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A radical approach to late-stage functionalization using photoredox and Diversinate™ chemistry on the Open Source Malaria (OSM) triazolopyrazine scaffold (Series 4) resulted in the synthesis of 12 new analogues, which were characterized by NMR, UV, and MS data analysis. The structures of four triazolopyrazines were confirmed by X-ray crystal structure analysis. Several minor and unexpected side products were generated during these studies, including two resulting from a possible disproportionation reaction. All compounds were tested for their ability to inhibit the growth of the malaria parasite Plasmodium falciparum (3D7 and Dd2 strains) and for cytotoxicity against a human embryonic kidney (HEK293) cell line. Moderate antimalarial activity was observed for some of the compounds, with IC50 values ranging from 0.3 to &gt, 20 µM, none of the compounds displayed any toxicity against HEK293 at 80 µM.

The BF3·OEt2-mediated disproportionate coupling reaction of sodium sulfinates was found for the first time. In this reaction, various S–S(O)2 bonds can be formed, efficiently giving thiosulfonates in moderate to excellent yields. As a convenient protocol for the synthesis of symmetrical and unsymmetrical thiosulfonates, its reaction mechanism involves the formation of a thiyl radical and sulfonyl radical via a sulfinyl radical disproportionation. What is more, this transformation can also be applied practically as a gram-scale reaction and to the two-step synthesis of sulfone and sulfonamide in one pot in situ using thiosulfonate as an intermediate.

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.

The past decade has witnessed the rapid development of radical generation strategies and their applications in C–C bond-forming reactions. Most of these processes require initiators, transition metal catalysts, or organometallic reagents. Herein, we report the discovery of a simple organic system (2-azaallyl anions) that enables radical coupling reactions under transition-metal-free conditions. Deprotonation of N-benzyl ketimines generates semistabilized 2-azaallyl anions that behave as “super-electron-donors” (SEDs) and reduce aryl iodides and alkyl halides to aryl and alkyl radicals. The SET process converts the 2-azaallyl anions into persistent 2-azaallyl radicals, which capture the aryl and alkyl radicals to form C–C bonds. The radical coupling of aryl and alkyl radicals with 2-azaallyl radicals makes possible the synthesis of functionalized amine derivatives without the use of exogenous radical initiators or transition metal catalysts. Radical clock studies and 2-azaallyl anion coupling studies provi...

Heteroatom-centered radicals show versatile reactivity and offer useful synthetic methods in organic chemistry. The development of new approaches for forming heteroatom-centered radicals has recently expanded the practicality of radical chemistry for synthesis. This review focuses on recent advances in reactions of representative heteroatom-centered radicals.1 Introduction2 Group 17 Elements: Chlorine and Bromine Radicals3 Group 15 and Group 16 Elements3.1 Nitrogen- and Oxygen-Centered Radicals3.2 Phosphorus- and Sulfur-Centered Radicals3.3 Other Radicals4 Group 14 Elements: Silicon-Centered Radicals5 Group 13 Elements: Boron-Centered Radicals6 Conclusion

Silver-mediated reactions have recently emerged as a frontier area in organic chemistry. Indeed, the remarkable property of silver as a single-electron oxidant has led to its exploitment into the field of radical chemistry. The excellent reactivity and high selectivity exhibited by silver salts in the activation of different organic substrates place them among the most prominent reagents in free radical reactions. The present review aims to provide the readers with a critical overview of silver-based radical reactions, especially emphasizing the reactivity, selectivity and a detailed description of the reaction mechanism with particular attention to the role of Ag(I)-based catalysts/reactants. The selected examples have been classified according to the type of radicals generated, namely carbon-centered radicals and heteroatom (such as nitrogen, oxygen, phosphorus, sulfur)-centered radicals. The generation of such radicals as well as their application in synthetic processes including oxidation, addition, coupling, and radical cascades have been described in detail.

