Diazonium ions coupling

Diazo coupling is expected to occur only with highly reactive systems, and experiment bears this out. Diazonium ions couple with the anions of N-unsubstituted imidazoles at the 2-position (e.g. 125 yields 126) and with indazoles (127) in the 3-position. In general, other azoles react only when they contain an amino, hydroxyl, or potential hydroxyl group, e.g. the 4-hydroxypyrazole (128), the triazolinone (129) and the thiazolidinedione (130) (all these reactions occur on the corresponding anions). 

Primary aromatic amines react with nitrous acid to give aryldiazonium ions, ArN2+, which are useful intermediates in synthesis of aromatic compounds. The process by which they are formed is called diazotization. The nitrogen in these ions can readily be replaced by various nucleophiles (OH, Cl, Br, I, CN). Diazonium ions couple with reactive aromatics, such as amines or phenols, to form azo compounds, which are useful as dyes. 

A pronounced kinetic isotope effect can also be seen in diazonium ion coupling reactions (Zollinger, H. Helv. Chim. Acta 1955,38,1617) and mercuration of benzene (Perrin, C. Westheimer, F. H. /. Am. Chem. Soc. 1963, 85, 2773). 

Catalysis by functional micelles continues to attract attention. A micellar diazonium ion couples to 2-naphthols up to 244-fold faster than a model diazonium ion at pH 7. This reaction is of interest as it represents the formation of a cationic intermediate catalysed by a cationic micelle. ... 

A reaction of aryl diazonium salts that does not involve loss of nitrogen takes place when they react with phenols and arylamines Aryl diazonium ions are relatively weak elec trophiles but have sufficient reactivity to attack strongly activated aromatic rings The reaction is known as azo coupling two aryl groups are joined together by an azo (—N=N—) function... 

The acid—base equiUbtia are fundamental to the kinetics of azo coupling and of practical significance for azo technology. Thus it is important that coupling reactions be carried out in a medium such that the acid—base equiUbtia of the diazo and coupling components favor as much as possible the diazonium ions and the phenolate ions or the free amine, respectively. 

Pyridine-2- and -4-diazonium ions are far less stable than benzenediazonium ions. Azolediazonium salts generally show intermediate stability provided diazotization is carried out in concentrated acid, many of the usual diazonium reactions succeed. Indeed, azolediazonium salts are often very reactive in coupling reactions. 

Certain aliphatic diazonium species such as bridgehead diazonium ions and cyclo-propanediazonium ions, where the usual loss of N2 would lead to very unstable carbocations, have been coupled to aromatic substrates. ... 

Diazonium coupling reactions are typical electrophilic aromatic substitutions in which the positively charged diazonium ion is the electrophile that reacts with the electron-rich, ring of a phenol or arylamine. Reaction usually occurs at the para position, although ortho reaction can take place if the para position is blocked. 

The high reactivity of heterocyclic diazonium ions in azo coupling reactions is the reason why in some cases the primary diazotization products cannot be isolated. For example, diazotization of 2-methyl-5-aminotetrazole (2.14) directly yields the triazene 2.15, i. e., the N-coupling product, since the intermediate diazonium ion is reactive enough to give the N-coupling product with the parent amine even under strongly acidic conditions (Scheme 2-8 Butler and Scott, 1967). 

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). 

If an aromatic o-diamine such as 1,2-diaminobenzene (2.24) is diazotized in dilute aqueous acid, the 2-aminobenzene-l-diazonium ion formed first (2.25) undergoes a rapid intramolecular N-azo coupling reaction to give 1,2,3-benzotri-azole (2.26). Both amino groups of 2.24 can, however, be diazotized in concentrated acid (Scheme 2-18), forming the bis-diazonium ion 2.27. 1,3- and 1,4-diamines must also be bisdiazotized in concentrated acids in order to avoid inter-molecular N- or C-coupling. 

