Mechanism alkene ozonolysis

Mechanism of Ozonolysis (Criegee mechanism) The initial step of the reaction involves a 1,3-dipolar cycloaddition of ozone to the alkene leading to the formation of the primary ozonide (molozonide or 1,2,3-trioxolane), which decomposes to give a carbonyl oxide and a carbonyl compound. The carbonyl oxides are similar to ozone in being 1,3-dipolar compounds and undergo 1,3-dipolar cycloaddition to the carbonyl compound with the reverse regio-chemistry, leading to a relatively stable secondary ozonide (1,2,4-trioxolane) (Scheme 5.47). 

As noted, the ozonide is not isolated, but a second chemical step is performed in the same fiask (such as treatment with hydrogen peroxide). Two products result from the ozonolysis of 2,3-dimethyl-2-butene 2-propanone (acetone, 2) and a second molecule of acetone. This statement is phrased this way because this is a symmetrical alkene, but an unsymmetrical alkene will give two different ketones. In effect, the C=C unit is cleaved and each carbon is oxidized to a C=0 unit. The mechanism of ozonolysis was described in Chapter 10. 

By use of matrix isolation infrared spectroscopy, it has been shown that the mechanism of ozonolysis of (Z)-3-methyl-2-pentene (mp) is similar to that for ozonolysis of simple alkenes. Indirect evidence for formation of one or both possible Criegee intermediates is presented. Eight fundamental vibrations of the c/s -isomer of the primary ozonide of mp are observed. UV irradiation led to the product arising from O atom addition to mp. Second-order rate coefficients for the ozonolysis of -butyl methacrylate, ethyl cro-tonate and vinyl propionate under atmospheric pressure have been determined and the effects of substituent groups on the overall rate coefficients have been analysed. Free energy relationships are presented and atmospheric lifetimes are discussed. ... 

The mechanisms of ozonolysis of volatile organic compounds such as alkenes and dienes are discussed and the products output is determined by matrix isolation FTIR spectroscopy [18],... 

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... 

With advances in technology, theoretical methods have gained importance as a powerful tool. 1,2,4-Trioxolanes have attracted the most interest, undoubtedly because of their pivotal role in the mechanism of the ozonolysis of alkenes. Apart from 1,2,4-trithiolane (2) nothing has been reported for the other five-membered rings (3)-(6) or their derivatives. 

The ozonolysis of alkenes has been comprehensively covered in several excellent reviews (see Section 4.16.1) and will therefore not be discussed in detail here. It is pertinent, given the importance of 1,2,4-trioxolane synthesis, to highlight the key points of the ozonolysis reaction mechanism and several other developments. 

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 . 

Carbon monoxide has been used to scavenge OH fonned from the ozonolysis of alkenes. The CO2 tints generated was detected by FTIR spectroscopy and the "OH yields for individual reactions were calculated.239 The significance of the OH-induced intramolecular transformation of glutathione thiyl radicals to a-aminoalkyl radicals has been discussed with respect to its biological implications.240 The kinetics and mechanism of the process indicated that it could be a significant pathway for the selfremoval of glutathione thiyl radicals in vivo. 

Depending on the alkene cation radical nature, open-chain oxygenation and epoxida-tion take place as well as the formation of other trivial ozonolysis products. Alkylaromatic compounds are also oxidized by ozone via the ion radical mechanism. Ethylbenzene, for example, undergoes ozone attack on the ring (80%) and on the alkyl group (20%). According to kinetic studies, the ozone consumption obeys the chain law (Galstyan et al. 2001). 

A thorough theoretical analysis of the Criegee mechanism for the ozonolysis of cis- and trans-symmetrical alkenes Rl IG—Cl IR has been performed by semiempirical AMI calculations <1997JOC2757>. The experimentally observed stereoselectivity for bulky groups (e.g., R = Bu ) is that from the m-alkene a cisltrans ratio of 7 3 is encountered while from a trans-alkene a 3 7 ratio for the cisltrans secondary ozonides resulted. With smaller R groups (e.g., R = Me) both as- and trans-alkenes lead preferentially to the trans secondary ozonide (Scheme 1). 

The mechanism proposed by Criegee for the ozonolysis of alkenes <1975AGE745> considers an initial it-complex between the alkene and ozone which decays via a 1,3-dipolar cycloaddition into a 1,2,3-trioxolane or primary ozonide, known also as the molozonide . These compounds are unstable, even at low temperatures, and due to cycloreversion decompose into a carbonyl fragment and a CO, which may recombine by another 1,3-dipolar cycloaddition step to form the more stable 1,2,4-trioxolane ( secondary ozonide or final ozonide (see also Section 6.06.2). 

We shall conclude this Section with an example of solvent cage effects of ion-molecule recombination reactions as found in the ozonolysis of alkenes in nonpolar solvents [739, 740]. According to the Criegee mechanism [424], unsymmetrically... 

Ethene, like other alkenes, reacts also with ozone in the atmosphere. The older work on ozone reactions has been reviewed by Leighton (1961) and by Bufalini and Altshuller (1965). More recent work has done much to clarify the principal reaction mechanisms involved. Criegee (1957, 1962, 1975), who had studied the ozonolysis of alkenes in solution, suggested that ozone adds to the C=C double bond, forming an unstable intermediate, which then decomposes toward a carbonyl compound and a zwitterion fragment, for example ... 

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