Ascorbic acid is a well-known antioxidant and radical scavenger. It can be oxidized by losing two protons and two electrons, but normally loses only one electron at a time. The reactivity of the ascorbate radical is unusual, in that it can either disproportionate or react with other radicals, but it reacts poorly with non-radical species. To explore the oxidation mechanism of ascorbic acid, the pKa's and reduction potentials have been calculated using the B3LYP/6-31+G(d,p) and CBS-QB3 levels of theory with the SMD implicit solvent model and explicit waters. Calculations show that the most stable form of dehydroascorbic acid in water is the bicyclic hydrated structure, in agreement with NMR studies. The possible oxidation reactions at different pH conditions can be understood by constructing a potential-pH (Pourbaix) diagram from the calculated pKa's and standard reduction potentials. At physiological pH disproportionation of the intermediate radical is thermodynamically favored. The calculations show that disproportionation proceeds via dimerization of ascorbate radical and internal electron transfer, as suggested by Bielski. In the dimer, one of the ascorbate units cyclizes. Then protonation and dissociation yields the fully reduced and bicyclic fully oxidized structures. Calculations show that this mechanism also explains the reaction of the ascorbic acid radical with other radical species such as superoxide. Ascorbate radical combines with the radical, and intramolecular electron transfer followed by cyclization and hydrolysis yields dehydroascorbic acid and converts the radical to its reduced form.

DFT (U)B3LYP/cc-pVTZ and (U)M06-2X/cc-pVTZ and multireference CASSCF/cc-pVTZ level of theories have been used to investigate the electronic structure of isomeric dehydrooxazole (1b–d, 3 isomers), dehydrothiazole (2b–d, 3 isomers) and dehydroimidazole (3a–d, 4 isomers) radicals. The ground state electronic structure of each radical isomer has been confirmed by predicting their doublet excited state structure and calculating the adiabatic energy difference. The stability order of the individual isomeric radicals has been estimated through the comparison of absolute energies. A hypothetical isodesmic reaction has been utilized to calculate radical stabilization energies (RSEs). The results show N-dehydro imidazole 3a as the thermodynamically most stable radical. In order to understand the structural and stability aspects of the radicals and interactions between the radical electron and the electron lone pairs, we have analysed spin densities and hybridisation changes and also performed MCSCF calculations. The reason for the higher stability of 3a has been attributed to the attainment of a π-character and subsequent delocalization, whereas, all other carbon centred radicals are found to be localized σ-radicals. Furthermore, the kinetic stability of the radicals has been investigated through unimolecular decomposition channels. All the studies showed a weak to strong coupling between the N3 and the radical centre depending on the location of the radical centre. NBO analysis suggests that through space coupling between N3 and σ* of the radical centre leads to a stabilising effect when the radical centre is adjacent to N3, whereas such interactions are absent when the radical centre is away from it. However, only a weak coupling is observed between the radical centre and X1 (O/S/NH). Particularly the interaction strength has the following trend: S < O < N–H. Indeed S, O and N–H show a stabilising effect through bond interactions with the antibonding orbitals of the alternate bonds on either side.

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.

We have performed a theoretical study, using density functional theory at the B3LYP/6-311G(d, p) level, of the possible radical types and their structures in coal and their stabilities once they come into contact with atmospheric O2 at ambient temperature. Eight possible radical types were proposed and explored through fully optimized electronic structure calculations. It was found that the radicals arising from dangling bonds on both the aromatic ring and the alkyl chain via covalent bond rupture are highly reactive toward oxidation, which resulted in the formation of stable oxygen-containing radicals. However, singlet occupied molecular orbital analysis suggested that the obtained radical via O2 addition on the alkyl group should be attributed to the carbon-centered radical. The aryloxy C–O bond scission actually forms a carbon-centered radical. This type of radical and the radical that originated from odd-numbered carbon fused aromatic rings show high stabilities toward oxidation on account of the posi...

Conversion of NO to other nitrogen oxides is an elementary step in its catalytic removal processes. On coinage metal surfaces, two kinds of NO activation mechanisms have been well documented: the unimolecular dissociation of NO generates two adsorbed atoms, and the dissociation of an adsorbed (NO)2 unit generates an adsorbed O and a free N2O. In this work, we observed a disproportionation mechanism involving three NO molecules on Au6– at a very low temperature (150 K), in which an adsorbed (NO)2 reacts with a free NO forming an adsorbed NO2 and a free N2O. The density functional theory (DFT) calculations indicated that this disproportionation step is significantly exothermic and has a very low activation barrier. The charge distributions on the involved cluster complexes and the correlation between the activity and the electronic properties of Au6– indicate the important role of extra negative charge in all reaction steps. The disproportionation mechanism revealed in this work could possibly exist in the ...

