3889-74-5Relevant 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.
An Electron Spin Resonance Study of the Regioselectivity of the Cyclization of Protonated and Unprotonated Pent-4-en-1-oxyl and Pent-4-en-1-peroxyl Radicals
Bloodworth, A. J.,Davies, Alwyn G.,Hay-Motherwell, Robyn S.
, p. 575 - 582 (2007/10/02)
The regioselectivity of the ring closure of protonated and unprotonated pent-4-en-1-oxyl radicals has been determined by monitoring by e.s.r. spectroscopy the radicals which are formed when the corresponding pent-4-enyl peroxides (2; R = H or Me) are photolysed in solution.Under neutral conditions, the spectra of the corresponding tetrahydrofuran-2-ylmethyl radicals (6) are observed, confirming the preferential 5-exo-mode of cyclization.When trifluoroacetic acid is present, the different spectra of the protonated tetrahydrofuran-2-ylmethyl radicals (8) are observed, and it is argued that this reflects the similar regioselectivity of the intramolecular addition of the protonated alkoxyl radicals.The identity of the radicals (6) and (8) was confirmed by generating them independently by abstracting bromine atoms from the corresponding bromides (9).Similar experiments with the hydroperoxides (1) failed to show the spectra of the dioxan-3-ylmethyl radicals (17), perhaps because they rapidly undergo γ-scission to the oxygen-centred radicals (18).However, (17) could be trapped with oxygen to afford, after Ph3P reduction, 1,2-dioxanes (4).No evidence could be found for the protonation of alkylperoxyl radicals by trifluoroacetic acid.
The Free Radical Reaction between Alkanes and Carbon Tetrachloride
Hawari, J. A.,Davis, S.,Engel, P. S.,Gilbert, B. C.,Griller, D.
, p. 4721 - 4724 (2007/10/02)
Product studies and kinetic electron paramagnetic resonance methods were used to investigate the free radical reaction between alkanes and carbon tetrachloride in solution.Trichloromethyl radicals abstracted hydrogen from simple alkanes with rate constants of ca. 60 M-1 s-1 at 300 K. in good agreement with gas-phase data.However, rate constants for chlorine abstraction by alkyl radicals from carbon tetrachloride were ca.E4 M-1 s-1 and were therefore ca. 2 orders of magnitude higher in solution than in the gas phase.Possibilities for the origin of this effect are discussed.
Deuterium-Hydrogen Exchange and Scrambling Reactions in the Pyrolysis of Labeled Cyclopentene. A Radical Mechanism
Kosnik, Kenneth G.,Benson, Sidney W.
, p. 2790 - 2795 (2007/10/02)
Inter- and intraradical mechanism that promote deuterium-hydrogen scrambling in labeled cyclopentene-d1 are investigated at 800 K and 1200 K.Rate constants are estimated for each step and for possible competing side reactions.The fast radical bimolecular exchange at 800 K and unimolecaular scrambling at 1200 K of labeled cyclopentene are shown to be faster than the Woodward-Hoffman allowed 1,4 concerted molecular elimination of hydrogen.The low-energy estimate of 8.0 kcal/mol by Lewis of the difference between the allowed 1,4 and disallowed 1,2 channels can thus be explained.No conclusions can be drawn concerning the 1,2 channel.The estimated rates of radical reactions are in agreement with experiments on the addition of D2 to cyclopentadiene at 300 deg C which shows only cis, 3-5 addition and with pyrolysis experiments at 500 +/- 20 deg C which show about 5percent contribution of a higher than first-order radical reaction.The mechanism of this radical decomposition is given and its steps are explicitly evaluated.
A Kineic Study of the Reaction of tert-Butoxyl with Alkenes: Hydrogen Abstraction vs. Addition
Wong, P. C.,Griller, D.,Scaiano, J. C.
, p. 5106 - 5108 (2007/10/02)
Rate constants and Arrhenius parameters have been measured for the reactions of tert-butoxyl radicals with alkenes by using laser flash photolysis in benzene-di-tert-butyl peroxide (1:2 v/v) as solvent.With norbornene and norbornadiene, additions to the double bonds were the only modes of reaction, whereas with cyclopenene and cyclopentadiene, hydrogen abstraction was a competitive process.The measured activation energies were quite low; thus for norbornene, norbornadiene, and cyclopentene, Ea (kcal/mol) = 2.36 +/- 0.40, 1.56 +/- 0.55, and 2.35 +/- 0.22, respectively, while the corresponding A factors were log (A/(M-1 s-1)) = 7.76 +/- 0.31, 7.65 +/- 0.43, and 8.56 +/- 0.18, For comparison, hydrogen abstraction by tert-butoxyl at cyclopentane proceeds with Ea = 3.47 +/- 0.59 and log (A/(M-1 s-1)) = 8.47 +/- 0.45.
