Azide ions carbocation reactivity

It is more difficult to interpret micellar effects upon reactions of azide ion. The behavior is normal , in the sense that k /kw 1, for deacylation, an Sn2 reaction, and addition to a carbocation (Table 4) (Cuenca, 1985). But the micellar reaction is much faster for nucleophilic aromatic substitution. Values of k /kw depend upon the substrate and are slightly larger when both N 3 and an inert counterion are present, but the trends are the same. We have no explanation for these results, although there seems to be a relation between the anomalous behavior of the azide ion in micellar reactions of aromatic substrates and its nucleophilicity in water and similar polar, hydroxylic solvents. Azide is a very powerful nucleophile towards carboca-tions, based on Ritchie s N+ scale, but in water it is much less reactive towards 2,4-dinitrohalobenzenes than predicted, whereas the reactivity of other nucleophiles fits the N+ scale (Ritchie and Sawada, 1977). Therefore the large values of k /kw may reflect the fact that azide ion is unusually unreactive in aromatic nucleophilic substitution in water, rather than that it is abnormally reactive in micelles. 

Fig. 1 A hypothetical plot of azide ion selectivity (M ) against the reactivity of the carbocation intermediate of solvolysis of R-X in aqueous solution (Scheme 4). The descending limb on the left hand side of this plot is for reactions where the value of ks(s ) is increasing relative to the constant value of s ) for diffusion-limited addition of...

In the presence of solvent alone, the lifetime of the intermediate of the stepwise reaction of X-l-Y in the narrow borderline between the S l and Sn2 substitution reactions of azide ion (—0.32 < a" " < —0.08, Fig. 2.2) is 1/ = 10 ° s. Azide ion is 10°-10 -fold more reactive than water toward triarylmethyl carboca-tions and related electrophiles, and this selectivity is independent of carbocation reactivity, so long as the reactions of both azide ion and solvent are limited by... 

The hydrates of phenanthrene (13), anthracene, and benzofuran are not sufficiently reactive to form carbocations at the mild pH of azide buffers. However, the cation may be generated by solvolysis of their acetate or chlor-oacetate esters. Trapping of the cation by azide ions then occurs in the normal way.25,75 Moreover, the solvolytically generated cations react in these cases not only through loss of a proton to form the aromatic product but by nucleophilic... 

Choride ion is considerably less reactive than the azide ion. Thus, although values of kc 1/ kn2o have been quite widely available from mass law effects of chloride ion on the solvolysis of aralkyl halides, normally the reaction of the chloride ion cannot be assumed to be diffusion controlled and the value of kn2o cannot be inferred, except for relatively unstable carbocations (p. 72). Mayr and coworkers251 have measured rate constants for reaction of chloride ion with benzhydryl cations in 80% aqueous acetonitrile and their values of logk are plotted together with a value for the trityl cation19 in Fig. 7. There is some scatter in the points, possibly because of some steric hindrance to reaction of the trityl cations. However, it can be seen that chloride ion is more... 

The dilemma presented by these conflicting results was resolved by TaShma and Rappoport.265 They pointed out that the apparent dependence of kAz/ knl0 upon the reactivity of the carbocation arose because even the most stable cation reacting with azide ion did so at the limit of diffusion control. Thus while kn2o remained dependent on the stability of the cation in the manner illustrated in Fig. 7 the rate constant for the azide ion remained unchanged. Thus the most stable cation formed in the solvolysis reactions was the trityl ion, for which direct measurements of kn2o = 1 -5 x 105 s 1 and kAz = 4.1 x 109 now show that even for this ion the reaction with azide ion is diffusion controlled.22... 

It seems clear therefore that more reactive cations than those for which Ritchie s N+ relationship was developed, show a distinct dependence of selectivity between nucleophiles upon the stability and reactivity of the carbocation. Richard has confirmed that for a very stable benzylic carbocation, represented by the bis-trifluoromethyl quinone methide 57, the N+ regime is restored and that a plot of log k against N+ for reactions of nucleophiles, including amines, oxygen and sulfur anions, the azide ion, and a-effect nucleophiles, shows a good correlation with N+.219... 

