Mechanisms concerted, ozone reactions

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

The initial steps of the reaction between chloro derivatives of ethylene and O3 were studied using UQCISD, UB3LYP, UMP2, and MRMP2 methods with 6-3H-G and aug-cc-PVDZ basis sets. Two possible mechanisms, concerted and stepwise additions of ozone to the double bond, were considered. The energies and geometries of the transition states, rate constants, and kinetic parameters were calculated. ... 

We have studied the kinetics of this reaction in CFCI3 at -4 C and have obtained the kinetic isotope effect and an approximate activation energy. Using these data, both an electron-transfer reaction and a concerted 1,3-dipolar insertion reaction can be eliminated as mechanistic possibilities for the peroxide-ozone reaction. We conclude the mechanism involves a molecule-assisted homolysis ... 

Alkylaziridines can be stereospecifically deaminated to alkenes by reaction with m-chioroperbenzoic acid (70AG(E)374). The reaction and work-up are carried out in the dark to avoid isomerization of the cw-alkene, and the mechanism is thought to involve an initial oxidation to an amine oxide followed by a concerted elimination. Aziridine oxides have been generated by treating aziridines with ozone at low temperatures (71JA4082). Two... 

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 . 

The ozonization of some a,j8-unsaturated acids, aldehydes, and ketones proceeds abnormally in the presence of water, the C—C bond adjacent to the carbonyl group being cleaved as well as the C—C double bond.8 Barton and Seoane123 formulated this reaction as formation of the ozonide, followed by decomposition by a concerted mechanism, in which water plays an important part see also Refs. 124 and 125. 

The arrow formalism lets us map out the formation of the ozonide. The 7t bond of the alkene reacts with ozone to produce the five-membered ring. These primary ozonides are extremely difficult to handle, and it took great experimental skill on the part of Rudolf Criegee (1902-1975) and his co-workers at the University of Karlsruhe in Germany to isolate the ozonide produced in the reaction between ozone and frarar-di-fcrf-butylethylene. A concerted mechanism predicts that the stereochemical relationship of the alkyl groups in the original alkene will be preserved in the ozonide, and this is what happens (Fig. 10.51). 

The primary step of acetylene ozonation has been studied by BILYP, PBEO, CASSCF, MRMP2, and CCSD methods using the 6-31-l-G, aug-cc-p VDZ, cc-pVTZ, and aug-cc-p VTZ basis sets. The study confirms that the intermediate complex, the transition states of the concerted addition (Criegee s mechanism) and unconcerted addition (DeMore s mechanism) are involved in this reaction. Although Criegee s mechanism is dominant, the DeMore mechanism also contributes. ... 

The mechanism of ozonolysis proceeds through initial electrophilic addition of ozone to the double bond, a transformation that yields the so-called molozonide. In this reaction, as in several others already presented, six electrons move in concerted fashion in a cyclic transition state. The molozonide is unstable and breaks apart into a carbonyl fragment and a carbonyl oxide fragment through another cychc six-electron rearrangement. Recombination of the two fragments as shown yields the ozonide. 

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