Ozonides to aldehydes

The reducing properties of organic compounds of sulfur, such as methyl mercaptan, show up in partial reduction of trigeminal to geminal dihalides [243]. Dimethyl sulfide reduces hydroperoxides to alcohols and ozonides to aldehydes while being converted to dimethyl sulfoxide [244]. 

The most common procedure is ozonolysis at -78 °C (P.S. Bailey, 1978) in methanol or methylene chloride in the presence of dimethyl sulfide or pyridine, which reduce the intermediate ozonides to aldehydes. Unsubstituted cydohexene derivatives give 1,6-dialdehydes, enol ethers or esters yield carboxylic acid derivatives. Oxygen-substituted C—C bonds in cyclohexene derivatives, which may also be obtained by Birch reduction of alkoxyarenes (see p. 103f.), are often more rapidly oxidized than non-substituted bonds (E.J. Corey, 1968 D G. Stork, 1968 A,B). Catechol derivatives may also be directly cleaved to afford conjugated hexa-dienedioic acid derivatives (R.B. Woodward, 1963). Highly regioselective cleavage of the more electron-rich double bond is achieved in the ozonization of dienes (W. KnOll, 1975). 

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. 

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. 

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. 

Trioxolanes are key intermediates in the ozonolysis of alkenes (Section 4.16.8.2). This reaction is of considerable importance in synthetic chemistry where ozonide intermediates are often reduced (to aldehydes or alcohols) or oxidized (to carboxylic acids) in situ. Advantage has been taken of the stability of certain derivatives to investigate selective chemical reactions. An example of selective reduction is shown in Scheme 47 <91TL6454> with other uses of the 1,2,4-trioxolane ring as a masked aldehyde or ester referred to in Section 4.16.5.2.1. 

Acid-catalyzed condensation of bicyclic ozonides with aldehydes and ketones, in the presence of hydrogen peroxide, leads to the formation of bicyclic peroxide analogs of acetals in low yields, as shown in equation 91 for the condensation of the ozonide of 1-phenylcyclopentene (266) with benzaldehyde. The structure of compound 267, with the preferred ring conformation as shown, was determined by XRD analysis . The same method served to demonstrate that the condensation compound 16c is unique, with structure 254 . 

Procedures. Chromatographic Purification of Ozonization Products. Ozonization products from ethyl 10-undecenoate and 1-octene were chromatographed on silica gel columns (Baker) and eluted with 15 or 25% ether in petroleum ether (b.p., 30°-60°). Fractions were examined by thin-layer chromatography (TLC) on silica gel G Chroma-gram sheet eluted with 40% ether in petroleum ether. For development of ozonide and peroxide spots, 3% KI in 1% aqueous acetic acid spray was better than iodine. The spots (of iodine) faded, but a permanent record was made by Xerox copying. Color of die spots varied from light brown (ozonide) to purple-brown (hydroperoxide), and the rate of development of this color was related to structure (diperoxide > hydroperoxide > ozonide). 2,4-Dinitrophenylhydrazine spray revealed aldehyde spots and also reacted with ozonides and hydroperoxides. Fractions were evaporated at room temperature or below in a rotary evaporator. 

Le Carrer-Le Goff and co-workers have reported an expeditious synthetic route to a-substituted statines by the well-defined metal-mediated allylation of V-protected a-amino aldehydes followed by the ozonolysis of the double bond (Scheme 11.3).16 Ozonolysis of the double bond in 13, carried out in dichloromethane/methanol, at -78°C in the presence of sodium hydroxide permits the intermediate ozonide to be converted directly to the statine methyl ester 14.17... 

Work on the pyridine-modified ozonization of tetramethylethylene showed that pyridine oxide is not-a product of ozonization (8). Most of the pyridine (— 90% ) remains unchanged during double bond cleavage. Only one mole of acetone, rather than two, is formed for each mole of olefin oxidized. Other work with a disubstituted olefin, trans-4-octene, showed that ozonides are formed in the reaction so that the reaction of pyridine with ozonide to form acid and aldehyde cannot occur (9). An NMR study of trans-4-octene ozonolysis in the presence of pyridine using 1,2-dichloroethane as the solvent shows that aldehyde and hydroxyl-containing material (carboxylic acid, peracid, and other OH species) are formed directly during double bond cleavage. 

This compound—known as an ozonide—is the first stable product of the reaction with ozone. It is the culmination of two 1,3-dipolar cycloadditions and one reverse 1,3-dipolar cycloaddition. It is still not that stable and is quite explosive, so for the reaction to be of any use it needs decomposing. The way this is usually done is with dimethylsulfide, which attacks the ozonide to give DMSO and two molecules of aldehyde. 

Sidewall-functionalized carbon nanotubes were prepared by Wong [4] using ozone with an oxygen carrier then postreacted with sodium hydride or DMS to decompose primary ozonides to form aldehydes and ketones. 

A much more frequently used reaction is the cleavage of unsaturated compounds to aldehydes (equations 98 and 99). Alkenes and cycloalkenes that possess one or two hydrogens at the double bonds are oxidized by ozone to ozonides, which have to be reduced to prevent a subsequent oxidation to acids by the excess oxygen atom. Reductions are carried out, usually without isolation of the ozonides, by catalytic hydrogenation over palladium catalyst [80, 81,1106] or Raney nickel [55] or by treatment with... 

The intermediate ozonide can then be reduced to aldehydes/ketones or be oxidised to carboxylic acids. 

Tertiary amines. Using a secondary amine to decompose an ozonide derived from 1-alkene effects its alkylation. The amine initiates an eliminative fragmentation of the ozonide to generate an aldehyde and dialkylammonium formate. Schiff base formation from the aldehyde and another molecule of the amine is then followed by reduction by the formate ion. 

Ozone is being promoted for use in the conversion of tertiary amines to amine oxides, of a-pinene to pinonic and pinic acids, of olefins to ozonides and these in turn to aldehydes and oxy-peroxides, of sulfides to sulfoxides and sulfones, and of various other organic substances. Such reactions are of practical interest in drug manufacture, and several drug companies are now commercially using ozone in their manufacturing operations, specifically in oxidation of sterols in hormone syntheses. 

C.ii. Ozonides. Ozonolysis is a powerful method for cleaving alkenes to carbonyl products (sec. 3.7.B). Ozonolysis of alkenes initially generates an ozonide that can be reduced by a variety of reagents. Dimethyl sulfide or zinc in acetic acid are the most common reagents for the reduction of an ozonide to an aldehyde or ketone. Reduction of the ozonide with the more powerful lithium aluminum hydride, however, gives direct conversion to the alcohol. 

In the spectra of the ozonized polybutadienes the appearance of bands at 1,111 and 1,735 cm, which are characteristic for ozonide and aldehyde groups, respectively, is observed [22, 31], It was found that the integral intensity of ozonide peak in the l,4-cz5-polybutadiene Emulsion Butadiene Rubber (E-BR) spectrum is greater and that of the aldehyde is considerably smaller in comparison with the respective peaks in the Diene 35 NFA (BR) spectrum at one and the same ozone conversion degree of the double bonds. The differences in the aldehyde yields indicate that, according to IR-analysis, the degradation efficiency of the BR solutions is greater. 

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. 

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