Showing papers on "Hydrogen atom abstraction published in 1989"
TL;DR: Although homolytic reactions of PUFA hydroperoxides have received the most attention, hydroper oxides are also susceptible to heterolytic transformations, such as nucleophilic displacement and acid-catalyzed rearrangement.
685 citations
TL;DR: In this paper, rate constants for the reactions of SO4− with a series of alcohols, including hydrated formaldehyde, have been determined over the range 10−45°C.
Abstract: Rate constants have been determined for the reactions of SO4− with a series of alcohols, including hydrated formaldehyde. The SO4− radical was produced by the laser-flash photolysis of persulfate, S2O82−. Rate constants for the reactions of SO4− with alcohols range from 1.0 × 107 for methanol to 3.4 × 108 M−1 s−1 for 1-octanol. Rate constants for the reactions of SO4− with deuterated methanol and ethanol are lower by about a factor of 2.5. For methanol, ethanol, and 2-propanol, the temperature dependence of the rate constant was determined over the range 10–45°C.
259 citations
TL;DR: In addition to the binuclear iron center, tyrosyl radical, and AdoCbl dependent systems discussed above, recent studies from Follmann' s labora tory indkate that ribonucleotide reductase isolated from Brevibacterium ammoniagenes requires manganese for activity is reason able to postulate the existence of a novel binuclear Mn(III)-dependent system.
Abstract: or is also warranted. While it is generally accepted that thiols are capable of transferring a hydrogen atom to carbon-centered radicals because of the relative weakness of their bond dissociation energies (91 + 1.5 kcallmol) (33, 89), their role as hydrogen atom abstractors is contraindicated by most commonly held beliefs. Hydrogen atom abstraction of the 31-H of a nucleoside by a thiyl radical is allowed on thermodynamic grounds because the C-H homolytic bond dissociation energy is -91 kcallmole. Akhlaq et al recently devised a method to detect thiyl radical-mediated hydrogen atom abstractions (34), which are usually masked in the presence of high RSH concentrations due to thiol-mediated repair. Akhlaq et al took either cis-2,5 Me2THF or trans-2,5 Me2THF and measured the production of a mixture of cis and trans 2,5 Me2THF (Eq. 4). Thiols in this case, as in the earlier studies H'C>(J0 CH, 103/M/, -aD CH, RS . H .====-H3C • + RSH H 108/M/s H 4. by Huyser & Kellogg (35), do in fact induce a chain reaction. Thus, the feasibility of thiyl radicals mediating hydrogen atom abstractions has reason able chemical precedent. The final step in the postulated mechanism, thiyl radical abstraction of a hydrogen atom from 51 deoxyadenosine, is thermo dynamically unfavorable by 1 8 kcal. The driving force for this reaction would have to be provided by reformation of the C-Co bond to produce AdoCbI. This proCI!SS has recently been shown to be exothermic by 25 to 30 kcallmole (91). The establishment of the role of any postulated thiol in an enzymatic reaction will be facilitated by the cloning, sequencing, and expression of any of these genes and use of site-directed mutagenesis methods to replace the cysteine residues. Efforts are presently under way with both RTPR and ethanolamine ammonia lyase to investigate the putative role of thiyl radicals in the chemistry . An interesting paper of Hartmanis & Stadtman provides further credibility to the postulated role of protein radicals (36). Specifically, a dioldehydrase that does not require AdoCbl has been partially purified from Chlostridium glycolicum and shown to possess a protein radical that is destroyed by reduction with hydroxyurea concomitant with loss of dioldehydrase activity. The properties of this radical are similar to those observed in E. coli RDPR. In addition to the binuclear iron center, tyrosyl radical, and AdoCbl dependent systems discussed above, recent studies from Follmann' s labora tory indkate that ribonucleotide reductase isolated from Brevibacterium ammoniagenes requires manganese for activity (37) . Based on recent chemi cal model systems from Lippard's and Wieghardt' s laboratories, it is reason able to postulate the existence of a novel binuclear Mn(III)-dependent system
197 citations
TL;DR: In this article, rate constants of 0.44, 4.0, and 10.5 × 107 M−1 s−1 were found for the SO4− radical produced by the laser-flash photolysis of persulfate.
