Ozonides cleavage

Ozonides are rarely isolated [75, 76, 77, 78, 79], These substances tend to decompose, sometimes violently, on heating and must, therefore, be handled with utmost safety precautions (safety goggles or face shield, protective shield, and work in the hood). In most instances, ozonides are worked up in the same solutions in which they have been prepared. Depending on the desired final products, ozonide cleavage is done by reductive or oxidative methods. Reductions of ozonides to aldehydes are performed by catalytic hydrogenation over palladium on carbon or other supports [80, 81, 82, S3], platinum oxide [84], or Raney nickel [S5] and often by reduction with zinc in acetic acid [72, 81, 86, 87], Other reducing agents are tri-phenylphosphine [SS], trimethyl phosphite [89], dimethyl sulfide (DMS) [90, 91, 92], and sodium iodide [93], Lithium aluminum hydride [94, 95] and sodium borohydride [95, 96] convert ozonides into alcohols. 

Addition compounds called ozonides are produced when alkenes react with ozone and reductive cleavage of these compounds is used extensively in preparative and diagnostic organic chemistry. 

This cleavage reaction is more often seen in structural analysis than in synthesis The substitution pattern around a dou ble bond is revealed by identifying the carbonyl containing compounds that make up the product Hydrolysis of the ozonide intermediate in the presence of zinc (reductive workup) permits aide hyde products to be isolated without further oxidation... 

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). 

Ozone cracking is a physicochemical phenomenon. Ozone attack on olefinic double bonds causes chain scission and the formation of decomposition products. The first step in the reaction is the formation of a relatively unstable primary ozonide, which cleaves to an aldehyde or ketone and a carbonyl. Subsequent recombination of the aldehyde and the carbonyl groups produces a second ozonide [58]. Cross-linking products may also be formed, especially with rubbers containing disubstituted carbon-carbon double bonds (e.g. butyl rubber, styrene-butadiene rubber), due to the attack of the carbonyl groups (produced by cleavage of primary ozonides) on the rubber carbon-carbon double bonds. 

It follows that tetrasubstituted alkenes do not normally give ozonides. However, they do give the normal cleavage products (ketones) by the other pathways. For the preparation of... 

Due to the retractive forces in stretched mbber, the aldehyde and zwitterion fragments are separated at the molecular-relaxation rate. Therefore, the ozonides and peroxides form at sites remote from the initial cleavage, and underlying mbber chains are exposed to ozone. These unstable ozonides and polymeric peroxides cleave to a variety of oxygenated products, such as acids, esters, ketones, and aldehydes, and also expose new mbber chains to the effects of ozone. The net result is that when mbber chains are cleaved, they retract in the direction of the stress and expose underlying unsaturation. Continuation of this process results in the formation of the characteristic ozone cracks. It should be noted that in the case of butadiene mbbers a small amount of cross-linking occurs during ozonation. This is considered to be due to the reaction between the biradical of the carbonyl oxide and the double bonds of the butadiene mbber [47]. 

This discussion of the structures of diene polymers would be incomplete without reference to the important contributions which have accrued from applications of the ozone degradation method. An important feature of the structure which lies beyond the province of spectral measurements, namely, the orientation of successive units in the chain, is amenable to elucidation by identification of the products of ozone cleavage. The early experiments of Harries on the determination of the structures of natural rubber, gutta-percha, and synthetic diene polymers through the use of this method are classics in polymer structure determination. On hydrolysis of the ozonide of natural rubber, perferably in the presence of hydrogen peroxide, carbon atoms which were doubly bonded prior to formation of the ozonide... 

Hydrolysis, or better, hydrogenolysis of the normal ozonide leads to the cleavage of the original carbon-carbon double bond with formation of carbonyl groups. The abnormal ozonide usually decomposes before it reaches room temperature and both the double bond and the adjacent carbon-carbon single bond are found to have cleaved. 

