- Hydrogen abstraction from unactivated hydrocarbons using a photochemically excited isoindoline nitroxide
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The photochemical! y excited nitroxide, 1,1,3,3-tetramethylisoindolin-2-yloxyl (1*), abstracts primary, secondary or tertiary hydrogens from unactivated hydrocarbons including cyclohexane, isobutane and n-butane. The resultant carbon-centred radicals are rapidly and efficiently trapped by ground state nitroxide. Hydrogen abstraction from methane may also occur, but is masked by secondary photochemical reactions.
- Bottle, Steven E.,Chand, Ugesh,Micallef, Aaron S.
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- Factors affecting the rates of addition of free radicals to alkenes - Determination of absolute rate coefficients using the persistent aminoxyl method
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The rate of coupling of alkyl radicals with the persistent aminoxyl radical 1,1,3,3-tetramethyl-isoindolin-N-oxyl (1) has been used as a kinetic probe to determine absolute rate coefficients for the addition of alkyl radicals to methyl acrylate. The results are discussed in terms of the role of the structure and functionalization of the attacking radical on the rates of addition, particularly as they affect steric, polar, and enthalpic factors. The aminoxyl method is assessed against other methods for determining free radical addition rate coefficients.
- Beckwith, Athelstan L. J.,Poole, James S.
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- Study of alkyl radicals fragmentation from 2-alkyl-2-propoxyl radicals
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The reaction of a series of 2-alkyl-2-propoxyl radicals 2 in cumene has been studied. Alkoxyl radicals 2 were generated from the thermolysis of the corresponding tert-alkyl peroxypivalates, and underwent several modes of unimolecular reactions, that is, β-scission to give methyl radical, β-scission to give alkyl radicals and 1,5-H shift, which competed with hydrogen abstraction from cumene. The ratios of the rate constant for alkyl radicals elimination to that for methyl radical elimination, kβ(R)/kβ(Me), were determined by using the radical trapping method employing TEMIO as the scavenger. The logarithm of the relative rate was satisfactorily correlated with the heat of formation of the leaving alkyl radicals containing the steric parameters of neopentyl radical 2d. Then the plot of ln [kβ(R)/kβ(Me)] vs. the SOMO energies of the corresponding alkyl radicals showed a linear relationship with a slope of -7.6 eV-1, which is comparable to that of formolysis of alkyl bromides. The mechanism involving a fairly polar transition state of the alkyl radicals fragmentation from tert-alkoxyl radicals 2 is discussed in terms of the MO diagrams and the solvent effects.
- Nakamura, Tomoyuki,Watanabe, Yasumasa,Suyama, Shuji,Tezuka, Hiroshi
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p. 1364 - 1369
(2007/10/03)
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- Thermal decomposition mechanisms of tert-alkyl peroxypivalates studied by the nitroxide radical trapping technique
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The thermolysis of a series of tert-alkyl peroxypivalates 1 in cumene has been investigated by using the nitroxide radical-trapping technique. tert-Alkoxyl radicals generated from the thermolysis underwent the unimolecular reactions, β-scission, and 1,5-H shift, competing with hydrogen abstraction from cumene. The absolute rate constants for β-scission of tert- alkoxyl radicals, which vary over 4 orders of magnitude, indicate the vastly different behavior of alkoxyl radicals. However, the radical generation efficiencies of 1 varied only slightly, from 53 (R = Me) to 63% (R = Bu(t)), supporting a mechanism involving concerted two-bond scission within the solvent cage to generate the tert-butyl radical, CO2, and an alkoxyl radical. The thermolysis rate constants of tert-alkyl peroxypivalates 1 were influenced by both inductive and steric effects [Taft-Ingold equation, log(rel k(d)) = (0.97 ± 0.14)Σσ* - (0.31 ± 0.04)ΣE(s)(c), was obtained].
- Nakamura,Busfield,Jenkins,Rizzardo,Thang,Suyama
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- Reactions of tert-Butoxyl Radicals with Cyclic Ethers Studied by the Radical Trapping Technique
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The pattern and relative rate of abstraction reactions of tert-butoxyl radicals with fourteen cyclic alkanes and cyclic ethers have been investigated by the aminoxyl radical trapping technique.Oxygen α to C-H strongly enhances abstraction, but β to C-H generally retards abstraction.The rate of abstraction of hydrogens α to oxygen in cyclic ethers was found to decrease with ring size in the order 5 > 7 >> 6 > 4.The results are discussed in terms of ring size, abstraction position relative to ethereal oxygen atoms in the ring and aliphatic substitution.Factors such as polarity, torsional strain, hybridisation of the oxygen (whether sp2 or sp3) and the anomeric effect are considered.
- Busfield, W. Ken,Grice, I. Darren,Jenkins, Ian D.