Abstract: Rate constants have been determined for the reactions of SO4− with a series of alkanes and ethers. The SO4− radical was produced by the laser-flash photolysis of persulfate, S2O82−. For methane, only an upper limit of 1 × 106 M−1 s−1 could be determined. For ethane, propane, and 2-methylpropane, rate constants of 0.44, 4.0, and 10.5 × 107 M−1 s−1 were found. For ethyl and n-propyl ether, rate constants of 1.3 × 108 and 2.2 × 108 M−1 s−1 were found and for 1,4-dioxane and tetrahydrofuran, rate constants of 7.2 × 107 and 2.8 × 108 were obtained. The reaction of SO4− with allyl alcohol was also studied and found to have a rate constant of 1.4 × 109 M−1 s−1.
136 citations
124 citations
TL;DR: The large-curvature semiclassical ground-state tunneling approximation, version 3 (abbreviated LCG3) has been implemented for variational transition state theory calculations on systems with more than three atoms as discussed by the authors.
109 citations
TL;DR: In this paper, the yields and chemical behavior of these intermediates were studied by pulse radiolysis of DCM solutions containing various solutes: phenols, anilines, dimethoxybenzene, hexamethylbenzenes, cyclohexene, dimethyl sulfoxide, and zinc tetratolylporphyrin.
Abstract: Radiolysis of dichloromethane (DCM) leads to formation of primary oxidizing radicals and carbon-centered radicals. The latter react with oxygen to yield peroxyl radicals. The yields and chemical behavior of these intermediates were studied by pulse radiolysis of DCM solutions containing various solutes: phenols, anilines, dimethoxybenzene, hexamethylbenzene, cyclohexene, dimethyl sulfoxide, and zinc tetratolylporphyrin. At low concentrations, some of these solutes were found to be oxidized by two peroxyl radicals, CH{sub 2}ClO{sub 2}{sup {sm bullet}} and CHCl{sub 2}O{sub 2}{sup {sm bullet}}, with different rate constants. At higher concentrations, the solutes react also with the primary radicals: Cl atoms and the radical cations CH{sub 2}Cl{sub 2}{sup +{sm bullet}}, with diffusion-controlled rate constants. The rates of these reactions were determined by competition kinetics because of the very short lifetimes of the species. Cl atoms were found to have a half-life of about 5 ns in DCM, reacting predominantly with the solvent by hydrogen abstraction. The radical cations decay within a fraction of a nanosecond. The total yield of these primary radicals was determined to be G = 3.6 and appears to be divided about equally between Cl and the radical cations. The total yield of oxidation, by the primary and the peroxyl radicals, wasmore » found to be G = 7.5. Cl atoms were found to be very reactive in electron transfer as well as addition and hydrogen abstraction reactions.« less
108 citations
TL;DR: Thiyl free radicals must consequently be considered as a potential source of lipid peroxidation.
106 citations
TL;DR: The mechanisms of degradation and oxidation reactions are in principle fairly well known: Extraneous groups and/or impurities in the polymer absorb light quanta and form excited states, initially singlets (short-lived) which are transformed to triplets (long-lived and reactive) as mentioned in this paper.
83 citations
TL;DR: Several fluorinated triplet aryl nitrenes have been generated in low-temperature polycrystals by photolysis of the corresponding azides as mentioned in this paper, and they have been detected by EPR.
Abstract: Several fluorinated triplet aryl nitrenes have been generated in low-temperature polycrystals by photolysis of the corresponding azides. Upon extended photolysis at {minus}196{degree}C the nitrenes abstract hydrogen from frozen toluene to give anilino-benzyl radical pairs, which subsequently combine to give CH insertion products. The radical pairs and the triplet nitrenes have been detected by EPR. In toluene solution, the major reaction products are tar, the corresponding fluorinated anilines, and azo compounds.