The reaction gave only the rearrangement products 333 and 334, and the side product 335, as expected from the reactivity of alkylidenecyclopropane derivatives (Scheme 49). Compound 333 might arise from the 0-0 bond cleavage followed by the rearrangement of a cyclopropyloxy cation to an oxoethyl cation (Scheme 49, path a). Spiro-hexanone 334 could arise from a different fragmentation of ozonide C-O bond and further cyclopropyloxy-cyclobutanone rearrangement (Scheme 49, path b). Oxirane 335 can eventually derive from the same path b or from other side processes [13b]. 

There is some information concerning the reaction of ozone with chemicals under aqueous conditions. The information available suggests that double-bond cleavage takes place, just as it does under nonaqueous conditions, except that ozonides are not formed. Instead, the zwitterionk intermediate reacts with water, producing an aldehyde and hydrogen peroxide. In addition to double-bond cleavage, a number of other oxidations are possible. Mudd et showed that the susceptibility of amino acids is in the order cysteine, tryptophan, methionine. 

Beside being a source of C-22 chemicals, erucic acid is a natural source of C-13 and C-9 chemicals made by cleavage at the site of the double bond. Ozonolysis of erucic acid followed by oxidative cleavage of the ozonide to form brassylic acid (BA) and pelargonic acid (Chart I) has been demonstrated on the pilot plant scale.[2,3] Yield of 99% pure brassylic acid was 72 to 82% of theoretical. 

In the ozonolysis of allylic compounds, varying amounts of products are formed that are due to cleavage of both the C=C double bond and the adjacent C—C single bond. These are the so called abnormal or anomolous products which arise from a fragmentation of the intermediate ozonide (59) (Scheme 10) <81TL1447>. 

Dichloroaluminium hydride in ether or sodium borohydride in TEA can lead to formation of ethers from ozonides by reductive cleavage of the two C—O bonds of the peroxide bridge (Equation (19)) <85JOC275>. 

Dimethylfuran has been chosen as a substrate for the photooxygenation reactions. This substrate yields the ozonide of 1,2-dimethyl cyclobutadiene which undergoes a facile cleavage by the alcoholic solvent to the corresponding 2-alkoxy-5-hydroperoxy-2,5-dimethylfuran derivative.104,105... 

One of the most common reactions for the complete cleavage of an exocyclic C-C double bond is ozonolysis. The reaction is typically performed at low temperature in dichloromethane, methanol or a mixture of both, followed by the usual reduction of the ozonide with dimethyl sulfide 15 17 80 82 i48,337 or thiourea83 to give excellent yields of the corresponding cyclobu-tanone. 

NOC1 and an alcohol 9-9 From ozonides 9-10 Oxidative cleavage of enol ethers 9-13 Reaction between carboxylic acids and lead tetraacetate 9-18 Oxidation of ethers 9-22 Oxidation of primary alcohols or aldehydes... 

Ozonation. Alkenes react readily with ozone to give oxidized compounds via cleavage of the n or the direct product of the reaction, are rarely isolated but are transformed in situ into synthetically useful products. Depending on the solvent used and the actual workup of the reaction mixture different products may be isolated.582-584,592 Although its use is inconvenient, ozone is widely employed in selective syntheses. A classical use of ozone is in the structure determination of natural compounds and polymers. 

Ozonolysis of cyclic olefins in the presence of carbonyl compounds gives the corresponding cross-ozonides.1329 In the ozonation of 1,2,4,5-tetramethyl-1,4-cyclohexadiene, oxidative dehydrogenation (formation of 1,2,4,5-tetramethylben-zene) was found to compete with oxidative cleavage because of steric hindrance.1330 Secondary ozonides (the 76 1,2,4-trioxolanes) are formed in high yields in the gas-phase, low-temperature ozonation of terminal and disubstituted alkenes.1331... 

Substrates suitable for oxidative conversion into carbonyl compounds are alkenes, primary or secondary alcohols, and benzyl halides. Polystyrene-bound alkenes have been converted into aldehydes (with the loss of one carbon atom) by ozonolysis followed by reductive cleavage of the intermediate ozonide (Entry 1, Table 12.3). 

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