Multichannel gas-phase reactions of CH3OCH2CH2Cl/CH3CH2OCH2CH2Cl with chlorine atom and hydroxyl radical have been investigated using ab initio method and canonical variational transition-state dynamic computations with the small-curvature tunneling correction. Further energetic information is refined by the coupled-cluster calculations with single and double excitations (CCSD)(T) method. Both hydrogen abstraction and displacement processes are carried out at the same level. Our results reveal that H-abstraction from the –OCH2– group is the dominant channel for CH3OCH2CH2Cl by OH radical or Cl atom, and from α-CH2 of the group CH3CH2– is predominate for the reaction CH3CH2OCH2CH2Cl with Cl/OH. The contribution of displacement processes may be unimportant due to the high barriers. The values of the calculated rate constants reproduce remarkably well the available experiment data. Standard enthalpies of formation for reactants and product radicals are calculated by isodesmic reactions. The Arrhenius expressions are given within 220–1200 K. The atmospheric lifetime, ozone depleting potential (ODP), ozone formation potential (OFP), and global warming potential (GWP) of CH3OCH2CH2Cl/CH3CH2OCH2CH2Cl are investigated. Meanwhile, the atmospheric fate of the alkoxy radicals are also researched using the same level of theory. To shed light on the atmospheric degradation, a mechanistic study is obtained, which indicates that reaction with O2 is the dominant path for the decomposition of CH3OCH(O•)CH2Cl, the C–C bond scission reaction is the primary reaction path in the consumption of CH3CH(O•)OCH2CH2Cl in the atmosphere. Ab initio method and canonical variational transition-state theory are employed to study the kinetic nature of hydrogen abstraction reactions of CH3OCH2CH2Cl/CH3CH2OCH2CH2Cl with Cl atom and OH radical and fate of alkoxy radicals (CH3OCH(O•)CH2Cl/CH3CH(O•)OCH2CH2Cl).

Based on our recent reinvestigation of the disproportionation reaction of 3,3′,5,5′-tetraphenyl-4,4′-biphenoquinone, which exhibited drastic color change, we performed the synthesis and characterization of 3,3′′,5,5′′-tetraphenyl-4,4′′-terphenoquinone with the aim of obtaining enhanced physical properties, such as lower reduction potentials and bathochromic shift in light absorption, and realizing a disproportionation reaction at a lower temperature. To construct the terphenyl and/or terphenoquinone skeletons, the Suzuki-Miyaura coupling, which involves coupling between 1,4-diiodobenzene and the boronic acid ester with phenyl and oxygen moieties attached was carried out. The subsequent hydroquinone formation reaction and oxidation reaction yielded a brilliant ultramarine-blue terphenoquinone. The quinone showed properties typical of π-expanded quinones, namely, positively shifted reduction potential, small on-site Coulomb repulsion, and lower excitation energy. DSC measurement of this product showed an exothermic process at 212.5 °C and this temperature was much lower than that of the corresponding biphenoquinone, 289.5 °C. The products of this reaction were similar to those of the biphenoquinone reaction, i. e., two benzofurans and the corresponding hydroquinone.

We have applied density functional calculations to study reactions SiX3 + Si20X20, and CX3 + Si20X20 (X = H, F) based on two reaction channels (H- or F-abstraction and H-or F-displacement from Si20H20 and Si20F20). Our results show that SiX3 radicals prefer the hydrogen or halogen atom abstraction from Si20H20 and Si20F20 fullerenes. The exothermic reaction channels are proceed via reactant-like transition states i.e. the elongation of the breaking bonds of transition states is smaller than that of their forming bonds. Among the mentioned reaction channels, the reactions of SiF3 radical with Si20F20 and CF3 radical with Si20H20 are most favorable both thermodynamically and kinetically with the lowest barrier height exothermic character. Generally, the reactions of Si20H20 and Si20F20 with SiF3 radical are more favorable than SiH3 radical and the reaction of CH3 and CF3 radicals with Si20H20 fullerenes is more favorable than SiH3 and SiF3 radicals. These results are in agreement with the electrostatic surface potentials of reactants.

The enthalpy change of disproportionation reaction of nitroxyl and peroxyl radicals was calculated by the DFT B3LYP/cc-pVDZ method. The regeneration of the corresponding hydroxylamine during the oxidation of unsaturated compounds inhibited by nitroxyl radicals is shown to be associated with the disproportionation of the nitroxyl and peroxyl radicals.

