Oxy-radicals

The mechanism of free radical polymerization of ethylene is outlined m Figure 6 17 Dissociation of a peroxide initiates the process m step 1 The resulting per oxy radical adds to the carbon-carbon double bond m step 2 giving a new radical which then adds to a second molecule of ethylene m step 3 The carbon-carbon bond forming process m step 3 can be repeated thousands of times to give long carbon chains... 

The radical X is formed by homolysis of the X—R bond either thermally or photolytically. In the reactions of alcohols with lead tetraacetate evidence suggests that the X—R bond (X = 0, R = Pb(OAc)3) has ionic character. In this case the oxy radical is formed by a one electron transfer (thermally or photochemically induced) from oxygen to lead. 

In this section we shall restrict ourselves to abstraction of hydrogen atoms by oxy radicals since comparative data are available. Many aspects of the conclusions reached will be valid for nitrogen radicals as well. 

The importance of the carbonyl-forming fragmentation varies according to the method of oxy radical production and is most pronounced in the lead... 

The homolysis of tertiary hypochlorites for the production of oxy radicals is well known." The ease with which secondary hypohalites decompose to ketones has hampered the application of hypohalites for transannular reactions. However the tendency for the base-catalyzed heterolytic decomposition decreases as one passes from hypochlorites to hypobromites tohypoidites. Therefore the suitability of hypohalites for functionalization at the angular positions in steroids should increase in the same order. Since hypoidites (or iodine) do not react readily with ketones or carbon-carbon double bonds under neutral conditions hypoiodite reactions are more generally applicable than hypochlorite or hypobromite decompositions. 

The low bond-strength of the O—0 bond renders peroxides susceptible to homolytic fission to give oxy radicals on heating. Diacyl peroxides give rise to acyloxy radicals which then decompose to aryl radicals and carbon dioxide, Eq. (5). For example, dibenzoyl... 

A general reaction mechanism for the grafting of MA onto EPM is given in Figure 13.3 [15,16]. Free-radical grafting of MA starts with the decomposition of the radical initiator, usually a peroxide [15,18]. The peroxide decomposes at elevated temperamres into the corresponding oxy radicals, which may further degrade to alkyl radicals and ketones. These oxy and alkyl radicals abstract... 

Figure 4.14 Diagrammatic representation of (a) oxy-radical>mediated S-thioiation and (b) thiol/disulphide-initiated S-thiolation of protein suiphydryl groups. Under both circumstances mixed disuiphides are formed between glutathione and protein thiois iocated on the ion-translocator protein resulting in an alteration of protein structure and function. Both of these mechanisms are completely reversible by the addition of a suitabie reducing agent, such as reduced glutathione, returning the protein to its native form.

Krinsky, N. I. and S. M. Deneke (1982). Interaction of oxygen and oxy-radicals with carotenoids. J. Natl. Cancer Inst. 69(1) 205-210. 

El-Agamey, A, Cantrell, A, Land, EJ, McGarvey, DJ, and Truscott, TG, 2004a. Are dietary carotenoids beneficial Reactions of carotenoids with oxy-radicals and singlet oxygen. Photochem Photobiol Sci 3, 802-811. 

Palozza, P, Calviello, G, Serini, S, Maggiano, N, Lanza, P, Ranelletti, FO, and Bartoli, GM, 2001. Beta-carotene at high concentrations induces apoptosis by enhancing oxy-radical production in human adenocarcinoma cells. Free Radic Biol Med 30, 1000-1007. 

Kinetic analysis of the results of ketone oxidation in the presence of amine II reveals that the velocity constant of the oxidation of amines by acyl per-oxy radicals must be greater (by a factor of 2 - 3) than that of the interaction of these radicals with the nitroxide-i. In this reaction, acyl peroxy radicals are captured and destroyed by amines. 

C6HuO-(Cyclohexyl -oxy- radical, formed from DCPD)... 

D Schulte-Frohlinde, R Anker, E Bothe. In M. A. J. Rogers, E. L. Powers, eds. Oxygen and Oxy-Radicals in Chemistry and Biology. New York Academic Press, 1981, p 61. 

RL Arudi, MW Sutherland, BHJ Bielski. In Oxy Radicals and Their Scavenger Systems. Vol 1, G Cohen, RA Greenwald (Eds). Elsevier, Amsterdam, 1983. 

Decomposition of azo compounds and peroxides provides the alkoxyamine by the nitroxide-trapping of the primary radicals [29]. The radicals produced by hydrogen abstraction with oxy radicals are also trapped by the nitroxide [242, 243]. In the photoreaction, alkoxyamines were isolated with high yields [244]. The reactions of Grignard reagents with nitroxides [215] and the coupling reaction of sodium nitroxides with bromo compounds [234,235] are also used. The hydrolysis of 56 followed by the reaction with acyl or alkyl halides afforded alkoxyamines with various functional groups,63 (Eq. 68) [245-251] ... 

The deoxygenation of epoxides with the metal complexes mentioned above all seem to proceed via intermediate /i-melal oxy radicals. The reaction path after their trapping seems, however, to depend on the Lewis acidity of the ET reagent. 

Heating cleaves a weak 0-0 bond homolytically to give two oxy radicals. Fragmentation of the C1-C6 and C9-C14 bonds gives two radicals which recombine to give a cyclic diacyl peroxide. 

The fragment ions at m/z 149, [CgHsOs], and 167, [C8H704], are especially prominent in the El spectra of phthalates. The formation of the [CgHsOs]" ion has initially been attributed to a McLafferty rearrangement followed by loss of an alk-oxy radical and final stabilization to a cyclic oxonium ion. [104] However, it has been revealed that four other pathways in total lead to its formation excluding the above one. [105,106] The two most prominent fragmentation pathways are ... 

An inner-sphere electron reduction has been proposed as a possible mechanism for the Fe(II)-induced decomposition of 1,2,4-trioxolanes (ozonides) (75) and (76). Benzoic acid was found to be the major product. The nucleophilic Ee(II) species attack the ozonide from the less hindered side of the electrophilic 0-0 a orbital to generate exclusively the Ee(III) oxy-complexed radical (inner-sphere electron transfer). After selective scission of the C-C bond, the resulting carbon-centred radical produced the observed product. The substituent effect determine the regioselective generation of one of the two possible Fe(III)-complexed oxy radicals. The bond scission shown will occur if R is bulkier than R. ... 

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