TL;DR: The rotational and vibrational state distributions of the H2 product from the reactions of translationally excited H atoms with HCl, HBr, and HI at 1.6 eV are probed by coherent anti-Stokes Raman scattering spectroscopy after only one collision of the fast H atom as discussed by the authors.
Abstract: The rotational and vibrational state distributions of the H2 product from the reactions of translationally excited H atoms with HCl, HBr, and HI at 1.6 eV are probed by coherent anti‐Stokes Raman scattering spectroscopy after only one collision of the fast H atom. Despite the high collision energy, only the very exoergic (ΔH=−1.4 eV) hydrogen atom abstraction involving HI leads to appreciable H2 product vibrational excitation. For this reaction the H2 vibrational distribution is strongly inverted and peaks in v’=1, with 25% of the total available energy partitioned to vibration. For the mildy exoergic (ΔH=−0.72 eV) reaction with HBr and the nearly thermoneutral (ΔH=−0.05 eV) reaction with HCl, very little energy appears in H2 vibration, 9% and 2%, respectively, and the vibrational state distributions peak at v’=0. However, in all three reactions a significant fraction, 18% to 21%, of the total energy available appears as H2 rotation. All three reactions show a strong propensity to conserve the translation...
TL;DR: The photolyses of 5α-cholestane-3β,6β-diol 3-acetate (1), 5α cholestan-2β-ol (4), 5β-choline-4β-OL (8), (20R)-pregn-5-ene-3 β,20-dioxioxioxyl-20diol (3-β,20dioxyl)-3-acetates (19), and dihydrotigogenin-3acetate(25) in the presence of
Abstract: The photolyses of 5α-cholestane-3β,6β-diol 3-acetate (1), 5α-cholestan-2β-ol (4), 5α-cholestan-4β-ol (8), (20R)-pregn-5-ene-3β,20-diol 3-acetate (19), (20S)-pregn-5-ene-3β,20-diol 3-acetate (21), and dihydrotigogenin 3-acetate (25) in the presence of iodine and various hypervalent organoiodine compounds lead to alkoxyl radicals which undergo intramolecular hydrogen abstraction to produce, in most cases, 1,4-iodohydrins and tetrahydrofuran derivatives.
TL;DR: In this article, high-temperature photochemistry kinetics studies of the reactions of hydrogen atom(1µS) and deuterium atom (1²S) with nitrous oxide were presented.
Abstract: Article on high-temperature photochemistry kinetics studies of the reactions of hydrogen atom(1²S) and deuterium atom (1²S) with nitrous oxide.
TL;DR: In this article, the rate constants for hydrogen abstraction from benzene, toluene, and cyclopentane by methyl and ethyl radicals have been measured relative to the corresponding abstraction reaction from ethyle...
Abstract: Rate constants for the abstraction of hydrogen from benzene, toluene, and cyclopentane by methyl and ethyl radicals have been measured relative to the corresponding abstraction reaction from ethyle...
Patent•
26 Sep 1989
TL;DR: In this article, the authors describe precursors for preparing polyimides polyisoindoloquinazolinediones, polyoxazinedionsiones or polyquinazolate compounds which when excited by UV radiation are capable of hydrogen abstraction.
Abstract: Radiation-reactive mixtures suitable for producing insulating layers and printed circuits are composed of certain precursors for preparing polyimides polyisoindoloquinazolinediones, polyoxazinediones, polyquinazolinediones or polyquinazolones and aryl-containing carbonyl compounds which when excited by UV radiation are capable of hydrogen abstraction, these mixtures undergoing solubility differentiation on irradiation with actinic light.
TL;DR: The cytochrome P-450 mediated activation of paracetamol to the reactive electrophilic intermediate N-acetyl-p-benzoquinone imine (NAPQI) has been studied by use of SV 6-31G ab initio energy calculations and spin distributions and indicates that a recombination reaction between a hydroxyl radical and these radicalized carbon atoms at the ortho or para positions could explain the formation of the minor metabolites.