Modeling of the addition of various radicals to C60 fullerene is currently an active research area. However, the radicals considered are not able to adequately model polymeric radicals. In this work, we have performed a theoretical study of the possible reactions of C60 fullerene with 1-n-phenylpropyl radicals, which are used to model polystyrene radicals. Several possible ways of subsequent addition of up to four 1-phenylpropyl radicals to C60 have been analyzed, the structures of the intermediates have been defined and thermal properties, such as the activation enthalpies of the corresponding reactions, have been calculated using density functional theory with the approximation of PBE/3z. It is shown that the topology of the spin density distribution on the fullerenyl radical causes regioselectivity for further radical addition. According to the energetic characteristics of the reactions, we assume the possibility of formation of products of one-, two-, three-, and four- addition of the growth radical to the fullerene core in radical polymerization of styrene in the presence of C60 fullerene. © 2016 Wiley Periodicals, Inc.

© 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.

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 thermokinetics for the tautomerization of a series of methylenedihydroacenes to the corresponding methylacenes (toluene to 6-methylpentacene) have been investigated by means of CBS-QB3 calculations. Only for 6-methylpentacene does the methylenedihydro form predominate at room temperature. The obtained equilibrium ratios are consistent with various theoretical methods, but the agreement with the scarce experimental data is only qualitative. The noncatalyzed thermal tautomerization of the methylenedihydroacene in an inert solvent may proceed by means of a reverse radical disproportionation reaction (RRD) as the rate-determining step. The benzylic BDE(C-H)s and the hydrogen atom affinities (HA) of the tautomers have been used to calculate the reaction enthalpy, ΔRRDH. It appears that the Ea,RRD is substantially higher than ΔRRDH. This implies that the opposite reaction (and the tautomer forming step), a radical-radical disproportionation (RD), is an activated process. This is an often ignored or overlooked kinetic feature. The consequence is that although the RRD reaction may be kinetically feasible at elevated temperatures, the products are not the tautomers but rather dimers stemming from radical-radical recombination reactions, with p-isotoluene as a clear exception. It is further shown that the RRD self-reaction of phenalene is too slow at 298 K, despite claims to the contrary.

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.

A range of arylmethyl-substituted pyrrolidines and tetrahydrofurans were produced by FeCl2-catalyzed tandem cyclization and cross-coupling reactions of alkyl iodides and aryl Grignard reagents. The substituents on alkenes had a profound effect on the progress of the tandem reactions, with di- and tri-substituted alkenes affording cyclized pyrrolidines. Several experimental results, such as cyclopropyl ring opening, the stereochemical outcome of the reaction with a secondary iodide substrate, and intermediate identification with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), support the conclusion that a radical process is involved in this catalytic system. Moreover, deuterium labeling experiments indicated that radical disproportionation could occur in the termination reactions of the cyclized alkyl radical species.

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.

Heteroatom-centered radicals are known to play critical roles in atmospheric chemistry, organic synthesis, and biology. While most studies have focused on the radical reactivity such as hydrogen abstraction, the base properties of heteroatom-centered radicals have long been overlooked, despite the profound consequences, such as their ability to participate in hydrogen-bonding networks. In this study, we use the sulfinyl radical (-SO(•)) as a model to show that the dual properties of heteroatom-centered radicals, that is, their ability to function as a radical and a base, can coexist in peptides and be differentiated by examining the loss of hydrosulfinyl radical (SOH) upon unimolecular dissociation of the peptide sulfinyl radical ions in the gas phase. The loss of SOH can result from two channels; one involves hydrogen atom abstraction, which reflects the radical property; the other is initiated by proton transfer to the sulfinyl radical, manifesting its base property. Tuning of the two properties of peptide sulfinyl radicals can be achieved by varying the chemical properties of the neighboring functional groups, which demonstrates the influence of the local chemical environment on the behavior of the radical species. The experimental approach established in this study to probe the dual chemical property of the peptide sulfinyl radical can be potentially applied to studying other types of heteroatom-centered radical species of biological significance.

The benzyl radical (1) is a key intermediate in the combustion and tropospheric oxidation of toluene. Because of its relevance, the reaction of 1 with molecular oxygen was investigated by matrix-isolation IR and EPR spectroscopy as well as computational methods. The primary reaction product of 1 and O2 is the benzylperoxyl radical (2), which exists in several conformers that can easily interconvert even at cryogenic temperatures. Photolysis of radical 2 at 365 nm results in a formal [1,3]-H migration and subsequent cleavage of the O-O bond to produce a hydrogen-bonded complex between the hydroxyl radical and benzaldehyde (4). Prolonged photolysis produces the benzoyl radical (5) and water, which finally yield the phenyl radical (7), CO, and H2O. Thus, via a sequence of exothermic reactions 1 is transformed into radicals of even higher reactivity, such as OH and 7. Our results have implications for the development of models for the highly complicated process of combustion of aromatic compounds.

