- Concerted proton-electron transfer oxidation of phenols and hydrocarbons by a high-valent nickel complex
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The high-valent nickel(iii) complex Ni(pyalk)2+ (2) was prepared by oxidation of a nickel(ii) complex, Ni(pyalk)2 (1) (pyalk = 2-pyridyl-2-propanoate). 2 and derivatives were fully characterized by mass spectrometry and X-ray crystallography. Electron paramagnetic resonance spectroscopy and X-ray photoelectron spectroscopy confirm that the oxidation is metal-centered. 2 was found to react with a variety of phenolic and hydrocarbon substrates. A linear correlation between the measured rate constant and the substrate bond dissociation enthalpy (BDE) was found for both phenolic and hydrocarbon substrates. Large H/D kinetic isotope effects were also observed for both sets of substrates. These results suggest that 2 reacts through concerted proton-electron transfer (CPET). Analysis of measured thermodynamic parameters allows us to calculate a bond dissociation free energy (BDFE) of ~91 kcal mol-1 for the O-H bond of the bound pyalk ligand. These findings may shed light onto CPET steps in oxidative catalysis and have implications for ligand design in catalytic systems.
- Fisher, Katherine J.,Feuer, Margalit L.,Lant, Hannah M. C.,Mercado, Brandon Q.,Crabtree, Robert H.,Brudvig, Gary W.
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p. 1683 - 1690
(2020/02/25)
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- Functional models of nonheme diiron enzymes: Reactivity of the μ-oxo-μ-1,2-peroxo-diiron(iii) intermediate in electrophilic and nucleophilic reactions
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The reactivity of the previously reported peroxo-adduct [FeIII2(μ-O)(μ-1,2-O2)(IndH)2(solv)2]2+ (1) (IndH = 1,3-bis(2-pyridyl-imino)isoindoline) has been investigated in nucleophilic (e.g., deformylation of alkyl and aryl alkyl aldehydes) and electrophilic (e.g. oxidation of phenols) stoichiometric reactions as biomimics of ribonucleotide reductase (RNR-R2) and aldehyde deformylating oxygenase (ADO) enzymes. Based on detailed kinetic and mechanistic studies, we have found further evidence for the ambiphilic behaviour of the peroxo intermediates proposed for diferric oxidoreductase enzymes.
- Kripli, Balázs,Szávuly, Miklós,Csendes, Flóra Viktória,Kaizer, József
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supporting information
p. 1742 - 1746
(2020/02/20)
-
- Iodine/DMSO promoted oxidation of benzylic Csp3–H bonds to diketones – A mechanistic investigation
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This article describes a mechanistic investigation into the I2/DMSO mediated benzylic Csp3–H oxidation of an α-methylene ketone. The electron paramagnetic resonance (EPR) spectrum centred at g = 2.0011 supports the involvement of iodine and benzylic radicals, as the α-iodinated compound 2-iodo-1,2-diphenylethanone was isolated as a key reactive intermediate. The oxidation reaction relies, primarily, on DMSO as a source of oxygen in benzil, proven by the reaction of benzyl phenyl ketone with diphenyl sulfoxide (DPSO).
- Jayram, Janeeka,Xulu, Bheki A.,Jeena, Vineet
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- Hydrogen Atom Abstraction by High-Valent Fe(OH) versus Mn(OH) Porphyrinoid Complexes: Mechanistic Insights from Experimental and Computational Studies
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High-valent metal-hydroxide species have been implicated as key intermediates in hydroxylation chemistry catalyzed by heme monooxygenases such as the cytochrome P450s. However, in some classes of P450s, a bifurcation from the typical oxygen rebound pathway is observed, wherein the FeIV(OH)(porphyrin) species carries out a net hydrogen atom transfer reaction to form alkene metabolites. In this work, we examine the hydrogen atom transfer (HAT) reactivity of FeIV(OH)(ttppc) (1), ttppc = 5,10,15-tris(2,4,6-triphenyl)-phenyl corrole, toward substituted phenol derivatives. The iron hydroxide complex 1 reacts with a series of para-substituted 2,6-di-tert-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, Et, H, Ac), with second-order rate constants k2 = 3.6(1)-1.21(3) × 104 M-1 s-1 and yielding linear Hammett and Marcus plot correlations. It is concluded that the rate-determining step for O-H cleavage occurs through a concerted HAT mechanism, based on mechanistic analyses that include a KIE = 2.9(1) and DFT calculations. Comparison of the HAT reactivity of 1 to the analogous Mn complex, MnIV(OH)(ttppc), where only the central metal ion is different, indicates a faster HAT reaction and a steeper Hammett slope for 1. The O-H bond dissociation energy (BDE) of the MIII(HO-H) complexes were estimated from a kinetic analysis to be 85 and 89 kcal mol-1 for Mn and Fe, respectively. These estimated BDEs are closely reproduced by DFT calculations and are discussed in the context of how they influence the overall H atom transfer reactivity.
- Zaragoza, Jan Paulo T.,Cummins, Daniel C.,Mubarak, M. Qadri E.,Siegler, Maxime A.,De Visser, Sam P.,Goldberg, David P.
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supporting information
p. 16761 - 16770
(2019/12/24)
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- Antiradical activity of benzazole-2-thiones
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Reaction of benzoxazole-2-thione with 3,5-di-tert-butyl-4-hydroxybenzylacetate in methanol affords S-(3,5-di-tert-butyl-4-hydroxybenzyl)-2-mercaptobenzoxazole as the major product. Antiradical activity of S- and N-3,5-di-tert-butyl-4-hydroxybenzyl derivatives of benzothiazole(oxazole, imidazole)-2-thione with respect to 2,2-diphenyl-1-picrylhydrazyl is varies widely. S-Benzyl derivatives exhibit higher reactivity at 30°C.
- Gataullina,Mogilevtseva,Nugumanova,Bukharov,Tagasheva,Deberdeev, R. Ya.
