Electrophilic addition ozone

The aromatic ring of a phenoxy anion is the site of electrophilic addition, eg, in methylolation with formaldehyde (qv). The phenoxy anion is highly reactive to many oxidants such as oxygen, hydrogen peroxide, ozone, and peroxyacetic acid. Many of the chemical modification reactions of lignin utilizing its aromatic and phenoHc nature have been reviewed elsewhere (53). 

How the initial electrophilic addition of ozone to alkene takes place is not completely known, but it is thought that in the first step of addition the bond connecting the two carbons remains intact. 

Selenoxide elimination is now widely used for the synthesis of a,p-unsaturated carbonyl compounds, allyl alcohols and terminal alkenes since it proceeds under milder conditions than those required for sulfoxide or any of the other eliminations discussed in this chapter. The selenoxides are usually generated by oxidation of the parent selenide using hydrogen peroxide, sodium periodide, a peroxy acid or ozone, and are not usually isolated, the selenoxide fragmenting in situ. The other product of the elimination, the selenenic acid, needs to be removed from the reaction mixture as efficiently as possible. It can disproportionate with any remaining selenoxide to form the conesponding selenide and seleninic acid, or undergo electrophilic addition to the alkene to form a -hydroxy selenide, as shown in... 

Electrophilic Addition.—Reactions of androsta-3,5-dienes with MCPBA gave complex mixtures the composition of which was dependent upon the level of peracid used (1 or 2 equivalents). Diepoxides were isolated in low yield only when 2 equivalents were used and in general products were derived from epoxide ring opening.34 In a study of the structures of withanolides G,H,I,J,K, and U which were all shown to possess the 14a-hydroxy-group, it was demonstrated that the 14a-hydroxy-group influenced the epoxidation of the 5,6-double bond.35 Incorporation of ozonizable dyes as internal standards facilitated selective ozonization of... 

Ozone and the alkene undergo a concerted cycloaddition reaction—the oxygen atoms add to the two sp carbons in a single step. The addition of ozone to the alkene should remind you of the electrophilic addition reactions of alkenes discussed in Chapter 4. An electrophile adds to one of the sp carbons, and a nucleophile adds to the other. The electrophile is the oxygen at one end of the ozone molecule, and the nucleophile is the oxygen at the other end. The product of ozone addition to an alkene is a... 

The (indirect) photodegradation - reaction with reactive species formed by photochemical processes - has been recognized as the major transformation pathway for chemicals in the troposphere. The electrophilic addition of tropospheric radicals constitutes the principal degradation pathway. The relevant reactive species are hydroxyl radicals (OH ) and ozone (O3) during day time and N03 radicals at night. The hydroxyl radicals result from reactions of oxygen atoms with water vapour, photolysis of HNO2 and reactions of HO2 (a... 

The first step of the ozonolysis mechanism is the initial electrophilic addition of ozone to the C=C double bond to form the molozonide intermediate. Its instability leads to a further reaction, producing a carbonyl and carbonyl oxide molecule (Scheme 2.10, II). The carbonyl and carbonyl oxide rearrange to create the stable ozonide intermediate (Scheme 2.10, III). A reductive workup is then undertaken to convert the ozonide specie into carbonyl products (Scheme 2.10,1) [19]. 

A general and easy approach to ozone reactivity is its reaction with unsaturated moieties, which occurs through the well-estabhshed Crigee mechanism. An electrophilic addition occurs over the dorrble bond, leading to the cleavage of the bond and to the formation of aldehydes or ketones (Bailey and Ferrell, 1978 Dowideit et al., 1998). The presence of accessible double bonds in the stractures of opiates and caimabinoids suggests a rapid reaction of these compoimds with ozone (Reaction 11.3). 

The mechanism of ozonolysis proceeds through initial electrophilic addition of ozone to the double bond, a transformation that yields the so-called molozonide. In this reaction, as in several others already presented, six electrons move in concerted fashion in a cyclic transition state. The molozonide is unstable and breaks apart into a carbonyl fragment and a carbonyl oxide fragment through another cychc six-electron rearrangement. Recombination of the two fragments as shown yields the ozonide. 

The reaction of ozone with allyl alcohol is expected to involve, as for other unsaturated alcohols and olefins, electrophilic addition at the carbon-carbon double bond leading to the formation of a trioxide followed by its unimolecular decomposition into two carbonyls and two biradicals (Criegee intermediates) ... 

