Self-terminating radical reactions oxygenations

On addition of S04 to the triple bond in the lO-member cycloalkyne 24 and cyclo-aUcynone 27, a nonchain, and anionic, self-terminating radical cyclization cascade is induced. In the former reaction (equation 22) the bicyclic ketones 25 and 26 are formed, and in the latter reaction (equation 23) the a,/3-epoxy ketones 28 and 29 are formed in good yields. Because of the difficulty of oxidizing isolated triple bonds, 804 does not react as an electron-transfer reagent in these reactions but acts as a donor of atomic oxygen. 

Cascade Reactions Initiated by Addition of O-Centered Radicals to Alkynes (Self-Terminating Radical Oxygenations)... 

Self-terminating radical oxygenations are not restricted to cyclic alkynes. Electro-and photochemically generated NO3 can be used for the oxidative cyclization of cycloalkyl-clamped alkynes 67 (Scheme 2.12). This reaction leads to formation of anelated tetrahydrofurans (68 with X = or pyrrolidines (68 with X = NTs,... 

The mechanisms behind lipid oxidation of foods has been the subject of many research projects. One reaction in particular, autoxida-tion, is consistently believed to be the major source of lipid oxidation in foods (Fennema, 1993). Autoxidation involves self-catalytic reactions with molecular oxygen in which free radicals are formed from unsaturated fatty acids (initiation), followed by reaction with oxygen to form peroxy radicals (propagation), and terminated by reactions with other unsaturated molecules to form hydroperoxides (termination O Connor and O Brien, 1994). Additionally, enzymes inherent in the food system can contribute to lipid oxidization. 

The self-reactivity of R and their reactivity with oxygen are serious competitive processes for the reaction (Eq. 16). Recombination of R is preferred in PE, disproportionation in PP [186]. Oxidation proceeds in both polymers. The low probability of the reaction (Eq. 16) in the air atmosphere has, unfortunately been, mostly not reported in discussions of the HAS mechanism. Serious doubts arise when analyzing the reaction possibilities of R and NO [177,178,184,185,187]. Aliphatic ] N0 react rapidly with R at ambient and elevated temperatures [65]. The rate constants for the coupling with R are influenced by the resonance stability of alkyls [187]. The five membered NO l,l,3,3-tetramethylisoindoline-2-oxyl (TMIO) was found to be more reactive than 2,2,6,6,-tetramethylpiperidine-l-oxyl (TEMPO). w-Pentyl, Cert-butyl and benzyl radicals were used at 20 2°C in deaerated isooctane (solvent) [178,184,185]. The rate constants for bimolecular reactions of R (disproportionation and recombination) were compared with those of R oxidation and TEMPO scavenging [178,184,185,188] (Scheme 15). At room temperature, the scavenging of R by N0 is, by about one order of magnitude, slower than the self-termination of R [178],... 

Oxidation is essentially the reaction of oxygen with radicals formed in the polymer. The first process is initiation, the production of free radicals at imperfections and reactive points in the molecules. Once the radical has been created, it can react with oxygen to form peroxides and hydroperoxides, -OOH. Hydroperoxides are moderately stable but can be broken down under the influence of Ught, heat or catalysts to continue a self-initiating process of oxidation. The radicals and peroxides formed can cause oxidation of the main or side chains - chain scission or cross-linking respectively. The chain reaction will be terminated by reactions of the radicals with molecules, which lead to stable, non-reactive products. Many... 

The experimentally measured direct chain termination constant was found to be 5.5 X 103 Mole"1 sec."1. However, this value was not considered very accurate because there is a large correction to the measured oxidation rates for oxygen evolved in the self-reactions of COO radicals at the relatively high photo-initiation rates required to reduce the importance of thermal initiation from the added COOH. A more accurate value of 2.9 X 103 mole"1 sec."1 was calculated from the limiting value of fcpC/[2fc fdirect)]"1/2 at high [COOH] for the AIBN thermally initiated reaction at 30 °C. combined with the measured value of Jcp for neat cumene (0.18 Mole"1 sec."1). 

In chain reactions involving three termination steps (two uncrossed and one crossed) the quantity = 1c /(1c 1c )1/2 is frequently used to interrelate the cross-termination constant with the two uncrossed termination constants. For many different types of radical < is found to be about 1 (or alternatively, if the statistical factor of 2 favoring the crossed termination process is ignored in the definition of the rate constants, < 2). In the present reaction system —3-6, in agreement with the value obtained by Russell at 90°C. (26). The crossed termination constant itself is somewhat less than half the value found for kt. This seems reasonable since only one hydrogen atom will be available for transfer in the crossed termination, compared with the two that are available in the self-reaction of two tetralylperoxy radicals. In addition, steric hindrance to reaction should be greater for the crossed termination than for Reaction 8. The products are presumably cumyl alcohol, a-tetralone [3,4-dihydro-l(2H)naphthalenone], and oxygen (28). 

Certain chain reactions involve steps which generate more free radicals than they consume. This is called chain branching. The inherent self-acceleration may or may not outrun termination. If it does, a detonation results. A classical example is the reaction of hydrogen with oxygen. 

In low temperature combustion of H2/O2HO2 radicals are formed from the pressure dependent combination of hydrogen atoms and oxygen. The relative unreactivity of the HO2 radical means that this reaction is effectively a termination step and determines the onset of the second explosion limit in the hydrogen oxygen system. One of the most important HO2 reactions is the self reaction to generate hydrogen peroxide... 

The aryloxyl radicals formed in the initial antioxidant reaction of phenols (equation 1) may undergo several different kinds of secondary reactions, including Type (1), rapid combination (termination) with the initiating oxygen-centered radicals (equation 11) Type (2), self-reactions Type (3), initiation of new oxidation chains by H-atom abstraction from the substrate, the so-called prooxidant effect and Type (4), reduction or regeneration by other H-atom donors resulting in synergistic inhibition. The relative importance of these secondary reactions will be considered briefly here, since they may affect the overall efficiency of the antioxidant, which includes the antioxidant activity, as measured by the rate constant, (equation 10), and the number of radicals trapped, n. 

Termination reactions such as (1.5) decrease the number of radicals and produce nonradical products such as alcohols and ketones. Mutual termination reactions of primary and secondary peroxyl radicals have near-zero activation energies but unusually low Arrhenius prefactors, suggesting a strained transition state. The exothermicity of this termination is sufficient to produce electronically excited states of either the carbonyl compound or oxygen. Besides the termination reaction (1.5), peroxyl radicals can also undergo a self-reaction without termination (2ROO 2RO -I- O2) that is often ignored but is equally important as the termination channel itself [9,10]. 

Acetyl peroxide, methyl acetate, dimethyl peroxide and ethane are not produced in these reactions indicating that there is no cage collapse of the radicals. Instead the methyl radicals diffuse out of the cage and react with oxygen to give methylperoxy. This means that autoxidation of acetaldehyde is terminated either by reaction of methylperoxy with acetylperoxy or by self-reaction of methylperoxy (23) and (2i+). 

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