1,3-dipolar cycloaddition reactions with ozone

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]. 

This compound—known as an ozonide—is the first stable product of the reaction with ozone. It is the culmination of two 1,3-dipolar cycloadditions and one reverse 1,3-dipolar cycloaddition. It is still not that stable and is quite explosive, so for the reaction to be of any use it needs decomposing. The way this is usually done is with dimethylsulfide, which attacks the ozonide to give DMSO and two molecules of aldehyde. 

Further cycloadditions include the 1,3-dipolar cycloadditions as well as the respective [2-i-l]-reactions. The latter have already been mentioned in the reaction of carbenes photochemically generated from diazirines (Section 6.5.2.4). Experimental examples for [3-i-2]-cycloadditions have not yet been reported. Still theoretical considerations gave rise to the assumption that such reactions with, for example, azides, diazomethane or other classical 1,3-dipoles should readily take place on the surface dimers. The calculations further revealed that the cleavage of nitrogen under formation of the respective azacyclopropane as known for normal alkenes should also take place for the [3-i-2]-adducts bound to diamond. For the reaction with ozone (Section 6.5.2.3) it has not yet been clarified whether or not an ozonide is initially formed by [3-i-2]-cycloaddition, only then to be transformed into the carbonyl compound. 

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). 

Dipolar cycloadditions are another important family, with the impressive sequence of reactions involved when ozone reacts with an alkene as an example here. At -78°, ozone adds 1.3 (arrows) to give the molozonide 1.4. On warming, this undergoes a 1,3-dipolar cycloreversion (1.4 arrows),... 

Because of this reactivity, the ozone molecule is able to react through two different mechanisms called direct and indirect ozonation. Thus, ozone can directly react with the organic matter through 1,3 dipolar cycloaddition, electrophilic and, rarely, nucleophilic reactions [40,41], In water, only the former two reactions have been identified with many organics [42]. On the contrary, the nucleophilic reaction has been proposed in only a few cases in non-aqueous systems [43] (see examples of these mechanisms in Fig. 3). 

Figure 3 Direct pathways of ozone reaction with organics. (A) Criegge mechanism. (B) Electrophilic aromatic substitution and 1,3-dipolar cycloaddition. (C) Nucleophilic substitution.

The reaction of ozone with a C=C double bond begins with a 1,3-dipolar cycloaddition. It results in a 1,2,3-trioxolane, the so-called primary ozonide ... 

As stated earlier, the rate constants of ozone with organic compounds differ greatly for both types of processes (Table 3). The first reaction is important in acid media and for solutes that react very fast with ozone such as, for example, unsaturated compounds and compounds containing amine or acid groups. The results support the electrophilic nature of the reaction, either by electrophilic substitution or by dipolar cycloaddition [37]. This route leads to a very limited mineralization of the organic compounds, and its use for the removal of pollutants must be reinforced by modification of the method. 

The effects contributed by alkyl groups to the relative rate constants, kreh for the reaction of ozone with cis- and trans-1,2-disubstituted ethylenes are adequately described by Taft s equation = k °reX -f pSo-, where So- is the sum of Taft s polar substituent constants. The positive p values (3.75 for trans- and 2.60 for cis-l,2-disubstituted ethylenes) indicate that for these olefins the rate-determining step is a nucleophilic process. The results are interpreted by assuming that the electrophilic attack of ozone on the carbon-carbon double bond can result either in a 1,3-dipolar cycloaddition (in which case the over-all process appears to be electrophilic) or in the reversible formation of a complex (for which the ring closure to give the 1,2,3-trioxolane is the nucleophilic rate-determining step). 

The reaction of ozone with alkenes is one of the most useful 1,3-dipolar cycloadditions. Ozone undergoes [3 + 2] cycloaddition to the alkene to give a... 

I- 3]-Cycloadditions, also known as 1,3-dipolar cycloadditions, are widely exploited in SPOS because of the operational simplicity of the reactions along witli the architectural complexity of the structures that can be prepared. Moreover, the regio- and stereochemical outcome of these reactions are generally predictable, and their suitability for combinatorial chemistry is now well recognized. With the exception of azides and ozone, at least one carbon-carbon bond is formed in a [2 -I- 3]-cycloaddition. 

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. 

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). 

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