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p. 1919 - 1923
(2017/10/27)
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- Myricetin, rosmarinic and carnosic acids as superior natural antioxidant alternatives to α-tocopherol for the preservation of omega-3 oils
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22 natural polyphenols are compared to 7 synthetic antioxidants including BHT, BHA, TBHQ and PG with regard to their ability to protect omega-3 oils from autoxidation. The antioxidant efficiency of phenols is assessed using the DPPH test and the measurement of oxygen consumption during the autoxidation of oils rich in omega-3 fatty acids. Also, the bond dissociation enthalpies (BDE) of the Ar-OH bonds are calculated and excellent correlations between thermodynamic, kinetic and oxidation data are obtained. It is shown that kinetic rates of hydrogen transfer, number of radicals scavenged per antioxidant molecule, BDE and formation of antioxidant dimers from the primary radicals play an important role regarding the antioxidant activity of phenols. Based on this, it is finally shown that myricetin, rosmarinic and carnosic acids are more efficient than α-tocopherol and synthetic antioxidants for the preservation of omega-3 oils.
- Guitard, Romain,Paul, Jean-Fran?ois,Nardello-Rataj, Véronique,Aubry, Jean-Marie
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p. 284 - 295
(2016/07/12)
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- A cobalt(ii) iminoiodane complex and its scandium adduct: Mechanistic promiscuity in hydrogen atom abstraction reactions
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In addition to oxometal [Mn+O] and imidometal [Mn+NR] units, transient metal-iodosylarene [M(n-2)+-OIPh] and metal-iminoiodane [M(n-2)+-N(R)IPh] adducts are often invoked as a possible second oxidant responsible for the oxo and imido group transfer reactivity. Although a few metal-iodosylarene adducts have been recently isolated and/or spectroscopically characterized, metal-iminoiodane adducts have remained elusive. Herein, we provide UV-Vis, EPR, NMR, XAS and DFT evidence supporting the formation of a metal-iminoiodane complex 2 and its scandium adduct 2-Sc. 2 and 2-Sc are reactive toward substrates in the hydrogen-atom and nitrene transfer reactions, which confirm their potential as active oxidants in metal-catalyzed oxidative transformations. Oxidation of para-substituted 2,6-di-tert-butylphenols by 2 and 2-Sc can occur by both coupled and uncoupled proton and electron transfer mechanisms; the exact mechanism depends on the nature of the para substituent.
- Kundu, Subrata,Chernev, Petko,Engelmann, Xenia,Chung, Chan Siu,Dau, Holger,Bill, Eckhard,England, Jason,Nam, Wonwoo,Ray, Kallol
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supporting information
p. 14538 - 14543
(2016/09/28)
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- O-H bond oxidation by a monomeric MnIII-OMe complex
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Manganese-containing, mid-valent oxidants (MnIII-OR) that mediate proton-coupled electron-transfer (PCET) reactions are central to a variety of crucial enzymatic processes. The Mn-dependent enzyme lipoxygenase is such an example, where a MnIII-OH unit activates fatty acid substrates for peroxidation by an initial PCET. This present work describes the quantitative generation of the MnIII-OMe complex, [MnIII(OMe)(dpaq)]+ (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate) via dioxygen activation by [MnII(dpaq)]+ in methanol at 25 °C. The X-ray diffraction structure of [MnIII(OMe)(dpaq)]+ exhibits a Mn-OMe group, with a Mn-O distance of 1.825(4) ?, that is trans to the amide functionality of the dpaq ligand. The [MnIII(OMe)(dpaq)]+ complex is quite stable in solution, with a half-life of 26 days in MeCN at 25 °C. [MnIII(OMe)(dpaq)]+ can activate phenolic O-H bonds with bond dissociation free energies (BDFEs) of less than 79 kcal mol-1 and reacts with the weak O-H bond of TEMPOH (TEMPOH = 2,2′-6,6′-tetramethylpiperidine-1-ol) with a hydrogen/deuterium kinetic isotope effect (H/D KIE) of 1.8 in MeCN at 25 °C. This isotope effect, together with other experimental evidence, is suggestive of a concerted proton-electron transfer (CPET) mechanism for O-H bond oxidation by [MnIII(OMe)(dpaq)]+. A kinetic and thermodynamic comparison of the O-H bond oxidation reactivity of [MnIII(OMe)(dpaq)]+ to other MIII-OR oxidants is presented as an aid to gain more insight into the PCET reactivity of mid-valent oxidants. In contrast to high-valent counterparts, the limited examples of MIII-OR oxidants exhibit smaller H/D KIEs and show weaker dependence of their oxidation rates on the driving force of the PCET reaction with O-H bonds. This journal is
- Wijeratne, Gayan B.,Day, Victor W.,Jackson, Timothy A.
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p. 3295 - 3306
(2015/03/05)
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- Importance of π-stacking interactions in the hydrogen atom transfer reactions from activated phenols to short-lived N-oxyl radicals
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A kinetic study of the hydrogen atom transfer from activated phenols (2,6-dimethyl- and 2,6-di-tert-butyl-4-substituted phenols, 2,2,5,7,8- pentamethylchroman-6-ol, caffeic acid, and (+)-cathechin) to a series of N-oxyl radical (4-substituted phthalimide-N-oxyl radicals (4-X-PINO), 6-substituted benzotriazole-N-oxyl radicals (6-Y-BTNO), 3-quinazolin-4-one-N-oxyl radical (QONO), and 3-benzotriazin-4-one-N-oxyl radical (BONO)), was carried out by laser flash photolysis in CH3CN. A significant effect of the N-oxyl radical structure on the hydrogen transfer rate constants (kH) was observed with kH values that monotonically increase with increasing NO-H bond dissociation energy (BDENO-H) of the N-hydroxylamines. The analysis of the kinetic data coupled to the results of theoretical calculations indicates that these reactions proceed by a hydrogen atom transfer (HAT) mechanism where the N-oxyl radical and the phenolic aromatic rings adopt a π-stacked arrangement. Theoretical calculations also showed pronounced structural effects of the N-oxyl radicals on the charge transfer occurring in the π-stacked conformation. Comparison of the kH values measured in this study with those previously reported for hydrogen atom transfer to the cumylperoxyl radical indicates that 6-CH3-BTNO is the best N-oxyl radical to be used as a model for evaluating the radical scavenging ability of phenolic antioxidants.