Abstract: The cytochrome P-450 mediated activation of paracetamol (PAR) to the reactive electrophilic intermediate N-acetyl-p-benzoquinone imine (NAPQI) has been studied by use of SV 6-31G ab initio energy calculations and spin distributions. A simplified model for cytochrome P-450 has been used by substituting the proposed biologically active ferric-oxene state of cytochrome P-450 by a singlet oxygen atom. The results indicate that an initial hydrogen abstraction from the phenolic hydroxyl group is favored by 30.1 kcal/mol over an initial hydrogen abstraction from the acetylamino nitrogen atom. Metabolic activation of PAR via primary formation of a phenoxy radical seems the most likely mechanism. The calculated ab initio spin densities indicate that the radical formed by hydrogen abstraction from the phenolic hydroxyl group stays predominantly localized at the phenolic oxygen. A second hydrogen abstraction from the acetylamino nitrogen atom, giving rise to the reactive intermediate NAPQI, is then favored in terms of energy differences. The unpaired electron of the phenoxy radical was found to delocalize only to a small extent toward the carbon atoms at the ortho and para positions relative to the hydroxyl-containing ring carbon, but nevertheless a recombination reaction between a hydroxyl radical and these radicalized carbon atoms at the ortho or para positions could explain the formation of the minor metabolites 3-hydroxy-PAR and p-benzoquinone plus acetamide.
TL;DR: In this paper, saddle-point geometries and energies for hydrogen abstractions from HO-sub 2 were calculated assuming direct attack at the hydrogen atom and showed that activation energies for these reactions should decrease with increasing exothermicity of reaction.
Abstract: Ab initio quantum mechanical methods are used to calculate saddle-point geometries and energies for hydrogen abstractions from HO{sub 2} by N, ClO, O, Cl, H, OH, and F assuming direct attack at the hydrogen atom. Results are consistent with bond energy-bond order principles and indicate that activation energies for these reactions should decrease with increasing exothermicity of reaction. In addition, the large rate constant for OH + HO{sub 2} {yields} H{sub 2}O + O{sub 2} is interpreted to be due to long-range dipole-dipole attraction, leading ultimately to formation of a hydrogen bond and to stabilization of an early transition state. Results for the ClO + HO{sub 2} {yields} HOCl + O{sub 2} reaction suggest a dual mechanism dominated by direct abstraction at high temperatures and by elimination from a stable intermediate at lower temperatures, perhaps the only reaction in this series which does so. These results are in good agreement with current experimental rate data.
TL;DR: The reaction of singlet oxygen with cis olefins is regioselective and shows a general preference for hydrogen abstraction on the larger alkyl group of the double bond.
TL;DR: In this article, the primayy step of the o-nitrobenzaldehyde-o-nitrogensobenzoic acid photorearrangement in solution has been studied by flash absorption with 35 ps 355 nm light pulses.
Abstract: The primayy step of the o-nitrobenzaldehyde-o-nitrosobenzoic acid photorearrangement in solution has been studied by flash absorption with 35 ps 355 nm light pulses. Flash photolysis of o-nitrobenzaldehyde in acetonitrile or THF solutions produces a transient absorption with a maximum at ca. 440 nm. Formation of the transient was < 35 ps, the laser pulse width, and within experimental error, no furthrr buildup was observed. The transient which decayed at nanosccond times is attributed to a remarkably reactive ketene intermediate formed by H abstraction of the aldehydic hydrogen by the excited state of the nitro group. Decay of the ketene was more rapid in water-acetonitrile, methanol-acetonitrile, tert-butyl alcohol and in THF than in acetonitrile solution. It is suggested that the intramolecular reaction of the ketene intermediate is enhanced in THF relative to acetonitrile because of the ability of THF to faciliaate proton transfer associated with the reaction.
