- Atmospheric chemistry of 1,4-dioxane. Laboratory studies
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A pulse radiolysis technique was used to measure the UV absorption spectra of c-C4H7O2 and (c-C4H7O2)O2 radicals over the range 220-320 nm, σ(c-C4H7O2)250 nm = (5.9 ± 0.6) × 10-18 and σ[(c-C4H7O2)O2]240 nm = (4.8 × 0.8) × 10-18 cm2 molecule-1. The observed self-reaction rate constants for the c-C4H7O2 and (c-C4H7O2)O2 radicals, defined as -d[c-C4H7O2]/dt = 2k4[c-C4H7O2]2 and -d[(c-C4H7O2)O2]/dt = 2k5 obs[(c-C4H7O2)O2]2 were k4 = (3.3 ± 0.4) × 10-11 and k5 obs = (7.3 ± 1.2) × 10-12 cm3 molecule-1 s-1. The rate constants for reactions of (c-C4H7O2)O2 radicals with NO and NO2 were k6 (1.2 ± 0.3) × 10-11 and k7 = (1.3 ± 0.3) × 10-11 cm3 molecule-1 s-1, respectively. The rate constants for the reaction of F atoms with 1,4-dioxane and the reaction of c-C4H7O2 radicals with O2, were k3 = (2.4 ± 0.7) × 10-10 and k2 = (8.8 ± 0.9) × 10-12 cm3 molecule-1 s-1, respectively. A relative rate technique was used to measure the rate constant for the reaction of Cl atoms with 1,4-dioxane, k17 = (2.0 ± 0.3) × 10-10 cm3 molecule-1 s-1. A long-pathlength FTIR spectrometer coupled to a smog chamber system was used to show that the sole atmospheric fate of the alkoxy radical (c-C4H7O2)O is decomposition via C-C bond scission leading to the formation of H(O)COCH2CH2OC(O)H (ethylene glycol diformate).
- Platz,Sehested,Mogelberg,Nielsen,Wallington
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- Rate Constants for Hydrogen Abstraction Reactions of the Sulfate Radical, SO4(1-). Experimental and Theoretical Results for Cyclic Ethers
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Rate constants have been determined as a function of temperature in aqueous solutions for the reactions of the SO4(1-) radical with the C3 to C6 cyclic monoethers and the diethers 1,4- and 1,3-dioxane.The reactions clearly are grouped into three sets: the C4 and C6 monoethers react the fastest, the C3 and C5 monoethers are slower, and the cyclic diethers are the lowest.Theoretical calculations also have been carried out to estimate the C-H bond strengths in these molecules and these results have been related to the experimental rate constants.
- Huie, Robert E.,Clifton, Carol L.,Kafafi, Sherif A.
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- Absolute Rate Constants for the Reactions of tert-Butoxyl with Ethers: Importance of the Stereoelectronic Effect
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Absolute rates of hydrogen atom abstraction by tert-butoxyl from variety of cyclic and acyclic ethers, orthoformates, and acetals have been measured at 27 deg C by laser flash photolysis techniques.High rates of abstraction from the C-H bond adjacent to oxygen and forming small dihedral angle (ca. 30 deg) with the ?-type orbital(s) on the oxygen(s) have been observed.For larger dihedral angles (ca. 90 deg) the rate of C-H abstraction is much lower.Arrehenius parameters have also been measured for a few representative cyclic ethers.
- Malatesta, V.,Scaiano, J. C.
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- REACTIONS OF ALKANETHIYL AND ALKOXYL RADICALS WITH UNSATURATED CYCLIC ETHERS
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Methanethiyl radicals (CH3S.) are shown to react with cyclic ethers containing double bonds to give addition and hydrogen abstraction from the allylic positions.The esr spectra of the corresponding radicals have been determined.In some cases radicals arising from rearrangement (probably 1,4 H-shift) were obserwed. t-Butoxyl radicals also give abstraction and addition with the same substrates.In two cases the barriers to ring inversion for six-membered ring radicals have been determined.
