Criegee mechanisms

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). 

Scheme 3. The Criegee mechanism for the osmium tetroxide mediated dihydroxylation reaction.

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. 

The above stereochemical results have been explained on the basis of the Criegee mechanism with the following refinements (1) The formation of 14 is stereospecific, as expected from a 1,3 dipolar cycloaddition. (2) Once they are formed, 16 and IS remain attracted to each other, much like an ion pair. (3) Compound 16 exists in syn and anti forms, which are produced in different amounts and can hold... 

The ozonolysis of ethylene in the liquid phase (without a solvent) was shown to take place by the Criegee mechanism.This reaction has been used to study the structure of the intermediate 16 or 17. The compound dioxirane (21) was identified in the reaetion mixture at low temperatures and is probably in equilibrium with the biradical 17 (R = H). Dioxirane has been produced in solution but it oxidatively cleaves dialky] ethers (such as Et—O—Et) via a chain radical process, so the choice of solvent is important. 

Ozonolysis in the gas phase is not generally carried out in the laboratory. However, the reaction is important because it takes place in the atmosphere and contributes to air pollution. There is much evidence that the Criegee mechanism operates in the gas phase too, though the products are more complex beeause of other reactions that also take plaee. ... 

There have been numerous theoretical and experimental efforts to explain the mechanism by which ozone reacts with double bonds of unsaturated substances (11,12,35). Perhaps the more widely accepted reaction is the Criegee mechanism which produces the two groups A and B (as shown below) ( 36-42) ... 

The Criegee mechanism should be drawn. The initially formed 1,2,3-trioxolane can be split up in two ways, one of which gives the desired aldehyde, but the mechanism can t stop there. 

A mechanism has been proposed recently by O Neal and Blumstein for the gas-phase ozone-olefin reaction. This mechanism postulates that molozonide-biradical equilibrium is reached fast and postulates a competition between a-, 8-, and y-hydrogen abstraction reactions and the classical mechanism proposed by Criegee for the liquid-phase reaction. The main features of the Criegee mechanism (Figure 3-9) are the formation, from the initial molozonide, of the major carbonyl products and a second biradical intermediate, the zwitterion. The decomposition pathways of the zwitterion comprise unimolecular re-... 

The Criegee mechanism, widely accepted for the liquid-phase reaction, does not adequately explain the available gas-phase data. O Neal and Blumstein suggested a biradical structure for the first gas-phase intermediate and proposed three types of unimolecular hydrogen abstraction reactions (Figure 3-10). 

The a-hydrogen abstraction leads to an a ketoperoxide, which has been tentatively identified by Pitts in the case of cM-2-butene. Further reactions of the ketoperoxide include the formation of the normal products, i.e., the carbonyl products that can also be explained by the Criegee mechanism. The /3-hydrogen abstraction accounts for the ob-... 

FIGURE 3-9 Liquid-phase ozone-olefln reaction the Criegee mechanism. ... 

All aerosol products identified in the sm( chamber can be reasonably explained in terms of the O Neal and Blumstein and Criegee mechanisms, as is illustrated in Figure 3-11 for Qrclohexene. The major difference between alkenes and cyclic olefins lies in the fact that, after opening of the ( clic olefin double bond, the original number of carbon atoms is conserved and the chain carries both the carbonyl group and the biradical intermediate, whose further reactions lead to the observed difunctional compounds. 

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. 

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,... 

Evidence that the basic Criegee mechanism operates even in these cases comes from l80 labeling experiments, making use of the fact, mentioned above, that mixed ozonides (e.g.. 18) can be isolated when an external aldehyde is added. Both the normal and modified Criegee mechanisms predict that if l80-labeled aldehyde is added to the ozonolysis mixture, the label will appear in the ether oxygen (see the reaction between 14 and 13), and this is what is found. tw There is evidence that the anti-14 couples much more readily than the syn-14.11 1... 

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