Ozonization of alkenes

Ozonation of alkenes involves a cycloaddition-cycloreversion sequence. A synthetically significant differentiation of an alkene is shown below [246]. 

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 ... 

Ozonization of alkenes has been studied extensively for many years, but there still is disagreement about the mechanism (or mechanisms) involved because... 

A number of useful reactions for the preparation of aldehydes and ketones, such as ozonization of alkenes and hydration of alkynes, have been considered in previous chapters. These and other methods of preparation are summarized in Tables 16-7 and 16-8 at the end of the chapter. Only a few rather general methods that we have not discussed will be taken up here. 

Ozonation Thiourea can be used to reduce the a-alkoxyhydroperoxides obtained on ozonation of alkenes in methanol to aldehydes and the dimethyl acetal. 

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. 

Dry ozonization of alkenes and alkynes has also been explored. Sometimes these results resemble those of the homogeneous ozonization in an aptotic solvent. ... 

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. 

Applications of tetracyanoethylene have been reviewed recent information on molecular complexes, ozonization of alkenes and alkynes (the Criegee reaction), and reactions with ketones is included. Dicyanoketene has been prepared in situ from 2,5-diazido-3,6-dicyano- and 2,6-diazido-3,5-dicyano-l,... 

New Applications of Tetracyanoethylene (TCNE) in Organic Chemistry, A. J. Fatiadi (1986). This review with 501 references deals with reactions of tetracyanoethylene used in organic synthesis. Information on molecular complexes, ozonization of alkenes and acetylenes, dehydrogenation and tricyanovinylation, reactions of TCNE oxide, reactions with ketones and diketones, synthesis of heterocycles and cationic polymerizations are included in this survey. Some industrial and analytical applications are also discussed. 

The products formed by oxidative deavage of an alkene depend on the alkene structure and the agent used Acid fluorides are formed by ozonization of per-fluoroalkenes in tnfluoroacetic acid [3t5] (equation 28)... 

The addition of ozone to alkenes to form ozonides, and the subsequent decomposition of the latter to yield carbonyl compounds, has long been known ... 

The initial addition of ozone to alkenes to form molozonides (p. 193) can be regarded as a 1,3-dipolar addition, and many other such additions are of great importance in the preparation of five-membered heterocyclic systems. Thus we have already seen the... 

Ozone vigorously reacts with the double bonds of alkenes and quite rapidly with the iT-bonds of aromatic rings. Its reaction with the C—H bonds of hydrocarbons occurs relatively slowly and is accompanied by the formation of free R and HCV radicals followed by the decomposition of the unstable radical H03 (see Chapter 3). 

How the initial electrophilic addition of ozone to alkene takes place is not completely known, but it is thought that in the first step of addition the bond connecting the two carbons remains intact. 

An elegant one-pot reduction of ozonides consisting of the treatment of a crude product of ozonization of an alkene in methanol with dimethyl sulfide (36% molar excess) gives 62-97% yields of very pure aldehydes [244. ... 

The reaction of alkenes with ozone constitutes an important method of cleaving carbon-carbon double bonds.138 Application of low-temperature spectroscopic techniques has provided information about the rather unstable species that are intermediates in the ozonolysis process. These studies, along with isotope labeling results, have provided an understanding of the reaction mechanism.139 The two key intermediates in ozonolysis are the 1,2,3-trioxolane, or initial ozonide, and the 1,2,4-trioxolane, or ozonide. The first step of the reaction is a cycloaddition to give the 1,2,3-trioxolane. This is followed by a fragmentation and recombination to give the isomeric 1,2,4-trioxolane. The first step is a... 

Like the parent compound (1), alkyl- and/or aryl-substituted 1,2,3-trioxolanes can be generated only at low temperatures by the cycloaddition of ozone to alkenes. The concerted nature of the cycloaddition is reflected in the stereochemistry of the products observed aliphatic cis alkenes afford only cis 4,5-dialkyl-1,2,3-trioxolanes, and the analogous trans alkenes yield only trans 4,5-dialkyl-... 

One of the most important features of the ozonolysis reaction of alkenes is one in which ozone adds to the C=C bond to form a primary ozonide (1,2,3-trioxolane). The Criegee mechanism suggests that this unstable intermediate decomposes into a carbonyl compound and a carbonyl oxide that recombine to form a final isomeric ozonide (1,2,4-trioxolane). Direct spectroscopic evidence for a substituted carbonyl oxide has only recently been reported by Sander and coworkers for the NMR characterization of dimesityl carbonyl oxide. Kraka and coworkers have theoretically modeled dimesityl carbonyl oxide and confirmed the structural aspects reported by Sander and coworkers on the basis of NMR data. 

Schindler and coworkers verified the formation of hydroxyl radicals kinetically and further RRKM calculations by Cremer and coworkers placed the overall concept on a more quantitative basis by verifying the measured amount of OH radical. An extensive series of calculations on substituted alkenes placed this overall decomposition mechanism and the involvement of carbonyl oxides in the ozonolysis of alkenes on a firm theoretical basis. The prodnction of OH radicals in solution phase was also snggested on the basis of a series of DFT calculations . Interestingly, both experiment and theory support a concerted [4 4- 2] cycloaddition for the ozone-acetylene reaction rather than a nonconcerted reaction involving biradical intermediates . 

Grosjean, E., and D. Grosjean, Gas Phase Reaction of Alkenes with Ozone Formation Yields of Primary Carbonyls and Biradicals, Emiron. Sci. TechnoL, 31, 2421-2427 (1997). 

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