Diazonium ions compounds

Aryl diazonium ions prepared by nitrous acid diazotization of primary arylamines are substantially more stable than alkyl diazonium ions and are of enormous synthetic value Their use m the synthesis of substituted aromatic compounds is described m the following two sections... 

Reaction with arenediazonium salts Adding a phe nol to a solution of a diazonium salt formed from a primary aromatic amine leads to formation of an azo compound The reaction is carried out at a pH such that a significant portion of the phenol is pres ent as its phenoxide ion The diazonium ion acts as an electrophile toward the strongly activated ring of the phenoxide ion... 

Schiemann reaction (Section 22 17) Preparation of an aryl fluonde by heating the diazonium fluoroborate formed by addition of tetrafluorobonc acid (HBF4) to a diazonium ion Schiffs base (Section 17 10) Another name for an imine a compound of the type R2C=NR ... 

Among the reagents that are classified as weak electrophiles, the best studied are the aromatic diazonium ions, which reagents react only with aromatic substrates having strong electron-donor substituents. The products are azo compounds. The aryl diazonium ions are usually generated by diazotization of aromatic amines. The mechanism of diazonium ion formation is discussed more completely in Section 11.2.1 of Part B. 

Because of the limited range of aromatic compounds that react with diazonium ions, selectivity data comparable to those discussed for other electrophilic substitutions are not available. Because diazotization involves a weak electrophile, it would be expected to reveal high substrate and position selectivity. 

In a protic solvent—glycols are often used, with the base being the corresponding sodium glycolate—the reaction proceeds via formation of a carbenium ion 5. The diazo compound 3 can be converted into the diazonium ion 4 through transfer of a proton from the solvent (S-H). Subsequent loss of nitrogen then leads to the carbenium ion 5 ... 

When an aprotic solvent is used, the reaction proceeds via an intermediate carbene 6. In the absence of a proton donor, a diazonium ion cannot be formed and the diazo compound 3 loses nitrogen to give the carbene 6 ... 

Blomstrand s formula, which had been largely disregarded, was rediscovered practically simultaneously by Bamberger (1895a) and Hantzsch (1895), who recognized how well it meets the experimental facts. Hantzsch, one of the precursors of physical organic chemistry, also realized the cationic character of these compounds and proposed the present name diazonium ion. 

Griess (1864a) had already observed that the diazo compounds obtained from primary aromatic amines in acid solution are converted by alkalis into salts of alkalis. The reaction is reversible. The compounds which Hantzsch (1894) termed sjw-diazotates exhibit apparently the same reactions as the diazonium ions into which they are instantaneously transformed by excess of acid. Clearly the reaction depends on an acid-base equilibrium. 

Compounds of the form RN2 X are named by adding the suffix -diazonium to the name of the parent compound RH, the whole being followed by the name of X- (Rule C-931.1, e.g., methanediazonium tetrafluoroborate, benzenediazonium chloride, not phenyldiazonium). Following RC- 82.2.2.3 (IUPAC, 1993), diazonium ions may also be named structurally on the basis of the parent cation diazenylium HNJ, e.g., benzenediazenylium ion. We name the substituent — NJ diazonio (not diazonium) following the same rule. Diazonio describes both mesomeric structures — N = N and — N = N. If one wants to describe one of these structures only, diazyn-l-ium-l-yl or diazen-2-ylium-l-yl has to be used for -N = N or -N = N, respectively. In the General Subject Index of Chemical Abstracts and in Beilstein, diazonium compounds as a class are indexed under this heading. 

Compounds containing the neutral (formally zwitterionic) group =N2 attached by one atom to carbon are named by adding the prefix diazo- to the name of the parent compound (Rule 931.4), e.g., diazomethane, ethyl diazoacetate. Diazo is a so-called characteristic group appearing only as a prefix in substitutive nomenclature. Chemical Abstracts and Beilstein indexing of diazo compounds is analogous to that mentioned above for diazonium ions and salts, but Diazo compounds is not... 

The radical and the anion, R-N2 and R-N2, derived (formally) from a diazonium ion by addition of one and two electrons respectively, are named as diazenyl ( radical at the end is not necessary ) and diazenide (IUPAC, 1993). The radical derived formally from a diazoalkane by addition of a hydrogen atom (R=N-NH) is named diazanyl . In order to be consistent with the nomenclature of diazonium ions, the name of the parent compound should precede the words mentioned, e. g., benzenediazenyl for C6H5 - NJ (the term phenyldiazenyl radical is, however, used by Chemical Abstracts). 

At low concentrations of hydrogen ions the diazonium ion formed reacts with the free base of an as yet unattacked amine to produce the triazene (diazoamino) compound. 

