Dediazoniation alkanediazonium ions

In this chapter we will discuss only the dediazoniation of arenediazonium ions (Group a). The dediazoniation of alkene- and alkanediazonium ions and of diazoalkanes (Groups b, c, and d) will be treated in the second book on diazo chemistry (Zollinger, 1995, Chs. 7-9). 

As discussed in Chapter 2, diazotization of primary aliphatic amines generally does not lead to diazoalkanes, because the intermediate alkanediazonium ion loses the diazonio group faster than a proton of the C(a)-atom. Diazoalkane formation is dominant if the deprotonation rate is increased by acidifying substituents in the a-position (see Sect. 2.3). Curtius synthesis of ethyl diazoacetate (1883) is the classical example. Hart and Brewbaker (1969) showed clearly that acidifying substituents favor diazoalkane formation over dediazoniation electron-donating substituents exert the opposite effect. 

The vast majority of diazoalkane reactions are based on the ambident nucleophilic character of diazoalkanes, as shown by the mesomeric structures of diazomethane 4.32 a and 4.32 b. Bronsted and Lewis acids can be added at the C- and the N()8)-atoms. The reaction with Bronsted acids is particularly important. Alkanediazonium ions are obtained by proton addition at the C-atom giving rise to dediazoniation and various reactions of carbocations (see Chapt. 7). 

Bis(trifluoromethyl)diazomethane (7.26) is protonated in fluorosulfonic acid and the equilibrium is essentially on the side of the corresponding diazonium ion (7.27) at -70°C (Mohrig et al., 1974). As mentioned in Section 2.1, this was the first reasonably stable alkanediazonium ion observed. At - 5 to +5 °C, it undergoes slow dediazoniation (7-12). The kinetics of this reaction were found to be of first-order both in the concentration of diazonium ion and of the nucleophile. This is consistent with an ANDjsr-type displacement of the diazonio group by the fluorosulfonate anion ki > k- < ki), but not with unimolecular dediazoniation of the... 

It is not an exaggeration to state that Brosch and Kirmse s contribution to the same reaction, but 34 years later (1991), was a sensation. At that time, theory and experiment (Ford and Scribner, 1983) indicated that the dediazoniation of 1-alkanediazonium ions is endothermic in the gas phase. Therefore, Brosch and Kirmse considered it as rather unlikely that racemates are really formed in such significant amounts as found by Streitwieser and Schaeffer for [l- H]butylamine (31%) and 2-methyl[l- H]propylamine (72%). 

We will discuss the dediazoniation of alkenediazonium ions separated from that of alkanediazonium ions (Chapt. 7) because alkenediazonium ions behave quite differently with respect to dediazoniation. As the diazonio group is bonded to an sp -C-atom, this difference is not surprising. The reactivity of alkenediazonium ions is, however, also significantly different from that of arenediazonium ions in... 

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