- Mazzonna, Marco,Bietti, Massimo,Dilabio, Gino A.,Lanzalunga, Osvaldo,Salamone, Michela
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supporting information
p. 5209 - 5218
(2014/06/23)
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- Antioxidant activity of hydrazones with sterically hindered phenol fragments
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Kinetic parameters of the antiradical activity of derivatives of hydrazones of 4-hydroxy-3,5-di-tert-butyl-benzaldehyde are determined photocolorimetrically in their reactions with a stable diphenylpicrylhydrazyl radical, and by chemiluminescence from the capture of peroxide radicals upon the initiated oxidation of ethylbenzene. It is found that during inhibited oxidation, the reactive centers (N-H and O-H) in hetaryl-and acylhydrazone molecules operate in parallel. Regularities of the compounds' inhibiting effect are studied in heterogeneous systems upon the initiated oxidation of ethylbenzene in emulsion, and in a waterlipid model of the oxidation of phosphatidylcholine dispersion. It is established that hydrazone derivatives are antioxidants of combined action in heterophase processes of the oxidation of unsaturated substrates, displaying properties of hydroperoxide deactivators in addition to their antiradical activity. Pleiades Publishing, Ltd., 2012.
- Nikolaevskii,Kniga,Khizhan,Tikhonova,Vinogradov,Khizhan
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p. 1816 - 1820
(2013/01/15)
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- Incorporation of ring nitrogens into diphenylamine antioxidants: Striking a balance between reactivity and stability
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The incorporation of nitrogen atoms into the aryl rings of conventional diphenylamine antioxidants enables the preparation of readily accessible, air-stable analogues, several of which have temperature-independent radical-trapping activities up to 200-fold greater than those of typical commercial diphenylamines. Amazingly, the nitrogen atoms raise the oxidation potentials of the amines without greatly changing their radical-trapping (H-atom transfer) reactivity.
- Hanthorn, Jason J.,Valgimigli, Luca,Pratt, Derek A.
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supporting information; experimental part
p. 8306 - 8309
(2012/07/13)
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- Inhibiting effect of phenolic compounds in initiated oxidation of tridecane
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Initiated oxidation of tridecane in chlorobenzene solution with molecular oxygen in the presence of a series of phenol derivatives was studied by chemiluminescence and gas-volumetric methods. The kinetic parameters of the antiradical and antioxidant activ
- Nikolaevskii,Morenko,Kniga,Ivleva
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experimental part
p. 649 - 654
(2011/07/07)
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- Spin trapping of radical intermediates generated by the oxidation of substituted 4-methylphenols
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A series of substituted 4-methylphenols 1 and 2 was oxidized with PbO 2 in the presence of nitroso compounds 3-10. The formation of adducts of benzyl radicals with the nitroso spin traps in the reaction mixture was established, suggesting the abstraction of an H-atom from the methyl substituent of 1 or 2. In the consecutive steps, the adducts underwent a further rearrangement to the corresponding nitrones. When the starting phenol contained bulky tBu groups in ortho-position (see 2,6-di(tert-butyl)-4- methylphenol (1a)), the stable 2,6-di(tert-butyl)-4R-phenoxy radicals (R=-CH=N+(O-)-X) were detected as the final radical products. The indirect evidence of nitrones in the reaction mixture was performed in one case by the reaction with a RO2 radicals. Copyright
- Majzlik, Petr,Omelka, Ladislav,Superatova, Renata,Holubcova, Petra
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experimental part
p. 1260 - 1268
(2011/10/09)
-
- Predicting organic hydrogen atom transfer rate constants using the Marcus cross relation
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Chemical reactions that involve net hydrogen atom transfer (HAT) are ubiquitous in chemistry and biology, from the action of antioxidants to industrial and metalloenzyme catalysis. This report develops and validates a procedure to predict rate constants for HAT reactions of oxyl radicals (RO ?) in various media. Our procedure uses the Marcus cross relation (CR) and includes adjustments for solvent hydrogen-bonding effects on both the kinetics and thermodynamics of the reactions. Kinetic solvent effects (KSEs) are included by using Ingold's model, and thermodynamic solvent effects are accounted for by using an empirical model developed by Abraham. These adjustments areshown to be critical to the success of our combined model, referred to as the CR/KSE model. As an initial test of the CR/KSE model we measured self-exchange and cross rate constants in different solvents for reactions of the 2,4,6-tri-tert-butylphenoxyl radical and the hydroxylamine 2,2′-6,6′-tetramethylpiperidin-1-ol. Excellent agreement is observed between the calculated and directly determined cross rate constants. We then extend the model to over 30 known HAT reactions of oxyl radicals with OH or CH bonds, including biologically relevant reactions of ascorbate, peroxyl radicals, and α-tocopherol. The CR/KSE model shows remarkable predictive power, predicting rate constants to within a factor of 5 for almost all of the surveyed HAT reactions.
- Warren, Jeffrey J.,Mayer, James M.
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scheme or table
p. 5282 - 5287
(2010/09/10)
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- Organochalcogen substituents in phenolic antioxidants
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Little is known about the ED/EW character of organochalcogen substituents and their contribution to the O-H bond dissociation enthalpy (BDE) in phenolic compounds. A series of ortho- and para-(S,Se,Te)R-substituted phenols were prepared and investigated by EPR, IR, and computational methods. Substituents lowered the O-H BDE by >3 kcal/mol in the para position, while the ortho-effect was modest due to hydrogen bonding (~3 kcal/mol) to the O-H group.
