Molozonide intermediate

While the oxygen-18 labeling results described here confirm the molozonide-aldehyde mechanism for the types of olefins considered, the ozonolysis reaction in general is quite complex and seems to vary widely depending especially upon the stereochemistry of the olefin. To sum up, the molozonide-aldehyde mechanism 14) considered here appears to be applicable to any important degree only to trans-disubstituted olefins, relatively unhindered cis olefins, and perhaps to unhindered terminal olefins. As pointed out, more hindered olefins seem to react by one or more different pathways, which differ most notably from the present system in the apparent absence of a molozonide intermediate (2, 8, i2,14). 

The first step of the ozonolysis mechanism is the initial electrophilic addition of ozone to the C=C double bond to form the molozonide intermediate. Its instability leads to a further reaction, producing a carbonyl and carbonyl oxide molecule (Scheme 2.10, II). The carbonyl and carbonyl oxide rearrange to create the stable ozonide intermediate (Scheme 2.10, III). A reductive workup is then undertaken to convert the ozonide specie into carbonyl products (Scheme 2.10,1) [19]. 

Unsaturated compounds undergo ozonization to initially produce highly unstable primary ozonides (15), ie, 1,2,3-trioxolanes, also known as molozonides, which rapidly spHt into carbonyl compounds (aldehydes and ketones) and 1,3-zwitterion (16) intermediates. The carbonyl compound-zwitterion pair then recombines to produce a thermally stable secondary ozonide (17), also known as a 1,2,4-trioxolane (44,64,125,161,162). 

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... 

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 ozonolysis of olefins may be analyzed as a sequence of two 1,3-dipolar cycloadditions initial electrophilic attack by ozone 18 to form the first intermediate, which decomposes into a carbonyl compound and a carbonyl oxide 14 followed by nucleophilic 1,3-dipolar addition of the carbonyl ylide 14 to the ketone, yielding the molozonide. 

Onsager inverted snowball theory (Com.) relation to Smoluchowski equation in, 35 relaxation time by, 34 rotational diffusion and, 36 Ozone in the atmosphere, 108 alkene reactions with, 108 Crigee intermediate from, 108 molozonide from, 108 ethylene reaction with, 109 acetaldehyde effect on, 113 formic anhydride from, 110 sulfur dioxide effect on, 113 sulfuric acid aerosols from, 114 infrared detection of, 108 tetramethylethylene (TME) reaction with, 117... 

First step is a 1,3-dipoIar cycloaddition of ozone to the alkene leading to the primary ozonide (molozonide, 1,2,3-trioxolane, or Criegee intermediate) which decomposes to give a carbonyl oxide and a carbonyl compound ... 

This intermediate is called an initial or primary ozonide (or sometimes a molozonide). One of the oxygen/oxygen bonds now breaks heterolytically, and then the carbon/carbon bond breaks to yield a carbonyl compound and a zwitterion. Write down these two steps. 

The reaction of alkenes with ozone at low temperature produces an intermediate molozonide (or primary ozonide) which rapidly rearranges to form an ozonide. This leads to the cleavage of the C=C double bond. 

Let us consider as an example the reaction of cyclohexene with ozone in the atmosphere. This reaction has been studied in laboratory chamber experiments by Kalberer et al. (2000). A potential reaction mechanism is depicted in Figure 14.11. The first steps of the reaction are the formation of an initial molozonide M, its transformation to a peroxy radical intermediate, and then to a dioxyrane-type intermediate, and finally to an excited Criegee biradical [CHO(CH2)4CHO O ]. A series of reactions then lead from the Criegee biradical to stable products, some with five and some with six carbon atoms (Figure 14.11). This rather complicated series of reactions leads to the stable gas-phase products listed in Table 14.10. 

Paul R. Story In the first place we cannot rule out some homolytic decomposition of the proposed intermediate. However, it is worth comparing our proposed seven-membered ring trioxide with the molozonide for which an ionic decomposition to zwitterion and carbonyl is readily envisioned our intermediate and the proposed ionic decomposition are quite analogous to Criegee s molozonide or primary ozonide and its decomposition. 

Using modern analytical methods, a number of transient intermediates and byproducts could be verified [19, 20]. The first step in the mechanism of ozonolysis is the 1,3-dipolar cycloaddition of the dipole ozone to the double bond of OA. A 1,2,3-trioxolane is formed, the unstable primary ozonide or molozonide. The primary ozonide collapses in a 1,3 dipolar cycloreversion to a carbonyl compound and a carbonyl oxide, the so-called Criegee zwitterion. Since OA is substituted with two diverse groups at the double bond, two different opportunities exist for the formation of carbonyl compound and carbonyl oxide. Again, a 1,3-dipolar cycloaddition of these intermediates leads to three different pairs of 1,2,4-trioxolane derivatives (cisltram), the secondary ozonides, which are more stable than the primary ones. Their oxidative cleavage results in AA and PA. 

A dioxetan intermediate (123) has been isolated from the ozonolysis of ethylidenecyclohexane in pinacolone and appears to be formed via reduction of the molozonide (124) by solvent. On heating, (123) gave cyclohexanone and acetaldehyde, which are the normal ozonolysis products. 

Certain aldehydes and ketones, when used as solvents, intercept and reduce a labile intermediate in the ozonolysis of olefins. The intermediate, which can be considered the progenitor of many other ozonolysis products, is formulated as the Staudinger molozonide, e.g. (577), and its reduction generates the corresponding dioxetan (578) with a Baeyer-Villiger oxidation of the aldehyde or ketone solvent. The dioxetan intermediate, normally cleaved to the carbonyl components, has now been isolated and characterized by using pinacolone as a solvent. Low-temperature infrared studies of simple alkene-ozone reactions have been made. ... 

Ozonolysis occurs in several steps. First, an unstable intermediate, called a molozonide, forms by a cyclic concerted addition of the terminal oxygen atoms of ozone to the 7i bond of the alkene. This step requires a total of three electron pair shifts, as shown below. 

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