Alkyl peroxides reduction

In this section, we will present examples of the concerted ET reduction of the 0—0 bond in a number of simple alkyl peroxides (RO-OR) and endo-peroxides. These systems provide the first clear examples of the parabolic activation/driving force relationships. The necessary background is provided in the section below. The data provided by the peroxide systems also allow us to introduce important new insights into concerted dissociative processes, particularly the importance of steric and nonadiabatic effects. Finally, data for perbenzoates, another peroxide system related to the early studies, provide evidence for the transition between the concerted and stepwise dissociative mechanisms. 

Nishinaga and co-workers isolated a series of stable cobalt(III)-alkyl peroxide complexes such as (170) and (171) in high yields from the reaction of the pentacoordinated Co"-Schiff base complex with the corresponding phenol and 02 in CH2C12. Complex (170 R=Bu ) has been characterized by an X-ray structure. These alkyl peroxide complexes presumably result from the homolytic addition of the superoxo complex Co111—02 to the phenoxide radical obtained by hydrogen abstraction from the phenolic substrate by the CoUI-superoxo complex. The quinone product results from / -hydride elimination from the alkyl peroxide complex (172)561,56,565,566 The quinol (169) produced by equation (245) has been shown to result from the reduction of the CoIU-alkyl peroxide complex (170) by the solvent alcohol which is transformed into the corresponding carbonyl compound (equation 248).561... 

Fig. 2. Thermodynamics of the reduction of oxygen species Potential diagram at pH 7. Figures are apparent standard redox potentials versus NHE, at pH 7. Figures used for the alkyl peroxide species at the top of the diagram are approximations derived from computations by Koppenol [19]. An upper value for the ROO /ROOH redox couple would be 1.1 V, i.e. the value derived from the trichloromethyl peroxyl radical, the strongest oxidant known among peroxyl radicals. See the discussion in the text. Note (see a) A aq -0.16 V (relative to 1 M 02) is a better reference than —0.33 V for the reversible...

We may note that the mechanisms of reactions included in the last two types are, in general, not the same for paraffins, on the one hand, and aromatic hydrocarbons, on the other hand, even if the products of these reactions are of the same type. For example, alcohols and phenols may be obtained from alkanes and arenes respectively by the reaction in air with hydroxyl radicals generated by the action of a metal complex. However, in the case of alkane, an alcohol can be formed by the reduction of alkyl peroxide, whereas hydroxyl is added to an arene with subsequent oxidation of a radical formed. Hence follows the possibility that arenes and alkanes may exhibit different reactivities in each specific reaction. 

Alkyl borane as the reductant in redox polymerization is well known. It has been used previously in conjunction with alkyl peroxide and peroxyester of carbonic acid. The mechanism of alkyl borane-peroxyester of carboxylic acid is similar to that previously described. Suspension polymerization or copolymerization of vinyl chloride by the redox system such as monotertiary butyl permaleate-EtsB or BU3B or iso-BusB has been reported in patent literature [198]. 

The reduction of alkyl-substituted siUcon and tin peroxides with sodium sulfite and triphenylphosphine has been reported (33,93). Alkyl-substituted aluminum, boron, cadmium, germanium, siUcon, and tin peroxides undergo oxygen-to-metal rearrangements (33,43,94), eg, equations 22 and 23. 

Methylsuccinic acid has been prepared by the pyrolysis of tartaric acid from 1,2-dibromopropane or allyl halides by the action of potassium cyanide followed by hydrolysis by reduction of itaconic, citraconic, and mesaconic acids by hydrolysis of ketovalerolactonecarboxylic acid by decarboxylation of 1,1,2-propane tricarboxylic acid by oxidation of /3-methylcyclo-hexanone by fusion of gamboge with alkali by hydrog. nation and condensation of sodium lactate over nickel oxide from acetoacetic ester by successive alkylation with a methyl halide and a monohaloacetic ester by hydrolysis of oi-methyl-o -oxalosuccinic ester or a-methyl-a -acetosuccinic ester by action of hot, concentrated potassium hydroxide upon methyl-succinaldehyde dioxime from the ammonium salt of a-methyl-butyric acid by oxidation with. hydrogen peroxide from /9-methyllevulinic acid by oxidation with dilute nitric acid or hypobromite from /J-methyladipic acid and from the decomposition products of glyceric acid and pyruvic acid. The method described above is a modification of that of Higginbotham and Lapworth. ... 

Use of mild conditions was crucial and the development of diimide reduction of singlet oxygenates, silver-salt-assisted displacement of halide by peroxide nucleophiles, peroxymercuration and demercuration, peroxide transfer from organotin to alkyl triflates, and oxygen trapping of azoalkane-derived diradicals have all played a part in providing the rich harvest of new bicyclic peroxides described herein. 

Kinetic Parameters of Reduction of Peroxides by Ketyl and Alkyl Radicals [68]... 

As we have seen earlier, alkyl radicals react with peroxides in two different ways substitution and reduction. The comparison of both these reactions is presented in Table 6.33. It is seen... 

Comparison of the Thermodynamic and the Kinetic Characteristics of Substitution and Reduction of Peroxides by Alkyl Radicals... 

---