- Amorati, Riccardo,Pedulli, Gian Franco,Valgimigli, Luca,Johansson, Henrik,Engman, Lars
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scheme or table
p. 2326 - 2329
(2010/07/20)
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- Absolute rate constants for some intermolecular reactions of α-aminoalkylperoxyl radicals. Comparison with alkylperoxyls
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(Graph Presented) Seven α-aminoalkylperoxyl radicals have been generated by 355 nm laser flash photolysis (LFP) of oxygen-saturated di-tert-butyl peroxide containing mono-, di-, and trialkylamines and a dialkylarylamine. All these peroxyls possess absorptions in the near-UV (strongest for the trialkylamine-derived peroxyls) which permits direct monitoring of the kinetics of their reactions with many substrates. The measured rate constants for hydrogen atom abstraction from some phenols and oxygen atom transfer to triphenylphosphine demonstrated that all seven α- aminoalkylperoxyls have similar reactivities toward each specific substrate. More importantly, a comparison with literature data for alkylperoxyls shows that α-aminoalkylperoxyls and these alkylperoxyls have essentially the same reactivities. The combination of LFP and alkylamines provides a quick, reliable method for determining absolute rate constants for alkylperoxyl radical reactions, an otherwise laborious task.
- Lalevee, Jacques,Allonas, Xavier,Fouassier, Jean-Pierre,Ingold
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p. 6489 - 6496
(2008/12/22)
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- Hydrogen-transfer reactions from phenols to TEMPO prefluorescent probes in micellar systems
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(Chemical Equation Presented) A nitroxide prefluorescent probe has been used to evaluate local reactivity of antioxidants in micellar systems. An apparent rate constant that directly reflects the relevance of antioxidant hydrophobicity on the reaction tow
- Aliaga, Carolina,Juarez-Ruiz, Juan M.,Scaiano,Aspee, Alexis
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supporting information; experimental part
p. 2147 - 2150
(2009/05/30)
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- Hydrogen atom transfer reactions of imido manganese(V) corroie: One reaction with two mechanistic pathways
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Hydrogen atom transfer (HAT) reactions of (tpfc)MnNTs have been investigated (tpfc = 5,10,-15-tris(pentafluorophenyl)corrole and Ts = p-toluenesulfonate). 9,10-Dihydroanthracene and 1,4-dihydrobenzene reduce (tpfc)MnNTs via HAT with second-order rate constants 0.16 ± 0.03 and 0.17 ± 0.01 M-1 s-1, respectively, at 22°C. The products are the respective arenes, TsNH2 and (tpfc)MnIII. Conversion of (tpfc)MnNTs to (tpfc)Mn by reaction with dihydroanthracene exhibits isosbestic behavior, and formation of 9,9′,10,10′- tetrahydrobianthracene is not observed, suggesting that the intermediate anthracene radical rebounds in a second fast step without accumulation of a MnIV intermediate. The imido complex (tpfc)-MnVNTs abstracts a hydrogen atom from phenols as well. For example, 2,6-di-tert-butyl phenol is oxidized to the corresponding phenoxyl radical with a second-order rate constant of 0.32 ± 0.02 M-1 s-1 at 22°C. The other products from imido manganese(V) are TsNH2 and the trivalent manganese corrole. Unlike reaction with dihydroarenes, when phenols are used isosbestic behavior is not observed, and formation of (tpfc)-Mn IV(NHTs) is confirmed by EPR spectroscopy. A Hammett plot for various p-substituted 2,6-di-tert-butyl phenols yields a V-shaped dependence on σ, with electron-donating substituents exhibiting the expected negative ρ while electron-withdrawing substituents fall above the linear fit (i.e., positive ρ). Similarly, a bond dissociation enthalpy (BDE) correlation places electron-withdrawing substituents above the well-defined negative slope found for the electron-donating substituents. Thus two mechanisms are established for HAT reactions in this system, namely, concerted proton - electron transfer and proton-gated electron transfer in which proton transfer is followed by electron transfer.
- Zdilla, Michael J.,Dexheimer, Jennifer L.,Abu-Omar, Mahdi M.
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p. 11505 - 11511
(2008/03/14)
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- Synthesis and inhibitory activity of alkyl(hydroxyaryl)amines
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The reaction of ω-(4-hydroxyaryl)haloalkanes with various nitrogen-containing agents afforded primary, secondary, and tertiary amino derivatives of 2,6-dialkylphenols. For the compounds synthesized, the reaction rate constants with peroxide radicals were
- Dyubchenko,Nikulina,Terakh,Prosenko,Grigor'ev
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p. 1149 - 1155
(2008/09/17)
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- Combined inhibitory effect of sulfur-containing phenol SO-4 with natural and synthetic antioxidants in the oxidation of methyl oleate
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In a model system of initiated oxidation of methyl oleate, the antioxidant activities of 3-hydroxy-2-ethyl-6-methylpyridinesuccinate (mexidol) and bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propyl] disulfide (SO-4) were studied and compared with those of α-tocopherol and 1-hydroxy-2,6-di-tert-butyl-4- methylbenzene (dibunol). A linear pattern of dependence of the inhibitory effect on the concentration of compounds was established. The ability of antioxidants to decompose hydroperoxides and inhibit their accumulation was revealed. The combined inhibitory effects of SO-4 with mexidol, α-tocopherol, and phospholipids were described for the first time. The rate constant for disproportionation of the SO-4 phenoxyl radicals, k 9 = 0.90.10 3 L mol-1 s-1, was determined by steady-state photolysis. The rate constant k 10 eff for the reactions of SO-4 phenoxyl radicals with lipids characterized by different unsaturation degrees were determined for methyl oleate, linolic and arachidonic acids containing one, two, and four multiple bonds, and phospholiopids containing polyunsaturated fatty acids.
