2025-55-0Relevant academic research and scientific papers
Homolytic dissociation of 1-substituted cyclohexa-2,5-diene-1-carboxylic acids: An EPR spectroscopic study of chain propagation
Jackson, Leon V.,Walton, John C.
, p. 1758 - 1764 (2007/10/03)
Hydrogen abstraction from 1-substituted cyclohexa-2,5-diene-1-carboxylic acids containing linear, branched and cyclic alkyl substituents, as well as allyl, propargyl (prop-2-ynyl), cyanomethyl and benzyl substituents, has been studied by EPR spectroscopy. For each carboxylic acid, EPR spectra of the corresponding cyclohexadienyl radicals were observed at lower temperatures, followed by spectra due to ejected carbon-centred radicals at higher temperatures. Rate constants, for release of the carbon-centred radicals from the cyclohexadienyl radicals, were determined from radical concentration measurements for the above range of substituents. The rate of cyclohexadienyl radical dissociation increased with branching in the 1-alkyl substituent and with electron delocalisation in the ejected carbon-centred radical; 3,5-and 2,6-dimethyl-substitution of the cyclohexadienyl ring led to reductions in the dissociation rate constants. Rate data for abstraction of bisallylic hydrogens from the cyclohexadienyl acids were also obtained for ethyl, n-propyl and isopropyl radicals. These results indicated a sharp drop in the rate of hydrogen abstraction as the degree of branching in the attacking radical increased. Small decreases in the hydrogen abstraction rate constants were observed for cyclohexadienes containing CO2R substituents.
Exploitation of aldoxime esters as radical precursors in preparative and EPR spectroscopic roles
McCarroll, Andrew J.,Walton, John C.
, p. 2399 - 2409 (2007/10/03)
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.
The efficiency of alkyl radical generation and hydrogen transfer from 1-alkylcyclohexa-2,5-diene-1-carboxylic acids
Jackson, Leon,Walton, John C.
, p. 7019 - 7021 (2007/10/03)
A novel EPR spectroscopic technique has been used to determine kinetic data for alkyl radical generation and hydrogen transfer from 1-alkylcyclohexa-2,5-diene-1-carboxylic acids; the implications of these data for preparative chain reactions of these reagents are inferred.
The reactivity of ketyl and alkyl radicals in reactions with carbonyl compounds
Denisov
, p. 2110 - 2116 (2007/10/03)
A parabolic model of bimolecular radical reactions was used for analysis of the hydrogen transfer reactions of ketyl radicals: >C+OH + R1COR2 → >C=O + R1R2C+OH. The parameters describing the reactivity of the reagents were calculated from the experimental data. The parameters that characterize the reactions of ketyl and alkyl radicals as hydrogen donors with olefins and with carbonyl compounds were obtained: >C+OH + R1CH=CH2 → >C=O + R1C+ HCH3; >R1CH=CH2 + R2C+HCH2R3 → R2C+HCH3 + R2CH=CHR3. These parameters were used to calculate the activation energies of these transformations. The kinetic parameters of reactions of hydrogen abstraction by free radicals and molecules (aldehydes, ketones, and quinones) from the C-H and O-H bonds were compared.
Kinetics and mechanisms of the reactions of chlorine atoms with ethane, propane, and n-butane
Tyndall,Orlando,Wallington,Dill,Kaiser
, p. 43 - 55 (2007/10/03)
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.
Reduction of 1-Substituted 2,4,6-Triphenylpyridinium Ions in Aprotic Media
Joergensen, Lars Viborg,Lund, Henning
, p. 759 - 764 (2007/10/02)
Some 1-substituted 2,4,6-triphenylpyridinium salts have been studied by cyclic voltammetry.The compounds exhibit two or three reduction peaks depending on the sweep rate and the rate of loss of the 1-substituent.The rate of cleavage of the one-electron reduction product (the radical) was measured by cyclic voltammetry and double potential step chronoamperometry, and the rate of cleavage of the two-electron reduction product (the anion) found by simulation of the voltammetric curves; the simulation suggested that the rate of cleavage of the radical was faster than the rate of cleavage of the anion.This unexpected finding is tentatively explained by different conformations of the phenyl groups in the 2 and 6 positions in the radical and in the anion.
Thermal and photochemical fragmentation of α,α-dialkoxybenzyl radicals: A comparison of the thermal reactions with laser induced fragmentations by using laser flash and laser-jet photolyses
Banks,Scaiano,Adam, Waldemar,Oestrich, Rolf Schulte
, p. 2473 - 2477 (2007/10/02)
The thermal and photochemical cleavage of α,α-dialkoxybenzyl radicals has been examined using a combination of techniques, including two-laser two-color laser flash photolysis and the laser-jet technique. For the parent α,α-dimethoxybenzyl radical photofragmentation occurs with a quantum yield of 0.80. The study of several unsymmetrically substituted radicals (e.g., methoxyisopropoxy) leads to the conclusion that the photoinduced fragmentation shows no selectivity. In contrast, the thermal decomposition of the radicals shows that fragmentation follows the expected radical stabilities, i.e., isopropyl > ethyl > methyl, the differences being almost exclusively due to changes in the activation energy. By comparing with literature data for methyl elimination it is possible to estimate the rate constants for fragmentation at 56°C as 213, 1380, and 16 600 s-1 for methyl, ethyl, and isopropyl elimination.
CIDEP Studies on Radicals Produced from Photochemical Reactions of Some Aromatic Carbonyl Compounds
Ikoma, Tadaaki,Akiyama, Kimio,Tero-Kubota, Shozo,Ikegami, Yusaku
, p. 1491 - 1494 (2007/10/02)
CIDEP spectra of radicals generated from photoinduced hydrogen abstraction and α-cleavage reactions of several alkyl phenyl ketones were studied.The participation of radical pair mechanism on the spectra was discussed from the magnitude of hyperfine interaction.
New General Processes of Homolytic Alkylation of Heteroaromatic Bases by t-BuOOH or (t-BuO)2 and Alkyl Iodides
Fontana, Francesca,Minisci, Francesco,Barbosa, Maria Claudia Nogueira,Vismara, Elena
, p. 995 - 999 (2007/10/02)
New general, selective processes of homolytic alkylation of protonated heteroaromatic bases have been developed using alkyl iodides and t-BuOOH or (t-BuO)2 as sources of alkyl radicals.Both processes are based on the generation of methyl radical from the peroxides, and on iodine abstraction from the alkyl iodide by the methyl radical.The selective processes are the result of combined enthalpic and polar effects.The enthalpic factor governs the equilibrium of iodine abstraction, whereas the polar effect governs the reactivity of the alkyl radicals with the protonated heteroaromatic ring.A redox chain is operative with t-BuOOH and an unusual free-radical chain process is involved with (t-BuO)2.Both chains are particularly effective because of the electron-transfer oxidation of the pyridyl radical intermediate, the ionization potential of which (5.4-6.0 eV) is close to that of lithium (5.39 eV) or sodium (5.14 eV).
Photoreactivity of σ-bonded metalloporphyrins. 1. Formation of zwitterionic indium and gallium porphyrin complexes in tetrahydrofuran
Kadish,Maiya,Xu
, p. 2518 - 2523 (2008/10/08)
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.
