The Structure of Diazonium Compounds

The packing arrangement of cation and anion in diazonium salts has important implications not only for the structure of diazonium ions, as discussed above, but also for the solid-state chemistry of these compounds, in particular with regard to halogeno-de-diazoniations such as the Schiemann reaction. TWo of the papers of Gougoutas (1978, with Johnson, and 1979) contain, in addition to the X-ray analyses, experimental results on bromo- and iodo-de-diazoniation, which can be interpreted on the basis of the structural information (see Secs. 10.4-10.6). 

Much earlier information on the structure of diazonium ions than that derived from X-ray analyses (but still useful today) was obtained by infrared spectroscopy. The pioneers in the application of this technique to diazonium and diazo compounds were Le Fevre and his school, who provided the first IR evidence for the triple bonds by identifying the characteristic stretching vibration band at 2260 cm-1 (Aroney et al., 1955 see also Whetsel et al., 1956). Its frequency lies between the Raman frequency of dinitrogen (2330 cm-1, Schrotter, 1970) and the stretching vibration frequency of the C = N group in benzonitrile (2255 cm-1, Aroney et al., 1955). In substituted benzenediazonium salts the frequency of the NN stretching vibration follows Hammett op relationships. Electron donor substituents reduce the frequency, whereas acceptor substituents increase it. The 4-dimethylamino group, for example, shifts it by 103 cm-1 to 2177 cm-1 (Nuttall et al., 1961). This result supports the hypothesis that... 

Ultraviolet spectra of diazonium salt solutions were recorded for the first time by Hantzsch and Lifschitz as early as 1912. However, electron spectra did not provide significant information on the structure of diazonium ions, either at that time or later. For example, Anderson and coworkers (Anderson and Steedly, 1954 Anderson and Manning, 1955), compared spectra of 4-amino-benzenediazonium salts with those of diphenylquinomethane (4.18). Their conclusion that the structures of these compounds are analogous is basically correct, but the arguments given by Anderson can easily be refuted, as shown by Sorriso (1978, p. 102). 

For a treatise, see Regitz Maas Diazo Compounds Academic Press New York. 1986. For reviews of the reactions of aliphatic diazo compounds with acids, sec Hcgarty. in Patai The Chemistry of Diazonium and Diazo Groups, pt. 2 Wiley New York, 1978, pp. 511-591. pp. 571-575 More O Fcrrall Adv. Phys. Org. Chem. 1967, 5. 331-399. For review of the structures of these compounds, see Studzinskii Korobitsyna Russ. Chem. Rev. 1970, 39. 834-843. 

Azo dyes and pigments have been an important commercial product for the past century. Following the discovery of diazonium compounds in 1858, and the first azcbenzene in 1861, this class of compounds was rapidly exploited for use as colorants. One of the reasons for the success iof this class is their relative ease of synthesis. Thus, large numbers of compounds can be readily prepared and their absorption characteristics systematically controlled by structural modifica-... 

Instead, these heterocycles and their derivatives most commonly undergo electrophilic substitution nitration, sulfonation, halogenation. Friedel-Crafts acylation, even the Reimer-Tiemann reaction and coupling with diazonium salts. Heats of combustion indicate resonance stabilization to the extent of 22-28 kcal/ mole somewhat less than the resonance energy of benzene (36 kcal/mde), but much greater than that of most conjugateci dienes (about Tlccal/mole). On the basis of these properties, pyrrole, furan, and thiophene must be considered aromatic. Clearly, formulas I, II, and III do not adequately represent the structures of these compounds. 

Owing to their particular interest two individual reactions will now be discussed separately. The reaction of methoxycarbonylhydrazine and 3-bromo-2,4-pentanedione affords, in addition to the expected pyrazole (608), a pyrazolium salt (609), the structure of which was established by X-ray crystallography (74TL1987). Aryldiazonium salts have been used instead of arylhydrazines in the synthesis of pyrazolines (610) and pyrazoles (611) (82JOC81). These compounds are formed by free radical decomposition of diazonium salts by titanium(n) chloride in the presence of a,/3-ethylenic ketones. 