- Perevozkina,Storozhok,Krysin,Borisenko
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p. 1380 - 1387
(2007/10/03)
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- Synthesis and antioxidant activity of alkyl 3-(4-hydroxyaryl)propyl sulfides
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4-Alkylthiopropylphenols were synthesized from corresponding 2,6-dialkylphenols via the allyl derivative. A comparative study of the antioxidant activity of the prepared compounds was carried out in five model systems. The rate constants for the reactions
- Prosenko,Markov,Khomchenko,Boiko,Terakh,Kandalintseva
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p. 442 - 446
(2007/10/03)
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- Synthesis and antioxidant activity of [60]fuIlerene-BHT conjugates
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Fullerene derivatives incorporating one or two 3,5-di-tert-butyl-4- hydroxyphenyl groups were synthesized by 1,3-dipolar cycloaddition of azomethine ylides to Q60. The O-H bond dissociation enthalpies (BDEs) of these compounds were estimated by
- Enes, Roger F.,Tome, Augusto C.,Cavaleiro, Jose A. S.,Amorati, Riccardo,Fumo, Maria Grazia,Pedulli, Gian Franco,Valgimigli, Luca
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p. 4646 - 4653
(2008/02/08)
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- Synthesis and study of the antiradical activity of substituted hydroxybenzylamines and their hydrogen chloride salts
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Different alkylated hydroxybenzylamines and their hydrogen chloride salts were synthesized. The rate constants of their reactions with peroxide radicals k1 and the stoichiometric factors of inhibition f were measured in the initiated oxidation of cumene and methyl oleate. Copyright
- Dyubchenko,Nikulina,Terakh,Kandalintseva,Markov,Grigor'Ev,Prosenko
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p. 330 - 334
(2007/10/03)
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- Alkyl substituted phenoxyl decay in a hydrogen transfer equilibrium
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The kinetics of radical decay in the equilibrium: 2,4.6-tri-tert- butylphenoxyl radical 1 + 2,6-di-tert-butyl-4-methylphenol 2 = 2.4,6-tri-tert-butylphenol 3 + 2,6-di-tert-butyl-4-methylphenoxyl radical 4 was studied at 298 and 273 K by means of EPR spectroscopy. At 298 K second order prevails, whereas at 273 K the best fit was order 3/2. The extinction of 4 takes place in two steps: dimerization followed by disproportionation of the dimer, but the stable radical 1 enters in crossed dimerization with 4 to yield nonradical products. The mechanism ensures a constant |4|/|1| ratio along the decay. 2004 Wiley Periodicals, Inc.
- Coronel, Marta E. J.
-
-
- Abnormal solvent effects on hydrogen atom abstractions. 1. The reactions of phenols with 2,2-diphenyl-1-picrylhydrazyl (dpph?) in alcohols
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Rate constants, kArOH/dpph?,s, for hydrogen atom abstraction from 13 hindered and nonhindered phenols by the diphenylpicrylhydrazyl radical, dpph?, have been determined in n-heptane and a number of alcoholic and nonalcoholic, hydrogen-bond accepting solvents. Abnormally enhanced values of kArOH/dpph?,s have been observed in alcohols. It is proposed that this is due to partial ionization of the phenols and a very fast electron transfer from phenoxide anion to dpph?. The popular assessment of the antioxidant activities of phenols with dpph? in alcohol solvents will generally lead to an overestimation of their activities.
- Litwinienko, Grzegorz,Ingold
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p. 3433 - 3438
(2007/10/03)
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- Hydrogen-bonding effects on the properties of phenoxyl radicals. An EPR, kinetic, and computational study
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The effect of 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) on the properties of phenoxyl radicals has been investigated. HFP produces large variations of the phenoxyl hyperfine splitting constants indicative of a large redistribution of electron spin density, which can be accounted for by the increased importance of the mesomeric structures with electric charge separation. The conformational rigidity of phenoxyl radicals with electron-releasing substituents is also greatly enhanced in the presence of HFP, as demonstrated by the 2 kcal/mol increase in the activation energy for the internal rotation of the p-OMe group in the p-methoxyphenoxyl radical. By using the EPR equilibration technique, we have found that in phenols the O-H bond dissociation enthalpy (BDE) is lowered in the presence of HFP because it preferentially stabilizes the phenoxyl radical. In phenols containing groups such as OR that are acceptors of H-bonds, the interaction between HFP and the substituent is stronger in the phenol than in the corresponding phenoxyl radical because the radical oxygen behaves as an electron-withdrawing group, which decreases the complexating ability of the substituent. In phenols containing OH or NH2 groups, EPR experiments performed in H-bond accepting solvents showed that the interaction between the solvent and the substituent is much stronger in the phenoxyl radical than in the parent phenol because of the electron-withdrawing effect of the radical oxygen, which makes more acidic, and therefore more available to give H-bonds, the OH or NH2 groups. These experimental results have been confirmed by DFT calculations. The effect of HFP solvent on the reactivity of phenols toward alkyl radicals has also been investigated. The results indicated that the decrease of BDE observed in the presence of HFP is not accompanied by a larger reactivity. The origin of this unexpected behavior has been shown by DFT computations. Finally, a remarkable increase in the persistency of the α-tocopheroxyl radical has been observed in the presence of HFP.
- Lucarini, Marco,Mugnaini, Veronica,Pedulli, Gian Franco,Guerra, Maurizio
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p. 8318 - 8329
(2007/10/03)
-
- Comparison of electron spin relaxation times measured by Carr-Purcell-Meiboom-Gill and two-pulse spin-echo sequences
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Electron spin relaxation times obtained by two-pulse spin-echo and Carr-Purcell-Meiboom-Gill (CPMG) experiments were compared for samples with: (i) low concentrations of nuclear spins, (ii) higher concentrations of nuclear spins and low concentrations of
- Harbridge, James R.,Eaton, Sandra S.,Eaton, Gareth R.
