Aromatic diazonium ion

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... 

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... 

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... 

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. 

While A -dimethylaniline is an extremely reactive aromatic substrate and is readily attacked by such weak electrophiles as aiyl diazonium ions and nitrosonium ion, this reactivity is greatly diminished by introduction of an alkyl substituent in the ortho position. Explain. 

R—N=N=. Aryl diazonium ions are formed by treatment of primary aromatic amines with nitrous acid. They are extremely useful in the preparation of aryl halides, phenols, and aryl cyanides. 

Experimental observations indicate that electron-rich aromatic nucleophiles, such as phenoxide, add to phenyl diazonium ion in the same way as dimethylamine. 

An attempt to cyclize the corresponding fully aromatic system (16) gave instead the indazole (17), Eq. (24), probably because the acidity of the bridging methylene group in (16) enables cyclization to occur on the diazonium ion through the carbanion. ... 

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. ... 

The nitrosation of primary aromatic amines 1 with nitrous acid 2 and a subsequent dehydration step lead to the formation of diazonium ions 3. The unstable nitrous acid can for example be prepared by reaction of sodium nitrite with aqueous hydrochloric acid. 

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 ... 

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. 

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. 

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). 

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. 

Other Reactions Involving Formation of Aromatic Diazonium Ions... 

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... 

This statement does not mean, however, that the mechanism of diazotization was completely elucidated with that breakthrough. More recently it was possible to test the hypothesis that, in the reaction between the nitrosyl ion and an aromatic amine, a radical cation and the nitric oxide radical (NO ) are first formed by a one-electron transfer from the amine to NO+. Stability considerations imply that such a primary step is feasible, because NO is a stable radical and an aromatic amine will form a radical cation relatively easily, especially if electron-donating substituents are present. As discussed briefly in Section 2.6, Morkovnik et al. (1988) found that the radical cations of 4-dimethylamino- and 4-7V-morpholinoaniline form the corresponding diazonium ions with the nitric oxide radical (Scheme 2-39). 

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. 

In the literature discussing these results, the coincidence of the NN bond lengths in diazonium ions with that in dinitrogen seems always to be regarded with complete satisfaction. In the opinion of the present author this close coincidence is somewhat surprising, firstly because of the fact that in diazonium ions one of the nitrogen atoms is bonded to another atom in addition to the N(2) atom, and secondly because work on dual substituent parameter evaluations of dediazoniation rates of substituted benzenediazonium ions clearly demonstrates that the nx orbitals of the N(l) nitrogen atom overlap with the aromatic 7t-electron system (see Sec. 8.4). 

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). 

In this section we discuss theoretical investigations that help us to understand the structures, electronic spectra, and reactions of aromatic and heteroaromatic diazonium ions. 

In the historical introduction to this book (Sec. 1.1) it was mentioned that the discoverer of diazo compounds, Peter Griess, realized quite early (1864 a) that these species could react with alkali hydroxides. Thirty years later Schraube and Schmidt (1894) found that the primary product from the addition of a hydroxide ion to a diazo compound can isomerize to form a secondary product. In this section we will discuss the equilibria of the first acid-base process of aromatic diazonium ions. In the following section additional acid-base reactions will be treated in connection with the isomerism of addition products of hydroxide ions to diazonium ions. 

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