Ozone electrophilic attack

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... 

At a pH less than 6, molecular ozone directly attacks the phenolic ring. Then, the ozone further oxidizes the dihydric phenol to either o- or p-quinone. Because ozone is a relatively less powerful oxidant, the selectivity can be clearly demonstrated, as in Figure 8.8. The Hammett plot was first reported by Hoigne (1982) and confirmed by Gurol and Nekoulnalni (1984) (Figure 8.9). At a pH greater than 6, however, ozone is decomposed as hydroxyl radicals, and substituted phenols are ionized to form phenolate anions, which are much stronger electrophilic species than the protonated forms at low pH. As a result, the measured rate constants for some substituted phe-nolates approach the diffusion-controlled limits. 

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]. 

Phenanthridine in methanol solution is unaffected by ozone, but in methylene dichloride quinoline-3,4-dicarboxylic acid (2%) and phenanthridone (23%) are formed a large part of the remaining phenanthridine can be recovered unchanged.335 The reaction thus proceeds much less readily than in the case of acridine. The mechanism by which the lactam is formed is not known with certainty, but an intermediate such as 233 has been proposed.335 However, an initial electrophilic attack producing 234, followed by loss of oxygen and a proton shift, seems equally likely. 

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). 

More recent studies by Nakagawa, et al. (8) confirm the interpretation of the ozone attack on carbon—carbon double bonds in terms of an electrophilic attack. Their investigations on the kinetics of ozonization of polyalkylbenzenes, in CC14 and CH3COOH solution, indicate that the logarithms of the rate constants for the ozonolysis of polymethylbenzenes increase linearly with the number of methyl substituents on the aromatic nucleus. 

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. 

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... 

When all the mechanistic evidence is taken into consideration, the following reaction sequence appears to best satisfy the data. The silane undergoes reversible complexation (A) with the ozone, the complex being present in only small concentrations. The rate step then involves electrophilic attack on the hydridic hydrogen, passing through a five-center transition state. This may decompose to either a silyl hydrotrioxide (Bi) or directly to the radical pair (B2). The silyl hydrotrioxide, if present, must decompose rapidly to the radical pair (C). This radical pair then recombines with retention of configuration to afford the ultimate product, the silanol (D). 

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. 

Since the electrophilic ozone preferentially attacks a more nucleophilic double, it is possible to achieve chemoselective cleavage in compounds containing two or more double bonds by limiting the amount of ozone. The relative reactivity of double bonds toward ozone decreases in the following order ... 

The well-controlled attack of ozone on the sulfur molecule can best be explained by the assumption that a terminal oxygen of the ozone molecule executes an electrophilic attack on the sulfur, forming a new bond with the sulfur. [Compare with Meinwald s work (4) for the analogy in the attack of ozone on the double bond.] Thereupon, the second and third atoms of the ozone are liberated as molecular oxygen. This theory explains the fact that only one oxygen from each ozone molecule is added to the sulfur. If one assumes an attack of the apex oxygen of the ozone molecule, it is difficult to understand why only one of the oxygen atoms stays associated with the sulfur. 

The reaction may be explained as an electrophilic attack of the terminal oxygen of ozone on the nucleophilic nitrogen to form a transitory intermediate which yields the amine oxide. 

Though the product 203 is a mixture of diastereoisomers, it does at least show that even ozone sometimes attacks an enol as an oxygen electrophile. The second example is even more remarkable. Attempted epoxidation of the tricyclic ketone 204 gave a mixture of four compounds two epoxides 205 in a 7 1 ratio from mCPBA attack at the alkene and two regioisomeric lactones 206 from the Baeyer-Villiger rearrangement of the ketone. 

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. 

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