Diazonio group

Arenediazonium salts are extremely useful because the diazonio group (N2) can be replaced by a nucleophile in a substitution reaction. 

The diazonio group can also be replaced by —OH to yield a phenol and by —H to yield an arene. A phenol is prepared by reaction of the arenediazonium salt with copper(I) oxide in an aqueous solution of copper(ll) nitrate, a reaction that is especially useful because few other general methods exist for introducing an -OH group onto an aromatic ring. 

Arylamines are converted by diazotization with nitrous acid into arenediazonium salts, ArN2+ X-. The diazonio group can then be replaced by many other substituents in the Sandmeyer reaction to give a wide variety of substituted aromatic compounds. Aryl chlorides, bromides, iodides, and nitriles can be prepared from arenediazonium salts, as can arenes and phenols. In addition to their reactivity toward substitution reactions, diazonium salts undergo coupling with phenols and arylamines to give brightly colored azo dyes. 

Aromatic diazonium salts are almost as important for reactions in which the diazonio group is lost as molecular nitrogen and in which aryl cations and radicals are the reagent proper (dediazoniation reactions, see Chs. 8 and 10). 

The introduction of two diazonio groups should be called a bisdiazotization and not a tetrazoti-zation. The latter refers to the introduction of a tetrazonio group ( —N2 —N=N). 

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). 

Balli and Felder (1978) and Balli and Ritter (1981) showed that diazo transfer can be applied advantageously to the diazotization of sufficiently nucleophilic heteroaromatic compounds such as 5-hydroxy- and 5-amino-3-methyl-l-phenyl-pyrazole if 3-ethyl-2-azido-benzthiazolium tetrafluoroborate (2.50) is used as diazo transfer reagent (for other applications of this diazo transfer reagent see Zollinger, 1995, Secs. 2.6-2.8). The diazonio group is introduced in the 4-position (2.51). 

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... 

In the present nomenclature to be called the diazonio group . In the IUPAC nomenclature for transformations this process is called diazonio-de-hydrogenation , or, in short, diazoniation . 

Azidochloromethylene)dimethylammonium chloride (2.62) is another reagent which can be used to introduce a diazonio group directly into a phenol (Kokel and Viehe, 1980). 

The experimental data are clearly consistent with the (modernized) formula of Blomstrand (4.1a), remembering of course, that in his time the concepts of bond angles and ionic bonding of the diazonio group were still unknown in organic chemistry. We will discuss the data in Table 4-1 in relation to the mesomeric structures 4.1a and 4.1c (X = H). 

The diazonio group of one zwitterion is stabilized by intermolecular interactions with the carboxylato oxygens of two neighbouring zwitterions. The same type of coordination is observed in crystals of benzene diazonium chloride, tribromide, and tetrafluoroborate (Andresen and Romming, 1962 Romming, 1963 Cygler et al., 1982). 

Wallis and Dunitz (1984a) also investigated another type of sterically induced interaction of a donor group with a diazonio group. Quinoline-8-diazonium-l-oxide tetrafluoroborate (4.14) was analyzed at 95 K (R = 0.034). 

The most important results are given in Figure 4-1. The oxygen atom lies 244 pm from the N(l) atom of the diazonio group, well within the sum of the van der Waals radii. The diazonio group deviates by 10.4° from linearity. It seems that the 0(1) N(l) interaction is attractive, as indicated by the angle of 169.6° (instead of 180°) at N(l), but the 0(1) N(2) interaction is not. The NN distance (109.9 pm) is, however, not different from normal values found in diazonium ions. The same authors demonstrated later (Wallis et al., 1993) that this result is not unique for the quino-line-8-diazonium-l-oxide salt, as it was found also for two 1-naphthalenediazonium tetrafluoroborates substituted in the 8-position with the electron donors -SCH3 and -N(CH3)2 and - perhaps unexpectedly - for 8-nitronaphthalene-l-diazo-... 