- Lunazzi, Lodovico,Placucci, Giuseppe,Grossi, Loris
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- On the mechanism of reaction of radicals with tirapazamine
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Ketyl radicals produced by photolysis of ketones or di-tert-butyl peroxide (DTBP) in alcohol solvents react rapidly with tirapazamine (TPZ). The acetone ketyl radical (ACOH) reacts with TPZ with an absolute second-order rate constant of (9.7 ± 0.4) × 108 M-1 s-1. The reaction kinetics can be followed by monitoring the bleaching of TPZ absorption at 475 nm or the formation of a reaction product which absorbs at 320 and 410 nm. The ACOD radical reacts with TPZ in 2-propanol-OD with an absolute rate constant of (6.7 ± 0.5) × 108 M-1 s -1 corresponding to a kinetic isotope effect (KIE) of 1.4. Deuteration of the radical on carbon (ACOH-d6) retards the reaction of the radical with TPZ even further (absolute rate constant = (4.8 ± 0.04) × 108 M-1 s-1). This result corresponds to a KIE of 2.0. Radicals derived from dioxane and diisopropyl ether by flash photolysis of DTBP in ethereal solvent react with TPZ more slowly than do ketyl radicals. It is concluded that ketyl radicals react, in part, with TPZ in organic solvents by transfer of a hydrogen atom from the OH and CH 3 groups of the ketyl radical to the oxygen atom at the N4 position of TPZ to form acetone or acetone enol and a radical derivative of TPZ (TPZH). The latter species absorbs at 320 and 405 nm, has a lifetime of hundreds of microseconds in alcohol solvents, and decays by disproportionation to form TPZ and a reduced heterocycle. The reduced heterocycle eventually forms a desoxytirapazamine by a polar mechanism. The results are supported by density functional theory calculations. It is proposed that dioxanyl radical will also react, in part, with TPZ by transfer of a hydrogen atom from the carbon adjacent to the radical center to the oxygen atom at the N4 position of TPZ. This produces the enol ether and the previously mentioned TPZH radical. It is further posited that ether radicals react a bit more slowly than ketyl radicals because they lack the second mode of hydrogen transfer (from the OH group) that is present in the ACOH radical. Our data are permissive of the possibility that ether radicals add to TPZ at a rate that is competitive with β-hydrogen atom transfer.
- Shi, Xiaofeng,Mandel, Sarah M.,Platz, Matthew S.
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- Palladium catalyzed Csp2-H activation for direct aryl hydroxylation: The unprecedented role of 1,4-dioxane as a source of hydroxyl radicals
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A novel strategy for direct aryl hydroxylation via Pd-catalysed Csp2-H activation through an unprecedented hydroxyl radical transfer from 1,4-dioxane, used as a solvent, is reported with bio relevant and sterically hindered heterocycles and various acyclic functionalities as versatile directing groups.
- Seth, Kapileswar,Nautiyal, Manesh,Purohit, Priyank,Parikh, Naisargee,Chakraborti, Asit K.
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supporting information
p. 191 - 194
(2015/01/09)
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- The interplay of electronic, steric and stereoelectronic effects in hydrogen-atom abstraction reactions of SO4-., revealed by ESR spectroscopy
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Attack by SO4-. (generated by photolysis and one-electron transfer on a number of carbohydrates and some model compounds has been followed directly by ESR spectroscopy. SO4-. is a reactive, electrophilic species but is more selective
- Gilbert, Bruce C.,Smith, John R. Lindsay,Taylor, Philip,Ward, Steven,Whitwood, Adrian C.
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p. 1631 - 1638
(2007/10/03)
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- Oxidation vs. fragmentation in radiosensitization. Reactions of α-alkoxyalkyl radicals with 4-nitrobenzonitrile and oxygen. A pulse radiolysis and product analysis study
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α-Monoalkoxyalkyl radicals produced from 1,4-dioxane (100percent), 1,3-dioxane (56percent), tetrahydrofuran (92percent) and dimethyl ether (100percent) by H-abstraction by hydroxyl radicals generated in the radiolysis of water were found to react with 4-nitrobenzonitrile (NBN) by addition to give N-alkoxyaminoxyl-type radicals, which have absorption maxima at about 310 nm and decay very slowly (k = 0.4 - 1.0 s-1).On the other hand, the reaction of the α-dialkoxyalkyl radical, 1,3-dioxan-2-yl 3 with NBN leads to the rapid formation of the radical anion NBN.The N-alkoxyaminoxyl-type radicals (A in the case of 1,4-dioxane and D in the case of dimethyl ether) react with ascorbate (k ca. 2*104 dm3 mol-1 s-1).They have a very low reactivity with oxygen (k 3 dm3 mol-1 s-1 in the case of