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p. 1079 - 1086
(2007/10/02)
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- Kinetics of nitroxide radical trapping. 2. Structural effects
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Laser flash photolysis and kinetic competition product demonstrated that in isooctane at ambient temperature the rate constant for coupling of carbon-centered with presistent nitroxides, kT, upon the of steric hindrance to coupling and upon the extent of resonance stabilization of the carbon radical. Sterically induced reductions in the magnitude of kT are observed for changes in both the structure of the nitroxide the structure of the carbon radical. Thus, for any particular carbon kT is largest for the Bredt's rule protected nitroxides, 9-azabicyclo[3.3.1]nonane-N-oxyl (ABNO) and nortropane-N-oxyl, while for the "usual" di-tert-alkyl nitroxides kT decreases along the series, 1,1,3,3-tetra-methylisoindoline-2-oxyl ≥ 2,2,5,5-tetramethylpiperidin-1-oxyl (Tempo) > di-tert-butyl nitroxide, i.e., kT decreased on going from a five-membered ring to a six-membered ring to a noncyclic structure. Cyclopropyl triphenylmethyl are trapped at the fastest and slowest rates, respectively, the corresponding kT value 3.0 × 109 and 1.2 × 108 M-1 s-1 for ABNO and 2.1 × 109 6 M-1 s-1 for Tempo. Steric effects in the carbon radicals are for Tempo than for ABNO. For example, the ratio of kT's for the trapping of nonyl tert-butyl is 1.7 for Tempo but 1.3 for ABNO, while for the trapping of benzyl cumyl the ratio of kT's is 4.1 for Tempo 0.9 for ABNO. The effect of resonance stabilization can be illustrated by the kT values for three sterically unhindered primary radicals, n-nonyl, benzyl, 2-naphthyhmethyl, viz., 1.2 × 109, 4.8 × 108, and 5.7 × 107 M-1 s-1, respectively, for Tempo and 2.2 × 109, 1.2 × 109, and 8.1 × 108 M-1 s-1, respectively, for ABNO.
- Bowry,Ingold
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p. 4992 - 4996
(2007/10/02)
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- Unexpected Products from the Reaction of tert-Butoxyl Radicals with Acetylenes in the Presence of an Aminoxyl Radical Scavenger
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The radical trapping technique employing 1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-2-yloxyl (1) as a scavenger has been used to study the reaction of tert-butoxyl radicals with phenylacetylene, methyl acetylenecarboxylate and dimethyl acetylenedicarboxylate.With phenylacetylene, an aromatic ketone (5) was formed in which both the tert-butoxy group and the radical trap were bound to the same carbon, whereas the acetylenecarboxylates gave vinylamine and vinyloxyamine products.The mechanism of formation of these unexpected products is discussed.Acetylene and diphenylacetylene did not appear to react with tert-butoxyl radicals.
- Bottle, Steven,Busfield, W. Ken,Jenkins, Ian D.,Skelton, Brian W.,White, Allan H.,et al.
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p. 1001 - 1007
(2007/10/02)
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- RECTIONS OF HYDROXYL RADICALS WITH POLYMERIZABLE OLEFINS
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The reactions of hydroxyl radicals with methyl acrylate, methyl methacrylate, styrene, and α-methylstyrene have been investigated by a trapping technique which uses 1,1,3,3-tetramethylisoindolin-2-yloxyl (1) as a radical scavenger.The major reaction pathway in each case was addition to the unsubstituted (tail) end of the monomer, although addition to the substituted (head) end and reaction with the double-bond substituents were also observed.Absolute rates of reaction were estimated by means of competition experiments using cyclohexane as reference substrate.The implications of the results for the structure of polymers initiated by hydroxyl radicals are discussed.
- Grant, Richard D.,Rizzardo, Ezio,Solomon, David H.
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p. 379 - 384
(2007/10/02)
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- 2-(t-Butylazo)prop-2-yl Hydroperoxide: A Convenient Source of Hydroxyl Radicals in Organic Media
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In the presence of a suitable nitroxide, 2-(t-butylazo)prop-2-yl hydroperoxide (1a) decomposes cleanly into hydroxyl and t-butyl radicals; the rate constant for its disappearance in cyclohexane is given by: k = 9.58*1014 exp.(-1.47*104/T)s-1.
- Grant, Richard D.,Rizzardo, Ezio,Solomon, David H.
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p. 867 - 868
(2007/10/02)
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- Solvent Effects on the Reaction of t-Butoxy Radicals with Methyl Methacrylate
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Examination of the title reaction in a range of solvents shows that the ratio of hydrogen abstraction to t-butoxy radical addition increases with increasing solvent polarity.In several cases there is a competition between solvent and methyl methacrylate for reaction with t-butoxy radicals.The implications of these findings for polymer structure are discussed.
- Grant, Richard D.,Griffiths, Peter G.,Moad, Graeme,Rizzardo, Ezio,Solomon, David H.
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p. 2447 - 2454
(2007/10/02)
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