-
-
- A New Method to Study Antioxidant Capability: Hydrogen Transfer from Phenols to a Prefluorescent Nitroxide
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(Equation presented) Rate constants for hydrogen abstraction from phenols by a prefluorescent-TEMPO probe are reported. The nitroxide is employed as a potential model of peroxyl radicals. The probe works by nitroxide suppression of the fluorescence of the
- Aliaga, Carolina,Aspee, Alexis,Scaiano
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p. 4145 - 4148
(2007/10/03)
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- A new method for the generation of stable phenoxyl radicals by the reaction of [(Me3Si)2N]2E (E = Ge, Sn) with sterically hindered phenols
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An unusual reaction of diaminogermylene and diaminostannylene with sterically hindered phenols which leads to the formation of stable phenoxyl radicals in high concentrations was found. The reaction mechanism was proposed. Amides [(Me3Si)2N]2E (E = Ge, Sn) were investigated electrochemically.
- Orlov,Moiseeva,Butin,Egorov
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p. 1961 - 1962
(2007/10/03)
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- Localized electron transfer in nonpoiar solution: reaction of phenols and thiophenols with free solvent radical cations
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Free electron transfer (FET) is understood as the reaction of free and uncorrelated solvent parent radical cations with solutes characterized by a lower ionization potential than those of the solvent. We studied electron transfer from phenols and thiophen
- Brede, Ortwin,Ganapathi, Mahalaxmi R.,Naumov, Sergej,Naumann, Wolfgang,Hermann, Ralf
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p. 3757 - 3764
(2007/10/03)
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- Free electron transfer from several phenols to radical cations of non-polar solvents
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Electron-transfer reactions from phenols to parent radical cations of solvents were studied using pulse radiolysis. Phenols bearing electron-withdrawing, electron-donating and bulky substituents were investigated in non-polar solvents such as cyclohexane, n-dodecane, n-butyl chloride and 1,2-dichloroethane. The experiments revealed the direct, synchronous formation of phenoxyl radicals and phenol radical cations in all cases and in nearly the same relative amounts. This was explained by two competing electron-transfer channels which depend on the geometry of encounter between the parent solvent radical cations and the solute phenol molecules. The mechanism is analysed at a microscopic level, treating diffusion as a slow process and the local electron transfer as an extremely rapid event. Furthermore, the effect of various phenol substituents and solvent types on the electron-transfer mechanism and on the decay kinetics of the solute phenol radical cations was analysed. The results were further substantiated using a quantum chemical approach.
- Ganapathi,Hermann,Naumov,Brede
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p. 4947 - 4955
(2007/10/03)
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- Reactivity of substituted phenols toward alkyl radicals
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The rate constants for the reaction of primary alkyl radicals with substituted phenolic compounds have been measured in benzene or toluene at room temperature by using the radical clock technique. With three representative phenols, containing in the ortho positions substituents of different size, the kinetics of the hydrogen transfer to alkyl radicals was studied at different temperatures to obtain the corresponding Arrhenius parameters. The kinetic solvent effect on the reaction with α-tocopherol was also investigated in six different solvents behaving as hydrogen bond acceptors, while the reaction with 2,4,6-trimethylphenol and 2,6-di-tert-butylphenol was studied in toluene and γ-valerolactone. For some phenols, the effect of self-aggregation on the kinetic parameters was also studied.
- Franchi, Paola,Lucarini, Marco,Pedulli, Gian Franco,Valgimigli, Luca,Lunelli, Bruno
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p. 507 - 514
(2007/10/03)
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- Media effects on antioxidant activities of phenols and catechols
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The H-atom donating activities of 2,6-di-tert-butyl-4-methylphenol (BHT), 2,6-di-tert-butyl-4-methoxyphenol (DBHA), 2,2,5,7,8-pentamethyl-6-hydroxychroman (PMHC), and 3,5-di-tert-butylcatechol (DTBC) toward the nitrogen-centered 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical were measured by stopped flow methods in hexane, 1-propanol, tert-butyl alcohol, and acetone. Decreases in these activities on transferring from hexane to the hydrogen bond accepting (HBA) solvents, the kinetic solvent effect (KSE), are attributed to hydrogen bonding from the phenolic group. Steric hindrance accounts for a lower decrease observed for the highly hindered BHT and DBHA compared to PMHC. The catechol, DTBC, a very active H-atom donor to DPPH in hexane, showed a dramatic loss of activity in HBA solvents, especially acetone. Higher H-atom donating activities of BHT, DBHA, and PMHC were observed toward the oxygen-centered radical of 2,6-di-tert-butyl-4-(4′-methoxyphenyl)phenoxyl (DBMP), and the decreases in activity in the HBA solvents paralleled those found with DPPH. Thus the KSE was found to be independent of the nature of the abstracting radical for DPPH and DBMP. The inhibition of the oxygen uptake (IOU) method was used to determine the antioxidant activities (kinh) of α-tocopherol, PMHC, catechol, and DTBC during free radical autoxidation of styrene and mixtures of styrene and tert-butyl alcohol. The kinh of α-tocopherol and PMHC dropped to one-tenth of the values with increasing tert-butyl alcohol content due to the HBA activity of the alcohol compared to styrene.
- Barclay,Edwards,Vinqvist
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p. 6226 - 6231
(2007/10/03)
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- Effect of Solvation on the Bond Dissociation Energies of Phenolic Antioxidants
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The effect of solvent on the bond dissociation energies (BDEs) of the oxygen-hydrogen bond in substituted phenolic antioxidants has been investigated by means of an EPR technique.On changing the solvent from benzene to tert-butanol the BDE's were found to increase by ca. 2.2 kcal/mol for phenols without ortho substituents, by ca. 1 kcal/mol for 2,6-dimethyl substituted phenols while in 2,6-di-tert-butyl phenols they seem to be substantially unaffected.This behaviour has been interpreted by admitting that the BDE increase observed in tert-butanol is essentially due to the solvation of the hydroxylic hydrogen which stabilises the phenol, leaving the energy of the phenoxyl radical unaltered.Thus, solvation effects are expected to be large with unhindered phenols and relatively unimportant in phenols containing bulky substituents in the proximity of the OH group.