New reaction scheme is suggested for the initiated nonbranched-chain addition of free radicals to the multiple bond of the molecular oxygen. The scheme includes the reaction competing with chain propagation reactions through a reactive free radical. The chain evolution stage in this scheme involves a few of free radicals, one of which (tetraoxyl) is relatively low-reactive and inhibits the chain process by shortening of the kinetic chain length. Based on the proposed scheme rate equations (containing one to three parameters to be determined directly) are deduced using quasi-steady-state treatment. The kinetic description with use the obtained rate equations is applied to the γ-induced nonbranched-chain processes of the free-radical oxidation of liquid o-xylene at 373 K and hydrogen dissolved in water containing different amounts of oxygen at 296 K. In these processes the oxygen with the increase of its concentration begins to act as an oxidation autoingibitor (or an antioxidant), and the rate of peroxide formation as a function of the dissolved oxygen concentration has a maximum. The heat effects are compared for the overall reactions of dissociation of simple alkylperoxyl (exothermic) and alkoxyl (endothermic) free radicals in the gas phase. Possible nonchain pathways of the free-radical oxidation of hydrogen and the routes of ozone decay from the energetic standpoint via the reaction with the hydroxyl free radical in the upper atmosphere (including the addition yielding the hydrotetraoxyl free radical, which can be an intermediate in the sequence of conversions of biologically hazardous UV radiation energy) were examined. The energetics of the key radical-molecule reactions is considered.

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.

End-brominated poly(methyl methacrylate) (PMMABr) was prepared by atom transfer radical polymerization (ATRP) and employed in a series of atom transfer radical coupling (ATRC) and radical trap-assisted ATRC (RTA–ATRC) reactions. When coupling reactions were performed in the absence of a nitroso radical trap—traditional ATRC condition—very little coupling of the PMMA chains was observed, consistent with disproportionation as the major termination pathway for two PMMA chain-end radicals in our reactions. When 2-methyl-2-nitrosopropane (MNP) was used as the radical trap, coupling of the PMMA chains in this attempted RTA–ATRC reaction was again unsuccessful, owing to capping of the PMMA chains with a bulky nitroxide and preventing further coupling. Analogous reactions performed using nitrosobenzene (NBz) as the radical trap showed significant dimerization, as observed by gel permeation chromatography (GPC) by a shift in the apparent molecular weight compared to the PMMABr precursors. The extent of coupling wa...

We have decomposed 2-methylbutyraldehye in single pulse shock tube experiments, at temperatures between 1075 K to 1250 K and pressures of 1.5 and 6 atm. Five reaction channels have been identified from the unimolecular products that have been detected. The products involve CC bond cleavage together with a retro-ene reaction. Particularly remarkable is the detection of small amount of butane; the first time that an alkane has been detected in these high-temperature experiments. The formation of butane represents evidence of the roaming radical reaction or more generally the high-temperature radical disproportionation reaction. Ethylene is present in large yields, and this is traced to a 1,2 H-transfer reaction involving the highly exothermic transfer of a carbonyl hydrogen to an alkyl site. The results are consistent with earlier single pulse shock tube experiments on the 2-ketones. In particular, they confirm the earlier observation that substituting a methyl and hydrogen atom with a doubly bonded oxygen atom have very little effect on the rate constant of the adjoining CC bond. The yields of ethylene are consistent with a very low value of the carbonyl resonance energy.

RATIONALE Introducing radicals onto gas-phase non-covalent complexes and studying their chemistry is a relatively unexplored frontier. In generating these radicals via bond homolysis reactions, it is important that the energy necessary for forming the radical does not exceed the energy required for dissociating the complex itself. Based on this consideration, new approaches for creating these radicals will probably have to involve incorporation of weak bonds that can easily undergo homolysis. METHODS The formation of a radical cation, via collision-induced dissociation, of protonated S-nitrosocysteine non-covalently bound to the crown ether 18-crown-6 is described here. The radical cation of this complex was isolated and subjected to collisional activation and ion-molecule reactions with allyl iodide. The results were compared with those of the radical cation of 'bare' cysteine. RESULTS Collisional activation of the radical cation of the cysteine/crown complex led to fragmentation of cysteine as well as of the crown ether. Ion-molecule reactions of the radical cation of the complex with allyl iodide led to products arising from I and allyl abstraction. Isolation and CID of the former product ion led to the loss of iodocysteine. CONCLUSIONS Cleavage of the weak S–NO bond has allowed the formation of a radical site onto a non-covalent complex. Ion-molecule reactions and collisional activation were utilized to probe the chemistry of this radical cation. The approach adopted here for incorporating a radical onto a cysteine/crown complex shows promise for the introduction of radical sites onto other biological non-covalent complexes. Copyright © 2013 John Wiley & Sons, Ltd.