This diazotization is typical of many aminoazoles the diazonium ions formed are relatively strong acids. The pATa values of five di-, tri-, and tetrazolediazonium ions are reported to be between 3 and 4, i. e., about 10 units lower (more acidic) than those of the respective unsubstituted heterocycles (Vilarrasa et al., 1974). Therefore, deprotonation of the diazonium ion is easy and, depending on reaction conditions, yields either the diazonium salt or its conjugate base, the diazo compound. The electrophilic reactivity of the P nitrogen atom in the diazo group of the base is lower than the reactivity of the diazonio group of the cation (Diener and Zollinger, 1986 see Sec. 12.2). 

The diazonium ions 2.13 with electron-withdrawing substituents are not hetero-aromatic compounds and therefore do not strictly come within the scope of this book. They are formally related to the alkenediazonium ions. Nevertheless, they are discussed here because in their properties they bear a close resemblance to heteroaromatic and arenediazonium ions rather than to alkenediazonium ions. In par-... 

The diazotization of amino derivatives of six-membered heteroaromatic ring systems, particularly that of aminopyridines and aminopyridine oxides, was studied in detail by Kalatzis and coworkers. Diazotization of 3-aminopyridine and its derivatives is similar to that of aromatic amines because of the formation of rather stable diazonium ions. 2- and 4-aminopyridines were considered to resist diazotization or to form mainly the corresponding hydroxy compounds. However, Kalatzis (1967 a) showed that true diazotization of these compounds proceeds in a similar way to that of the aromatic amines in 0,5-4.0 m hydrochloric, sulfuric, or perchloric acid, by mixing the solutions with aqueous sodium nitrite at 0 °C. However, the rapidly formed diazonium ion is hydrolyzed very easily within a few minutes (hydroxy-de-diazonia-tion). The diazonium ion must be used immediately after formation, e. g., for a diazo coupling reaction, or must be stabilized as the diazoate by prompt neutralization (after 45 s) to pH 10-11 with sodium hydroxide-borax buffer. All isomeric aminopyridine-1-oxides can be diazotized in the usual way (Kalatzis and Mastrokalos, 1977). The diazotization of 5-aminopyrimidines results in a complex ring opening and conversion into other heterocyclic systems (see Nemeryuk et al., 1985). 

Salts of diazonium ions with certain arenesulfonate ions also have a relatively high stability in the solid state. They are also used for inhibiting the decomposition of diazonium ions in solution. The most recent experimental data (Roller and Zollinger, 1970 Kampar et al., 1977) point to the formation of molecular complexes of the diazonium ions with the arenesulfonates rather than to diazosulfonates (ArN2 —0S02Ar ) as previously thought. For a diazonium ion in acetic acid/water (4 1) solutions of naphthalene derivatives, the complex equilibrium constants are found to increase in the order naphthalene < 1-methylnaphthalene < naphthalene-1-sulfonic acid < 1-naphthylmethanesulfonic acid. The sequence reflects the combined effects of the electron donor properties of these compounds and the Coulomb attraction between the diazonium cation and the sulfonate anions (where present). Arenediazonium salt solutions are also stabilized by crown ethers (see Sec. 11.2). 

The so-called transdiazotizations are mechanistically related to the introduction of diazonio groups using sulfonic acid azides. An aromatic diazonium ion forms a triazene (diazoamino compound) with an aromatic amine the triazene tautomerizes and dissociates at the Na-Np bond of the original diazonium ion. This reaction is important for the synthesis of the 4-aminobiphenyl-4,-diazonium ion, which cannot be obtained by direct (mono-)diazotization of 4,4 -diaminobiphenyl (Allan and... 

We now know that Hammett s explanation is correct in all its aspects. This result is especially noteworthy because Hammett arrived at his conclusions not through extensive experimentation in his laboratory, but by the consistent application of the newer theories of organic chemistry to kinetic results already published by others. This is not the only example of such anticipation of views (now generally accepted) to be found in Hammett s book, and it is worth remembering that Hammett expressly postulates the diazonium ion as the reactive form of the diazo compound in coupling, in contrast to the then current opinion that the diazohydroxide was the effective species. 

The reversibility of aromatic diazotization in methanol may indicate that the intermediate corresponding to the diazohydroxide (3.9 in Scheme 3-36), i. e., the (Z)-or (is)-diazomethyl ether (Ar — N2 — OCH3), may be the cause of the reversibility. In contrast to the diazohydroxide this compound cannot be stabilized by deprotonation. It can be protonated and then dissociates into a diazonium ion and a methanol molecule. This reaction is relatively slow (Masoud and Ishak, 1988) and therefore the reverse reaction of the diazomethyl ether to the amine may be competitive. Similarly the reversibility of heteroaromatic amine diazotizations with a ring nitrogen in the a-position may be due to the stabilization of the intermediate (Z)-diazohydroxide, hydrogen-bonded to that ring nitrogen (Butler, 1975). However, this explanation is not yet supported by experimental data. 

The X-ray investigation of lH-3,5-dimethylpyrazole-4-diazonium chloride (4.7) demonstrates that heteroaromatic diazonium ions are very similar to those of the aromatic compounds (NN distance 111.3 pm, NC 136.9 pm, indicating a greater degree of NC double bond character (Brint et al., 1985). 

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). 

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). 

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