As inert as the C-25 lactone carbonyl has been during the course of this synthesis, it can serve the role of electrophile in a reaction with a nucleophile. For example, addition of benzyloxymethyl-lithium29 to a cold (-78 °C) solution of 41 in THF, followed by treatment of the intermediate hemiketal with methyl orthoformate under acidic conditions, provides intermediate 42 in 80% overall yield. Reduction of the carbon-bromine bond in 42 with concomitant -elimination of the C-9 ether oxygen is achieved with Zn-Cu couple and sodium iodide at 60 °C in DMF. Under these reaction conditions, it is conceivable that the bromine substituent in 42 is replaced by iodine, after which event reductive elimination occurs. Silylation of the newly formed tertiary hydroxyl group at C-12 with triethylsilyl perchlorate, followed by oxidative cleavage of the olefin with ozone, results in the formation of key intermediate 3 in 85 % yield from 42. 

In contrast with the behaviour of typical vinylphosphonic acid derivatives, the carbon-carbon double bond in the 1,2-oxa-phospholene (167) is remarkably unreactive towards a broad spectrum of reagents including electrophiles, most epoxidizing and organometallic reagents, as well as to dipolar addition reactants. Exceptional reagents are, however, N-bromoacetamide (NBA), ozone, dimethyllithiumcuprate, and sodium-naphthalene. 

The ozonolysis of olefins may be analyzed as a sequence of two 1,3-dipolar cycloadditions initial electrophilic attack by ozone 18 to form the first intermediate, which decomposes into a carbonyl compound and a carbonyl oxide 14 followed by nucleophilic 1,3-dipolar addition of the carbonyl ylide 14 to the ketone, yielding the molozonide. 

These free valence numbers in the S method run parallel to the self-polarizability from the M.O. description which has still to be discussed. A larger free valence therefore also means a smaller activation energy for (electrophilic) substitution and probably for radical substitution (p. 284). A high bond order runs parallel to a low bond localization energy of a tu electron pair in this bond and thus with a low activation energy for molecular addition and ozonization. 

The process of ozone cycloaddition (path 1) implies postulates similar to those discussed by Huisgen (12) in terms of a 1,3-dipolar cycloaddition. Although the extent of simultaneity in the formation of the two C—O bonds is an open question, it is assumed that the transition state closely resembles the final state—the primary ozonide—and that its final conversion to give the primary ozonide occurs rapidly. The ratedetermining step is thus the addition of ozone on the olefin. The electrophilic tendency of ozone, which is shown in several cases to play a domi-... 

Ozone has long been assigned the role of electrophile in addition and substitution reactions (2). In this respect the reagent has been reported to cleave various carbon-metal bonds (3-9). 

Ozone-mediated nitration (Kyodai nitration) of imidazole under neutral condition gave poor yields of 4-nitroimi-dazole. However, the yield improved dramatically with the addition of methanesulfonic acid. The combination NO2-O3 is a much more reactive electrophilic reagent since a reaction temperature of 0°C was sufficient (Scheme 56), <1996JCM244>. The 4-nitro product 238 could also be converted into 1,4-dinitroimidazole 239 over a long period. 

After a decision between these two broad, general mechanisms is reached, additional questions must be answered. If the first mechanism is in effect, does the attack involve the terminal oxygen atoms or adjacent oxygen atoms If the second mechanism is in effect, does the first step involve an electrophilic, nucleophilic, or radical attack Does the first step involve a terminal or a central atom If it involves a terminal atom, is the attack completed in the second step by the middle or the other terminal atom Further, one must explain why the double bond is so easily cleaved and why no intermediate from the initial ozone attack has ever been isolated. 

Olefins. Olefins are the most reactive class of hydrocarbons in photochemical smog and have been studied extensively (I, 17, 18, 19). In general, as was perhaps first noted by Schuck and Doyle (20), the mechanism for olefin decomposition apparently involves electrophilic attack (by atomic oxygen, ozone, and other species) on the double bond. Thus, for most of the chemical reactions related to smog formation, olefin reactivity generally increases with additional alkyl (or other electron-donating) groups attached to the two carbon atoms joined by the double bond. 

---