Alkenes ozonation

Hull, L. A., I. C. Hisatsune, and J. Hekklen. Low-temperature infrared studies of simple alkene-ozone reactions. I. Amer. Chem. Soc. 94 4856-4864, 1972. 

In short, the Criegee intermediate from alkene-ozone reactions can contribute, in principle, to the gas-phase oxidation of S02. In practice, it is likely less important than reaction with OH. In addition, as we shall see, even the OH-SOz gas-phase reaction is, under many conditions, swamped out by reactions occurring in the liquid phase found in clouds and fogs. As a result, the CI-S02 reaction may contribute in some circumstances but is unlikely to be a major contributor to S02 oxidation as a whole. 

On the basis of deuterium labeling, 1,3-dipolar cycloaddition to the bridge C—C bond to form a cyclic trioxide was suggested. Rearrangements similar to those in alkene ozonation yield the products. 

Compared to ozonation of alkenes, much less is known about the ozonation of alkynes,710 which yields 1,2-dicarbonyl compounds, carboxylic acids, and anhydrides. 1,2,3-Trioxolene (91), analogous to 74 in alkene ozonation mechanism (Scheme 9.14), and zwitterionic intermediates (92) were formulated on the basis of IR studies and trapping experiments ... 

Brown s crotyl borane 158 (chapter 24) provides reagent control through a chair-like six-membered ring in the formation of 159. Oxidation of the alkene (ozone with oxidative workup) gives the free acid marked in 160 and on removal of the benzylic group the freed amine (circled) cyclises to it to give the pyrrolidone required (147 as its methyl ester). The synthesis can easily be completed from there. 

Cleavage of alkenes. Ozonization of alkenes in aqueous acetone (H2O Me2CO = 5 95) at 0 gives carbonyl products directly, additional reagents for decomposition of ozonides are not needed. 

A peroxyacid, OSO4, and (cold basic) KMn04 break only the tt bond of the alkene. Ozone and acidic (or hot basic) KMn04 break both the ot bond and the o- bond. 

This type of dipolar addition reaction was introduced in chapter 3 in connection with ozonolysis (sec. 3.7.B), as well as permanganate (sec. 3.5.A),346 and osmium (sec. 3.5.B) oxidation of alkenes. Ozone is a classical example of a dipolar molecule (see canonical forms of 424 +0—O—0 and 0—O—0+). Fleming showed that the HOMO/LUMO orbitals of ozone interact with those of ethene (as shown in Figure 11.21).347... 

Ozone (O3) readily oxidizes unsaturated compounds. For example, with alkenes, ozone reacts to produce a malozonide... 

The mechanism of the ozonolysis reaction of alkenes has been investigated in the gas phase and solid state using matrix isolation spectroscopy. While alkene/ozone 7C-complexes and the primary ozonides are readily observed by IR und UV/vis spectroscopy, there is no direct spectroscopic evidence for the Criegee intermediate (carbonyl O oxide) in these reactions. However, these elusive species can be synthesized and characterized via the carbene/oxygen route. Comparison of experimental and calculated spectroscopic data allows for the prediction of the spectroscopic properties of carbonyl oxides which are not accessible by this method. 

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 main results of our investigations are (i) The first step in the alkene/ozone reaction is the formation of a 7i-complex with absorptions in the near UV or visible range, (ii) At 50-70 K this complex reacts to the primary ozonide (POZ) and, depending on substituents, traces of the secondary ozonide (SOZ). (iii) No carbonyl oxide was observed under any conditions used in our ozonolysis experiments, (iv) Some of the partially oxidized products formed are not in accordance with the Criegee mechanism and thus alternative mechanisms have to be considered. 

The reaction of ozone and alkenes is sufficiently fast that it can compete with other removal processes and provide sinks for both ozone and alkenes in the troposphere. While kinetic data for a series of alkene/ozone reactions have been reported, not much is known about details of the reaction mechanisms, the role that carbonyl O oxides play, and the role that free radicals play in these processes. Our laboratory experiments provide the spectroscopic data (both infrared and UV/visible) that are important for the spectroscopic identification of Criegee intermediates in the troposphere. In addition, we were able to characterize secondary partially oxidized products (aldehydes, peroxides etc.) that are produced during the gas-phase ozonolysis. These products might lead to a net increase of ozone, if oxygen atoms are formed during their decomposition. 

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. ... 

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