A mechanism that explains some of the more important observations in the acid-catalyzed hydrolysis of epoxides 49a-d is outlined in Scheme 15. The cis/trans diol product ratios from the acid-catalyzed hydrolysis of 49a-c, which have either hydrogen- or electron-donating groups in the para position of the phenyl ring, are 74 26, 83 17 and 65 35, respectively. An intermediate carbocation 52a is trapped by azide ion in the acid-catalyzed hydrolysis of 49a and the rate constant for reaction of 52a with water in 10 90 dioxane-water solvent is estimated, by the azide clock technique, to be 1.7 x 108 s 1. Azide ion also traps an intermediate 52b in the acid-catalyzed hydrolysis of 49b, but somewhat less efficiently. The rate constant ks for reaction of 52b with solvent is estimated to be 2 x 109 s-1. The somewhat greater reactivity of 52b compared to that of 52a is consistent with the observation that... 

In water, N3 is much less reactive in aromatic nucleophilic substitution than expected from its reactivity toward carbocations, that is, its N+value. Ritchie (43) initially developed his N+ scale from nucleophilicities toward preformed carbocations and the scale fits the data for nucleophilicities toward many electrophiles, regardless of their charge. However, in water, and similar hydroxy lie solvents, the nucleophilicity of azide ion, relative to that of other anions, seems to be related to the carbocation-like character of the electrophile. An acyl derivative with its sp2 carbonyl group is somewhat akin to a carbocation stabilized by an alkoxide group, >C=0 <-— >C+-0 , just as a triarylmethyl carbocation is stabilized by electron delocalization into the aryl groups and azide ion is a good nucleophile toward these electrophiles. As compared with anions such as OH- or CN , azide ion, in water, is very reactive toward carbocations and in deacylation but is relatively unreactive toward dinitrohaloarenes (44). 

The reactivity of the ethenediazonium salt 9.100 towards the nucleophiles mentioned shows that it has the properties of the corresponding carbocation, since it can ethylate the nucleophile and is prone to attack at the C()ff)-atom of the original ethene-l-diazonium ion. The thermal decomposition pattern is typical of that for an oxonium salt. Reactions with amines are similar to those of ketene acetals. No product that could be explained in terms of an azo coupling reaction, e.g., with 2-naphthol, could be observed. The electrophilicity of the diazonio group is, therefore, low. N-Azo coupling products with azide ions have been postulated with good arguments, however, by Kirmse and Schnurr (1977) with certain short-lived ethene diazonium intermediates produced from nitroso oxazolidones. 

Additional evidence in favor of the intermediacy of a carbocation in the acid solvolysis of styrene oxides could be obtained from a trapping experiment. In order to increase the lifetime of the carbocation, it would be convenient to use a -MeO-sustituted styrene oxide as substrate and very little nucleophilic solvent. The trapping agent should be very reactive and much more nucleophile than the solvent, to avoid any undesirable competition reaction between both species. The highly nucleophile azide ion could be suitable to trap the intermediate which has sufficiently long lifetimes in aqueous solutions to be trapped by the reagent (Scheme 11.8). 

If the rate of the reaction is unaffected by the concentration of the nucleophile, then the reaction must go by the Sj. mechanism. So in the RDS of the reaction, the carbon-chlorine bond is broken to give the carbocation. This is not surprising as this is a stable tertiary carbocation. The carboca-tion is very reactive, so in the second, fast, step, it will be attacked by both azide ion and water ... 

The value of kp obtained in this way for the phenanthreneonium ion is not far from the limit set by the rotational relaxation of water. For such fast reactions, Richard has pointed out that azide trapping could be influenced by preassociation.6 Preassociation has been well characterized in a number of nucleophilic reactions of reactive carbocations with water6 but its impact on deprotonation has not been fully clarified.5,6 In so far as preassociation... 

This concludes the discussion of the stabilities of carbocations with hydrocarbon-based structures and also of different methods for deriving equilibrium constants to express these stabilities. The remainder of the chapter will be concerned mainly with measurements of stabilities for oxygen-substituted and metal ion-coordinated carbocations. Consideration of carbocations as conjugate acids of carbenes and derivations of stabilities based on equilibria for the ionization of alkyl halides and azides will conclude the major part of the chapter and introduce a discussion of recent studies of reactivities. 

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