Klasinc and Schulte-Frohlinde (1968), who also used HMO for the investigation of substituted benzene diazonium ions, were the first to realize that the remarkable distortion of bond lengths and angles of the benzene ring by the diazonio group (Sec. 4.2) has a significant influence on its reactivity. They found that the sp2-hybridization at the carbon atoms of the benzene ring varied from 1.75 for C(l) to 2.16 for C(2). 

For arene- and heteroarenediazonium ions with substituents that are subject to their own acid-base equilibria the situation is more complex. For example, the hydroxy group of the 4-hydroxybenzenediazonium ion has a pAfa value of 3.40 (Lewis and Johnson, 1959) whereas the 2-hydroxy-5-sulfo-benzenediazonium zwit-terion has a pATa value of only —0.04 (Jermini et al., 1970). The 0 group of the conjugate base greatly reduces the acidity of the diazonio group, as indicated by the mesomeric quinonediazide structure in Scheme 5-13. 

A novel application of a phenyl aryldiazosulfone was found by Kessler et al. (1990). l-[4-(7V-Chlorocarbonyl-7V-methylamino)phenyl]-2-(phenylsulfonyl)diazene (6.18) is an acid chloride with a potential diazonio group. The above authors showed that in organic solvents (THF, etc.) this compound reacts easily, as expected, with nucleophiles (HNu), e.g., with aliphatic, aromatic, or heterocyclic amines, with cystine dimethyl ester, or with 4-methoxyphenol at the carbonyl function, yielding... 

The reaction of azide ions with aliphatic diazo compounds was investigated by Kirmse et al. (1979 for a discussion see Zollinger, 1995, Sec. 6.1). Here we mention only that cyclopropanediazonium ions react similarly to the aromatic diazonium ions, i.e., by TV-coupling to 1-cyclopropylpentazene and dediazoniation to cyclopropyl azide. In about 60% of the 1-cyclopropylpentazene the cyclopropyl azide is formed directly by dediazoniation of the original diazonio group, while in 40% the route is via the cyclopropylpentazole. 

In intramolecular N-coupling the diazonio group interacts with a nitrogen-containing substituent in the ortho-position of the aromatic ring. This is the cause of the difficulties in the diazotization and bisdiazotization of 1,2-diaminobenzene (6.45,... 

The fundamental understanding of the diazonio group in arenediazonium salts, and of its reactivity, electronic structure, and influence on the reactivity of other substituents attached to the arenediazonium system depends mainly on the application of quantitative structure-reactivity relationships to kinetic and equilibrium measurements. These were made with a series of 3- and 4-substituted benzenediazonium salts on the basis of the Hammett equation (Scheme 7-1). We need to discuss the mechanism of addition of a nucleophile to the P-nitrogen atom of an arenediazonium ion, and to answer the question, raised several times in Chapters 5 and 6, why the ratio of (Z)- to ( -additions is so different — from almost 100 1 to 1 100 — depending on the type of nucleophile involved and on the reaction conditions. However, before we do that in Section 7.4, it is necessary to give a short general review of the Hammett equation and to discuss the substituent constants of the diazonio group. 

The effect of the diazonio group on the reactivity of benzene derivatives was studied quantitatively by Lewis and Johnson (1959) by measuring the acidity constants of 3- and 4-diazoniobenzoic acid, of the 4-diazonio-anilinium ion, of 3- and 4-diazo-niophenol, and of 4-diazoniophenylacetic acid. The results are given in Table 7-3. 

The diazonio group is a somewhat more complex substituent for such evaluations because it is charged, in contrast to the majority of substituents on which the Hammett treatment is based. The electrostatic interaction of the diazonio and other charged groups was calculated by Hoefnagel et al. (1978) and by Exner (1978). The substituent constants they obtained, including the effects of coulomb interactions, are only slightly different from those of Lewis and Johnson (1959). 

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