Ozone cycloaddition

The activation energies of ozone cycloaddition to the two double bonds of isoprene were found to be comparable (3.3—3.4 kcal mol ) by DFT and ab initio calculations <2002JA2692>. The reaction energies are between —47 and —48 kcal mol-1. 

The process of ozone cycloaddition (path 1) implies postulates similar to those discussed by Huisgen (12) in terms of a 1,3-dipolar cycloaddition. Although the extent of simultaneity in the formation of the two C—O bonds is an open question, it is assumed that the transition state closely resembles the final state—the primary ozonide—and that its final conversion to give the primary ozonide occurs rapidly. The ratedetermining step is thus the addition of ozone on the olefin. The electrophilic tendency of ozone, which is shown in several cases to play a domi-... 

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). 

It was not their reactivity but their chemical inertness that was the true surprise when diazirines were discovered in 1960. Thus they are in marked contrast to the known linear diazo compounds which are characterized by the multiplicity of their reactions. For example, cycloadditions were never observed with the diazirines. Especially surprising is the inertness of diazirines towards electrophiles. Strong oxidants used in their synthesis like dichromate, bromine, chlorine or hypochlorite are without action on diazirines. Diazirine formation may even proceed by oxidative dealkylation of a diaziridine nitrogen in (186) without destruction of the diazirine ring (75ZOR2221). The diazirine ring is inert towards ozone simple diazirines are decomposed only by more than 80% sulfuric acid (B-67MI50800). 

A well-known example for a 1,3-dipolar compound is ozone. The reaction of ozone with an olefin is a 1,3-dipolar cycloaddition (see ozonolysis). 

Ozone lives to do 1,3-dipolar cycloadditions. After the cycloaddition to give the C6 011 and C5 09... 

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-... 

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 . 

R. L. Kuczkowski, in Ozone and Carbonyl Oxides in 1,3-Dipolar Cycloaddition Chemistry, Vol. 2 (Ed. A. Padwa), Chap. 11, Wiley, New York, 1984. 

The reaction of singlet oxygen with conjugated double bonds usually is a 1,4-cycloaddition leading to formation of derivatives of the 1,2-diox -ene ring system. This can be achieved either by photooxidation or by reaction in the presence of triphenyl phosphite-ozone adduct (Section Vin.D.2), shown in equations 85 and 86 . ... 

Arylidene-,6-ionones, triplet oxygen cycloaddition, 199, 201 Aryl phosphites, ozone adducts, 732 Aryl-substituted olefins, selenide-catalyzed epoxidation, 384-5 Ascaridole... 

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

The ozonolysis of olefins may be analyzed as a sequence of two 1,3-dipolar cycloadditions initial electrophilic attack by ozone 18 to form the first intermediate, which decomposes into a carbonyl compound and a carbonyl oxide 14 followed by nucleophilic... 

On the basis of deuterium labeling, 1,3-dipolar cycloaddition to the bridge C—C bond to form a cyclic trioxide was suggested. Rearrangements similar to those in alkene ozonation yield the products. 

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