TL;DR: It is shown that the protection efficiency under anoxia, and concomitantly the oxygen enhancement, increase with dose rate and with salt concentration, when DNA is irradiated in presence of DS2.
Abstract: SummaryThe ring-closed disulphide radical anion D/SS· −, formed radiolytically either by hydrogen abstraction from dithiothreitol (D(SH)2), or by one-electron reduction of the corresponding cyclic dithiane (DS2), is proposed to engage efficiently in ‘chemical repair’ of ·OH-induced DNA intermediates (DNA·) under gamma-irradiation of aqueous DNA solutions, Evidence for this concept derives from the observation that radioprotection of DNA by DS2 or D(SH)2 can be enhanced in anoxic N2O-saturated solution by the addition of formate, at a constant total ·OH scavenger capacity: carbon dioxide radical anions (CO· −2) actually promote the generation of D/SS· −, by interacting both with DS2 and D(SH)2. Oxygen enhancement of radiation-induced DNA damage in systems containing DS2 and/or D(SH)2 can be easily explained with the above concept, as previously proposed in the case of cysteine (Prutz and Monig 1987), by O2-induced annihilation of the protective disulphide radical anion. In further support of the mechanism ...
TL;DR: In this article, the reaction of trans-di-(n -pyridyl)ethylenes ( n,n′ -DPEs, n, n′ = 2,3,4) and 4,4′-bipyridsyl (BPY) following excitation by UV laser pulses were studied at room temperature in organic solvents.
Abstract: Reactions of trans -di-( n -pyridyl)ethylenes ( n,n′ -DPEs, n,n′ = 2,3,4) and 4,4′-bipyridyl (BPY) following excitation by UV laser pulses were studied at room temperature in organic solvents. Upon direct laser excitation a triplet was observed for BPY (λ max = 340 nm) and 2,2′-DPE (λ max = 360 nm) but not for 3,3′-DPE and 4,4′-DPE. The transient of 4.4′-DPE in 2-propanol (λ max = 455 nm) and a second transient of BPY (λ max = 370 nm) are assigned to radicals (half-life ⩾ 0.1 ms) of the type H-DPE • and H-BPY • respectively. The structure of H-DPE • is that of a hydrogen atom adduct to one of the nitrogens. The radicals are formed by hydrogen atom abstraction from the solvent involving the excited n,π* singlet state of trans -4,4′-DPE and the n,π* triplet state of BPY.
TL;DR: In this article, the rate coefficients were found to be 2.7 × 1012, 2.8 × 1013 cm3/mol-s, and 1.26 × 1.5 cm 3/mol s for the reactions of the hydroxyl radical with hydrogen, methane, cyclopentane, and isobutane, respectively.
Abstract: Reactions of the hydroxyl radical, OH, with several reactants have been studied near 1200 K in shock tube experiments in which UV absorption was used to monitor the OH concentration. The values of the rate coefficients were found to be 2.7 × 1012, 2.6 × 1012, 2.8 × 1013, and 1.26 × 1013 cm3/mol-s for the reactions of OH with hydrogen, methane, cyclopentane, and isobutane, respectively. These measured values are compared with previous experimental results and transition-state theory calculations.
TL;DR: In this paper, an anomalous intensity pattern in the biradical CIDEP spectrum is observed in an intramolecular hydrogen abstraction reaction system, and the intensity pattern is successfully interpreted by the BiradicalCIDEP theory proposed by Buckley et al.
TL;DR: The hypothesis that the alpha-nitrosamino radical is a common intermediate in enzymatic denitrosation versus dealkylation of NDMA is supported.
