Concerted reactions ozonolysis

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 . 

Table 11.4 summarizes the reactions in which the electrophile and the nucleophile are linked in the same molecule (hydroboration, hydroxylation, ozonolysis). These additions occur in a concerted manner. The regiochemistry of the addition is such that the nucleophile attaches to the carbon that would be more stable as a carbocation and the addition occurs with syn stereochemistry. 

The conversion of the initial ozonide to a carbonyl compound and a carbonyl oxide can be considered a retro-l,3-cycloaddition, and the combination of the carbonyl oxide and the carbonyl compound is another 1,3-cycloaddition (Figure 11.76). Nonconcerted mechanisms are possible for each of the three steps in the Criegee ozonolysis mechanism, but Kuczkowski has summarized evidence suggesting that the reactions are concerted. 

The three most common alkene oxidation reactions are epoxidation, dihydrox-ylation, and ozonolysis. Epoxides are formed when an alkene is treated with a peroxyacid, such as mCPBA. Since both C-O bonds are formed in the same step (described as a concerted mechanism), the stereochemistry of the starting alkene is preserved in the product. 

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. 

The relative stability of six cyclically overlapping orbitals accounts in a simple way for several reactions that proceed readily by what has seemed to be a complicated, concerted movement of three electron pairs the Diels-Alder reaction (Section 14-8), osmium tetroxide addition to aUcenes (Section 12-11), and the first step of ozonolysis (Section 12-12). In all three processes, a transition state occurs with cyclic overlap of six electrons in tt orbitals (or orbitals with tt character). This electronic arrangement is similar to that in benzene and is energetically more favored than the alternative, sequential bond breaking and bond making. Such transition states are called aromatic. 

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