Initial ozonides

Ozone reacts vigorously with alkenes to form unstable initial ozonides (molozonides) which rearrange spontaneously to form ozonides. 

The rearrangement is thought to go through dissociation of the initial ozonide into reactive fragments that recombine to give the ozonide. 

Ozone adds to the alkene to form an initial ozonide. 

The reaction of alkenes with ozone constitutes an important method of cleaving carbon-carbon double bonds.138 Application of low-temperature spectroscopic techniques has provided information about the rather unstable species that are intermediates in the ozonolysis process. These studies, along with isotope labeling results, have provided an understanding of the reaction mechanism.139 The two key intermediates in ozonolysis are the 1,2,3-trioxolane, or initial ozonide, and the 1,2,4-trioxolane, or ozonide. The first step of the reaction is a cycloaddition to give the 1,2,3-trioxolane. This is followed by a fragmentation and recombination to give the isomeric 1,2,4-trioxolane. The first step is a... 

Initiators, ozonides for polymerization, 622 Inorganic peroxides, commercial availability, 620-1, 622... 

Considering that an alkene on oxidation with ozone (Following fig.) that proceeds with the formation of an initial ozonide which... 

Path a involves unimolecular decomposition of an initial ozonide (VI). It is important to note that since other methylenecyclopropane derivatives, such as 2,3-dimethylmethylenecyclopropane, do not undergo ring opening of the cyclopropane ring, both ring strain and the presence of the carbomethoxy groups must be necessary for such an unusual decomposition of an initial ozonide. The following sequence rationalizes... 

Primary and secondary nitroalkanes.1 A new route to nitro compounds involves conversion of azides to phosphine imines by reaction with triphenyl- or tri-w-butylphos-phine at 25-35° followed by ozonolysis in CH2C12 at - 78°. Presumably the initial ozonide is converted into a nitroso compound, which is then oxidized to the nitro compound... 

Ozone preferentially reacts with the most unsaturated fatty acids present (187) arachidonic acid and higher PUFAs are particularly sensitive. T ranj -double bonds and fatty acids have been reported to react with ozone much more slowly than cis-double bonds (21), but this observation may be an artifact of measuring only initial ozonides. In fact, tran -fatty acids do react with ozone, but the initial ozonides decompose and rearrange more rapidly to generate peroxy-epoxide or peroxy-ozo-nide complexes and free acids (188). This is another example of how, as in lipid oxidation itself, downstream as well as initial products must be measured to obtain a full and accurate picture of reaction. 

The first intermediate, which never has been isolated and therefore must be very unstable, is called the Primdrozonid or initial ozonide (1). Nothing can be said with certainty at this moment about its special structure. 

The most important intermediate seems to be the zwitterion which arises from the spontaneous cleavage of the initial ozonide. From the zwitterion all reaction products which have been found from different starting materials under different conditions can be explained. 

These experiments give some further evidence about the formation of the ozonides from the initial ozonides. Concerning the formation of the initial ozonides from the olefins and ozone, the latter seems to be an electrophilic reagent. Meinwald (4) proposed that it reacts in the following manner with the polarized olefin ... 

Usually this first addition product instantaneously gives the initial ozonide by a nucleophilic attack of the second or third oxygen atom on the carbonium ion. 

Considering that an alkene on oxidation with ozone (Following fig.) that proceeds with the formation of an initial ozonide which then rearranges to an isomeric ozonide. This second ozonide is unstable and potentially explosive and so it is not generally isolated. Instead, it is reduced with zinc and... 

The Criegee mechanism for the ozonolysis of alkenes (Figure 11.73) can be analyzed in terms of a series of three 1,3-dipolar cycloadditions. The addition of ozone to an alkene leads first to a 1,2,3-trioxacyclopentane structure known variously as an initial ozonide, primary ozonide, or molozonide,... 

The conversion of the initial ozonide to a carbonyl compound and a carbonyl oxide can be considered a retro-l,3-cycloaddition, and the combination of the carbonyl oxide and the carbonyl compound is another 1,3-cycloaddition (Figure 11.76). Nonconcerted mechanisms are possible for each of the three steps in the Criegee ozonolysis mechanism, but Kuczkowski has summarized evidence suggesting that the reactions are concerted. 

Retro-[ 4, + and [ 4,+ 2J reactions in the conversion of the initial ozonide to final ozonide. 

The mechanism of ozone addition to alkenes begins with formation of unstable compounds called initial ozonides (sometimes called molozonides). The process occurs vigorously and leads to spontaneous (and sometimes noisy) rearrangement to compounds known as ozonides. The rearrangement is believed to occur with dissociation of the initial ozonide into reactive fragments that recombine to yield the ozonide. Ozonides are very unstable compounds, and low-molecular-weight ozonides often explode violently. 

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