Ozone nucleophilic reactions

This initial attack of the ozone molecule leads first to the formation of ortho- and para-hydroxylated by-products. These hydroxylated compounds are highly susceptible to further ozonation. The compounds lead to the formation of quinoid and, due to the opening of the aromatic cycle, to the formation of aliphatic products with carbonyl and carboxyl functions. The nucleophilic reaction is found locally on molecular sites showing an electronic deficit and, more frequently, on carbons carrying electron acceptor groups. In summary, the molecular ozone reactions are extremely selective and limited to unsaturated aromatic and aliphatic compounds as well as to specific functional groups. 

Nucleophilic reaction — The nucleophilic reaction is found locally on molecular sites showing electronic deficits and, more frequently, on carbons carrying electron-withdrawing groups. The molecular ozone reactions are extremely selective and limited to unsaturated aromatic and aliphatic compounds, as well as to specific functional groups. 

Because of this reactivity, the ozone molecule is able to react through two different mechanisms called direct and indirect ozonation. Thus, ozone can directly react with the organic matter through 1,3 dipolar cycloaddition, electrophilic and, rarely, nucleophilic reactions [40,41], In water, only the former two reactions have been identified with many organics [42]. On the contrary, the nucleophilic reaction has been proposed in only a few cases in non-aqueous systems [43] (see examples of these mechanisms in Fig. 3). 

Perhaps the most interesting point which emerges from the results is that in ethylenes bearing electron-releasing alkyl substituents the ratedetermining step appears to be a nucleophilic process, as indicated by the positive p values. This does not contradict the assumption that the first step in the ozone—olefin reaction is an electrophilic attack of ozone on the carbon-carbon double bond. The present observations also agree with some of the results obtained recently by Pritzkow et al. (16) for alkyl mono-substituted ethylenes in ethanol solution at — 60 °C. 

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. 

Tetrafluoroethylene Oxide TFEO has only been prepared by a process employing oxygen or ozone because of its extreme reactivity with ionic reagents. This reactivity may best be illustrated by its low temperature reaction with the weak nucleophile, dimethyl ether, to give either of two products (47) (eq. 10). 

In contrast to reaction of ozone with nucleophilic haUde and hypohaUte ions, reaction of ozone with electrophilic hypohalous acids is very slow. 

As inert as the C-25 lactone carbonyl has been during the course of this synthesis, it can serve the role of electrophile in a reaction with a nucleophile. For example, addition of benzyloxymethyl-lithium29 to a cold (-78 °C) solution of 41 in THF, followed by treatment of the intermediate hemiketal with methyl orthoformate under acidic conditions, provides intermediate 42 in 80% overall yield. Reduction of the carbon-bromine bond in 42 with concomitant -elimination of the C-9 ether oxygen is achieved with Zn-Cu couple and sodium iodide at 60 °C in DMF. Under these reaction conditions, it is conceivable that the bromine substituent in 42 is replaced by iodine, after which event reductive elimination occurs. Silylation of the newly formed tertiary hydroxyl group at C-12 with triethylsilyl perchlorate, followed by oxidative cleavage of the olefin with ozone, results in the formation of key intermediate 3 in 85 % yield from 42. 

Barrett and coworkers have explored hetero-substituted nitroalkenes in organic synthesis. The Michael addition of nucleophiles to 1-alkoxynitroalkenes or 1-phenylthionitroalkenes followed by oxidative Nef reaction (Section 6.1) using ozone gives a-substituted esters or thiol esters, respectively.41 As an alternative to nucleophilic addition to l-(phenylthio)-nitroalkenes, Jackson and coworkers have used the reaction of nucleophiles with the corresponding epoxides (Scheme 4.4).42 Because the requisite nitroalkenes are readily prepared by the Henry reaction (Chapter 3) of aldehydes with phenylthionitromethane, this process provides a convenient tool for the conversion of aldehydes into ot-substituted esters or thiol esters. 

Reaction of ozone with a double bond is not surprisingly a function of the nucleophilicity or electron density of the double bond. Therefore, in ozonolysis of octamethylsemibullvalene208 (122) as well as for hexamethylbicyclo[2.2.0]-2,5-hexadiene209 and octamethyltricyclo-octadiene210 the diozonides, e.g. 123, are formed as the major product (equation 33). On the other hand, for hexachlorobicyclopentadiene211 (124), hexachlorobicycloheptadiene212 and 2-chloro-3-methyl-l,3-butadiene213 attack takes place at the nonchlorinated double bond only to form the ozonide 125 (equation 34). 

Finally, the idea of the coupling between nucleophilic attack and proton transfer in the reaetions just discussed provides an interpretive framework for another important atmospheric reaction, namely the hydrolysis of dinitrogen pentoxide N2O5, thought to play an important role in mid-latitude global ozone depletion. 28,29 Indeed a related mechanism was suggested in ref 5 for the low acidify condition hydrolysis. 

While the Henry s law constant for ozone is fairly small (Table 8.1), there is sufficient ozone present in the troposphere globally to dissolve in clouds and fogs, hence presenting the potential for it to act as a S(IV) oxidant. Kinetic and mechanistic studies for the 03-S(IV) reaction in aqueous solutions have been reviewed and evaluated by Hoffmann (1986), who shows that it can be treated in terms of individual reactions of the various forms of S(IV) in solution. That is, S02 H20, HSOJ, and SO2- each react with 03 by unique mechanisms and with unique rate constants, although in all cases the reactions can be considered to be a nucleophilic attack by the sulfur species on 03. 

Ozonium Ion (H03+). Ozone is a resonance hybrid of canonical structures 50a-50d.135 Ozone does in fact act as a 1,3-dipole—that is, either as an electrophile or a nucleophile. The electrophilic nature of ozone has been recognized for a long time in its reactions with alkenes, alkynes, arenes, amines, phosphines,... 

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