Alkanediazonium ion

As explained in the preceding section, we will discuss the structure of aromatic diazonium salts on the basis of evidence from X-ray investigations. We will supplement those results with data obtained by other physical methods, in particular NMR and IR spectroscopy. Earlier experience with the more stable arenediazonium salts enabled those scientists who first obtained alkanediazonium ions in solution to characterize them by NMR spectroscopy (see Zollinger, 1995, Sec. 2.1). 

However, we have to criticize more specifically the paper by Lown et al. (1984), who characterized alkanediazonium ions, as well as (E)- and (Z)-alkanediazoate ions, by 15N NMR spectroscopy. They also report NMR data on the (E)- and (Z)-benzenediazohydroxides as reference compounds, describing the way they obtained these compounds in only three lines. Obviously the authors are not familiar with the work on the complex system of acid-base equilibria which led 30 years earlier to the conclusion that the maximum equilibrium concentration of benzenediazohydroxide is less than 1 % of the stoichiometric concentration in water (see Ch. 5). The method of Lown et al. consists in adding 10% (v/v) water to a mixture of benzenediazonium chloride and KOH in dimethylsulfoxide. In the opinion of the present author it is unlikely that this procedure yields the (Z)- and CE>benzenediazohydroxides. Such a claim needs more detailed experimental evidence. 

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). 

Fora discussion of evidence for tbe existence of alkanediazonium ions as species possessing a finite lifetime, see Ref. 12. It has been suggested that diazonium ions are not mandatory intermediates see later discussion (p. 14). 

Alkanediazonium ions may lose a proton to give a diazoalkane when loss of nitrogen to form a carbonium ion is unfavorable. Product formation by way of diazoalkanes can be detected by deuterium labelling, and has been shown to be most likely for primary alkanedia-... 

Kirmse and coworkers have studied the reaction of alkanediazonium ions with amines and with lithium azide Cyclopropanediazonium ions give azo coupling products 6 and 7 with dimethylamine and with ethylamine, respectively (1) K However, no azo coupling of 1 with phenols was observed. In the reaction... 

Other alkanediazonium ions, including the bridgehead bicyclo[2.2.1]heptane-l-diazonium ion 9, do not undergo azo coupling reactions but give only nucleophilic substitution productsIn addition to the examples given above, some alkene-diazonium ions generated from nitrosooxazolidones can also add azide ions ... 

Remember that reactions in which arenediazonium ions are involved must be carried out at 0 °C because they are unstable at higher temperatures. Alkanediazonium ions are even less stable. They lose molecular N2—even at 0 °C—as they are formed, reacting with whatever nucleophiles are present in the reaction mixture by both Sn1/E1 and Sn2/E2 mechanisms. Because of the mixture of products obtained, alkanediazonium ions are of limited synthetic use. 

Alkanediazonium ions (R—N = N) were identified only after the introduction of superacid media and the stabilizing effect of electron-withdrawing substituents like fluorine was taken into account. The first such compound was the 2,2,2-trifluoro-ethanediazonium ion CF3CH2Ni, prepared by Mohrig and Keegstra (1967) by protonation of the corresponding diazoethane in FSO3H at 78°C. 

This result is rather surprising as H/D exchange of diazo ketones in acidic D2O indicates reversible C-protonation. The explanation may be that the 2-hydroxy-alkenediazonium ions 2.11 and 2.12 are thermodynamically much more stable than the alkanediazonium ion 2.13 (see also Sect. 2.10). 

The methanediazonium ion has been generated in an ion cyclotron and its gas-phase reactions have been studied (Foster and Beauchamp, 1972 Foster et al., 1974 McMahon et al., 1988). Complexes of alkanediazonium ions with transition metals (Mo and W) are stable in the solid state (Lappert and Poland, 1969 Day et al., 1975 Herrmann, 1975a Herrmann et al., 1975b Hillhouse et al., 1979 see discussion in Sect. 10.3). 

The first method corresponds essentially to aromatic diazotization. As discussed in Section 2.1 (Scheme 2-1), this method is applicable for the synthesis of a diazo compound only if loss of a proton from the C(a)-atom of the primarily formed alkanediazonium ion is faster than the loss of dinitrogen. 

As already indicated in the preceeding section, nitrosation of aliphatic amines yields alkanediazonium ions with a considerable life-time only in superacids and at very low temperature. Under more usual conditions, the diazonium ion either loses N2 and the carbocation formed yields solvolysis products (in water the corresponding alcohol) and various rearrangement products or, alternatively, a proton is eliminated from the C(a)-atom to give a diazoalkane (2.3 in Scheme 2-1). 

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). 

Table 5-1. and NMR data for alkanediazonium ions, alkenediazenium ions, and related compounds. 

Azo coupling reactions are frequently observed in diazo transfer processes in which 4-toluenesulfonyl and other azides react with highly reactive methylene compounds, e. g., with dicarbonyl compounds (see Sect. 2.6, Schemes 2-56, 2-59, and 2-63). Such reactions were not, however, investigated mechanistically. Thus, although it is likely that alkanediazonium ions are intermediates, there is no direct evidence for their intermediacy. 

Before we discuss those mechanisms in detail (Sects. 7.3-7.6), we will review in the following section the various methods developed for the formation of alkanediazo-nium ions, because most of these routes contributed substantially to our understanding of the reaction steps following formation of the alkanediazonium ion. 

The most important routes to alkanediazonium ions are shown in Scheme 7-3, which is an extended version of a scheme published by Kirmse (1976, 1979). 

The l-alkyl-3-aryltriazenes (7.15 see Scheme 7-3) are easily obtained from aromatic diazonium salts and alkylamines. They exist in a tautomeric equilibrium (see Zollinger, 1994, Sect. 13.4) and, under acid catalysis, they dissociate into both possible combinations of amine and diazonium ion. The aliphatic amine and aromatic diazonium ion will, however, react further with each other, whereas in the combination alkanediazonium ion -h aromatic amine the diazonium ion will decompose rapidly into the carbocation and dinitrogen. This system has been used little for mechanistic or preparative deamination studies, obviously because a very complex product pattern is inherent in it. The carbocation may react with the aromatic and the aliphatic amine at the amino group. A modified method was described by Southam and Whiting (1982) using anhydrous acetonitrile as medium at —10 to -5°C. ... 

The protonation of diazoalkanes (7.14) is also a versatile method for forming alkanediazonium ions and ion pairs. A classical example is Huisgen and Riichardt s investigation of 1-diazopropane (1956). In benzene, the reaction of 1-diazopropane with benzoic acid yields almost exclusively propyl benzoate, whereas in water with perchloric acid a propanol mixture containing 28% propan-2-ol was found. The 1,2-H shift leading to propan-2-ol seems to be minimized by the formation of the diazonium-benzoate ion pair. 

Dauben and Willey (1962) found an interesting photochemical method for the generation of alkanediazonium ions in neutral or alkaline aqueous systems. The anions of sulfonyl hydrazones (7.13) eliminate sulfinate ions (R — SOf") photochemically and the corresponding diazoalkane is formed. In most cases, the... 

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