Whereas there are numerous experimental and computational studies of electrochemical reduction leading to the formation of solid-electrolyte interface (SEI) in lithium-ion batteries, so far there have been no direct spectroscopic observations of radical intermediates involved in the SEI formation. In Part 1 of this series, radiolysis and laser photoionization of carbonate electrolytes are used to observe and identify these reaction intermediates using electron paramagnetic resonance spectroscopy. Our study indicates that the suggested scenarios for electrolyte reduction require elaboration. In particular, we establish the occurrence of efficient H abstraction and 1,2-migration involving radicals generated through the reductive ring-opening. Instead of the primary radicals postulated in the current models, secondary and tertiary radicals are generated, biasing the subsequent chemistry to radical disproportionation. The consequences of this bias for radical and anionic polymerization are examined, and it is...

Photolysis of an aryl sulfide containing 5,6-dihydropyrimidine (1) at 350 nm produces high yields of thymidine and products resulting from trapping of 5,6-dihydrothymidin-5-yl radical by O2 or thiols. Thymidine is believed to result from disproportionation of the radical pair originally generated from C-S bond homolysis in 1 on the microsecond timescale, which is significantly shorter than other photochemical transformations of modified nucleotides into their native forms. Duplex DNA containing 1 is destabilized, presumably due to disruption of π-stacking. Incorporation of 1 within the binding site of the restriction endonuclease EcoRV, provides a photochemical switch for turning on the enzyme's activity. In contrast, 1 is a substrate for endonuclease VIII and serves as a photochemical off switch for this base excision repair enzyme. Modification 1 also modulates the activity of the 10–23 DNAzyme despite its incorporation into a non-duplex region. Overall, dihydropyrimidine 1 shows promise as a tool that provides spatiotemporal control over DNA structure on the miscrosecond tiimescale.

We use tight-binding quantum chemical molecular dynamics to investigate the crystal growth mechanisms of H-terminated Si(001)-(2 × 1) during plasma-enhanced chemical vapor deposition of SiH3 and SiH2 radicals. We find that crystal growth by SiH3 radical deposition consists of two stages: (1) the first SiH3 radical abstracts a surface-terminating H atom and produces a dangling bond, and (2) a second SiH3 radical is adsorbed on the dangling bond. Thus, at least two SiH3 radicals are required for generating a new Si–Si bond. Interestingly, during SiH2 deposition, a SiH2 radical can be directly adsorbed onto a H-terminated site without H abstraction by another SiH2 radical. Thus, one SiH2 radical is sufficient for generating a new Si–Si bond. This SiH2 radical crystal growth mechanism is different from the SiH3 radical mechanism. The direct adsorption process consists of a two-step chemical reaction: (1) the SiH2 radical abstracts a surface-terminating H atom and produces a dangling bond and a SiH3 radical, a...

Compressed CO2 could promote the disproportionation reactions of aryl alcohols in water medium significantly. The control experiments indicated that the effect of CO2 on the properties of the reactant/water emulsions was the main reason for the acceleration of the reactions rate.

The reactivity of five transition metal dimers toward photochemical, in-solvent-cage disproportionation has been investigated using picosecond time-resolved infrared spectroscopy. Previous ultrafast studies on [CpW(CO)3]2 established the role of an in-cage disproportionation mechanism involving electron transfer between 17- and 19-electron radicals prior to diffusion out of the solvent cage. New results from time-resolved infrared studies reveal that the identity of the transition metal complex dictates whether the in-cage disproportionation mechanism can take place, as well as the more fundamental issue of whether 19-electron intermediates are able to form on the picosecond time scale. Significantly, the in-cage disproportionation mechanism observed previously for the tungsten dimer does not characterize the reactivity of four out of the five transition metal dimers in this study. The differences in the ability to form 19-electron intermediates are interpreted either in terms of differences in the 17/19-electron equilibrium or of differences in an energetic barrier to associative coordination of a Lewis base, whereas the case for the in-cage vs diffusive disproportionation mechanisms depends on whether the 19-electron reducing agent is genuinely characterized by 19-electron configuration at the metal center or if it is better described as an 18 + δ complex. These results help to better understand the factors that dictate mechanisms of radical disproportionation and carry implications for radical chain mechanisms.