- Pedrielli, Pamela,Pedulli, Gian Franco
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p. 509 - 512
(2007/10/03)
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- Aryl oxalate derivatives as convenient precursors for generation of aryloxyl radicals
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The use of aryloxy oxalyl chlorides (AOCs), aryloxy oxalyl tert-butyl peroxides (AOBs), and diaryl oxalates (DAOs) for unimolecular generation of phenoxyl-based radicals under solution and rigid matrix conditions is described. AOCs are usable for photochemical generation of phenoxyl radicals, but are only conveniently stable as precursors when 2,6-di-tert-butylated derivatives are used. AOBs may be used as thermal precursors to aryloxyl radicals, since they typically decompose within 2-3 h at 60-85 °C to give phenols. 1H-NMR solution kinetic studies find that ΔH? = 31 kcal/mol, and ΔS? = +3.4 cal/mol-K for decomposition of phenoxyoxalyl tert-butyl peroxide, consistent with substantial concertedness in peroxide bond cleavage. AOBs and the more stable DAOs are also convenient photochemical phenoxyl radical precursors. AOBs yield phenoxyl radicals more readily by photolysis than do corresponding DAOs, but the DAOs have fewer side reactions that can quench the product phenoxyl radicals.
- Lahti, Paul M.,Modarelli, David A.,Rossitto, Frank C.,Inceli, Ahmet Levent,Ichimura, Andrew S.,Ivatury, Shyamala
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p. 1730 - 1738
(2007/10/03)
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- Rate constants for the reaction of peroxyl free radical with flavonoids and related compounds as determined by the kinetic chemiluminescence method
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The kinetic chemiluminescence method was applied to determine rate constants (k5) for the reaction of the peroksyl radical from diphenylmethane, ROO-radical, with a number of flavonoids, catechol derivatives as well as two standard phenolic ant
- Belyakov, Vladimir A.,Roginsky, Vitalij A.,Bors, Wolf
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p. 2319 - 2326
(2007/10/03)
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- Negative activation energies and compensation effects for the reactions of diarylanimyl radicals with phenols
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The temperature dependence of the rate constants of the reaction of 4,4'-disubstituted diphenylaminyl radicals (ArAr'N.) with phenols has been studied by a laser pulse photolysis technique.The linear relationships between activation energies E1 (negative in the majority of cases) and logarithms of pre-exponential factors logA1 have been established.These results coincide with the two-parametric correlations of logk1 with ?+ constants of substituents in phenol and aminyl radicals under isothermal conditions.The reaction rate constants decrease considerably when toluene is used instead of n-decane. - Key words: aminyl radicals, phenols, negative temperature coefficient, isoparametric correlations.
- Varlamov, V. T.,Denisov, N. N.,Nadtochenko, V. A.
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p. 2282 - 2286
(2007/10/03)
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- Bond Dissociation Enthalpy of α-Tocopherol and Other Phenolic Antioxidants
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The equilibrium constants, K1, for the reaction between galvinoxyl and a series of phenolic antioxidants have been determined by means of EPR spectroscopy.With aroxyl radicals decaying at appreciable rates, K1 was obtained by performing kinetic analyses of the time dependence of the concentrations of the equilibrating radicals after mixing the reactants.In two cases the temperature dependence of K1 was also studied and the entropy change for the equilibration reaction was determined.Bond dissociation enthalpies, DH, of the ArO-H bond of the examined phenols were calculated by comparison with the known value of 2,4,6-tri-tert-butylphenol (81.24 kcal mol-1).A larger than expected DH value was found for probucol (81.03 kcal mol-1) and an explanation of this behavior was given in terms of the preferred conformation adopted by the para alkylthio group.The DH value of α-tocopherol (78.93 kcal mol-1) was found to be very close to that of the phenolic precursor of galvinoxyl (78.80 kcal mol-1) and somewhat larger than that of 2,6-di-tert-butyl-4-methoxyphenyl (77.61 kcal mol-1).
- Lucarini, Marco,Pedulli, Gian Franco,Cipollone, Marta
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p. 5063 - 5070
(2007/10/02)
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- CONVENIENT UNIMOLECULAR SOURCES OF ARYLOXYL RADICALS.III - PHOTOLYSIS OF BIS(ARYLOXY)PHOSPHINE AZIDES
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Photolysis of sterically hindered bis(aryloxy)phosphine azides in solution, frozen matrix, or neat solid states leads to facile unimolecular production of aryloxyl radicals.
- Kalgutkar, Rajdeep,Ionkin, Alexey S.,Quin, Louis D.,Lahti, Paul M.
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p. 3889 - 3892
(2007/10/02)
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- Entropic and Enthalpic Effects of 4-Methoxy Substitution in Phenoxyl Radicals
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Values of ΔH3 = (-12.4 +/- 1.6) kJ mol-1, ΔS3 = (-18.5 +/- 5.6) J K -1 mol-1 for reaction (3) (see text), corresponding to an O-H bond energy of 322.2 kJ mol-1 in 1, and to a 14.5 J K-1 mol-1 entropy loss for the CH3O-Ar (Ar = aromatic) libration in 2 relative to 1, are derived from the temperature dependence of the equilibrium constant K3, determined by EPR spectrometry in benzene-toluene media, between 251 and 304 K.These results allow, for the first time, discrimination between enthalpic and entropic effects on the rates of (O)H-atom abstraction by peroxyl radicals from 4-methoxyphenols and related species.
- Coronel, Marta E. J.,Colussi, Agustin J.
