Electrophilic attack by ozone

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

Quinoline 1-oxide undergoes nucleophilic attack by ozone to yield a hydroxamic acid (128), and 40% of the starting iV-oxide is recovered (Scheme 74). When an excess of ozone is employed the aldehydes (129) and (130) are obtained. Formation of these products has been attributed to electrophilic attack by ozone rather than further oxidation of (128), because in a separate experiment (128) yielded carbostyril on treatment with ozone. Isoquinoline 2-oxide yields 2-hydroxyisoquinolin-l-one, and acridine 10-oxide gives 10-hydroxyacridone and acridone in a similar manner to the above. Likewise, phenanthridine 5-oxide affords mainly 5-hydroxyphenanthridone. Quinoline 1-oxide undergoes oxidation by lead tetraacetate as shown (Scheme 75). 

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... 

Step A—Association of Ozone with the Silicon Atoms. The linear Hammett-type relationship of Figure 1 and Equation 1 indicates a slope, /o, of —1.25. This negative value denotes electrophilic attack by the ozone and/or the development of a partial positive charge on silicon in the transition state. Since the silicon is relatively electropositive, an electrophilic attack by ozone on silicon seems unlikely. The hydrogen bound to silicon, however, is hydridic in character and is the likely site of attack... 

A kinetic study of the reaction was also performed in which NMR-obtained rate data were correlated with mercurial structure changes (12). This study revealed a quite distinct reactivity order which, coupled with a 1 1 reactant stoichiometry, indicates a 1,3-dipolar electrophilic attack by ozone via a SE2 or four-center process. Although the exact mechanism was not conclusively proved, it is certain that neither the SE1 or SEi processes were operative during these reactions. 

Sequence 16 clearly demonstrates electrophilic attack by ozone in these reactions. As noted by Jensen and Rickborn (2), the rate of electrophilic cleavage of a carbon-metal bond increases as the polarization of that bond increases. This, in turn, is a direct consequence of the electronegativity of the second atom attached to mercury. The following representations, based on Pauling electronegativity values, illustrate this relationship. 

The relative reaction rates of RsSiH are also consistent with electrophilic attack by ozone. Ozone has been regarded as an electrophile or a nucleophile (i), but the fact that electron-withdrawing groups decrease the rate of reaction would indicate that electrophilic attack is involved. 

Figure 1 shows a plot of krei against Taft s o- constants (13) the slope (p ) is —0.40. Such a low value might indicate that the reactions were run near the isokinetic temperature. However, krei for EtaSiH -(CF3CH2CH2)3SiH at —60°C. was 100 55 compared with 100 63 at — 10°C., which would indicate that genuine substituent effects are being observed and that the reaction involves electrophilic attack by ozone. 

This is essentially an insertion reaction with the electrophilic attack by ozone on the hydridic hydrogen being assisted by coordination to the silicon, possibly using the silicon 3d orbitals. 

Such processes are always accompanied by a DP loss, either by electrophilic attack of ozone, by an ozone-catalyzed cleavage of the glycosidic bond or by attack of secondary radical species [15]. Residual lignin also plays a crucial role in ozone bleaching. Model studies showed that lignin with free phenolic hydroxyl groups accelerated carbohydrate oxidation, probably by activation of oxygen via phenoxyl radicals, whereas etherified phenolic model compounds had a protective effect [16,17]. 

The effects contributed by alkyl groups to the relative rate constants, kreh for the reaction of ozone with cis- and trans-1,2-disubstituted ethylenes are adequately described by Taft s equation = k °reX -f pSo-, where So- is the sum of Taft s polar substituent constants. The positive p values (3.75 for trans- and 2.60 for cis-l,2-disubstituted ethylenes) indicate that for these olefins the rate-determining step is a nucleophilic process. The results are interpreted by assuming that the electrophilic attack of ozone on the carbon-carbon double bond can result either in a 1,3-dipolar cycloaddition (in which case the over-all process appears to be electrophilic) or in the reversible formation of a complex (for which the ring closure to give the 1,2,3-trioxolane is the nucleophilic rate-determining step). 

Perhaps the most interesting point which emerges from the results is that in ethylenes bearing electron-releasing alkyl substituents the ratedetermining step appears to be a nucleophilic process, as indicated by the positive p values. This does not contradict the assumption that the first step in the ozone—olefin reaction is an electrophilic attack of ozone on the carbon-carbon double bond. The present observations also agree with some of the results obtained recently by Pritzkow et al. (16) for alkyl mono-substituted ethylenes in ethanol solution at — 60 °C. 

The reaction of a hydrosilane with ozone results in the rapid, quantitative conversion of the Si-H bond to the Si-OH moiety. The mechanism of this conversion has now been elucidated. It involves a fast, reversible complexation of ozone (acting as a nucleophile) with the silicon atom, followed by rate-determining electrophilic attack by the bound ozone upon the hydridic hydrogen, and decomposition into a RsSi OH radical pair which recombine to produce the silanol. Extensive data concerning the relative rates and other structure-dependent properties in the ozonation of a number of mono-, di-, and trihydrosilanes are presented. 

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. 

Interestingly, addition of BF3 etherate to the ozonolysis of o-dimethoxybenzene derivatives results in increased yields of (Z,Z)-dienes (eq 27, compare to eq 20).In this case, it is thought that coordination of the Lewis acid to the diene reduces its electron density and suppresses further attack by ozone. Also, the fact that the BF3 is already coordinated to ether may limit its ability to coordinate to ozone and increase its electrophilic reactivity. 

The mechanism of amine oxide formation has not been studied closely. The reaction has a first-order dependence on both amine and ozone concentrations and almost certainly involves a slow electrophilic attack by the terminal oxygen on the amine lone-pair electrons (equation 88) as the conjugate acid of the amine is unreactive . 

An ozone-oxygen stream passed at 0° into a soln. of N-cyclohexylideneisobutyl-amine in anhydrous ethyl acetate until 1 molar equivalent has been absorbed -> crude cyclohexanone. Y 48%.—Gontrary to reports in the literature, carbon-nitrogen double bonds, at least in Schiff bases and nitrones, are attacked by ozone, which behaves as a nucleophilic reagent, whereas in most other cases it acts as an electrophilic reagent. F. e. s. A. H. Riebel et al.. Am. Soc. 82, 1801 (1960). 

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