Rearrangement reaction molozonide

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). 

The molozonide was unstable and would either rearrange into the isozonide or form polymers. While Staudinger s theory explained the formation of the major products, some of the by-products could not be accounted for. The greatest step toward complete elucidation of the ozonolysis reaction was made by Criegee (Ref 3) In the 1950s. From a study of ozonolysis in various solvents and the constitution of the products, Criegee proposed these reactions ... 

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... 

Direct calorimetric measurements of two ozonides have been reported. Both the enthalpy of combustion of the ozonides and the direct enthalpies of ozonation of the precursor olefin were measured. The first species to be studied was the purported ozonide of A °octalin (l,2,3,4,5,6,7,8-octahydronaphthalene) . It is doubtful that the product of the octalin ozonation reaction would be the molozonide formed by direct addition with no subsequent rearrangement (i.e. ll,12,13-trioxabicyclo[4.4.3]tridecane) but perhaps even less likely is the rearranged and hence normal ozonide, the ll,12,13-trioxa[4.4.2.1]paddlane. From the published enthalpy of combustion of —5628 kJmoU, we derive an enthalpy of formation of this species, whatever it is, of —593.7 kJmoU. ... 

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. 

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]. 

The initial product of reaction of an alkene with ozone is an adduct called a molozonide, which rearranges under the conditions of the reaction to an isomeric compound called an ozonide. Low-molecular-weight ozonides are explosive and are rarely isolated. They are treated directly with a weak reducing agent to give the carbonyl-containing products. 

The addition of ozone (O3) to alkenes to give a primary ozonide (molozonide), which rearranges to an ozonide and eventually leads, on reduction, to carbonyl compounds (aldehydes and/or ketones), has already been mentioned and the reaction itself is shown in Scheme 6.11. However, it is important to recognize that this is only one example of a 4th- 2n electrocyclic addition and that orbital overlap for many sets of these reactions dictates their courses as well. Thus, to show the similarity of some of these dipolar 3 -f 2 addition reactions Equations 6.53-6.56 are provided. Although any alkene might be used as an example, (Z)-2-butene is used in each to emphasize that aU of them occur with retention of stereochemistry and, in the first (Equation 6.53), the reaction with ozone to form the primary ozonide (molozonide) is presented again (i.e., see Scheme 6.11). In a similar way, with a suitable azide, R-N3, readily prepared from an alkyl halide (Chapter 7), the same alkene forms a triazoline (Equation 6.54) and with nitrous oxide (N2O) the heterocycle (Chapter 13) cis -4,5-dimethyl-A -l,2,3-oxadiazoline (ds-4,5-dihydro-4,5-dimethyl-l,2,3-oxadiazole) (Equation 6.55). Finally, with a nitrile oxide, such as the oxide derived from ethanenitrile (acetonitrile [CH3ON]), the same alkene yields a different heterocycle, the dihydroisoxazole, 3,4,5-trimethyl-4,5-dihydroisoxazole (Equation 6.56). 

The mechanism of ozonolysis proceeds through initial electrophilic addition of ozone to the double bond, a transformation that yields the so-called molozonide. In this reaction, as in several others already presented, six electrons move in concerted fashion in a cyclic transition state. The molozonide is unstable and breaks apart into a carbonyl fragment and a carbonyl oxide fragment through another cychc six-electron rearrangement. Recombination of the two fragments as shown yields the ozonide. 

In subsequent steps, the molozonide rapidly rearranges when the n bond of the alkene and an O—O peroxide bond break. The fragments then recombine to give an ozonide. The individual fragments are reoriented to illustrate the addition reaction in the second step, which is shown below. 

The molozonide has two weak peroxide bonds, but the ozonide has only one. This difference accounts for the direction of the reaction. The rearrangement of the molozonide is exothermic. However, ozonides are explosively unstable compounds. For that reason, the reaction mixture is maintained at low temperatures and immediately reduced or oxidized after the reaction is complete. 

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