Radical species isomerization

The allylic resonance may give rise to formation of a mixture of isomeric allylic bromides, e.g. 6 and 8 from but-l-ene. The product ratio depends on the relative stability of the two possible allylic radical species 5 and 7 ... 

Isomerizations are important unimolecular reactions that result in the intramolecular rearrangement of atoms, and their rate parameters are of the same order of magnitude as other unimolecular reactions. Consequently, they can have significant impact on product distributions in high-temperature processes. A large number of different types of isomerization reactions seem to be possible, in which stable as well as radical species serve as reactants (Benson, 1976). Unfortunately, with the exception of cis-trans isomerizations, accurate kinetic information is scarce for many of these reactions. This is, in part, caused by experimental difficulties associated with the detection of isomers and with the presence of parallel reactions. However, with computational quantum mechanics theoretical estimations of barrier heights in isomerizations are now possible. 

The OOQOOH radical may isomerize further, similar to the reactions of RO2. The isomer-ized product decomposes into a ketohydroperoxide5 species and one OH radical. The keto-hydroperoxide is fairly stable below about 800 K, but at higher temperatures it decomposes to yield two additional radicals [426]. Thus it is not until this final decomposition step of the ketohydroperoxide that chain branching is finally achieved in the low-temperature mechanism, yielding three radicals from the initial peroxide radical. 

In the case of linoleic acid, two isomeric peroxyl radicals are formed (Fig. 3). Such peroxyl radical species are expected to abstract H-atoms from other fatty acid molecules (reaction (3)) giving lipid hydroperoxides LO2H (9 and 13-hydroperoxyoctadecadienoicacid in the case of linoleic acid (Fig. 3)), which are the initial non-radical products formed during lipid peroxidation. 

Previously, we have examined the formation of amino acid hydroperoxides following exposure to different radical species [100]. We observed that valine was most easily oxidised, but leucine and lysine are also prone to this modification in free solution. Scheme 12 illustrates the mechanism for formation of valine hydroperoxide. However, tertiary structure becomes an important predictor in proteins, where the hydrophobic residues are protected from bulk aqueous radicals, and lysine hydroperoxides are most readily oxidised. Hydroperoxide yield is poor from Fenton-derived oxidants as they are rapidly broken down in the presence of metal ions [101]. Like methionine sulphoxide, hydroperoxides are also subject to repair, in this case via glutathione peroxidase. They can also be effectively reduced to hydroxides, a reaction supported by the addition of hydroxyl radical in the presence of oxygen. Extensive characterisation of the three isomeric forms of valine and leucine hydroxides has been undertaken by Fu et al. [102,103], and therefore will not be discussed further here. 

The reaction of a mercurio ketone with NaBRt produces a radical species which can be trapped in situ by a reactive acceptor such as a vinyl ketone. - Treatment of a mixture of a mercurio ketone and an electron-deficient terminal alkene (or fumarate) in CH2Q2 with concentrated aqueous NaBH4 gives a conjugate adduct (Scheme 19). Isomerization of such a homoenolate radical, presumably via an intermediate cyclopropanoxy radical, has been observed in the reaction of a mercurio aldehyde (Scheme 20).37.38 direction of the isomerization is opposite to that observed in the reactions of anionic ho-moenolates. Such a rearrangement has been used for a ring expansion reaction. ... 

The catalysed isomerizations of ethylenes are usually ascribed to addition of positive, negative or radical species to the double bond (71). This lowers the barrier to rotation (around the central C=C bond) energy barrier. 

ECF of morpholines and piperidines yields the corresponding perfluorinated compounds. However, isomerization and fragmentation of the intermediate radical species can slightly decrease the efficiency of this process. 

The oxidation of cyclobutanemethanol and a,a-dimethylcyclobutanemethanol with lead tetra-acetate in benzene gave the same product distributions within the isomeric homoallylic, cyclobutyl and cyclopropylmethyl actates. The distribution was the same in the oxidation of cyclobutane carboxylic acid, suggesting that alcohol p-cleavage and acid decarboxylation gave rise to the same radical species. 

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