Ozonolysis steric effects

Bicyclic 1,2,4-trioxolanes can also be formed by intramolecular trapping of a carbonyl oxide with an ester as in the ozonolysis of (167) (Equation (30)). With (167), the steric effect of the gem-d.i-t-... 

Similar conclusions are drawn by Cvetanovic et al. from their results of ozonization of alkenes in the gas phase (9) and in CC14 solution (10). The rate constants for the ozonolysis of chloroethylenes and allyl chloride, in CC14 solution, indicate (11) that the rate of ozone attack decreases rapidly as the number of chlorine atoms in the olefin molecules is increased. However, to explain the departures from simple correlations, in some cases steric effects and the dipolar character of ozone had to be invoked (10). The relevance of the dipolar character of ozone in its reactions has also been stressed by Huisgen (12), who provided evidence that the ozone—olefin reaction is usually a 1,3-dipolar cycloaddition. 

According to Refs. [2, 10] two isomeric forms of 1,2,4-trioxolanes exist. The ratio between them is a function of the double bond stereochemistry, steric effect of the substituents, and the conditions of ozonolysis. It was found out only on the low molecular weight alkenes [19, 21]. The H-NMR spectroscopy is the most powerful method for determination of the cis/tram ratio of ozonides (in the case of pol5miers it is practically the only one method that can be applied). The measuring is based on the differences in the chemical shifts of the methine protons of the two isomers the respective signal of the cis form appears in lower field as compared to the trans one [19, 21]. 

A new structure-activity relationship, Xjj =yS+l, where y is a negative constant, S is the total steric effect, and 4 is the total inductive effect, correlated strongly with available measurements of ozonolysis. New rate coefficients were measured for ozonolysis of a number of unsaturated heteroatomic compounds and it has been emphasized that the inductive effect rather than the steric effect is important in predicting their reactivity %, the inductive effect index, was compared with the Taft a constant and rates of reaction of hydroxyl radical with a given species it correlated strongly in both cases (which should be unaffected by steric factors) suggesting a universal response by olefinic species towards electrophilic addition. ... 

The kinetics and product distributions of ozonolysis of vinylcyclohexane and methylene cyclohexane have been investigated.162 Steric hindrance of the cyclic substituent largely offsets electronic effects hi determining the rate of reaction. The main products of ozonation of catechols were quinones, while catechol acetals gave rise to compounds with an opened benzene ring.163 The ozonolysis of azoles such as pyrroles, oxazoles, and imidazoles has been reviewed.164... 

Ozonolyses of various dienes containing an enol carbonate afford products in which the isolated double bonds rather than the enol ones are cleaved. Rate measurements for ozonolysis of a variety of cyclohexene derivatives indicate that the enol carbonate function has a very low reactivity towards ozone. NMR and computational data indicate that the enol carbonate is not particularly electron deficient in its double bond. It is therefore suggested that the retardation is instead caused by ozone association with the carbonate group. The steric and electronic effects of alkyl and aryl substituents on the rate of ozonation of cyclic hydrocarbons have been studied. ... 

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