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 rate constants in Figure 5-3 were measured by injecting the solution of the (E)-diazoate into a buffer solution that also contained a highly reactive coupling component (2-naphthol-3,6-disulfonic acid, except at pH values below 2.5, where l,8-dihydroxynaphthalene-3,6-disulfonic acid was used instead). The diazonium ion formed reacts rapidly with these naphthols, and the concentration of the corresponding azo compounds was determined spectrophotometrically. 

An analogous regioselective effect of silver ions on an addition reaction of arene-diazonium ions was found by Ignasiak et al. (1975) with cyanide salts. Potassium cyanide yields diazocyanides (Ar— N2 — C = N see Sec. 6.6), i.e., C-coupling products, but with silver cyanide - albeit in low yield (7-9%) - diazoisocyanides (Ar—N2 — N = C) are formed (a better synthesis of diazoisocyanides is described in Sec. 6.4). 

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). 

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. 

C-coupling is of outstanding importance in the azo coupling reaction for the synthesis of azo dyes and pigments. An aromatic or heteroaromatic diazonium ion reacts with the so-called coupling component, which can be an aromatic primary, secondary, or tertiary amine, a phenol, an enol of an open-chain, aromatic, or heteroaromatic carbonyl compound, or an activated methylene compound. These reactions at an sp2-hybridized carbon atom will be discussed in Chapter 12. In the... 

First of all, there are the two products of O-coupling addition of methoxide ion to the diazonium ion, the (Z)- and (jE)-diazo methyl ethers. As discussed in Section 6.2, they are formed in reversible reactions with half-lives of the order of a fraction of a second (Z) to a minute (E). The two diazo ethers are, however, decomposed rapidly to the final dediazoniation products. We show in Scheme 8-47 the products obtained by Broxton and McLeish (1983 b) in the dediazoniation of 4-chloro-3-nitrobenzenedi-azonium ion (8.64) with methoxide ion in CH3OH. The products are 4-chloro-3-nitro-anisole (8.65, 49 9o), 2-chloro-nitrobenzene (8.66, 449o), and 2-nitroanisole (8.67). 

In the first paper on arenediazonium salt/crown ether complexes, Gokel and Cram (1973) mention that they were not able to synthesize the rotaxane 11.14 by an azo coupling reaction of the complexed diazonium ion with Af,Af-dimethylaniline. 

The replacement of an electrofugic atom or group at a nucleophilic carbon atom by a diazonium ion is called an azo coupling reaction. By far the most important type of such reactions is that with aromatic coupling components, which was discovered by Griess in 1861 (see Sec. 1.1). It is a typical electrophilic aromatic substitution, called an arylazo-de-hydrogenation in the systematic IUPAC nomenclature (IUPAC 1989c, see Sec. 1.2). 

As discussed in Sections 2.6 and 4.2, 5-diazo-l,2,3,4-tetracyanocyclopentadiene (2.52) has many properties that are characteristic of an aromatic diazonium ion. It is therefore not surprising that it forms azo compounds with A A-dimethylaniline or 2-naphthol as coupling components (Webster, 1966). 

A quantitative study of the azo coupling reactions of heterocyclic diazo compounds was made by Sawaguchi et al. (1971), who measured the coupling rates of seventeen heterocyclic diazonium ions with R-acid (2-naphthol-3,6-disulfonic acid) and compared them with those of the benzenediazonium ion with the same coupling component. All the heterocyclic diazonium ions investigated react faster with R-acid than does the benzenediazonium salt. More recently, Diener and Zollinger (1986) confirmed Sawaguchi s results and interpreted them in more detail. 

The pK values for azolediazonium ions (Scheme 12-4) refer to the heterolysis of the NH bond, not to the addition of a hydroxy group. Therefore, these heteroaromatic diazo components may react either as a cation (as shown in Scheme 12-4) or as the zwitterion (after loss of the NH proton). Diener and Zollinger (1986) investigated the relative reactivities of these two equilibrium forms (Scheme 12-5) in the azo coupling reaction of l,3,4-triazole-2-diazonium ion with the tri-basic anion of 2-naphthol-3,6-disulfonic acid. 

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