Arenediazonium ions

Arenediazonium ions 1 can undergo a coupling reaction with electron-rich aromatic compounds 2 like aryl amines and phenols to yield azo compounds 3. The substitution reaction at the aromatic system 2 usually takes place para to the activating group probably for steric reasons. If the para position is already occupied by a substituent, the new substitution takes place ortho to the activating group. 

Arenediazonium ions are stable in acidic or slightly alkaline solution in moderate to strong alkaline medium they are converted into diazohydroxides 4 ... 

The optimal pH-value for the coupling reaction depends on the reactant. Phenols are predominantly coupled in slightly alkaline solution, in order to first convert an otherwise unreactive phenol into the reactive phenoxide anion. The reaction mechanism can be formulated as electrophilic aromatic substitution taking place at the electron-rich aromatic substrate, with the arenediazonium ion being the electrophile ... 

For aryl amines the reaction mixture should be slightly acidic or neutral, in order to have a high concentration of free amine as well as arenediazonium ions. Aryl ammonium species—ArNH3+—are unreactive. The coupling of the diazonium species with aromatic amines proceeds by an analogous mechanism ... 

Arenediazonium ions are relatively weak electrophiles, and therefore react only with electron-rich aromatic substrates like aryl amines and phenols. Aromatic compounds like anisole, mesitylene, acylated anilines or phenolic esters are ordinarily not reactive enough to be suitable substrates however they may be coupled... 

Aliphatic primary amines also undergo the diazotization reaction in weakly acidic solution however the resulting aliphatic diazonium ions are generally unstable, and easily decompose into nitrogen and highly reactive carbenium ions. The arenediazonium ions are stabilized by resonance with the aromatic ring ... 

However even arenediazonium ions generally are stable only at temperatures below 5°C usually they are prepared prior to the desired transformation, and used as reactants without intermediate isolation. They can be stabilized more effectively through complexation by crown ethers." ... 

An arenediazonium ion 1 in aqueous alkaline solution is in equilibrium with the corresponding diazohydroxide 4 The latter can upon deprotonation react with diazonium ion 1, to give the so-called anhydride 5. An intermediate product 5 can decompose to a phenyl radical 6 and the phenyldiazoxy radical 7, and molecular nitrogen. Evidence for an intermediate diazoanhydride 5 came from crossover experiments " ... 

The reaction mechanism is not rigorously known, but is likely to involve the following steps." " First the arenediazonium ion species 1 is reduced by a reaction with copper-(l) salt 2 to give an aryl radical species 4. In a second step the aryl radical abstracts a halogen atom from the CuXa compound 5, which is thus reduced to the copper-1 salt 2. Since the copper-(l) species is regenerated in the second step, it serves as a catalyst in the overall process. 

Mechanistically, these diazonio replacement reactions occur through radical rather than polar pathways. In the presence of a copper(I) compound, for instance, it s thought that the arenediazonium ion is first converted to an aryl radical plus copper(II), followed by subsequent reaction to give product plus regenerated copper(l) catalyst. 

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 reaction of molecular nitrogen with aryl cations, i. e., the reverse reaction of (heterolytic) dediazoniations of arenediazonium ions, is a direct introduction of the... 

Bagal (1974) studied the influence of substituents on the ground and first excited states of arenediazonium ions. With regard to compounds in which mesomeric structures such as 4.1b are important, these authors are skeptical about the validity of the PP method. Later, Bagal et al. (1982) used CNDO/2. The calculated 7r-electron densities at all the nitrogen and carbon atoms were similar to those in the earlier PP results. 

A new period in theoretical work on arenediazonium ions began with Vincent and Radom s paper in 1978. This was the first ab initio study of the methane- and benzenediazonium ions, and was carried out with a minimal (STO-3G) basis set, subject only to some (specified) symmetry constraints and a fixed CH bond length (108.3 pm). The optimized structure of the benzenediazonium ion is given in Figure 4-2. 

The experimental work of the groups of Swain and Zollinger on the dediazoniation mechanism of arenediazonium ions, which started in 1975, provided good evidence for the existence of aryl cations as steady state intermediates (see Sec. 8.3). These results also initiated theoretical work on aryl cations, in part combined with further calculations on the structure and reactivity of arenediazonium ions. Publications that contain data on arenediazonium ions and aryl cations will therefore be discussed in the chapter on dediazoniation reactions (Sec. 8.4). In the rest of this section we will concentrate on investigations that are concerned with the geometries and electron densities of diazonium ions but not, or only marginally, with energetics of the dediazoniation reaction. 