Experimental measurements of the rate coefficients for several types of gas phase radical reactions are reviewed. They include radical isomerization and dissociation, radical + radical association, and unimolecular reactions of peroxy radicals. Some reactions of methylene in its lowest singlet state are also considered. The links to theories of chemical reaction rates and especially of capture rates for radical + radical reactions are examined. Many of the reactions involve formation of adducts, which can isomerize and dissociate. Such reactions frequently involve energy distributions of the adducts that are far from Boltzmann, and the interpretation of measurements requires the use of master equation techniques. The basis of these methods and the use of matrix diagonalization and eigenvector/eigenvalue analysis to extract phenomenological rate coefficients are discussed. The relevance of the measurements to applications in atmospheric chemistry and, especially, in combustion is briefly considered.

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.

Interaction between adsorbed t-butyl peroxybenzoate and photoexcited graphene rendered trapped phenyl and t-butoxy radicals. Post-irradiation thermal desorption showed benzene, t-butanol, and isobutylene oxide as the end products. The required hydrogen atoms were obtained via the radical disproportionation. Graphene enabled radical species to be captured and their on-surface chemistry to be revealed.

Radical chain reactions leading to C-C bond formation are widely used in organic synthesis, and initiation of the radical chain process usually requires thermolabile radical initiators. Recent studies on transition-metal-free cross-coupling reactions between aryl halides and arenes have demonstrated an unprecedented initiation system for radical chain reactions, where the combination of simple organic additives and a base was used in place of conventional radical initiators. Among them, the combination of N,N'-dimethylethylenediamine (DMEDA) and t-BuOK is one of the most efficient and representative reaction systems, and the radical initiation mechanism of this system has attracted considerable research interest. In this study, through the combination of kinetic studies, deuterium labeling experiments, and DFT calculations, the radical initiation mechanism of the diamine-promoted cross-coupling reaction was carefully reinvestigated. In light of the present study, a mechanistic network of radical initiation in the DMEDA/t-BuOK system was revealed, which differs dramatically from the previously realized single radical initiation pathway. In this mechanism, the diamine acts as a hydrogen atom donor and plays a dual role as both "radical amplifier" and "radical regulator" to initiate the radical chain process as well as to control the concentration of reactive radical species. This represents a rare example of a structurally simple molecule playing such a subtle role in the radical chain reaction system. The present study sheds some light on the novel radical initiation mode in transition-metal-free cross-coupling reactions following a base-promoted homolytic aromatic substitution (BHAS) mechanism, and may also help to understand the mechanism of relevant reactions.

The intramolecular ring closure reactions of unsaturated hydrocarbon radicals potentially play an important role for the formation of molecular weight growth species, especially during the pyrolysis and oxidation of alkenes under low to intermediate temperatures. In this work we investigated a series of intramolecular cycloaddition reactions of both allylic- and alkyl-type dienyl radicals. In the first set of reactions, a resonant linear radical is converted into a non-resonant cyclic radical. In the second set, a non-resonant linear alkenyl radical isomerizes to either a resonant cyclic radical or a cyclic carbinyl radical. In both cases, three different reaction schemes are examined based on the location of the partially-formed resonance structure in the cyclic transition state. For each reaction scheme, both the endo- and exo-pathways were investigated. High pressure rate parameters are obtained from the results of CBS-QB3 electronic structure calculations combined with canonical transition state theory calculations. The results are discussed in the context of a Benson-type model to examine the impact of the partially-formed resonance stabilization on both the activation energies and pre-exponential factors. The results are compared to previously reported rate parameters for cycloaddition reactions of alkenyl radicals. The differences in the activation energies are primarily due to the bimolecular component of the activation energy. However, in some cases, the presence of the partial resonance structure significantly increases the strain energy for the ring that is formed in the transition state. The pre-exponential factors are also impacted by the formation of a partial resonance structure in the transition state. Lastly, the C6H9 potential energy surface is examined to show how the trends that are outlined here can be used to estimate rate parameters, which are needed to analyze pressure-dependent reaction systems.