tetrahydrofuran).On the other hand, they are rapidly reduced by NBN radical anion (k ca. 109 dm3 mol-1 s-1 as observed with A and with B derived from 1,3-dioxane).The products -7 mol J-1> in the γ-radiolysis of N2O-saturated solution of 1,4-dioxane in the presence of NBN are 1,4-dioxan-2-one (0.3), 2-hydroxy-1,4-dioxane (2.5), ethane-1,2-diol monoformate (2.1), ethane-1,2-diol diformate (0.7), formaldehyde (2.1), 4-nitrosobenzonitrile and other reduction products of 4-nitrobenzonitrile.These products are accounted for as resulting from the fragmentation of the aminoxyl radical A by (a) heterolysis of the C-O bond (45percent leading to the one-electron oxidation of the 1,4-dioxan-2-yl radical) and (b) homolysis of the N-O bond (55percent leading to the formation of the 1,4-dioxan-2-oxyl radical which undergoes further fragmentation.The products from the reaction of methoxymethyl radicals with NBN under γ-radiolysis conditions are formaldehyde (5.7), methanol (2.5) and methyl formate (1.3).It is concluded that also in this case the decay of the aminoxyl radical D occurs by two pathways: the heterolysis route (46percent) and the homolysis route (54percent).In the presence of oxygen the 1,4-dioxan-2-yl radicals are converted into the corresponding peroxyl radicals.Their bimolecular decay (2k = 2.0*108 dm3 mol-1 s-1) yields the same products as in the case of NBN (albeit with a different product distribution and the formation of some peroxides): 1,4-dioxan-2-one (0.4), 2-hydroxy-1,4-dioxane (0.4), ethane-1,2-diol monoformate (0.6), ethane-1,2-diol diformate (2.8) and formaldehyde (0.6).These results indicate that fragmentation reactions involving the carbon-skeleton of organic radicals are important not only in the case of peroxyl radicals but they can also be induced by nitroaromatic sensitizers.In cells, reduction of the long-lived sensitizer adduct radicals by reducing agents such as ascorbate to give (toxic) hydroxylamine type products may compete with the homolytic or heterolytic fragmentation of the N-alkoxyaminoxyl radicals.
- Nese, Chandrasekhar,Schuchmann, Man Nien,Steenken, Steen,Sonntag, Clemens von
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p. 1037 - 1044
(2007/10/02)
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- Laser flash photolysis determination of absolute rate constants for reactions of bromine atoms in solution
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The photodecomposition of vicinal dibromides at 266 nm produces bromine atoms with a quantum yield of ~2.0. This results from an efficient primary photocleavage of a C-Br bond, followed by rapid elimination of a second bromine atom from radicals of the ty
- Scaiano,Barra,Krzywinski,Sinta,Calabrese
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p. 8340 - 8344
(2007/10/02)
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- Fragmentation, Ring-opening, and Addition Reactions of Oxygen-conjugated Alkenyl Radicals derived by Rapid 1,5-Hydrogen Shifts in Vinyl Radicals. An Electron Spin Resonance and Kinetic Investigation
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The addition od ether-derived oxygen-conjugated radicals .CR1R2OCHR1R2 to alkynes R3CCR4 leads to vinyl radicals, most of which undergo a rapid 1,5-shift (k>105 s-1) to give radicals .CR1R2OCR1R2CR3=CHR4.The reactions of the latter species, as revealed by e.s.r. spectroscopy, include fragmentation (to give R1R2C=O and .CR1R2CR3=CHR4), ring-opening (e.g. for radicals from tetrahydrofuran and dioxane and their derivatives), and trapping via reaction with a further molecule of alkyne.A kinetic analysis is presented and results are interpreted in terms of the electronic and steric effects of substituents in the intermediates.
- Gilbert, Bruce C.,Parry, David J.
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p. 875 - 886
(2007/10/02)
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- Kinetic Applications of Electron Paramagnetic Resonance Spectroscopy. 36. Stereoelectronic Effects in Hydrogen Atom Abstraction from Ethers
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Relative rates of hydrogen atom abstraction by photogenerated tert-butoxyl from a variety of cyclic and acyclic ethers, acetals, and orthoformates have been measured at -60 deg C by an EPR spectroscopic technique.There is a pronounced stereoelectronic effect which produces high rates of abstraction from those C-H bonds adjacent to oxygen which have a relatively small dihedral angle (ca. 30 deg) with respect to the p-type orbital(s) on the oxygen(s).For C-H bonds which have a large dihedral angle (ca. 90 deg) abstraction is very much slower.The barrier to inversion of the 2-methoxy-1,3-dioxolan-2-yl radical is 10 kcal/mol.
- Malatesta, V.,Ingold, K. U.
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p. 609 - 614
(2007/10/02)
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