1.3- Dipolar cycloadditions ozonolysis

Most ozonolysis reaction products are postulated to form by the reaction of the 1,3-zwitterion with the extmded carbonyl compound in a 1,3-dipolar cycloaddition reaction to produce stable 1,2,4-trioxanes (ozonides) (17) as shown with itself (dimerization) to form cycHc diperoxides (4) or with protic solvents, such as alcohols, carboxyUc acids, etc, to form a-substituted alkyl hydroperoxides. The latter can form other peroxidic products, depending on reactants, reaction conditions, and solvent. 

A well-known example for a 1,3-dipolar compound is ozone. The reaction of ozone with an olefin is a 1,3-dipolar cycloaddition (see ozonolysis). 

Deprotection of N-2 by ozonolysis furnishes triazoles 1225 (Scheme 202) <2003JA7786>. Finding that 1,3-dipolar cycloaddition of alkynes 1222 to trimethylsilyl azide, carried out in DMF/MeOH in the presence of Cul as a catalyst, leads directly to products 1225 with much higher yields provides a significant progress to the synthesis of N-unsubstituted 1,2,3-triazoles <2004EJO3789>. 

Sha et al. (45) reported an intramolecular cycloaddition of an alkyl azide with an enone in an approach to a cephalotaxine analogue (Scheme 9.45). Treatment of the bromide 205 with NaN3 in refluxing methanol enabled the isolation of compounds 213 and 214 in 24 and 63% yields, respectively. The azide intermediate 206 underwent 1,3-dipolar cycloaddition to produce the unstable triazoline 207. On thermolysis of 207 coupled with rearrangement and extrusion of nitrogen, compounds 213 and 214 were formed. The lactam 214 was subsequently converted to the tert-butoxycarbonyl (t-Boc)-protected sprrocyclic amine 215. The exocyclic double bond in compound 215 was cleaved by ozonolysis to give the spirocyclic ketone 216, which was used for the synthesis of the cephalotaxine analogue 217. 

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... 

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. 

Ozonolysis of a double bond leads first to a so-called primary ozonide 40 through 1,3-dipolar cycloaddition. Rearrangement of primary ozonide 40 with ring cleavage produces a carbonyl oxide 42 and a carbonyl compound 41, which then recyclize to secondary ozonide 43. The reaction terminates with a redox process involving... 

The mechanism proposed by Criegee for the ozonolysis of alkenes <1975AGE745> considers an initial it-complex between the alkene and ozone which decays via a 1,3-dipolar cycloaddition into a 1,2,3-trioxolane or primary ozonide, known also as the molozonide . These compounds are unstable, even at low temperatures, and due to cycloreversion decompose into a carbonyl fragment and a CO, which may recombine by another 1,3-dipolar cycloaddition step to form the more stable 1,2,4-trioxolane ( secondary ozonide or final ozonide (see also Section 6.06.2). 

Ozonolysis of the cyclic vinyl ethers 71 (n = 2, 3 or 4) in the presence of M-benzylidene-benzylamine affords the dioxazoles 73 by dipolar cycloaddition to transient carbonyl oxides 72. The dioxazoles fragment to mixtures of phenylcycloalkenes, benzaldehyde and N-formyl-benzylamine by the action of silica gel (94JCS2449). Carbonyl oxides 75 (E = C02Me), generated by thermolysis of the peroxides 74, add to phenyl isocyanate to yield 1,2,4-dioxa-zolidin-3-ones 76 (94JCS3295). 

Dipolar Cycloadditions and 1,3-Dipolar Cycloreversions as Steps in the Ozonolysis of Alkenes... 

