- Exploitation of aldoxime esters as radical precursors in preparative and EPR spectroscopic roles
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Photolyses of aldoxime esters, containing a considerable range of alkyl groups, lead to cleavage of their N-O bonds and formation of aryliminyl and alkyl radicals. The process was found to be favoured by 4-methoxyacetophenone as a photosensitiser and by methoxy substituents in the aryl rings. 4-Nitro- and pentafluoro-substitutions of the aryl rings were, on the other hand, deleterious. The intermediate iminyl radicals, together with primary, secondary and tertiary alkyl radicals were characterised by 9 GHz EPR spectroscopy. Cyclopropyl, CF3, and CCl3 radicals were probably also formed, but were too reactive for direct EPR spectroscopic detection. Photosensitised reaction of benzophenone oxime O-nonanoyl ester produced the diphenylmethaniminoxyl, as well as the expected n-octyl and iminyl radicals. This indicated that O-C bond scission accompanied O-N scission for this ketoxime ester. At higher temperatures the C-centred radicals added to the starting oxime esters to produce alkoxyaminyl radicals that were also spectroscopically detected in some cases. No evidence for abstraction of the iminyl hydrogen by tertbutoxyl radicals was obtained. Instead, the t-BuO radicals added to the C=N double bonds of the oxime esters. Similarly, chlorine abstraction from alkylbenzohydroximoyl chlorides by trimethyltin radicals did not take place. Preparative scale experiments with oxime esters containing suitably unsaturated alkyl groups showed that good yields of cyclised products could be obtained in the presence of the photosensitiser. This process constitutes a general method by which carboxylic acids or acid chlorides can be converted into alkyl radicals and hence to cyclised derivatives.
- McCarroll, Andrew J.,Walton, John C.
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p. 2399 - 2409
(2007/10/03)
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- Kinetics and mechanisms of the reactions of chlorine atoms with ethane, propane, and n-butane
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Absolute (flash photolysis) and relative (FTIR-smog and GC) rate techniques were used to study the gas-phase reactions of Cl atoms with ethane (k1), propane (k3), and n-butane (k2). Experiments performed at 298-540 K give k2÷k1=(2.0±0.1)exp((183±20)÷T). At 296 K the reaction of Cl atoms with propane yields of 43 % 1-propyl and 57 % 2-propyl radicals, while the reaction of Cl atoms with n-butane produces 29% 1-butyl and 71% 2-butyl radicals. Butyl radicals were found to react with Cl2 with rates which are 3 times greater than the corresponding reactions with O2.
- Tyndall,Orlando,Wallington,Dill,Kaiser
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- The Nucleophilicity of Superoxide towards Different Alkyl Halides Estimated from Kinetic Measurements
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Values of the rate constant ksub are measured for the substitution reaction between superoxide O2 anion-radical and the alkyl halides butyl chloride, 2-butyl chloride, benzyl chloride, ethyl bromide, butyl bromide, 2-butyl bromide, neopentyl bromide, benzyl bromide, (1-bromo-2,2-dimethylpropyl)benzene and 1-iodoadamantane.These rate constants are compared with the expected rate constant kET for the electron transfer reaction between the same alkyl halides and an aromatic anion radical A anion-radical with the same standard oxidation potential as O2 anion-radical.The ksub/kET ratios show that the mechanism of the substitution reaction amy shift from SN2-like to ET-like on changes in the steric hindrance and the acceptor ability of the alkyl halide.The influence on ksub/kET of the difference in self-exchange reorganization energy λ(0) between O2 anion-radical/O2 and A anion-radical/A is discussed.
- Daasbjerg, Kim,Lund, Henning
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p. 597 - 604
(2007/10/02)
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- Synthesis and spectroscopic and electrochemical characterization of ionic and σ-bonded aluminum(III) porphyrins. Crystal structure of methyl(2,3,7,8,12,13,17,18-octaethylporphinato)aluminum(III), (OEP)Al(CH3)
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The synthesis and characterization of SO different ionic and σ-bonded aluminum(III) porphyrins are reported. These compounds were studied by mass spectrometry and IR, UV-visibte, and 1H NMR spectroscopy as well as by electrochemistry. The spectroscopically investigated compounds are represented by (P)AlCl an (P)Al(R), where P is the dianion of tetraphenylporphyrin (TPP) or octaethylporphyrin (OEP) and R is CH3, n-C4H9, C6K5, or C6F4H. The molecular structure of (OEP)Al(CH3) was determined by X-ray diffraction and provides the first structural data for an aluminum porphyrin complex. The Al(III) atom in (OEP)Al(CH3) is pentacoordinated and is located 0.465 (1) A? from the mean nitrogen plane. The electrochemically investigated compounds are represented by (P)AlCl and (P)Al(R), where P is OEP or TPP and R is CH3, n-C4H9, or C6H5. An overall mechanism for the oxidation and reduction of each derivative is presented, and data for the σ-bonded complexes are compared to results obtained under the same experimental conditions for oxidation and reduction of (P)M(R), where M = Ga, In, or Tl.
