Criegee ozonide

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

If the Criegee mechanism operated as shown above, the cis/trans ratio for each of the two cross ozonides would have to be identical for the cis and trans alkenes, since in this mechanism they are completely cleaved. 

Even ketones can react with 16 to form ozonides, provided they are present in large excess Criegee, R. Korber, H. Chem. Ben, 1971, 104, 1812. 

It has been shown that abstraction of an a or p hydrogen from the ozonide also can occur. In fact, a-hydrogen abstraction can occur up to 2.5 times faster than Criegee fragmentation [36], As an example, the proposed mechanism for the reaction of ds-2-butene with ozone is shown in Figure 4. 

Another reaction in which an oxygen cation is plausible as an intermediate is in the ozonization of olefins. Ozonides are now known to have many structures, but the molozonide precursor of the classical" or most common ozonide is believed to have a four-membered, cyclic structure. Criegee and the author have independently proposed a mechanism in which heterolytic fission of the cyclic peroxide bond leads to an intermediate that can rearrange either to the classical ozonide or to an "abnormal ozonide 816 328... 

These are only the suggested mechanisms. Less is known about how the ozonides are decomposed, at least in case of triple bonds. The knowledge about ozonolysis at present available is largely due to the work of R.C. Criegee (in Peroxide Reaction Mechanisms, interscience, New York, 1962, p. 29.). 

FIGURE 8-2 Mechanism of ozonolysis. A, Criegee zwitterion B, aldehyde C, ozonide. 

Ozonolysis of styrene and ethylidenecyclohexane in the presence of [ 0]benzal-dehyde yields stable secondary ozonides incorporating 0. O NMR showed that labelled oxygen appeared as the ether oxygen, not the peroxo bridge, thus confirming the Criegee mechanism as opposed to the so-called unified concept. ... 

One of the most important features of the ozonolysis reaction of alkenes is one in which ozone adds to the C=C bond to form a primary ozonide (1,2,3-trioxolane). The Criegee mechanism suggests that this unstable intermediate decomposes into a carbonyl compound and a carbonyl oxide that recombine to form a final isomeric ozonide (1,2,4-trioxolane). Direct spectroscopic evidence for a substituted carbonyl oxide has only recently been reported by Sander and coworkers for the NMR characterization of dimesityl carbonyl oxide. Kraka and coworkers have theoretically modeled dimesityl carbonyl oxide and confirmed the structural aspects reported by Sander and coworkers on the basis of NMR data. 

SCHEME 16. Criegee s mechanism of ozonide formation and ozonolys... 

This primary ozonide is not stable. One of the two peroxy 0-0 bonds (marked a or b in Eq. 43) and the C-C bond (marked c) cleave simultaneously to give an aldehyde or ketone and an intermediate called the Criegee intermediate, named after the German chemist who originally proposed this mechanism (e.g., see Criegee, 1975) ... 

Although a large amount of work has been done on the mechanism of ozonization (formation of 11), not all the details are known. The basic mechanism was formulated by Criegee.I7 l The first step of the Criegee mechanism is a 1,3 dipolar addition (5-46) of ozone to the substrate to give the initial or primary ozonide, the structure of which has been shown to be the 1,2,3-trioxolane 12 by microwave and other spectral methods.174 However,... 

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