Ozone, protonated, electrophilic

Because the protonation of ozone removes its dipolar nature, the electrophilic chemistry of HOs, a very efficient oxygenating electrophile, has no relevance to conventional ozone chemistry. The superacid-catalyzed reaction of isobutane with ozone giving acetone and methyl alcohol, the aliphatic equivalent of the industrially significant Hock-reaction of cumene, is illustrative. 

It should be mentioned that with superacidic electrophilic oxygenation of methane either to methanol (with protonated hydrogen peroxide) or to formaldehyde (with protonated ozone), the products formed are indeed the protonated products (CH3OH2 and CH2=OH+, respectively), which are protected from further electrophilic oxygenation, which happens only too readily in conventional oxidations. 

In electrophilic oxygenation with ozone in superacidic media or in dry ozonation over silica gel, protonated ozone, namely, +C>3H, was suggested to be the de facto electrophile. These reactions are fundamentally different from conventional ozonations with O3, which is a highly dipolar molecule. In 03H+ the dipole is removed by protonation. Cacace and Sporenza were able to directly identify +C>3H in the gas phase and also measure the proton affinity of ozone.107... 

Ozone has been shown to be protonated in the superacid media to ozonium ion H03+ 51 [Eq. (4.28)], which reacts with alkanes as a powerful electrophilic oxygenating agent.125 Similarly, ozone reacts with carbocations, giving alkylated ozonium ion that undergoes further cleavage reactions. These reactions are well-covered in Chapter 5. 

Oxenium Ions. Oxenium ions similar to nitronium ions are, in general, too reactive to be observed. The parent ion—that is, the hydroxyl cation HO+—is elusive, and it is improbable that it can be observed in its free form in the condensed state. However, the incipient hydroxyl cation is involved in acid-catalyzed electrophilic hydroxylation with protonated (or Lewis acid complexed) hydrogen peroxide (HO—OH2+) or ozone (HO—O—0+).125 Nitrous oxide is also apotential precursor for the hydroxyl cation (in its protonated form). The hydroxy diazonium ion HON2+ has not yet been observed. 

Since ozone is a strong 1,3-dipole,635 or at least has a strong polarizability (even if a singlet biradical structure is also feasible), it is expected to be readily protonated in superacids, in manner analogous to its alkylation by alkylcarbenium ions. Protonated ozone HC>3+, once formed, should have a much higher affinity (i.e., be a more powerful electrophile) for cr-donor single bonds in alkanes than neutral ozone. 

At a pH less than 6, molecular ozone directly attacks the phenolic ring. Then, the ozone further oxidizes the dihydric phenol to either o- or p-quinone. Because ozone is a relatively less powerful oxidant, the selectivity can be clearly demonstrated, as in Figure 8.8. The Hammett plot was first reported by Hoigne (1982) and confirmed by Gurol and Nekoulnalni (1984) (Figure 8.9). At a pH greater than 6, however, ozone is decomposed as hydroxyl radicals, and substituted phenols are ionized to form phenolate anions, which are much stronger electrophilic species than the protonated forms at low pH. As a result, the measured rate constants for some substituted phe-nolates approach the diffusion-controlled limits. 

Phenanthridine in methanol solution is unaffected by ozone, but in methylene dichloride quinoline-3,4-dicarboxylic acid (2%) and phenanthridone (23%) are formed a large part of the remaining phenanthridine can be recovered unchanged.335 The reaction thus proceeds much less readily than in the case of acridine. The mechanism by which the lactam is formed is not known with certainty, but an intermediate such as 233 has been proposed.335 However, an initial electrophilic attack producing 234, followed by loss of oxygen and a proton shift, seems equally likely. 

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