Intermediate Peroxides

Two major pathways exist for this reaction, one bypassing hydrogen peroxide (first pathway) and the other involving intermediate peroxide formation via reaction (15.21) (second pathway). The peroxide formed is either electrochemically reduced to water via reaction (15.22) or decomposed catalytically on the electrode surface via reaction (15.23), in which case half of the oxygen consumed to form it reemerges [in both cases the overall reaction corresponds to Eq. (15.20)]. 

These halides can be used for the preparation of large macrocycles as the reactions in Scheme 28 indicate. In contrast, oj-hydroxyal kyl tetrall uorostannatcs are obtained from stannacycloalkanes by rapid oxidation via an intermediate peroxide, detected by polaro-graphic analysis, followed by cleavage of the Sn—Ph bonds353 (Scheme 29). 

Experimental evidence of the formation of intermediate peroxide moieties in photo-oxidation of nitrones has been provided (463). Oxidation with singlet oxygen was observed in the photoirradiation of DMPO (464). 

For practical applications it is important to minimize the production of the intermediate peroxide, and to ensure that the reaction goes all the way to water. Sometimes this can be ensured by the addition of a suitable catalyst. A case in point is oxygen reduction on gold from alkaline solutions. At low and intermediate overpotentials the reaction produces only peroxide in a two-electron process at high overpotentials the peroxide is reduced further to water. The addition of a small amount of Tl+ ions to the solution catalyzes the reaction at low overpotentials, and makes it proceed to water. Thallium forms a upd layer at these potentials it seems that a surface only partially covered with T1 is a good catalyst, but the details are not understood [3]. 

The higher voltage is not only due to better kinetics but also due to the fact that oxygen reaction via the intermediate peroxide (H02 in alkaline electrolytes) is more facile. 

In developing oxidation processes a major source of free radical formation was found to be degenerate chain branching. Among the products derived from the branching were intermediate peroxides ROOH. Formation of radicals from the hydroperoxides proceeded not only by monomolecular breakdown of hydroperoxides ... 

Trialkyl derivatives of boron, and in fact many other molecules such as boroxines with carbon-boron bonds, react readily with oxygen. The initial products are peroxy derivatives with BOOR bonds, which tend to react further to form borate esters. The ease of the initial reaction is shown by the fact that reported examples of vinyl polymerization induced by trialkyl borons require oxygen and are actually radical processes induced by the boron oxygen reaction or intermediate peroxides (7). 

Oxidation results from the interactions between atmospheric oxygen and the double bonds of unsaturated fatty acids. Several parameters can catalyze lipid oxidation, while others can prevent or slow down the reactions. Metals, light, moisture and heat can all enhance oxidation, while antioxidant compounds (e.g., BHT and vitamin E) can be utilized to retard oxidation. Oxidation of double bonds leads to intermediate peroxides that eventually break down into a variety of stable compounds. 

Photosensitized photooxidation of pyrrole (412) by irradiation in the presence of eosin gives rise to the photoproduct (413), presumably via the intermediate peroxide (414).440 Transannular peroxides are... 

The photochemical oxidation of other heterocycles has been interpreted in terms of intermediate hydroperoxides. Thus, for example, 2,4,5-triphenylimidazole (418) forms448,449 the stable hydroperoxide (419), and hydroperoxides are probably involved in the photooxidation of 2,3-diethylindole,450 2,3,4,5-tetraphenyl-pyrrole [Eq. (117)],451 and 2,3,4,5-tetraphenylfuran.451 Pentaphenyl-pyrrole (420) is converted into the lactam (421), presumably by rearrangement of the intermediate peroxide.452... 

The last reaction of (17.2) may proceed by the formation of an intermediate peroxide ... 

The intermediate peroxide is reduced further to 2O2- as another pair of electrons is absorbed by the complex. Note that NADH, FADH2/ and UQH2 are carriers of two electrons each. The heme molecules and copper are carriers of only one. 

Further evidence in support of zwitterionic intermediates in the [2 + 2]-cycloaddition of singlet oxygen to electron-rich alkenes has been obtained by Jefford et al. [684]. The photo-oxygenation of 2-(methoxymethylidene)adamantane creates a zwitterionic intermediate (peroxide or perepoxide), which can be captured by acetaldehyde to give 1,2,4-trioxanes in addition to 1,2-dioxetanes cf. Eq. (5-147). 

Also, intermediate peroxides are formed in the oxidation of perfluorinated alkenes, e.g. in the photo-oxidation of perfluoroethene and perfluoropropene for the formation of Fomblin (Ausimont Co.) perfluoro-polyether fluids [198, 199]. 

Fio. 4. Autooxidation of hydrocarbons and chemiluminescent decomposition of intermediate peroxide. 

In related fashion, 1 lj8-hydroxy-l 1-epidihydroartemisinin (112) evolves to the hydroxytetra-hydrofuran (115) (Scheme 16). The presumed intermediate peroxide (113) then undergoes Baeyer-Villiger rearrangement to the formate (114). Acetalization by elimination of formic acid produces (115). 

Formation of potentially toxic intermediate peroxides and hydroperoxides ... 

Oxazoles are extremely susceptible to the action of singlet molecular oxygen and behave as 1,3-dienes, as they do in the Diels-Alder reaction. The wide variety of reactions observed with singlet oxygen and oxazoles take place, not by diverse modes of attack of the excited oxygen species with the substrate, but rather by a multitude of paths that appear to be open for the decomposition of the intermediate peroxide or hydroperoxide. The secondary decompositions are highly dependent on the structure of the oxazole, the nature of the functional groups in the immediate environment of the newly formed peroxide, the solvent, temperature, and other conditions. 

All catalyses studied so far can be accounted for qualitatively by either the existence of intermediate peroxides or the occurrence of oxidation-reduction reactions. Of course the actual reactions concerned are in general found to be more complex than those given in the simple schemes written above. Thus in the compensating reactions scheme the overall reduction and oxidation of the catalyst usually involve two or more consecutive steps, and the same probably holds in general for the formation and dissociation of intermediate peroxides. The aim of kinetic studies is of course to elucidate these details, but the difficulties involved in this can be judged from the fact that, among the systems investigated and to be described subsequently, there is not one for which the kinetics of the catalysis in all conditions have been accounted for satisfactorily and quantitatively in terms of a detailed reaction mechanism. 

Yoshida reported the oxygenative conjugate addition of perfluoroalkyl radicals to styrene derivatives (Scheme 4) [1 Ij. The reaction is photochemically initiated and used hexabutyldistannane as radical chain mediator and as reducing agent for the intermediate peroxide. 

As a reminder, in the case of liquid phase heterogeneous catalytic oxidation with a controlled pH in the presence of silver oxide, no intermediate peroxidic species is isolated. The non-radical reaction leads directly to the acid which is obtained in the form of a salt [19,20]. This highly special oxidation method will be dealt with in a separate section. 

Dehydrogenation. Tetraethynylethylenes (2) can be prepared from tetra-ethynylethanes (1) by conversion to the dhithio derivative followed by reaction with 2 eq. of f-butyl hypochlorite. Intermediate peroxides are decomposed by ferrous sulfate. 

Figure 11 Absorption spectrum of TlHg Lc oxygen intermediate/peroxide-level intermediate (a) compared to spectra of peroxo-dicopper(II) model complexes (b)-(e). Structural types indicated (after Solomon and...

---