Coote, Michelle/0000-0003-0828-7053; Coote, Michelle L/0000-0003-0828-7053; DEGIRMENCI, ISA/0000-0002-2708-7930 WOS: 000372562200013 PubMed: 26932454 High-level ab initio calculations have been used to compare prototypical thiyl, alkoxyl, and alkyl radical addition reactions. Thiyl radical addition to the sulfur center of thioketones is exothermic and rapid, occurring with negative enthalpic barriers and only weakly positive Gibbs free energy barriers. In stark contrast, alkoxyl radical addition to the oxygen center of ketones is highly endothermic and occurs with very high reaction barriers, though these are also suppressed. On the basis of analysis of the corresponding alkyl radical additions to these substrates and the corresponding reactions of these heteroatom radicals with alkenes, it suggested that addition reactions involving thiyl radicals have low intrinsic barriers because their unpaired electrons are better able to undergo stabilizing resonance interactions with the pi* orbitals of the substrate in the transition state. Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [2219]; Australian Research Council Centre of Excellence for Electromaterials ScienceAustralian Research Council I.D. gratefully acknowledges the Scientific and Technological Research Council of Turkey (TUBITAK) under 2219 grant and Naomi Haworth for her valuable support. M.L.C gratefully acknowledges generous allocations of supercomputing time on the National Facility of the Australian National Computational Infrastructure and financial support from the Australian Research Council Centre of Excellence for Electromaterials Science.

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 majority of free radicals are highly reactive species which participate in bimolecular reactions with each other. Validation of the theory of molecular diffusion and reactivity in the liquid state requires knowledge of rate constants of radical–radical reactions (recombination, disproportionation) and their viscosity dependencies. An accurate comparison of theory and experiment has become available due to experimentally measured diffusion coefficients of reactive radicals by transient grating technique. Initial distribution of radicals in solution can be not random but pair-wise as in photo- or thermoinitiation of free radical polymerization reactions. Probability of a radical escape of a partner (cage escape) characterizes the initiator efficiency. Despite decades of measurement of cage effect values, cage effect dynamics with free radicals have only been investigated quite recently. The present tutorial review considers the effect of viscosity of Newtonian liquid on two types of recombination—in the solvent bulk and in a cage. Further, since radicals are paramagnetic species, external magnetic field affects probability of their reactions in pairs. These effects are also observed in viscous liquids, and reasons for such observations are explained. The recently discovered low magnetic field effect is also observed on radical pairs in viscous liquids.

Organic reactions involving radical cation and radical anion intermediates are synthetically powerful umpolung processes that enable electronically mismatched couplings between pairs of electron-rich or pairs of electron-poor organic fragments. Nevertheless, the adoption of these reactions as synthetic methods has been relatively slow in comparison with that of reactions involving more conventional reactive intermediates such as carbanions, carbocations, and neutral radicals. This Microreview provides a brief survey of radical ion chemistry and highlights the use of transition metal photocatalysis as a convenient means to investigate radical-ion-mediated transformations.

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.

sec-Alkyl radicals are key reactive intermediates in the hydrocarbon combustion and atmospheric decomposition mechanisms that are formed by the abstraction of hydrogen from an alkane, or as a second generation product of n-alkyl H-migrations, C-C bond scissions in branched alkyl radicals, or the bimolecular reaction between olefins and n-alkyl radicals. Since alkanes and branched alkanes, which the sec-alkyl radicals are derived from, make up roughly 40-50% of traditional fuels an understanding of their chemistry is essential to improving combustion systems. The present work investigates all H-migration reactions initiated from an sec-alkyl radical that involve the movement of a secondary hydrogen, for the 2-butyl through 4-octyl radicals, using the CBS-Q, G2, and G4 composite methods. The resulting thermodynamic and kinetic parameters are compared to similar reactions in n-alkyl radicals in order to determine underlying trends. Particular attention is paid to the effect of cis/trans and 1,3-diaxial interactions on activation energies and rate coefficients. When combined with our previous work on n-alkyl radical H-migrations, a complete picture of H-migrations in unbranched alkyl radicals is obtained. This full data set suggests that the directionality of the remaining branched chains has a minimal effect on the rate coefficients for all but the largest viable transition states, which is in stark contrast to the differences predicted by the structurally similar dimethylcycloalkanes. In fact the initial location of the secondary radical site has a greater effect on the rate than does the directionality of the remaining alkyl chains. The activation energies for secondary to secondary reactions are much closer to those of the secondary to primary H-migrations. However, the rate coefficients are found to be closer to the corresponding primary to primary reaction values. A significant ramification of these results is that there will be multiple viable reaction pathways for these reactions instead of only one dominant pathway as previously believed.

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.