Oxidative Cleavage of an Alkene

There are many reagents that will add across an alkene and completely cleave the C=C bond. In this section, we will learn about one such reaction, called ozonolysis. Consider the following example  

Notice that the C=C bond is completely split apart to form two C=0 double bonds. Therefore, issues of stereochemistry or regiochemistry become irrelevant. In order to understand how this reaction occurs, we must hrst explore the reagents. 

Ozone is a compound with the following resonance structures  

Ozone is formed primarily in the upper atmosphere, where oxygen gas (O2) is bombarded with ultraviolet light. 

As we did in the previous sections, we will only explore the hrst step of the mechanism  

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Oxidative cleavage of an alkene with Klvln04 gives a carboxylic acid if the alkene has at least one vinylic hydrogen (Section 7.9). 

The products formed by oxidative cleavage of an alkene depend on the alkene structure and the agent used Acid fluorides are formed by ozomzation of per-fluoroalkenes in trifluoroacetic acid [36] (equation 28)... 

Oxidative cleavage of an alkene breaks both the a and n bonds of the double bond to form two carbonyl groups. Depending on the number of R groups bonded to the double bond, oxidative cleavage yields either ketones or aldehydes. 

Oxidative cleavage of an alkene with ozone leads to an ozonide. Reductive workup with dimethyl sulfoxide or zinc and acetic acid gives ketones and/or aldehydes. Oxidative workup with hydrogen peroxide gives ketones and/or carboxylic acids. Oxidative cleavage of 1,2-diols with periodic acid or with lead tetraacetate gives aldehydes or ketones. 

Cyclopentadiene Diels-Alder adducLs such as 11 are highly strained. Huw M.L. Davies of the State University of New York, Buffalo, has established (J. Am. Chem. Soc. 2004,126,2692) an important strategic connection between Diels-Alder adducts such as 13 and the enantiomerically-enriched cycloheptane derivative 14. Oxidative cleavage of the alkene of 14 would lead to a cyclohexane derivative that would be difficult to access by other means. The same adduct 14 (albeit in racemic form) is also available directly by AIC1,-mediated addition of mcthacrolein 15 to cyclopentadiene. 

Funk and Abelman used the Ireland-Claisen rearrangement in a formal synthesis of the tetracyclic terpenoid quadrone (Scheme 4.140) [136]. Treatment of the lactone under typical conditions resulted in facile rearrangement at or below room temperature to give the bicyclic acid after sUyl ester hydrolysis. Oxidative cleavage of the alkenes then intercepted an intermediate previously employed by Schles-singer in a total synthesis of quadrone. 

An important source of cyclic dicarboxylic acids involves oxidative cleavage of bicyclic alkenes. cis-Cyclopentane dicarboxylic acid was prepared by ozonolysis of norbomene and then converted to its dimethyl ester (7.36). Kinetic resolution of... 

The carbon-carbon double bond of an alkene 8 can be cleaved oxidatively, by a dihydroxylation reaction-glycol cleavage sequence ... 

The initial step of the coupling reaction is the binding of the carbonyl substrate to the titanium surface, and the transfer of an electron to the carbonyl group. The carbonyl group is reduced to a radical species 3, and the titanium is oxidized. Two such ketyl radicals can dimerize to form a pinacolate-like intermediate 4, that is coordinated to titanium. Cleavage of the C—O bonds leads to formation of an alkene 2 and a titanium oxide 5 ... 

Alkynes, like alkenes, can be cleaved by reaction with powerful oxidizing agents such as ozone or KMnC, although the reaction is of little value and we mention it only for completeness. A triple bond is generally less reactive than a double bond and yields of cleavage products are sometimes low. The products obtained from cleavage of an internal alkyne are carboxylic acids from a terminal alkyne, CO2 is formed as one product. 

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