92,959 research outputs found
6,835,216 - Chain-breaking antioxidants
No full textCompounds, preferably 5-pyrimidinol and 3-pyridinol derivatives, that act as effective chain breaking antioxidants of both the lipid and water-soluble variety (analogous to the natural Vitamins E and C), many of which are more reactive toward peroxyl radicals than the most potent form of Vitamin E. These compounds may exhibit many chemopreventive effects associated with conditions in which free radical-mediated cellular damage or disruption is implicated and Vitamins E and C are shown to have protective effects. Additionally, these compounds should be excellent oxidation inhibitors as additives to fuels, lubricants, rubber, polymers, chemicals, solvents and foodstuffs
Novel Chain-breaking Antioxidants
No full textCompounds, preferably 5-pyrimidinol and 3-pyridinol derivatives, that act as effective chain breaking antioxidants of both the lipid and water soluble variety (analogous to natural Vitamins E and C), many of which are more reactive than the most potent form of Vitamin E. These compounds may exhibit many chemopreventive affects. Additionally, these compounds should be excellent oxidation inhibitors as additives to fuels, lubricants, rubber, polymers, chemicals, solvents and food stuffs
The Electron Transfer Properties of Alkoxyl Radicals. A Time-Resolved Kinetic Study of the Reactions of the tert-Butoxyl, Cumyloxyl and Benzyloxyl Radicals with Alkyl Ferrocenes
No full textA time-resolved kinetic study on the reactions of the tert-butoxyl (t-BuO(center dot)), cumyloxyl (CumO(center dot)), and benzyloxyl (BnO(center dot)) radicals with alkylferrocenes has been carried out in MeCN solution. With all radicals, clear evidence for an electron transfer (ET) process has been obtained, and with the same ferrocene donor, the reactivity has been observed to increase in the order t-BuO(center dot) < CumO(center dot) < BnO center dot(,) with the difference in reactivity approaching 3 orders of magnitude on going from t-BuO(center dot) to BnO(center dot). With BnO(center dot), an excellent fit to the Marcus equation has been obtained, from which a value of the reduction potential of BnO(center dot) (E degrees(BNO center dot/BNO-) = 0.54 V/SCE) has been derived. The latter value appears, however, to be significantly higher than the previously determined reduction potential values for alkoxyl radicals and in contrast with the differences in the computed solution-phase electron affinities determined for t-BuO(center dot), CumO(center dot), and BnO(center dot), indicating that the reaction of BnO(center dot) with ferrocene donors may not be described in terms of a straightforward outer sphere ET mechanism. From these data, and taking into account the available value of the reduction potential for CumO(center dot), a value of E degrees(BnO center dot/BnO-) = -0.10 V/SCE has been estimated. On the basis of computational evidence for the formation of a pi-stacked prereaction complex in the reaction between BnO(center dot) and DcMFc, an alternative ET mechanism is proposed for the reactions of both CumO(center dot) and BnO(center dot). In these cases, the delocalized nature of the unpaired electron allows for the aromatic ring to act as an electron relay by mediating the ET from the ferrocene donor to the formal oxygen radical center. This hypothesis is also in line with the observation that both BnO(center dot) and CumO(center dot) react with the ferrocene donors with rate constants that are in all cases at least 2 orders of magnitude higher than those measured for t-BuO(center dot), wherein the radical is well-localized
Hydrogen atom abstraction reactions from tertiary amines by benzyloxyl and cumyloxyl radicals: Influence of structure on the rate-determining formation of a hydrogen-bonded prereaction complex
No full textA time-resolved kinetic study on the hydrogen atom abstraction reactions from a series of tertiary amines by the cumyloxyl (CumO \u2022) and benzyloxyl (BnO \u2022) radicals was carried out. With the sterically hindered triisobutylamine, comparable hydrogen atom abstraction rate constants (k H) were measured for the two radicals (k H(BnO \u2022)/k H(CumO \u2022) = 2.8), and the reactions were described as direct hydrogen atom abstractions. With the other amines, increases in k H(BnO \u2022)/k H(CumO \u2022) ratios of 13 to 2027 times were observed. k H approaches the diffusion limit in the reactions between BnO \u2022 and unhindered cyclic and bicyiclic amines, whereas a decrease in reactivity is observed with acyclic amines and with the hindered cyclic amine 1,2,2,6,6-pentamethylpiperidine. These results provide additional support to our hypothesis that the reaction proceeds through the rate-determining formation of a C-H/N hydrogen-bonded prereaction complex between the benzyloxyl \u3b1-C-H and the nitrogen lone pair wherein hydrogen atom abstraction occurs, and demonstrate the important role of amine structure on the overall reaction mechanism. Additional mechanistic information in support of this picture is obtained from the study of the reactions of the amines with a deuterated benzyloxyl radical (PhCD 2O \u2022, BnO \u2022- d 2) and the 3,5-di-tert-butylbenzyloxyl radical. \ua9 2011 American Chemical Society.Peer reviewed: YesNRC publication: Ye
Hydrogen atom abstraction selectivity in the reactions of alkylamines with the benzyloxyl and cumyloxyl radicals. the importance of structure and of substrate radical hydrogen bonding