- Guilard,Zrineh,Tabard,Endo,Han,Lecomte,Souhassou,Habbou,Ferhat,Kadish
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p. 4476 - 4482
(2008/10/08)
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- Single-Pulse Shock Tube Studies on the Thermal Decomposition of n-Butyl Phenyl Ether, n-Pentylbenzene, and Phenotole and the Heat of Formation of Phenoxy and Benzyl Radicals
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n-Butyl phenyl ether, n-pentylbenzene, and phenotole have been decomposed in single-pulse shock tube experiments.The main reaction for all these processes involves bond cleavage leading to the formation of an alkyl and a resonance-stabilized radical.In the case of n-butyl phenyl ether decomposition, a molecular mechanism involving the direct formation of 1-butene has also been detected.The rate expressions for the decomposition processes are k(n-C4H9-OC6H5 -> n-C4H9 + C6H5O) = 1.0 * 1016 exp(-33000/T) s-1, k(n-C4H9-OC6H5 -> 1-C4H8 + C6H5OH) = 4.0 * 1013 exp(-28900/T) s-1, k(n-C4H9-CH2C6H5 -> n-C4H9 + CH2C6H5) = 1 * 1016 exp(-36500/T) s-1, and k(n-C2H5-OC6H5 -> C2H4 + C6H5OH and C2H5 + C6H5O) = 5.7 * 1015 exp(-32300/T) s-1 over the temperature range of 940-1100 K and pressures of 2-3 atm.The assumption that the primary C-H bond strength for butane is 421.5 kJ mol-1 leads to heat of formation of the phenoxy radical of 55.3 kJ mol-1 and that of benzyl radical, 203 kJ mol-1.The A factors for the bond cleavage reactions are all significantly higher than previously reported for analogous processes.In the case of the benzyl-forming process they are now in accord with combination rates and the geometric mean rule.
- Walker, James A.,Tsang, Wing
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p. 3324 - 3327
(2007/10/02)
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- Photoreactivity of σ-bonded metalloporphyrins. 1. Formation of zwitterionic indium and gallium porphyrin complexes in tetrahydrofuran
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Continuous irradiation and laser-flash photolysis experiments were carried out in THF on σ-bonded indium and gallium porphyrins of the type (P)M(R), where P is the dianion of tetraphenylporphyrin (TPP) or octaethylporphyrin (OEP), M is In or Ga, and R is CH3, C2H5, C4H9, CH(CH3)2, C(CH3)3, CH=CHC6H5, or C≡CC6H5. Steady-state photolysis of (P)M(R), where R = CH3, C2H5, C4H9, CH(CH3)2, or C(CH3)3, leads to photodissociation of the metal-carbon bond. In contrast, indium and gallium porphyrins with σ-bonded CH=CHC6H5 or C≡CC6H5 groups do not undergo this photodissociation reaction. Laser photolysis studies were carried out both in the presence and in the absence of ferrocene and indicate that the photoreaction occurs via a triplet state originating from the porphyrin macrocycle. The resulting ESR spectra obtained after photolysis of (TPP)In(C2H5) in THF are compared with ESR spectra obtained after electrochemical reduction of (TPP)InCl. On the basis of these results, a photochemical generation of zwitterionic metalloporphyrin radicals of the type (P)-In+ is demonstrated to occur in THF. Similarly, the (P)Ga(R) complexes also form a zwitterionic photoproduct upon irradiation as evidenced by ESR and UV-visible spectra. The photoreactivity of (P)In(R) and (P)Ga(R) is discussed with respect to the porphyrin macrocycle, the σ-bonded R group, and the solvent polarity.
- Kadish,Maiya,Xu
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p. 2518 - 2523
(2008/10/08)
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- Absolute Rate Constants for Some Intermolecular and Intramolecular Reactions of α-, β-, and γ-Silicon-Substituted Radicals
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Rate constants for hydrogen atom abstraction from n-Bu3GeH (kGeH) by Me3SiCH2. (1.), Me3SiCH2CH2. (2.), and Me3SiCH2CH2CH2. (3.) and from n-Bu3SnH (kSnH) by 1. and 3. have been determined at ambient temperatures.The order of decreasing radical reactivity is 1. > n-alkyl > 3. > 2..However, for bromine abstraction from the parent bromides by n-Bu3Sn. and n-Bu3Ge., the order of decreasing reactivity is 1-Br > 2-Br > 3-Br ca. n-alkyl bromide.The Arrhenius equations for reaction of 1. and 3. with n-Bu3SnH were also determined: log (kSnH(1.)/(M-1 s-1)) = (10.2 +/- 0.5) - (3.90 +/- 0.62)/Θ and log (kSnH(3.)/(M-1 s-1)) = (8.4 +/- 0.7) - (2.81 +/- 0.95)/Θ, where Θ = 2.3RT kcal/mol.These kinetic data are discussed in relation to previously measured rate constant ratios, kc5+6/kSnH and kexo5/kendo6, where kc5+6 corresponds to the cyclization of α-, β-, and γ-dimethylsilyl-substituted 5-hexenyl radicals to form 5-membered (kexo5) and 6-membered (kendo6) silacycloalkylmethyl radicals.
- Wilt, James W.,Lusztyk, J.,Peeran, Mehoob,Ingold, K. U.
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p. 281 - 287
(2007/10/02)
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- ON THE REMOVAL OF METALLIC MIRRORS BY FREE RADICALS.
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Large radicals can be formed by passing chlorinated organic compounds at pressures of a few mm. , through a furnace containing a pellet of sodium and heated to 350-400 degree C. It is found that the only radicals that will remove metallic mirrors (of tellurium or antimony, etc. , previously deposited beyond the furnace) are those that can decompose into methyl or ethyl radicals plus an unsaturated molecule, without undergoing any transmigration of atoms. The authors also found, especially in the case of larger monochlorinated molecules, that there was some decomposition, approximately half, even in the absence of metallic sodium.
- Rice,Tweedell
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p. 995 - 997
(2007/10/02)
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