Ozonization of olefin

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... 

The chemistry of the ozonization of olefins has been reviewed (I, 2, 5). Although the exact details of the mechanism(s) have not been elucidated, the Criegee zwitterion mechanism plays an important role in olefin ozonization. According to this mechanism, ozonization occurs in the manner shown in Figure 2. 

Complexes and Radicals Produced during Ozonation of Olefins... 

Tn previous work it has been shown that a competition exists during - ozonation of olefins between ozonolysis and epoxide formation (I). As steric hindrance increases around the double bond, the yield of epoxide or subsequent rearrangement products increases. This is illustrated with both old (1) and new examples in Table I for purely aliphatic olefins and in Table II for aryl substituted ethylenes. It was suggested that the initial attack of ozone on an olefinic double bond involves w (pi) complex formation for which there were two fates (a) entrance into 1,3-dipolar cycloaddition (to a 1,2,3-trioxolane adduct), resulting in ozonolysis products (b) conversion to a o- (sigma) complex followed by loss of molecular oxygen and epoxide formation (Scheme 1). As the bulk... 

Pi complexes have frequently been proposed in initial ozone—olefin interactions (I, 8, 9,10). To our knowledge, however, the ones reported here are the first ever observed and characterized. The fact that complexes but no radicals were observed during ozonation of the ether and ester of trimesitylvinyl alcohol indicates that the hydroxy proton was essential to dissociation of the complexes to radicals. Further discussion of the significance of these complexes to ozonation of olefins, as well as a discussion of the ir systems involved, will be given elsewhere. 

In [17], two approaches were taken to assess the role of each reaction pathway quantum-chemical calculations and parabolic simulation of the reaction of addition (semiempirical method of intercrossing parabolas, MIP) [34-36]. Using these approaches in combination, the authors could evaluate independently the reaction rate constants for each pathway and compare their contributions to the total ozonation of olefins of different structures. The comparison results are listed in Table 8. We note a good agreement between the calculated and experimental constants values. 

Thus, reactions of carbonyl compounds with hydrogen peroxide and acids lead to products similar to those obtained by the ozonization of olefins (Section III), which also yields 1,2,4-trioxolans and 1,2,4,5-tetroxans. The similarity of the two reactions is understandable, since the intermediates )C+—OO- in the ozonization27a and )C+—OOH in the hydrogen peroxide reactions are related as a conjugate base-acid pair. A number of cyclic peroxides of structure 7 are prepared from bis(hydroperoxy)dialkyl peroxides (5) by reaction with lead(IV) acetate, as described by Criegee et al.la This reaction is also thought to involve a carbonium ion intermediate,31 which reacts with the second OOH group. 

The ozonization of olefins yields not only the ozonides (which were first isolated by Harries,82 and whose structures were subsequently elucidated by Staudinger83 and Rieche),2,23 but also other cyclic peroxides. The formation of these products is explained by the ozonization mechanism proposed by R. Criegee64 In 1958, Bailey8 published a review of ozonization reactions in general. 

Olefins with hindered double bonds may be transformed stereospecifically to oxiranes by treatment with ozone. The epoxidation of propylene has been achieved with alkoxyalkyl-hydroperoxides obtained by the ozonization of olefins in the presence of alcohol.The yield depends on whether the alcohol is a primary, secondary, or tertiary one. The low-temperature (—70°) epoxidation of olefins with a yield of about 30% has been performed with electrophilic intermediates produced in the course of the ozonization of alkynes these intermediates are probably five-membered cyclic trioxides. This epoxidation is almost totally stereospecific. 

The use of tetracyanoethylene as buffer in the ozonization of olefins is described in the section on this reagent. 

Yang and Carr,19 it is questionable whether ozonization of olefins constitutes a feasible method of preparing 1,2-dioxetanes. 

As we have pointed out already, the claim17,19 that dioxetanes are formed in the ozonization of olefins has been refuted.18 Recently, however, the chemiluminescence observed in the gas phase ozonization of 2-butene, which leads to complex cleavage products, has been explained in terms of intermediary 1,2-dioxetanes.3 ... 

---