Epoxidation with ozone

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into lTans-l,2-diols by acid-catalyzed epoxide hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with 0s04. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. 

In a simulated atmosphere, direct epoxidation by ozone led to the formation of benzo[a]pyrene-4,5-oxide. Benzo [a] pyrene reacted with benzoyl peroxide to form the 6-benzoyloxy derivative (quoted, Nikolaou et al, 1984). It was reported that benzo [a] pyrene adsorbed on fly ash and alumina reacted with sulfur dioxide (10%) in air to form benzo[a]pyrene sulfonic acid (Nielsen et al., 1983). Benzo [a] pyrene coated on a quartz surface was subjected to ozone and natural sunlight for 4 and 2 h, respectively. The compounds 1,6-quinone, 3,6-quinone, and the 6,12-quinone of benzo[a]pyrene were formed in both instances (Rajagopalan et al., 1983). 

Uncatalysed cleavage of epoxides has been reported [e.g. tetrahydro-(39)- dihydro-(64 X = OH)]," and oxidation of tetrahydrogeranyl acetate with ozone on silica gel yields dihydro-(65 R = H) predominantly with some 8-acetoxy-6-methyloctan-2-one (cf. Vol. 7, p. 7)." ... 

A detailed study271 on the reaction of 2,3-dimethyl-2-butene with ozone revealed that epoxide formation strongly depends on alkene concentration and temperature. Under appropiate reaction conditions (neat alkene, 0°C) the corresponding tetra-methyloxirane was the main product. Dimethyldioxirane formed from energy-rich acetone oxide (a cleavage product of the alkene) was postulated to be responsible for epoxidation. 

Highly enantioselective epoxidation of unfunctionalized alkenes was developed by using chiral metalloporphyrin catalysts.1214-1218 Remarkable anion axial ligand effects were observed with [Fe(TPFPP)X] complexes (X = triflate, perchlorate, nitrate).1219 Hexafluoroacetone was found to be an efficient cocatalyst with H202,1220 and alkenes could be epoxidized by ozone at ambient temperature.1221... 

The rate constants for oxidation of a series of cycloalkenes with ozone have been determined using a relative rate method. The effect of methyl substitution on the oxidation of cycloalkenes and formation of secondary organic aerosols has been analysed.155 Butadiene, styrene, cyclohexene, allyl acetate, methyl methacrylate, and allyl alcohol were epoxidized in a gas-phase reaction with ozone in the absence of a catalyst. With the exception of allyl alcohol, the yield of the corresponding epoxide ranged from 88 to 97%.156 Kinetic control of distereoselection in ozonolytic lactonization has been (g) reported in the reaction of prochiral alkenes.157... 

Epoxidation of bullvalene with ozone gave racemic trisepoxide. The enantiomers were separated and the absolute configuration determined by X-ray analysis. Acid-catalysed isomerization of the enantiomerically pure (+)-(44) proceedes without... 

Less regularly used reagents are tert-hu y hydroperoxide, tf/Y-butyllithium, ozone. dioxiranes, fluorine/water/acetonitrile, or A, A -diethylhydroxylamine. Alkenes carrying a donor substituent can also be epoxidized with pcracids. Fluorinated allylic alcohols give, under Sharpless conditions, epoxides in good yield and enantiosclcclivity. ... 

Ozone preferentially reacts with the most unsaturated fatty acids present (187) arachidonic acid and higher PUFAs are particularly sensitive. T ranj -double bonds and fatty acids have been reported to react with ozone much more slowly than cis-double bonds (21), but this observation may be an artifact of measuring only initial ozonides. In fact, tran -fatty acids do react with ozone, but the initial ozonides decompose and rearrange more rapidly to generate peroxy-epoxide or peroxy-ozo-nide complexes and free acids (188). This is another example of how, as in lipid oxidation itself, downstream as well as initial products must be measured to obtain a full and accurate picture of reaction. 

Olefins with hindered double bonds may be transformed stereospecifically to oxiranes by treatment with ozone. The epoxidation of propylene has been achieved with alkoxyalkyl-hydroperoxides obtained by the ozonization of olefins in the presence of alcohol.The yield depends on whether the alcohol is a primary, secondary, or tertiary one. The low-temperature (—70°) epoxidation of olefins with a yield of about 30% has been performed with electrophilic intermediates produced in the course of the ozonization of alkynes these intermediates are probably five-membered cyclic trioxides. This epoxidation is almost totally stereospecific. 

Unique successful hydroxylations with ozone and mCPBA First Successful Method Epoxidation of Silyl Enol Ethers (Rubottom Oxidation) Method, mechanism, and isolation of intermediates Chemo- regio- and stereoselectivity synthesis of fused y-laclones Hydroxylation of Amino-ketones via Silyl Enol Ethers Synthesis of vinca alkaloids and model compounds... 

These reactions proceed by initial ozone attack on the C = C bond of the olefin. An intermediate ozonide is formed, which rapidly decomposes to a carbonyl and a biradical. The biradical can be stabilized, or it can decompose. Paulson et al. (1991b) found the products methacrolein, methyl vinyl ketone, and propene, in yields of 68%, 25%, and 7%, respectively. Based on the presence of epoxides in the ozone/isoprene system, Paulson et al. concluded that 0(3P) was being formed. Calculations indicated that 0.45 0(3P) radicals were formed for every ozone/isoprene reaction. However, Atkinson et al. (1993) recently showed that the epoxides were formed directly from the reaction with ozone rather than the reaction with 0(3P). The epoxides formed were l,2-epoxy-2-methy 1-3-butene and l,2-epoxy-3-methyl-3-butene, in yields of 0.028 and 0.011, respectively. There was also definite evidence for the formation of OH radicals in the ozone system, thus causing difficulties in product analyses. Each ozone/isoprene reaction yielded 0.68 OH radicals (Paulson et al., 1991b). 

Epoxidation of unsaturated compounds is usually carried out in the industry by means of anhydrous solutions of peracetic acid (PAA) in ethyl acetate. The PAA solutions are manufactured by the oxidation of acetaldehyde with ozonized oxygen. 

Oxidation of an alkene with a peroxyacid leads to an epoxide, with a carboxylic acid as the by-produet 18,37,38,40,42,44, 51. 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 earboxylic acids 19,20, 21, 22,36,39,41,44. 

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