Abstract: The microsomal metabolism of the carcinogen N-nitrosodimethylamine (NDMA) was suggested to be initiated by hydrogen atom abstraction to form an alpha-nitrosamino radical, which either oxidizes further to an alpha-hydroxy nitrosamine as the initial product of the activating dealkylation pathway or fragments to the nitric oxide radical and N-methylformaldimine as the first step of the presumably inactivating denitrosation route. To examine the chemistry of the alpha-nitrosamino radical in a nonenzymatic setting, we exposed NDMA to the Fenton reagent, which is known to be capable of abstracting hydrogen atoms from organic species. The products observed were those expected of a denitrosation model. Solutions containing 13 mM [14C]NDMA, 15 mM FeSO4, 15 mM H2O2, and 7.5 mM H2SO4 were kept at 4-10 degrees C for 1 h and then basified to yield methylamine (3.2 +/- 0.5 mM, mean +/- SD, n = 8), formaldehyde (3.1 +/- 0.9 mM), and unreacted nitrosamine (10.2 +/- 0.7 mM) as the only radioactive species detected, with total nitrate/nitrite also being found at a level of 2.8 +/- 0.5 mM. N-Methylformaldiminium ion was identified as an intermediate. The parallels between these results and those seen in the microsomal reaction support the hypothesis that the alpha-nitrosamino radical is a common intermediate in enzymatic denitrosation versus dealkylation of NDMA.(ABSTRACT TRUNCATED AT 250 WORDS)
TL;DR: In this paper, the rate coefficient for hydrogen abstraction from (2) by ButO˙ to give the aminel alkylboryl radical Me3N→BHThx, has been estimated to be 4.7 × 107 dm3 mol−1 s−1 at 189 K in cyclopropane.
Abstract: E.s.r. spectroscopy has been used to show that abstraction of electron-deficient hydrogen atoms from α-C-H groups in esters, ketones, and acetic anhydride by electrophilic t-butoxyl radicals is catalysed by amine–alkylboranes, in particular by trimethylamine–thexylborane (Me3N→BH2Thx; Thx =–CMe2CMe2H)(2). The single-step abstraction by ButO˙ is replaced by a two-step catalytic cycle in which ButO˙ first abstracts electron-rich hydrogen from the amine–alkylborane to give a nucleophilic amine-alkylboryl radical, which subsequently abstracts with high regioselectivity the electron-deficient α–hydrogen from the carbonyl compound. Both steps of this catalytic cycle are promoted by favourable polar effects, while such effects militate against direct α-hydrogen abstraction by ButO˙. The rate coefficient for hydrogen abstraction from (2) by ButO˙ to give the aminel alkylboryl radical Me3N→BHThx, has been estimated to be 4.7 × 107 dm3 mol–1 s–1 at 189 K in cyclopropane. Relative reactivities of carbonyl compounds towards α-hydrogen-atom abstraction by amine-alkylboryl radicals have been determined in competition experiments at 189 K and shown to depend on enthalpic, polar, and steric factors. Towards Me3N→BHThx, the relative molar reactivities of MeCO2Et, MeCH2CO2Et, and Me2CHCO2Et are 1 : 4.5 : 0.4, while towards the less bulky Me3N→BHMe they are 1 : 6.7 : 7.3, showing the importance of steric effects in determining the rate of hydrogen abstraction. Ab initio molecular-orbital calculations have been carried out at the MP3/6-31 G**//HF/6-31 G** level for H3N→BH2Me and H3N→BHMe and the strengths of the B–H and B–C bonds in ammonia–methylborane are estimated to be 432 and 404 kJ mol–1, respectively.
TL;DR: In this article, it was found that the rate constant for hydrogen abstraction from these compounds by 3BZP may be related to the number of secondary, allylic and doubly allylic hydrogens in each molecule.
Abstract: Laser photolysis techniques have been used to measure the reactivity of benzophenone triplet (3BZP) toward various fatty acids and two glycerides in benzene solution. Eight compounds varying both in number and in the configuration of olefinic bonds have been examined. It has been found that the rate constant for hydrogen abstraction from these compounds by 3BZP may be related to the number of secondary, allylic and doubly allylic hydrogens in each molecule by the equation: kH = [0.023[H sec] + 0.112[H allylic] + 1.78[H doubly allylic]] x 10(7) M-1s-1