Ethers, Peroxides and Ozonides

One instance in which structural changes result in a very considerable frequency shift of the C—O vibration is in compounds containing the group =C—O—. In the case of alcohols, such as phenol, a high-frequency shift towards 1250 cm is found as a 

Aryl ethers and others with the group =C—O-Cyclic ethers (1) Epoxy compounds 

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Following this work on NMR spectra of ozonides, there is an extensive paper by the Griesbaum group" where 35 ozonides (6-14 with different stereochemistries) have been studied. The widely different structures examined allowed the influences of structural features on "O NMR spectra of ozonides to be shown. Five structurally different types of ozonides can be recognized symmetrically tetrasubstituted (type 6), unsymmetrically tetrasubstituted (type 7), unsymmetrically tri- and tetrasubstituted (type 8), unsymmetrically disubstituted (type 9-13) and bicyclic ozonides (type 14). "O NMR chemical shifts of peroxidic and ethereal oxygens are collected in Table 3. All spectra were obtained at natural isotopic abundance, in benzene-dg solution mainly at 25 °C, although in some cases higher temperatures had to be used. These experimental conditions make for an easy comparison with the previously discussed data. 

The chemistry of primary ozonides is more varied if they are less highly alkylated than the primary ozonide of Figure 15.47. This is particularly true if the primary ozonide is unsym-metrical, like the one shown in Figure 15.48. This is because its decay may involve two different 1,3-dipolar cycloreversions. Both of them result in one carbonyl oxide and one carbonyl compound. If the reaction is carried out in methanol, the two carbonyl oxides can react with the solvent (as in Figure 15.47) whereby each of them affords a hydroperoxide (an ether peroxide analog). 

The ozonide is dissolved in 100 ml acetic acid, the methylene chloride removed carefully by suction, and the solution is slowly dropped into a mixture of 114 gm of 30% hydrogen peroxide, 5 ml of concentrated sulfuric acid, and 200 ml of water. Behind a shield, cautious heating is applied progressively. This phase needs careful attention because the reaction becomes vigorous and requires intermittent cooling. Refluxing is continued for 2 hr. After cooling to ice temperature, extraction with ether is performed. The ether layer is then extracted with a solution of sodium hydroxide. The latter is acidified, extracted with ether, dried, and distilled. The acid is collected at 200°-210°C. Redistillation yields a fraction bp 204°-207°C (752 mm), ng 1.4220, 0.9162 amide mp 99.5°-100°C p-bromophenacyl ester... 

Explosion. Use shielding when working with explosive classes such as acetylides, azides, ozonides, and peroxides. Peroxidizable substances such as ethers and alkenes, when stored for a long time, should be tested for peroxides before use. Only sparkless flammable storage refrigerators should be used in laboratories. 

When ozonolysis is done in alcoholic solvents, the carbonyl oxide fragmentation product can be trapped as an a-hydroperoxy ether.146 Recombination to the ozonide is then prevented, and the carbonyl compound formed in the fragmentation step can also be isolated. If the reaction mixture is treated with dimethyl sulfide, the hydroperoxide is reduced and the second carbonyl compound is also formed in good yield.147 This procedure prevents oxidation of the aldehyde by the peroxidic compounds present at the conclusion of ozonolysis. 

O NMR resonances for several 1,2,4-trioxolanes have been reported <91CC816> (Table 6). The ether and peroxide signals are very distinct proving the value of O NMR as an analytical tool for characterization of ozonides. It can be used to determine unequivocally whether a compound is an ozonide (peroxide 6 295-327 ppm) or tetroxane ((26), for example d = 256 ppm). This is often a competing product from the ozonolysis reaction of alkenes (Section 4.16.8.2). 

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. 

In the envelope conformation (A) the peroxide bond and the two carbon atoms are all coplanar (with the C-O-O-C dihedral angle being close to 0°) while the ethereal oxygen atom can be displaced by as much as 0.65 A to either side of this plane. In conformation B the peroxide bond straddles the plane of the remaining three atoms and this dihedral is around 50°. While conformation A is achiral, B has C.y symmetry. Usually ozonides crystallize in chiral space groups however, both enantiomorphic forms of B are usually encountered in the crystal lattice. Furthermore, disorder of the peroxide oxygen atoms over several occupancies is frequent, and in recent analyses, due mostly to improvement in the structure refinement algorithms, this disorder could be taken into account and suitably refined models could be built from the diffraction data. 

Rieche and Meister34 obtained bis(l-hydroxyethyl) peroxide (21) from 2 moles of acetaldehyde and 1 mole of hydrogen peroxide. Treatment of 21 with phosphorus pentoxide in ether followed by distillation gives 3,5-dimethyl-l,2,4-trioxolan (2,3-butylene ozonide) (22) as the distillate and dimeric butylene ozonide with a ten-membered ring structure (23) as the residue.23-34 Thermal decomposition of (23) at 60 to 80° in vacua yields polymeric ethylidene peroxide (25), 3,5,7-trimethyl-l,2,4,6-tetroxepan (mono-... 

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