Hydroxy radical cleavage

Chemical/Physical. In the gas phase, cycloate reacts with hydroxyl and NO3 radicals but not with ozone. With hydroxy radicals, cleavage of the cyclohexyl ring was suggested leading to the formation of a compound tentatively identified as C2H5(Cff0)NC(0)SC2H5. The calculated photolysis lifetimes of cycloate in the troposphere with hydroxyl and NO3 radicals are 5.2 h and 1.4 d, respectively. The relative reaction rate constants for the reaction of cycloate with OH and nitrate radials are 3.54 x lO " and 3.29 x 10 cm /molecule-sec, respectively (Kwok et al., 1992). 

Several studies have been performed on the photodecomposition of diaryl sulfones and polysulfones Khodair, et. al., (21) demonstrated that the photodecomposition of diaryl sulfones proceeds by a free-radical mechanism with initial carbon-sulfur bond cleavage. This gives an aryl radical and an aromatic sulfonyl radical. The latter radical can react with oxygen and a hydrogen donor to eventually form the hydroxyl radical. The hydroxy radical may attack the aromatic nucleus in PET and forms the hydroxyterephthaloyl radical. 

Hydroxy radicals are intermediates in the reaction of Ti3+ and H2O2 (175). This system is also capable of hydroxylation of aromatics and alkanes but, in contrast to reactions with Fenton s reagent (Fe2+ + H202, reductive, homolytic cleavage, Eq. (11)), only non-chain processes are possible, because Ti4+ is not usually an oxidant. Hence, relatively high selectivities are feasible. 

A third possible fate of the alkoxy radical is shown in Scheme 18.3 (lower reaction) and was postulated by Grossetete et al. [11], In this reaction, the hydroxy radical in the cage from hydroperoxide cleavage yields an anhydride by extraction of the a-hydrogen to the radical. These workers cite IR evidence in support of this reaction which seems quite reasonable. In addition, the report by Valk et al. [21] of glycolic acid would seem to confirm this path as well. 

This is believed to involve a free radical cleavage of the isoxazoline to the [3-hydroxy imine 11, which then undergoes hydrolysis (Scheme 15.5).5... 

In this section, cleavage reactions will be discussed from the point of view of the metal complex. To cleave DNA, an oxidant must be generated. Numerous redox reactions of metal complexes have been used to generate appropriate DNA oxidants (4). The simplest comes from Fenton chemistry, in which peroxide is used to generate hydroxy radicals from Fe2+ (3) ... 

The oxidant can be formulated either as a hydroxy radical bound to Cu(II) or as a hydroxide ligand on Cu(III). In either case, the activated complex is capable of strand scission, and the cleavage pattern suggests that the oxidant is nondiffusible and therefore bound to the metal (17). Cleavage by this reagent involves sugar oxidation, as discussed in subsequent paragraphs. 

Aziridines, azomethine ylids from, cycloadditions, 60, 35 Aziridines, /3-hydroxy-, radical induced cleavage, 58, 41... 

Hydroperoxides of unsaturated fatty acids formed by autoxidation are very unstable and break down into a wide variety of volatile flavor compounds as well as nonvolatile products. It is widely accepted that hydroperoxide decomposition involves homolytic cleavage of the -OOH group, giving rise to an alkoxy radical and a hydroxy radical (5). 

Chemical reactions with the radicals formed during the primary processes are also found to be responsible for synergistic effects, e.g. in a combination of 1 and 1 -benzoyl cyclohexanol 3. in aerated medium. A possible explanation [23] is based on the decomposition by 1 of the hydroperoxides arising from the oxidation of the radicals generated through cleavage of 3- The weak reactivity of these peroxy radicals compared to alkoxy and hydroxy radicals (Scheme 2) and the concommittant reduction of air inhibition due to the consumption of dissolved oxygen by excited benzophenone can be also invoked to account for the experimental observations. 

However, the intense fragment peak immediately below the molecular ion has an even mass and corresponds not to a simple cleavage product but to the multicenter elimination of water. Such an ortho effect is not uncommon in mass spectrometry and can be rationalized by a six-membered transition state. The subsequent loss of CO may occur by expulsion of the group from the ring or from the new ketene function (Equation 2.99). Meta and para isomers of this compound fragment in a manner parallel to that of benzoic acid loss of a hydroxy radical followed by loss of CO (Equation 2.100). 

Thermal cleavage of the above peroxides results in macromolecules with free-radicals sites. Hydroxy radicals also form and initiate formations of homopolymers. Decompositions of the peroxides by redox mechanisms, however, increase the yields of graft copolymers, but do not stop all formations of hydroxy radicals ... 

Fe [129-138], Fe [126, 138-144], Ni° [145, 146], and v [147-150]) were employed to promote site-selective oxidative degradation of folded proteins under non-denaturing conditions. In the presence of oxidants (e.g., O2 or H2O2), hydroxy radicals or in situ-generated metal-oxo species abstracted an H atom at Ca. This event was followed by oxidation at Ca and cleavage of the Ca-N or Ca-C (O) bond [118, 126, 141, 142]. In those methods, backbone cleavage sites were determined by the affinity to metal complexes [127]. 

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