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p. 785 - 788
(2007/10/02)
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- FORMATION OF RADICAL PAIRS IN THE PHOTOLYSIS OF 2,6-DI(TERT-BUTYL)-1,4-BENZOQUINONEDIAZIDE IN SINGLE CRYSTALS OF 2,6-DI(TERT-BUTYL)-4-METHYLPHENOL
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In the photolysis of single crystals of 2,6-di(tert-butyl)-4-methylphenol with the addition of 2,4-di(tert-butyl)-1,4-benzoquinonediazide it was found that they form carbenes and radical pairs with r1 = 6.0 Angstroem, which are secondary products of the photolysis of the quinonediazide and which form as a result of transfer of a hydrogen atom from the phenol to the carbene.The high thermal stability of these radical pairs makes it possible to use them as two-spin probes.Also found were radical products that are annealed by light with wavelength greater than 560 nm and are hypothetically classified as ion-radical pairs with rav = 8.9 Angstroem.
- Lazarev, G. G.,Kuskov, V. L.,Lebedev, Ya. S.
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p. 483 - 486
(2007/10/02)
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- The Reaction of the 2,6-Di-tert-butyl-4-methylphenoxy Radical with Phenols
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Dimer 3 dissociates in solution to give the phenoxy radical 4.When solid 3 is dissolved in a solution of phenol 11a, radical 4 dehydrogenates the phenol to afford phenol 5 and phenoxy radical 12.Radical 12 couples with more 4 to afford principally bis(cyclohexadienone) 16a and a minor amount of the dimeric bis(cyclohexadienone) 13.The preferential formation of 16a over 13 is explained as the result of a solvent-cage reaction.Compounds 13 and 16a are isomerized in the presence of silica gel or triethylamine to 4,4'-dihydroxybiphenyl 14 and phenolic dienone 17a, respectively.Compound 13 is relatively stable in hexane at 30 deg C, while 16a slowly dissociates under these conditions to afford parent radicals 4 and 12.If 11a is present, radical 4 either recombines with 12 or dehydrogenates the phenol.Two radicals of 12 generated by the dissociation of 16a and the subsequent dehydrogenation of 11a couple to give 13.Therefore, dissolving 3 in hexane containing excess 11a and keeping the resulting solution at 30 deg C results in the preferential formation of 13.Dissolving 3 in triethylamine containing phenols 11 or 21 similarly affords dienones 17 or 23.The yields of these dienones increase with increasing electron-donating capability of the substituent of the phenol ring.
- Omura, Kanji
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p. 921 - 927
(2007/10/02)
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- KINETIC STUDY OF THE REACTION OF BENZOYL PEROXIDE WITH PHENOLIC COMPOUNDS. A TECHNIQUE FOR THE EVALUATION OF PHENOLIC O-H BOND DISSOCIATION ENERGIES
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From kinetic data concerning the reaction of benzoyl peroxide (POB) with several phenolic antioxidants (ArOH): POB + ArOH -> C6H5CO2. + C6H5CO2H + ArO., it is shown that differences in the phenolic O-H bond dissociation energies may be obtained.In the particular case of α-tocopherol (α-TH), a stabilization energy defined with respect to phenol: D(C6H5O-H) - D(α-T-H) = 10 kcal.mol-1 has been estimated in heptanol as solvent.It would correspond for α-TH to an O-H bond dissociation energy around 78 kcal.mol-1.
- Rousseau-Richard, Claire,Richard, Claude,Martin, Rene
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p. 2057 - 2066
(2007/10/02)
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- CONVENIENT UNIMOLECULAR SOURCES OF ARYLOXYL RADICALS I-ARYLOXYOXALYL CHLORIDES
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UV photolysis of readily synthesized aryloxyoxalyl chloride half-esters provides a new unimolecular source of aryloxyl radicals, as shown by ESR and UV-vis spectroscopy.
- Modarelli, David A.,Rossitto, Frank C.,Lahti, Paul M.
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p. 4473 - 4476
(2007/10/02)
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- CONVENIENT UNIMOLECULAR SOURCES OF ARYLOXYL RADICALS II-ARYLOXYOXALYL tert-BUTYLPEROXIDES
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UV photolysis and mild thermolysis of aryloxyoxalyl tert-butylperoxides provides a new unimolecular source of aryloxyl radicals, as shown by ESR and UV-vis spectroscopy.
- Modarelli, David A.,Rossitto, Frank C.,Lahti, Paul M.
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p. 4477 - 4480
(2007/10/02)
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- Fotosensitized Fading of Azomethine Dyes
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The quantum yields of photofading of the azomethine dyes 1a-h were determined in the presence of the sensitizers benzophenone (B), michler's ketone (MK) and anthracen (A), in isopropanol and benzene.In isoproponol, excluding oxygen, the leuco dyes 5 are formed by benzophenone in high quantum yield (ΦBAr=0.4).Photofading also takes place using Michler's ketone (ΦMKAr=3E-3) and anthracene (ΦAAr=1E-4), but 5 was not found.With benzophenone (ΦBO2=7E-3), Michler's ketone (ΦMKO2=1E-3) and anthracene (ΦAO2=1E-4) as sensitizers the chromophore of 1a-h were destroyed, when oxygen was present.The photofading with benzophenone and Michler's ketone is a result of radicalic processes.Radical scavangers 8 and 9, therefore, give a strong decrease of the quantum yield of photofading.The photofading in benzene corresponds to the results in isopropanol considering the lower potential of this solvent for photoreduction processes.
- Grossmann, N.,Wehner, V.,Weise, A.,Winnig, B.,Fanghaenel, E.
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p. 204 - 212
(2007/10/02)
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- Primary Processes of Stabilizer Action in Radiation-induced Alkane Oxidation
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Results of pulse-radiolytic model experiments on the auto-oxidation of long-chain n-alkanes and auto-oxidation retardation are presented.In pure n-heptadecane and solutions of di-t-butyl-p-cresol or diphenylamine the reaction of alkyl radicals with oxygen and the corresponding competition reactions with the stabilizer prototypes have been characterized by spectroscopic and kinetic data.A reaction mechanism describing the radiation-induced processes within the system long-chain n-alkane-oxygen-stabilizer is discussed.
- Brede, Ortwin,Hermann, Ralf,Mehnert, Reiner
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p. 2365 - 2380
(2007/10/02)
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