Peroxonium ions

Selective hydroxylation of phenol with hydrogen peroxide was reported on acid zeolite catalysts [91-92]. Peroxonium ions, formed by H2O2 protonation, are the oxidizing species. When the reaction is carried out on a faujasite catalyst, a mixture of hydroxybenzenes and tars is obtained [91]. In the presence of H-ZSM-5 on the other hand, no tar formation was mentioned (which does not necessarily mean that it was absent) and p-selectivities close to 100% were reported for the hydroxylation [92]. These superior selectivities reflect the shape selective properties of ZSM type zeolites. 

Clearly, TPP-sensitized oxygenation of disilirane proceeds in a stereospecific manner via peroxonium ion intermediate 89 (Scheme 23). The potential energy surface of the... 

In contrast to the radical processes described before, high selectivity is characteristic of electrophilic oxidation of methane.61 It reacts, for instance, with H2O2 in superacidic media to give methanol.1,62 The reaction is best explained by electrophilic insertion of the hydrogen peroxonium ion (H3Oj) into the methane C—H bond ... 

The synthetic utility of hydrogen peroxonium ion reagent is discussed in Chapter 5. Similarly, protonation and cleavage reactions of hydroperoxides and peroxides have been investigated extensively125 and are also discussed in Chapter 5. 

The hydrogen peroxonium ion may be considered as an incipient OH+ ion capable of electrophilic hydroxylation of single (effect reactions similar to such previously described electrophilic reactions as protolysis, alkylation, chlorination (chlorolysis), and nitration (nitrolysis). 

All these reactions involve hydroxylation of aromatics by hydrogen peroxonium ion [Eq. (5.215)]. 

Since the relative rate of formation of dimethyhnethylcarboxonium ion from isobutane is considerably faster than that of rert-butyl cation from isobutane in the absence of ozone under the same conditions,it is, however, likely that protonated ozone inserts into the C-H bond of the alkane to form a pentacoordinate peroxonium ion 70 that decomposes to a very reactive tert-alkyloxenium ion (71) that subsequently undergoes Baeyer-Villiger oxidation [Eq. (6.52)]. 

When hydrogen peroxide is protonated in superacid media, hydrogen peroxonium ion (HaOa ) is formed. Certain peroxonium salts have been well characterized and even isolated as stable salts ". Hydrogen peroxonium ion is considered as the incipient OH ion, a strong electrophile for electrophilic hydroxylation at single (cr) bonds of alkanes. The reactions are thus similar to those described previously for electrophilic protonation (protolysis), alkylation and formylation. 

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