Aldehydes ozonides

Methylcyclopentene co-ozonolyzed with formaldehyde, acetyl cyanide, or benzoyl cyanide afforded only the normal 1,2,4-trioxolane (secondary ozonide, 88) by contrast, 1-methylcyclohexene co-ozonolyzed with formaldehyde or acetyl cyanide gave no such ozonide, but almost equal amounts of the aldehyde-ozonide 86 and the diozonide 87, as shown in Equation (8) and Table 10. 

Norbornene co-ozonized with formaldehyde, acetyl cyanide, or benzoyl cyanide gave similarly the aldehydic ozonide 91, which then on co-ozonolysis with vinyl acetate (i.e., the source of formaldehyde oxide) afforded a diozonide 92, as indicated in Scheme 27 and Table 12. 

Phenanthrene has also a reactive 9,10-double bond, in agreement with the Clar structure having two aromatic sextets and a C=C fixed double bond in the median ring. On co-ozonolysis with formaldehyde, acetyl cyanide, or benzoyl cyanide, phenanthrene reacted accordingly, affording an aldehydic ozonide 112, which in a separate co-ozonolysis with vinyl acetate that produced formaldehyde oxide (H2C-0-0) gave rise to a diozonide 113 (Scheme 35 and Table 14). 

The final polycyclic aromatic hydrocarbon that was investigated <2000EJ0335> is benzo[fixed double bond like phenanthrene. Its cross-ozonolysis with formaldehyde gave none of the normal ozonide 120, but mainly the aldehydic ozonide 117. At room temperature, a substantial amount of opening of the ozonide ring occurred with the formation of the acid aldehyde 121. Both products 117 and 121 could be stabilized by treatment with O-methylhydroxylamine, yielding products 118 and 122, respectively. The separate co-ozonolysis of compound 117 with vinyl acetate afforded the diozonide 119 (Scheme 37 and Table 16). The cross-ozonolysis with acetyl cyanide followed by treatment of the crude reaction mixture with O-methylhydroxylamine yielded the O-methyloxime of the cross-product. Cross-ozonolysis with benzoyl cyanide was not successful, and only the normal mono-ozonide 120 was formed. 

The basic functional groups— products from the rubbers ozonolysis were identified and quantitatively characterized by means of IR-spectros-copy and H-NMR spectroscopy. The aldehyde—ozonide ratio was 11 89 and 27 73 for E-BR and BR, respectively. In addition, epoxide groups were detected, only in the case of BR, their yield was about 10 per cent of that of the aldehydes. On polyisoprenes the ozonide—ketone—aldehyde ratio was 40 37 23 and 42 39 19 for E-IR and Z-IR, respectively. Besides the already-specified functional groups, epoxide groups were also detected, their yields being 8 and 7 per cent for E-IR and Z-IR, respectively, with respect to reacted ozone. In the case of 1,4-rrara-polychloroprene, the chloroanhydride group was found to be the basic carbonyl product. 

Aldehydes are easily oxidized to carboxylic acids under conditions of ozonide hydroly SIS When one wishes to isolate the aldehyde itself a reducing agent such as zinc is included during the hydrolysis step Zinc reduces the ozonide and reacts with any oxi dants present (excess ozone and hydrogen peroxide) to prevent them from oxidizing any aldehyde formed An alternative more modem technique follows ozone treatment of the alkene m methanol with reduction by dimethyl sulfide (CH3SCH3)... 

The dipolar ion can react in several ways according to the solvent and the stmcture of the olefin. In inert solvents, if the carbonyl compound is highly reactive (eg, an aldehyde), the dipolar ion can be added to the carbonyl fragment to give the normal ozonide or 1,2,4-trioxolane (7) for example, 1,1-and 1,2-dialkylethylenes react in this manner. Tri- or tetraalkyl-substituted olefins produce a smaH, if any, yield of an ozonide when the ozonolysis is... 

Commercially, pure ozonides generally are not isolated or handled because of the explosive nature of lower molecular weight species. Ozonides can be hydrolyzed or reduced (eg, by Zn/CH COOH) to aldehydes and/or ketones. Hydrolysis of the cycHc bisperoxide (8) gives similar products. Catalytic (Pt/excess H2) or hydride (eg, LiAlH reduction of (7) provides alcohols. Oxidation (O2, H2O2, peracids) leads to ketones and/or carboxyUc acids. Ozonides also can be catalyticaHy converted to amines by NH and H2. Reaction with an alcohol and anhydrous HCl gives carboxyUc esters. 

Unsaturated compounds undergo ozonization to initially produce highly unstable primary ozonides (15), ie, 1,2,3-trioxolanes, also known as molozonides, which rapidly spHt into carbonyl compounds (aldehydes and ketones) and 1,3-zwitterion (16) intermediates. The carbonyl compound-zwitterion pair then recombines to produce a thermally stable secondary ozonide (17), also known as a 1,2,4-trioxolane (44,64,125,161,162). 

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. 

This cleavage reaction is more often seen in structural analysis than in synthesis. The substitution pattern around a double 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 aldehyde products to be isolated without further oxidation. 

Ozone adds readily to unsaturated organie eompounds and ean eause unwanted eross-linking in rubbers and other polymers with residual unsaturation, thereby leading to brittleness and fraeture. Addition to alkenes yields ozonides whieh ean be reduetively eleaved by Zn/H20 (or I /MeOH, ete.) to yield aldehydes or ketones. This smooth reaetion, diseovered by C. D. Harries in 1903, has long been used to determine the position of double bonds in organie moleeules, e.g. ... 

Low -molecular-weight ozonides are explosive and are theretore not isolated. Instead, the ozonide is immediately treated with a reducing agent such as zinc metal in acetic acid to convert it to carbonyl compounds. The net result of the ozonolysis/reduction sequence is that the C=C bond is cleaved and oxygen becomes doubly bonded to each of the original alkene carbons. If an alkene with a letrasubstituted double bond is ozonized, two ketone fragments result if an alkene with a trisubstituted double bond is ozonized, one ketone and one aldehyde result and so on. 

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]. 

The levulinic aldehyde and acid obtained by Harries on hydrolyzing the ozonide of rubber demonstrated recurrence of the structure... 

Ozonoiysis is a reaction used with unsaturated hydrocarbons when preparing aldehydes and ketones, by reducing intermediate ozonide or acids by oxidation. The reducing agents used include hydrogen in the presence of palladium, and zinc in acid medium. 

The zwitterion (59) is thereby prevented from reacting with the ketone (58) to form the ozonide in the normal way, and both (58) and (60) may now be isolated and identified. In preparative ozonolysis it is important to decompose the ozonide (57a) by a suitable reductive process, as otherwise H202 is produced (on decomposition of the ozonide with H20, for example) which can further oxidise sensitive carbonyl compounds, e.g. aldehydes— carboxylic acids. 

The above pathway accounts satisfactorily for the main features of ozonolysis but requires modification in detail to account for the observed stereochemistry of the reaction. Thus while a trans- (or cis-) alkene is often found to lead to a mixture of cis- and trans-ozonides as might have been expected, the trans-alkene (55) leads only to the trans-ozonide (57). The latter example demands a high degree of stereoselectivity in both the decomposition of (54) to aldehyde + peroxyzwitterion and in their subsequent recombination to (57) a demand that is not implicit in the pathway as we have written it. 

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