Ozone dipolar structure

The direct oxidation (M + O-,) of organic compounds by ozone is a selective reaction with slow reaction rate constants, typically being in the range of kD = 1.0 - 103 M 1 s. The ozone molecule reacts with the unsaturated bond due to its dipolar structure and leads to a splitting of the bond, which is based on the so-called Criegee mechanism (see Figure 2-2). The Criegee mechanism itself was developed for non-aqueous solutions. 

Cyclo-addition (Criegee mechanism) — As a result of its dipolar structure, an ozone molecule may lead to three dipolar cyclo-additions on unsaturated bonds, with the formation of primary ozonide (I) corresponding to the reaction shown in Figure 4.8. In a protonic solvent such as water, this primary ozonide decomposes into a carbonyl compound (aldehyde or ketone) and a zwitterion (II) that quickly leads to a hydroxy-hyperoxide (III) stage that, in turn, decomposes into a carbonyl compound and hydrogen peroxide (see Figure 4.9). 

Although a large amount of work has been done on the mechanism of ozonization (formation of 11), not all the details are known. The basic mechanism was formulated by Criegee.I7 l The first step of the Criegee mechanism is a 1,3 dipolar addition (5-46) of ozone to the substrate to give the initial or primary ozonide, the structure of which has been shown to be the 1,2,3-trioxolane 12 by microwave and other spectral methods.174 However,... 

Fig. 2.3 shows the core structures of the most important 1,3-dipoles, and what they are all called. As with dienes, they can have electron-donating or withdrawing substituents attached at any of the atoms with a hydrogen atom in the core structure, and these modify the reactivity and selectivity that the dipoles show for different dipolarophiles. Some of the dipoles are stable compounds like ozone and diazomethane, or, suitably substituted, like azides, nitrones, and nitrile oxides. Others, like the ylids, imines, and carbonyl oxides, are reactive intermediates that have to be made in situ. Fig. 2.4 shows some examples of some common 1,3-dipolar cycloadditions, and Fig. 2.5 illustrates two of the many ways in which unstable dipoles can be prepared. 

Ozone is known as a very reactive agent in both water and air. The high reactivity of the ozone molecule is due to its electronic configuration. Ozone can be represented as a hybrid of four molecular resonance structures (see Fig. 2). As can be seen, these structures present negative and positively charged oxygen atoms, which in theory imparts to the ozone molecule the characteristics of an electrophilic, dipolar and, even, nucleophilic agent. 

A kinetic study of the reaction was also performed in which NMR-obtained rate data were correlated with mercurial structure changes (12). This study revealed a quite distinct reactivity order which, coupled with a 1 1 reactant stoichiometry, indicates a 1,3-dipolar electrophilic attack by ozone via a SE2 or four-center process. Although the exact mechanism was not conclusively proved, it is certain that neither the SE1 or SEi processes were operative during these reactions. 

Ozone first reacts with the olefin to form a primary ozonide, which probably has the five-membered ring (1,2,3-trioxolan) structure (90) as a result of a 1,3 dipolar addition.84... 

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 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,... 

Dipolar reagents, 1,3-dipoles (Section 10.5) Molecules for which a good neutral structure cannot be written. Ozone is a typical example. These species undergo addition to It systems to give five-membered rings. 

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