Ozonation in superacids

When methane is reacted with ozone in superacidic media,61,67 formaldehyde is directly formed through a pathway that is considered attack by +03H into a C—H bond, followed by cleavage of H2O2 to give very reactive methyloxenium ion (2), which instantly rearranges to protonated formaldehyde ... 

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... 

Carbon—carbon bond cleavage is also characteristic of branched saturated hydrocarbons reacting with ozone in superacid media.1 Depending on the structure of the reacting alkanes, different mechanisms can be operative. 

When methane is reacted with ozone in superacidic media, formaldehyde is directly formed. The pathway is considered an attack by +O3H into a C-H bond to form a very reactive methyloxenium ion (69) through the pentacoordinate transition state [Eq. (6.50)]. 

In contrast, alcohols may also be formed when straight-chain alkanes react with an equimolar amount of ozone in superacidic media (i.e., with +O3H) at -78°C, followed by raising the temperature of the reaction mixture.In this case H2O2 formed in situ in primary ozonolysis participates in hydroxyl-ation as shown in Equation (6.49). 

The characteristic transformation of branched alkanes with ozone in superacidic media is C-C bond cleavage. When a stream of oxygen containing 1-5% ozone was passed through a solution of isobutane in FSO3F-SbF5-SO2ClF solution held at -78°C, the H and C NMR spectra of the resultant solution were consistent with the formation of dimethylmethylcarboxonium ion 67 [Eq. (6.52)] in 45% yield. Similar treatment of isopentane, 2,3-dimethylbutanc, and 2,2,3-trimethylbutane resulted in the formation of related carboxonium ions as the major products. 

The results of dry ozonation, namely, regioselectivities and stereoselectivities, are very similar to those in superacidic liquid-phase ozonation. Tertiary C—H bonds in strained systems such as norbomane are inert to dry ozonation.93 Such compounds are oxidized at the secondary carbon to yield a mixture of alcohols and ketones.93 104 Similarly, substituted cyclopropanes exhibit a general preference for the oxidation of the secondary C—H bond in the a-position to the ring 104... 

Alkylated carboxonium ions have also been prepared by direct electrophilic oxygenations of alkanes, alcohols, and so on, by ozone or hydrogen peroxide in superacidic media606 [Eq. (3.82)]. 

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. 

Formation of an intermediate alkylcarbenium ion which is the key step in superacid-catalyzed reaction of ozone with alkanes is considered to proceed by two mechanistic pathways as illustrated in Scheme 12. The carbenium ions subsequently undergo nucleophilic reaction with ozone as discussed previously. Reactions of ozone with alkanes giving ketones and alcohols as involved in mechanism b have been reported in several instances " . The products obtained from isobutane and isoalkanes (Table 6) are in accordance with the mechanism discussed above. 

Scheme 13). Since ozone has a strong 1,3-dipole or at least a strong polarizability , it is expected to be readily protonated in superacid media. Attempts to directly observe protonated ozone 03 by NMR spectroscopy were rather inconclusive because of probable fast hydrogen exchange with the acid system. 

Novel oxidations of hydrocarbons in superacids with ozone or hydrogen peroxide have been investigated. Proto-nated ozone (O H) or hydrogen peroxide (H3OJ) attacks the single a-bond, resulting in oxygen insertion. These can be followed by protolytic transformation, such as the conversion of isobutane into acetone and methyl alcohol. 

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. 

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. 

No NMR evidence ( H and 13C) could be obtained for the vicinal-dication (221). Calculations at the MP2/6-31G level however show dication 221 to be a stable minimum. Its structure has D2symmetry with the two COC units in an almost perpendicular arrangement (dihedral angle between two COC units is 83.7°). Protonated ozone, O3H+, was studied by Olah et al. and by Italian researchers.111 In the superacid catalyzed reactions of ozone, diprotonated ozone is however possibly involved as the de facto intermediate. 

For the reaction of ozone with alkanes under superacidic conditions, two mechanistic pathways may be considered. The first is the formation of an alkylcarbenium ion via protolysis of the alkane prior to quenching of the ion by ozone (Scheme 6.26, route a). Alkylcarbenium ions may also be generated via initial transformation of the alkane to an alcohol followed by protonation and ionization (Scheme 6.26, route b). There have already been a number of reports of ozone reacting with alkanes to give alcohols and ketones. °" In both cases, intermediary alkylcarbenium ions would then undergo nucleophilic attack by ozone as mentioned earlier. 

Superacid-catalyzed reaction of ozone with straight-chain alkanes has also been investigated at low temperature. Magic Acid catalyzed ozonization of ethane is shown in equation 56. Reaction of methane under similar conditions was also investigated and discussed previously. Reactions of cycloalkanes have similarly been studied. ... 

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