The Combined System of Acid-Base Addition, (Z)/(E) and Prototropic Isomerisation Reactions of Arenediazonium Ions... 

As discussed in Sections 5.1-5.3, arenediazonium ions are Lewis acids in which the (3-nitrogen forms the center of electrophilic character. This was demonstrated by the addition of hydroxide ions and water molecules. Other nucleophiles can also be added and, in principle, these reactions display the same mechanistic characteristics as those with OH and H20. According to the nature of the atom of the nucleophile that provides the lone pair of electrons, O-, S-, Se-, N-, P-, or C-coupling can occur. With N- and C-coupling, important and large groups of compounds are formed, namely azo compounds (mainly important as azo dyes) and triazenes, respectively. These compounds will be discussed in Chapters 12 and 13, respectively. 

The addition of nucleophiles with charge n (Nu", n = 0, -1, -2) to arenediazonium ions can be summarized as shown in Scheme 6-1. In the primary adduct (6.1) Nu" is covalently bonded to the (3-nitrogen. In most - perhaps all - cases the azo group is in the (Z)-configuration. The stability of the primary adduct is critically... 

The volumes of activation for some additions of anionic nucleophiles to arenediazonium ions were determined by Isaacs et al. (1987) and are listed in Table 6-1. All but one are negative, although one expects — and knows from various other reactions between cations and anions — that ion combination reactions should have positive volumes of activation by reason of solvent relaxation as charges become neutralized. The authors present various interpretations, one of which seems to be plausible, namely that a C — N—N bond-bending deformation of the diazonium ion occurs before the transition state of the addition is reached (Scheme 6-2). This bondbending is expected to bring about a decrease in resonance interaction in the arenediazonium ion and hence a charge concentration on Np and an increase in solvation. 

Table 6-1. Volumes of activation for reactions of arenediazonium ions with nucleophiles (Isaacs et al., 1987).

The addition of acetate ion to an arenediazonium ion is also an O-coupling. As the equilibrium of this reaction lies very much to the side of the starting ions (Suschitzky, 1967), and since diazoacetates play an important role as transient intermediates in homolytic reactions of A-nitrosoacetanilides, we will discuss diazoacetates in that context (Sec. 10.10). 

As expected, the addition of arenediazonium ions to thiophenols does not involve the thiophenol molecule, but rather the thiophenoxide ion (Price and Tsunawaki, 1963). Dediazoniation with the formation of a diarylsulfide (6.16) is competitive with the formation of the diazo thioether 6.15 (Scheme 6-9 van Zwet et al., 1970). Whereas the early investigators detected only one isomer, (Z)- and (ii)-forms were... 

Recently Noble s group (Haub et al., 1992) showed that sulfidomolybdenum dimer anion complexes react with arenediazonium ions and form complexes (Mo)2S — N2 — Ar. Synthesis and (homolytic) dediazoniation reflect characteristics of arene-diazosulfide anions (see Zollinger, 1995, Sec. 10.1). 

The conductometric results of Meerwein et al. (1957 b) mentioned above demonstrate that, in contrast to other products of the coupling of nucleophiles to arenediazonium ions, the diazosulfones are characterized by a relatively weak and polarized covalent bond between the p-nitrogen and the nucleophilic atom of the nucleophile. This also becomes evident in the ambidentate solvent effects found in the thermal decomposition of methyl benzenediazosulfone by Kice and Gabrielson (1970). In apolar solvents such as benzene or diphenylmethane, they were able to isolate decomposition products arising via a mechanism involving homolytic dissociation of the N — S bond. In a polar, aprotic solvent (acetonitrile), however, the primary product was acetanilide. The latter is thought to arise via an initial hetero-lytic dissociation and reaction of the diazonium ion with the solvent (Scheme 6-11). 

Ammonia and its inorganic and organic derivatives (HNR R2) couple readily with arenediazonium ions to give triazenes (Ar — N2—NR R2). Originally these compounds were called diazoamino compounds. Nowadays IUPAC nomenclature (IUPAC, 1979, Rule 942.2) recommends that the prefix diazoamino should be used only for compounds with the same organic residue at each end of the — N2 —NH — group. 

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