Criegee ozonolysis mechanism

Mechanism of Ozonolysis (Criegee mechanism) The initial step of the reaction involves a 1,3-dipolar cycloaddition of ozone to the alkene leading to the formation of the primary ozonide (molozonide or 1,2,3-trioxolane), which decomposes to give a carbonyl oxide and a carbonyl compound. The carbonyl oxides are similar to ozone in being 1,3-dipolar compounds and undergo 1,3-dipolar cycloaddition to the carbonyl compound with the reverse regio-chemistry, leading to a relatively stable secondary ozonide (1,2,4-trioxolane) (Scheme 5.47). 

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. ... 

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. 

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... 

The Criegee mechanism for ozonolysis a dramatic sequence with successively a 1,3-dipolar cycloaddition, a 1,3-dipolar cycloreversion and another 1,3-dipolar cycloaddition, all taking place below room temperature. 

A thorough theoretical analysis of the Criegee mechanism for the ozonolysis of cis- and trans-symmetrical alkenes Rl IG—Cl IR has been performed by semiempirical AMI calculations <1997JOC2757>. The experimentally observed stereoselectivity for bulky groups (e.g., R = Bu ) is that from the m-alkene a cisltrans ratio of 7 3 is encountered while from a trans-alkene a 3 7 ratio for the cisltrans secondary ozonides resulted. With smaller R groups (e.g., R = Me) both as- and trans-alkenes lead preferentially to the trans secondary ozonide (Scheme 1). 

The calculated barriers determine the final structure of the secondary ozonide but as can be seen from Figure 2, the differences between different routes tend to be rather small. The dipolar complex, a slight modification to the Criegee mechanism, when tightly bound, seems to explain well the stereochemical outcome although different product ratios may be encountered in ozonolysis reactions where for instance the heating rates are varied. 

Enol ethers of 1,2- and 1,3-diketones afford on ozonolysis products that are not in full agreement with the Criegee mechanism, because in some cases products of the Baeyer-Villiger rearrangement are formed. The main product in the ozonolysis of the enol ether 180 is a mixture of spiranic stereoisomers 181 involving a lactone and a 1,2,4-trioxolane ring (Scheme 62) <2004HCA2025>. 

A theoretical study of the ozonolysis of ethylene at the MP(SDQ)/6-31G(d,p) level has suggested a variation of the Criegee mechanism (Scheme 5). These results suggest that the primary ozonide 98 (1,2,3-trioxolane) cleaves to give a closely associated dipole pair 99 which does not dissociate but reconfigers to give a dipole-dipole stabilized complex... 

T he Criegee (1) mechanism of ozonolysis postulates that unsymmetrical olefins should give two zwitterions and two carbonyl compounds and hence postulates the possible formation of three different ozonides. This prediction has now been realized in a number of cases (2-9). It has also been shown that in many cases the ozonide stereoisomer ratio depends upon olefin stereochemistry in both normal (3, 6-12) and cross (6-9) ozonides. Since the original Criegee mechanism did not provide for these stereochemical results, a number of additional suggestions for the mechanism have been made (6,9,13, 14), all of which retain the fundamentals of the Criegee mechanism. 

During the past seven or eight years some excellent and novel studies have cast doubt upon the Criegee mechanism as the sole mechanism of ozonolysis. All of the newer mechanisms proposed, however, have ac-... 

Ozonolysis of pinenes contributes to OH radical and SOA formation. The latter has been speculated to be responsible for the formation of "blue haze". Hence, it is of vital importance to quantitatively understand the roles of ozonolysis of a- and jS-pinenes on both OH and SOA formation in the troposphere. The O3 reaction with both pinenes follows the Criegee mechanism, similar to that of iso-prene. The initial step proceeds through cycloaddition of O3 to the C=C double bond of each pinene, forming a primary ozonide (POZ). The available reaction energy is retained as the internal energy of the product, resulting in formation... 

We shall conclude this Section with an example of solvent cage effects of ion-molecule recombination reactions as found in the ozonolysis of alkenes in nonpolar solvents [739, 740]. According to the Criegee mechanism [424], unsymmetrically... 

The primary step of the propylene ozonolysis was taken to be the generally accepted Criegee mechanism 46, 47), which leads to the formation of zwitterions and aldehydes ... 

According to the Criegee mechanism, formyl and acetyl zwitterions, HC HOO and CHaC HOO, are reactive intermediates in the ozonolysis of propylene. Among several thermochemically feasible reactions of the zwitterions, the following scheme provides a chain oxidation mechanism which is consistent with the observed data. 

The reactions of O3 with C3H6, 9 and 10, were based on a review of the mechanism of ozonolysis by Murray (15). The accepted mechanism in the gas phase is similar to that commonly referred to as the Criegee mechanism in the liquid phase. The attack of O3 on C3H6 is electrophilic, and Vrbaski and Cvetanovic (16) have correlated the electrophilic behavior with that of oxygen atom-olefin reactions. Both reaction rates correlate with the ionization potentials of a series of olefins. 

The Criegee mechanism 13-15, 17) for ozonolysis has been of tremendous importance in giving a better understanding of the course of the ozonolysis reaction—in particular, the nature and fate of the active oxygen-containing ozonolysis products. It leaves in doubt, however, the nature of the initial attack of ozone on an unsaturated system and of the intermediates leading to the formation of the primary cleavage products, the zwitterion (IV) and the aldehyde or ketone (V). 

Targely based on our finding that the cis/trans ratios of cross ozonides 10) formed from unsymmetrical olefins depended on olefin geometry 11, 12), we have proposed a new mechanism of ozonolysis which takes account of this effect 14). The new mechanism, which considers only a limited type of olefin, namely trans-disubstituted and relatively unhindered cis olefins, differs significantly from the generally accepted Criegee mechanism 1,5). 

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