr) provides a convenient source of CpM(CO) 3 °, an organometallic free radical with 17 valence electrons. It is a transient and highly reactive species. Depending on the circumstances and the other reagents present, the radical will dimerize, undergo halogen and hydrogen atom abstraction reactions, and electron transfer reactions. With tetramethyl-phenylenediamine, there is a cyclic process of electron transfer steps, the net result of which is the catalyzed disproportionation of the metal radical.

The formation of α-dicarbonyl compounds seems to be an important step for cross-linking proteins in the glycation or Maillard reaction. To elucidate the mechanism for the cross-linking reaction, we studied the reaction between a three-carbon α-dicarbonyl compound, methylglyoxal, and amino acids. Our results showed that this reaction generated yellow fluorescent products as formed in some glycated proteins. In addition, three types of free radical species were also produced, and their structures were determined by EPR spectroscopy. These free radicals are 1) the cross-linked radical cation, 2) the methylglyoxal radical anion as the counterion, and 3) the superoxide radical anion produced only in the presence of oxygen. The generation of the cross-linked radical cations and the methylglyoxal radical anions does not require metal ions or oxygens. These results indicate that dicarbonyl compounds cross-link free amino groups of protein by forming Schiff bases, which donate electrons directly to dicarbonyl compounds to form the cross-linked radical cations and the methylglyoxal radical anions. Oxygen can accept an electron from the radical anion to generate a superoxide radical anion, which can initiate damaging chain reactions. Time course studies suggest that the cross-linked radical cation is a precursor of yellow fluorescent glycation end products.

The mercury-sensitized photolysis of Me3SiH was studied as a function of the exposure time, substrate pressure and light intensity, and in the presence of the additives McOH and H2. Two primary processes were observed: hydrogen abstraction from the SiCH bond and, to a minor extent, from the CH bond. The sum of the quantum yields of the two primary processes is only 0.8. The main part of the reaction mechanism, which concerns the reactions of the Me3Si radical, can be quantitatively explained by a previous investigation of the direct photolysis of Me4Si (Ahmed et al., J. Photochem. Photobiol. A: Chem., 86 (1995) 33). With the rate constants given by Ahmed et al., the experimental values can be satisfactorily reproduced by computer simulations. In particular, it is confirmed that silaethylene reacts in an almost collision-controlled manner with radicals and the disproportionation reactions of Si-centred radicals leading to an SiC double bond play only a minor role. The ratio of disproportionation to combination of the Me3Si radical was determined to be 0.07 ± 0.01.

Time-resolved EPR was applied to detect short-lived alkyl radicals in pulse radiolysis of liquid alkanes. Two problems were addressed: (i) the mechanism of radical formation and (ii) the mechanism of chemically-induced spin polarization in these radicals. (i) The ratio of yields of penultimate and interior radicals in n-alkanes at the instant of their generation was found to be ≈ 1.25 times greater than the statistical quantity. This higher-than-statistical production of penultimate radicals indicates that the proton transfer reaction involving excited radical cations must be a prevailing route of radical generation. The relative yields of hydrogen abstraction and fragmentation for various branched alkanes are estimated. It is concluded that the fragmentation occurs prior to the formation of radicals in an excited precursor species. (ii) The analysis of spin-echo kinetics in n-alkanes suggests that the alkyl radicals gain the emissive polarization in spur reactions. This initial polarization increases with shortening of the aliphatic chain. We suggest that the origin of this polarization is the ST − mechanism operating in the pairs of alkyl radicals and hydrogen atoms generated in dissociation of excited alkane molecules. It is also found that a long-chain structure of alkyl radicals results in much higher rate of Heisenberg spin exchange relative to the recombination rate (up to 30 times). That suggests prominent steric effects in recombination or the occurrence of through-chain electron exchange. The significance of these results in the context of cross-linking in polyethylene and higher paraffins is discussed.

Using competitive methods, the rates of hydrogen-atom abstraction by a typical perfluoro-n-alkyl radical, n-C7F15·, have been determined for a series of silane and stannane reductants including nBu3SnH (2.03×108 M−1 s−1), Et3SiH (7.5×105), (TMS)2SiMeH (1.63×107) and (TMS)3SiH (5.1×107), all of which exhibited substantial rate enhancements relative to their analogous reductions of hydrocarbon radicals. The reduction by PhSH is, in contrast, ∼400-times slower than for hydrocarbon radicals. Transition state polar effects are invoked to rationalize the relative reactivity of perfluoro- versus hydrocarbon radicals in these hydrogen-transfer reactions. A Hammett study for H-atom transfer from arene thiols (p+ = −0.56) provided further substantiation for this conclusion. A discussion of the relative reactivity of t-butoxyl and perfluoro-n-alkyl radicals is presented.

Reactions of 3,3′,5,5′-tetra- tert -butylstilbene-4,4′-quinone with free radicals from the decomposition of azo compounds and peroxides (R . ) have been studied by ESR spectroscopy. Relatively stable phenoxyl-type free radicals are formed via addition of R . across the exo CC bond. Their coupling constants with the methine proton reflect steric effects. Further addition products lead to sterically crowded molecules which spontaneously dissociate into radicals via C(4)C(7) bond cleavage.