Ozonides, formation and

SCHEME 16. Criegee s mechanism of ozonide formation and ozonolys... 

The lower mobility of the pol5mier parts impedes the further ozonide formation and causes the zwitterion to leave the cage and pass into the volume, which in turn accelerates the degradation process. The latter is associated with either its monomolecular decomposition or its interaction with low molecular components in the reaction mixture. The efficiency of degradation is determined by the C=C bonds location in the macromolecule, for example, at C=C bond location from the macromolecule center to its end, is in the range from 2 to 1. 

The reactions describing the ozonides formation and the ozone interaction with ROOH are included in the scheme pointed above ... 

For analysis of dienes and polyenes via oxidations one has to distinguish between the formation of an oxidized product of the target molecule (epoxide, peroxide, ozonide etc.) and the oxidative fragmentation of the molecule as in the case of ozonolysis30. Both... 

Thermal decomposition of allylbenzene ozonide (58) at 37°C in the liquid phase gave toluene, bibenzyl, phenylacetaldehyde, formic acid, (benzyloxymethyl)formate, and benzyl formate as products. In chlorinated solvents, benzyl chloride is also formed and in the presence of a radical quench such as 1-butanethiol, the product distribution changes. Electron spin resonance (ESR) signals are observed in the presence of spin traps, adding to the evidence that suggests radicals are involved in the decomposition mechanism (Scheme 9) <89JA5839>. 

Diaryl- and tetraaryl-substituted furans such as tetraphenylfuran (384) for example, yield generally cw-diaroylethylenes such as 380, probably via intermediate ozonide formation.257 Secondary reactions seem to depend very much on the nature of the solvents. Thus, cis-dibenzoylstilbene (380) has been observed by direct photooxygenation of 384 in CS2 as well as by methylene blue-sensitized photooxygenation of 384 in methanol.257,258 However, when the latter reaction was carried out in acetone, epoxide 386 and the enolbenzoate 387 were obtained.258... 

This indicates no evidence of stereorandomization about the carbon—carbon double bond. It means that 74 cleaves in a concerted fashion to yield syn and anti zwitter-ions produced in different amounts depending on the alkene geometry. The stereo-isomeric zwitterions remain attached to each other like an ion pair and recombine stereoselectively with aldehydes. The subtle nature of ozonation, however, may be illustrated by the observation that ozonide formation may be nonconcerted under... 

In studies of the reactions of 1.1 -difluoroethene, trifluoroethene, and perfluoroethene, the mechanism of ozonide, epoxide, and cyclopropane formation have been studied.84 Unusually stable ozonides 46A and B are prepared by treating mono- and bis(fluoroalkyl)cthenes with ozone85 at room temperature or on heating the substrate to which ozone is introduced. The rates are approximately 1()10 slower than that for the nonfluorinated analogs. Mono-perfluoroalkylated ethene seems to react several times more rapidly than the bis-perfluoroalkylated ethene.85... 

The study was continued with diastereoisomeric vinyl ethers. The ( )-isomer afforded on ozonolysis at —70 °C in ether a ry -carbonyl oxide, and with or without trifluoroacetophenone only the intramolecular ozonide 167, while the (Z)-isomer led to an anti-carbonyl oxide and to co-ozonolysis in the presence of trifluoroacetophenone, because the tf //-carbonyl oxide cannot adopt a suitable conformation for the intramolecular ozonide formation (Scheme 56) < 1996JOC5953, 1998JOC5617>. 

This chapter deals with the formation and behavior of peroxides in which the 0—0 group forms part of a ring. The most important of these heterocycles are peroxides of carbonyl compounds, which may also contain two or three peroxy groups in the same ring ozonides, which are also peroxides of carbonyl compounds, i.e., peroxidic acetals and endoperoxides, as cyclic dialkyl peroxides. 

In particular the autoxidation of benzaldehyde was investigated. Its choice as the initial subject for study was unfortunate, as the use of an impure sample of perbenzoic acid for determination of its absorption spectrum, not previously recorded, led to erroneous conclusions. These were later rectified after taking new measurements on a pure crystalline sample of the peracid (12). For the present study the two main bands at 1728 to 1730 and at 1270 cm.- are to be borne in mind. These bands made possible a demonstration of the acceleration of the autoxidation due to ozone and the influence of such acceleration in ozonide formation. Three spectral series (Figures 1, 2, and 3), obtained in collaboration with E. Dallwigk, are discussed below. 

When the oxides M205 are supported on other oxides or materials, surface M species can become either acidic or redox centers.670,694 The supported oxide often shows different catalytic activity from its bulk form. The formation and location of surface oxo species are controlled usually by the substrate s surface hydroxyl chemistry. For example, Ta205 supported on silica via impregnation exhibited a tetrahedral (=Si—0)3Ta=0 species which was photoactive.695 This center showed catalytic photoactivity in the oxidation of CO. The Ta=0 bond was activated to an exciton Ta4+-C)- which reacted with 02 to form a T-type ozonide [Ta-0-02]-. This in turn reacted with CO forming the intermediate product [03-C0]. ... 

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