Tn previous work it has been shown that a competition exists during - ozonation of olefins between ozonolysis and epoxide formation (I). As steric hindrance increases around the double bond, the yield of epoxide or subsequent rearrangement products increases. This is illustrated with both old (1) and new examples in Table I for purely aliphatic olefins and in Table II for aryl substituted ethylenes. It was suggested that the initial attack of ozone on an olefinic double bond involves w (pi) complex formation for which there were two fates (a) entrance into 1,3-dipolar cycloaddition (to a 1,2,3-trioxolane adduct), resulting in ozonolysis products (b) conversion to a o- (sigma) complex followed by loss of molecular oxygen and epoxide formation (Scheme 1). As the bulk... 

Similar conclusions are drawn by Cvetanovic et al. from their results of ozonization of alkenes in the gas phase (9) and in CC14 solution (10). The rate constants for the ozonolysis of chloroethylenes and allyl chloride, in CC14 solution, indicate (11) that the rate of ozone attack decreases rapidly as the number of chlorine atoms in the olefin molecules is increased. However, to explain the departures from simple correlations, in some cases steric effects and the dipolar character of ozone had to be invoked (10). The relevance of the dipolar character of ozone in its reactions has also been stressed by Huisgen (12), who provided evidence that the ozone—olefin reaction is usually a 1,3-dipolar cycloaddition. 

Ozonolysis is widely used both in degradation work to locate the position of double bonds and in synthesis for the preparation of aldehydes, ketones, and carboxylic acids. Ozone is a 1,3-dipole and undergoes 1,3-dipolar cycloadditions with alkenes. 

The reaction of ozone with alkenes is one of the most useful 1,3-dipolar cycloadditions. Ozone undergoes [3 -I- 2] cycloaddition to the alkene to give a 1,2,3-trioxolane, which immediately decomposes by a [3 -I- 2] retro-cycloaddition to give a carbonyl oxide and an aldehyde. When the ozonolysis is carried out in the presence of an alcohol, the alcohol adds to the carbonyl oxide to give a hydroperoxide acetal. In the absence of alcohol, though, the carbonyl oxide undergoes another [3 -b 2] cycloaddition with the aldehyde to give a 1,2,4-trioxolane. 

Another example demonstrating the difference in reactivity is the ozonolysis reactions of acetylene and ethylene. Ozonolysis of ethylene is a classical 1,3-dipolar cycloaddition reaction with an activation energy of 5 kcal/mol [106], whereas a larger activation energy of 11 kcal/mol was measured for the reaction of ozone with acetylene [107]. The 1,3-dipolar cycloaddition adduct, 1,2,3-trioxolene, has not been definitively observed as an intermediate involved in the acetylene ozonolysis. Nevertheless, according to the combined microwave and ab-initio calculation studies, the formation of similar van der Waals complexes in the course of ozonolysis has been established for both acetylene and ethylene [108]. 

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. 

The ozonolysis reaction has been the subject of considerable mechanistic study. It is likely that in most cases the reaction proceeds by breakdown of the 1,3-dipolar cycloaddition product to a carbonyl oxide 99 and an aldehyde (or ketone) (5.99). The fate of the carbonyl oxide depends on the solvent and on its structure and the structure of the carbonyl compound. In an inert (non-participating) solvent, the carbonyl compound may react with the carbonyl oxide to form an ozonide 100 otherwise the carbonyl oxide may dimerize to the peroxide 101 or give ill-defined polymers. In nucleophilic solvents such as methanol or acetic acid, hydroperoxides of the type 102 are formed. 

Ozone plays a major role in the degradation of unsaturated VOCs in the troposphere, especially during night-time. The rate constants of the ozonolysis of a variety of alkenes have been reported [1]. However, in most instances the fate of the primary products of the ozonolysis is unknown, although the secondary reaction products are of crucial importance for the overall understanding of the alkene/ozone chemistry. The classical Criegee mechanism of the ozonolysis reaction involves the primary ozonide (POZ, 1,2,3-trioxolane), which cleaves to the Criegee intermediate (carbonyl O oxide) and a carbonyl compound [2, 3]. The secondary ozonide (SOZ, 1,2,4-trioxolane) is formed from these components in a [l,3]-dipolar cycloaddition reaction. 

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,... 

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