No full textFigure Presented A time-resolved kinetic study on the hydrogen abstraction reactions from a series of primary and secondary amines by the cumyloxyl (CumO \u2022) and benzyloxyl (BnO \u2022) radicals was carried out. The results were compared with those obtained previously for the corresponding reactions with tertiary amines. Very different hydrogen abstraction rate constants (k H) and intermolecular selectivities were observed for the reactions of the two radicals. With CumO \u2022, k H was observed to decrease on going from the tertiary to the secondary and primary amines. The lowest k H values were measured for the reactions with 2,2,6,6-tetramethylpiperidine (TMP) and tert-octylamine (TOA), substrates that can only undergo N-H abstraction. The opposite behavior was observed for the reactions of BnO \u2022, where the k H values increased in the order tertiary < secondary < primary. The k H values for the reactions of BnO \u2022 were in all cases significantly higher than those measured for the corresponding reactions of CumO \u2022, and no significant difference in reactivity was observed between structurally related substrates that could undergo exclusive \u3b1-C-H and N-H abstraction. This different behavior is evidenced by the k H(BnO \u2022)/k H(CumO \u2022) ratios that range from 55-85 and 267-673 for secondary and primary alkylamines up to 1182 and 3388 for TMP and TOA. The reactions of CumO \u2022 were described in all cases as direct hydrogen atom abstractions. With BnO \u2022 the results were interpreted in terms of the rate-determining formation of a hydrogen-bonded prereaction complex between the radical \u3b1-C-H and the amine lone pair wherein hydrogen abstraction occurs. Steric effects and amine HBA ability play a major role, whereas the strength of the substrate \u3b1-C-H and N-H bonds involved appears to be relatively unimportant. The implications of these different mechanistic pictures are discussed. \ua9 2011 American Chemical Society.Peer reviewed: YesNRC publication: Ye
5-Pyrimidinols: Novel chain-breaking antioxidants more effective than phenols
No full textnot availabl
Extremely Fast Hydrogen Atom Transfer between Nitroxides and HOO · Radicals and Implication for Catalytic Coantioxidant Systems
No full textWe report a novel coantioxidant system based on TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) that, in biologically relevant model systems, rapidly converts chain carrying alkylperoxyl radicals to HOO center dot. Extremely efficient quenching of HOO center dot by TEMPO blocks the oxidative chain. Rate constants in chlorobenzene were measured to be 1.1 x 10(9) M-1 s(-1) for the reductive reaction TEMPO + HOO center dot -> TEMPOH + O-2 and 5.0 x 10(7) M-1 s(-1) for the oxidative reaction TEMPOH + HOO center dot -> + TEMPO + H2O2. These rate constants are significantly higher than that associated with the reaction of HOO center dot with alpha-tocopherol, Nature's best lipid soluble antioxidant (k = 1.6 x 10(6) M-1 s(-1)). These data show that in the presence of ROO center dot-to-HOO center dot chain-transfer agents, which are common in lipophilic environments, the TEMPO/TEMPOH couple protects organic molecules from oxidation by establishing an efficient reductive catalytic cycle. This catalytic cycle provides a new understanding of the efficacy of the antioxidant capability of TEMPO in nonaqueous systems and its potential to act as a chemoprotective against radical damage
The unusual reaction of semiquinone radicals with molecular oxygen
No full text(Chemical Equation Presented) Hydroquinones (benzene-1,4-diols) are naturally occurring chain-breaking antioxidants, whose reactions with peroxyl radicals yield 1,4-semiquinone radicals. Unlike the 1,2-semiquinone radicals derived from catechols (benzene-1,2-diols), the 1,4-semiquinone radicals do not always trap another peroxyl radical, and instead the stoichiometric factor of hydroquinones varies widely between 0 and 2 as a function of ring-substitution and reaction conditions. This variable antioxidant behavior has been attributed to the competing reaction of the 1,4-semiquinone radical with molecular oxygen. Herein we report the results of experiments and theoretical calculations focused on understanding this key reaction. Our experiments, which include detailed kinetic and mechanistic investigations by laser flash photolysis and inhibited autoxidation studies, and our theoretical calculations, which include detailed studies of the reactions of both 1,4-semiquinones and 1,2-semiquinones with O2, provide many important insights. They show that the reaction of O2 with 2,5-di-tert-butyl-1,4-semiquinone radical (used as model compound) has a rate constant of 2.4 \ub1 0.9
7 105 M -1 s-1 in acetonitrile and as high as 2.0 \ub1 0.9
7 106 M-1 s-1 in chlorobenzene, i.e., similar to that previously reported in water at pH 3c7. These results, considered alongside our theoretical calculations, suggest that the reaction occurs by an unusual hydrogen atom abstraction mechanism, taking place in a two-step process consisting first of addition of O2 to the semiquinone radical and second an intramolecular H-atom transfer concerted with elimination of hydroperoxyl to yield the quinone. This reaction appears to be much more facile for 1,4-semiquinones than for their 1,2-isomers. \ua9 2008 American Chemical Society.Peer reviewed: YesNRC publication: Ye
Modeling noncovalent radical-molecule interactions using conventional density-functional theory: Beware erroneous charge transfer
No full textConventional density-functional theory (DFT) has the potential to overbind radical-molecule complexes because of erroneous charge transfer. We examined this behavior by exploring the ability of various DFT approximations to predict fractional charge transfer and by quantifying the overbinding in a series of complexes. It is demonstrated that too much charge is transferred from molecules to radicals when the radical singly unoccupied molecular orbitals are predicted to be erroneously too low in energy relative to the molecule highest occupied molecular orbitals, leading to excessive Coulombic attraction. In this respect, DFT methods formulated with little or no Hartree-Fock exchange perform most poorly. The present results illustrate that the charge-transfer problem is much broader than may have been previously expected and is not limited to conventional (i.e., molecule-molecule) donor-acceptor complexes. \ua9 2013 American Chemical Society.Peer reviewed: YesNRC publication: Ye
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