Abstract:Can. J. Chem. 60, 2566Chem. 60, (1982. The major products of the self-reaction of l,l-diphenylethylperoxyl have been determined from product studies of the autoxidation of l,l-diphenylethane, induced decomposition of l,l-diphenylethyl hydroperoxide, and decomposition of 2,2,3,3-tetraphenylbutane under an atmosphere of oxygen. Overall self-reaction is a complex free-radical process involving the intermediacy of I, l-diphenylethoxyl and l-phenyl-1-phenoxyethoxyl whichundergo H-atom abstraction, p-scissionand, … Show more
Theoretical calculations were carried out to provide a framework for understanding the free radical oxidation of unsaturated lipids. The carbon[bond]hydrogen bond dissociation enthalpies (BDEs) of organic model compounds and oxidizable lipids (R[bond]H) and the carbon[bond]oxygen bond dissociation enthalpies of peroxyl radical intermediates (R[bond]OO*) have been calculated. The carbon[bond]hydrogen BDEs correlate with the rate constant for propagation of free radical autoxidation, and the carbon[bond]oxygen BDEs of peroxyl radicals correlate with rate constants for beta-fragmentation of these intermediates. Oxygen addition to intermediate carbon radicals apparently occurs preferentially at centers having the highest spin density. The calculated spin distribution therefore provides guidance about the partitioning of oxygen to delocalized carbon radicals. Where the C[bond]H BDEs are a function of the extent of conjugation in the parent lipid and the stability of the carbon radical derived therefrom, C[bond]OO* BDEs are also affected by hyperconjugation. This gives way to different rates of beta-fragmentation of peroxyl radicals formed from oxygen addition at different sites along the same delocalized radical. We have also studied by both theory and experiment the propensity for benzylic radicals to undergo oxygen addition at their ortho and para carbons which, combined, possess an equivalent unpaired electron spin density as the benzylic position itself. We find that the intermediate peroxyl radicals in these cases have negative C[bond]OO* BDEs and, thus, have rate constants for beta-fragmentation that exceed the diffusion-controlled limit for the reaction of a carbon-centered radical with oxygen.
Theoretical calculations were carried out to provide a framework for understanding the free radical oxidation of unsaturated lipids. The carbon[bond]hydrogen bond dissociation enthalpies (BDEs) of organic model compounds and oxidizable lipids (R[bond]H) and the carbon[bond]oxygen bond dissociation enthalpies of peroxyl radical intermediates (R[bond]OO*) have been calculated. The carbon[bond]hydrogen BDEs correlate with the rate constant for propagation of free radical autoxidation, and the carbon[bond]oxygen BDEs of peroxyl radicals correlate with rate constants for beta-fragmentation of these intermediates. Oxygen addition to intermediate carbon radicals apparently occurs preferentially at centers having the highest spin density. The calculated spin distribution therefore provides guidance about the partitioning of oxygen to delocalized carbon radicals. Where the C[bond]H BDEs are a function of the extent of conjugation in the parent lipid and the stability of the carbon radical derived therefrom, C[bond]OO* BDEs are also affected by hyperconjugation. This gives way to different rates of beta-fragmentation of peroxyl radicals formed from oxygen addition at different sites along the same delocalized radical. We have also studied by both theory and experiment the propensity for benzylic radicals to undergo oxygen addition at their ortho and para carbons which, combined, possess an equivalent unpaired electron spin density as the benzylic position itself. We find that the intermediate peroxyl radicals in these cases have negative C[bond]OO* BDEs and, thus, have rate constants for beta-fragmentation that exceed the diffusion-controlled limit for the reaction of a carbon-centered radical with oxygen.
“…ke radical with oxygen. This reaction is nearly diffusion controlled, and the equilibrium lies toward the peroxy radical (23)(24)(25)(26)(27). Competition kinetic experiments have yielded values from 25 to 400 s"1 (30 °C) for ß (27).…”
A new method has been developed to analyze the primary products of phospholipid peroxidation. The procedure utilizes the ability of phospholipase C to hydrolyze phospholipid hydroperoxides to their corresponding diacylglycerol derivatives. 1-Palmitoyl-2-linoleoylphosphatidylcholine (1P,2L-GPC), 1-stearoyl-2-linoleoylphosphatidylcholine (1S,2L-GPC), and 1-stearoyl-2-arachidonylphosphatidylcholine (1S,2A-GPC) were autoxidized. The diacylglycerol hydroxides derived from the phosphatidylcholine hydroperoxides were separated by reverse-phase high-pressure liquid chromatography (RP-HPLC) and normal-phase high-pressure liquid chromatography (NP-HPLC). 1P,2L-diglyceride (1P,2L-DG) and 1P,2A-DG products were easily separated from 1S,2L-DG and 1S,2A-DG products by RP-HPLC. The linoleate diglyceride oxidation mixture was separated into the 13-trans/cis, 13-trans/trans, 9-trans/cis, and 9-trans/trans isomers by NP-HPLC. Likewise, 1P,2A-DG and 1S,2A-DG oxidation products were resolved into the 15-trans/cis, 15-trans/trans, 12-trans/cis, 11-trans/cis, 9-trans/cis, 8-trans/cis, and 5-trans/cis isomers. In both of the above cases, the 1,2-diacylglycerol isomers could be separated from the 1,3 isomers. Moreover, the diastereomers of the 9-, 8-, and 5-hydroxides could be separated. Each of the diacylglycerol oxidation products was characterized by (1) proton nuclear magnetic resonance (proton NMR), (2) electron ionization-mass spectrometry (EI-MS), and (3) NP-HPLC of the corresponding fatty acids. The diacylglycerol analysis provided the same results for the autoxidation of 1P,2L-GPC as the fatty acid methyl ester analysis. In addition, when 1S,2A-GPC was autoxidized in the presence of 5% alpha-tocopherol, both diastereomers of the 5-hydroxide were observed in the same proportions as the other hydroxides.
“…The mechanism for self-reaction of tertiary alkylperoxyls has been well estabished (2)(3)(4). These radicals exist in dynamic equilibrium with a tetroxide which decomposes to give two caged alkoxyls and oxygen.…”
Section: Introductionmentioning
confidence: 99%
“…These radicals exist in dynamic equilibrium with a tetroxide which decomposes to give two caged alkoxyls and oxygen. A fraction (-10%) of these alkoxyls combine to give di-tert-alkyl peroxide while the remainder escape the solvent cage and undergo H-atom abstraction, p-scission, or rearrangement (3,4).…”
The major initial products of the self-reaction of α-tetralylperoxyls (C10H11O2•) in chlorobenzene at 303–353 K are equal concentrations of α-tetralol and α-tetralone in ~90% yield based on the number of initiating radicals. These yields are consistent with the non-radical (Russell) mechanism for self-reaction. Low concentrations of bis(α-tetralyl) peroxide are produced, indicating that there is a small but detectable free-radical contribution towards termination. C10H11O2• undergoes β-scission in this temperature range but steady-state concentrations of C10H11• are too low to influence the termination rate constant 2kt, or react with C10H11O2• to give (C10H11O2. α-Tetralol to α-tetralone ratios and total yields of these products are significantly less than 1 and 100%, respectively, in methanol and acetonitrile. Formaldehyde is produced in methanol indicating the involvement of α-hydroxymethylperoxyls, derived from the solvent, in termination. There is no evidence for a chain reaction or a zwitterion intermediate for self-reaction of C10H11O2• in solution.
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