The structure of 1,2,3-benzoxadiazole (4.16) bears some resemblance to Wallis and Dunitz s structure (4.14, Fig. 4-1) for quinoline-8-diazonium-l-oxide, as the latter structure has a tendency towards forming a five-membered heteroaromatic ring. The two compounds are, however, different with respect to the involvement of an N(2) and an N(l) diazo atom. The 1,2,3-benzoxadiazole structure is consistent with the bands observed in the 9.45 to 12.37 eV range in the photoelectron spectrum,... 

We include here a short discussion of diazocyclopentadiene (4.21), its 2,3,4,5-tetra-cyano derivative (4.22), and the analogous heterocyclic compound 2-diazo-4,5-di-cyanoimidazole (4.23). Their synthesis and properties are discussed in Sections 2.6 and 12.2, where it is mentioned that they show in some respects the behavior of diazonium ions, i.e., of the mesomeric structure 4.21b (also analogously for 4.22 and 4.23). 

In studies aimed at understanding the influence of structure on the reactivity of diazonium ions, Diener and Zollinger (1986) found that the NMR chemical shifts of the aromatic or heteroaromatic parent compounds provided a novel probe. This method can be applied both to substituted benzenediazonium ions and to various heteroaromatic diazonium ions, and it also provides semiquantitative information on the relative reactivities of the l,3,4-triazole-2-diazonium ion (12.5) and its deprotonated zwitterion (12.6). 

The products formed in deamination depend on the lifetimes of the different ion-pair species, which, in turn, depend on the structure of the amine and the nature of the solvent. The counter-ion in such reactions can conveniently be varied by using N-nitrosoamides as starting materials. These compounds react to give diazonium ion-pairs in reactions that closely resemble amine deamination, and the lifetime of the ion-pairs can be studied by using lsO-labelled ni-trosoamides (see Scheme 2, XH = solvent or added nucleophile). 

Aminoindoxazenes, which are stable towards acids and bases, can be diazotized in acetic acid Lindemann and Ciss e46,70 reported the conversion of 3-amino-5-nitro- and 3-amino-5-acetamido-indoxazenes into the corresponding 3-hydroxy compounds (indoxazenones), via the diazonium salts. Attempts to prepare 3-hydroxyindoxazene and its 6-chloro derivative by the same route were unsuccessful.46 The structures of these 3-hydroxy compounds would seem doubtful since Boshagen reports a much higher melting point for 3-hydroxy-5-nitro-indoxazene (223.5,46 85-88°,70). 

Phenyl Nitroso Amine.—The iso-diazotate differs in a noticeable way from the diazotate. Not only is it more stable, but on treatment with acids it does not yield diazonium salts, but an entirely different compound, viz., phenyl nitroso amine, CeHs—NH—NO. This compound has the same composition as the diazo hydroxide and the relationship means that the isomeric diazo hydroxide undergoes structural rearrangement into a new compound. This is the reason for the view that this same compound, phenyl nitroso amine, is an intermediate product in the formation of diazo. compounds as already referred to (p. 587). These relationships may be shown as follows ... 

The chemical properties of nitroestrones are much the same as those of simpler aromatic compounds. The nitro group in 19-1 can, for instance, be converted to the aniline 21-1 for example by reduction with tin and acid treatment of the resulting aniline with nitrous acid then gives the diazonium salt 21-2 (Scheme 3.21). Photolysis of that salt in methanol leads to formation of 2-methoxyestrone, (21-3), in this case confirming the structure of one of the metabolites of estradiol. 

Give the structure of the activated benzene ring and the diazonium ion used in the synthesis of the following compounds ... 

This discussion of the potential existence of enols of a-diazocarbonyl compounds brings us to the structure of alkenediazonium ions. The mesomeric structures 5.13 b and 5.13 c of an a-diazocarbonyl compound have a similarity with alkenediazonium ions (5.29a-5.29b). Scheme 5-6 shows one of Bott s synthetic routes to alkene diazonium salts (see Sect. 2.10). 

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