Diazonium ions, aromatic substitution reactions

A second group of aromatic substitution reactions involves aryl diazonium ions. As for electrophilic aromatic substitution, many of the reactions of aromatic diazonium ions date to the nineteenth century. There have continued to be methodological developments for substitution reactions of diazonium intermediates. These reactions provide routes to aryl halides, cyanides, and azides, phenols, and in some cases to alkenyl derivatives. 

Chapter 11 focuses on aromatic substitution, including electrophilic aromatic substitution, reactions of diazonium ions, and palladium-catalyzed nucleophilic aromatic substitution. Chapter 12 discusses oxidation reactions and is organized on the basis of functional group transformations. Oxidants are subdivided as transition metals, oxygen and peroxides, and other oxidants. 

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 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). 

Diarylamines couple with considerably more difficulty than alkylarylamines. In contrast to primary aromatic amines and alkylarylamines, dialkylarylamines are substituted by diazonium ions only in the 4-, not in the 2-position. This is due to the considerable sensitivity of azo coupling reactions to steric hindrance. 

The first widely used intermediates for nucleophilic aromatic substitution were the aryl diazonium salts. Aryl diazonium ions are usually prepared by reaction of an aniline with nitrous acid, which is generated in situ from a nitrite salt.81 Unlike aliphatic diazonium ions, which decompose very rapidly to molecular nitrogen and a carbocation (see Part A, Section 4.1.5), aryl diazonium ions are stable enough to exist in solution at room temperature and below. They can also be isolated as salts with nonnucleophilic anions, such as tetrafluoroborate or trifluoroacetate.82 Salts prepared with 0-benzenedisulfonimidate also appear to have potential for synthetic application.83... 

This chapter is concerned with reactions that introduce or replace substituent groups on aromatic rings. The most important group of reactions is electrophilic aromatic substitution. The mechanism of electrophile aromatic substitution has been studied in great detail, and much information is available about structure-reactivity relationships. There are also important reactions which occur by nucleophilic substitution, including reactions of diazonium ion intermediates and metal-catalyzed substitution. The mechanistic aspects of these reactions were discussed in Chapter 10 of Part A. In this chapter, the synthetic aspects of aromatic substitution will be emphasized. 

In the electrophilic substitution reactions of coordinated aromatic ligands for which rate data are presently available, there is every indication that the mechanism of the reaction is unchanged in its essentials. Following the lead of the physical organic chemists the course of the reaction of the complex and the diazonium ion can be depicted as ... 

Not all reactions of diazonium ions involve cleavage of the C-N bond. An important group of reactions of arenediazonium ions involves aromatic substitution by the diazonium ion acting as an electrophilic agent to yield azo compounds, Ar—N=N—Ar ... 

Azo coupling is the most widely used industrial reaction in the production of dyes, lakes and pigments. Aromatic diazonium ions acts as electrophiles in coupling reactions with activated aromatics such as anilines or phenols. The substitution normally occurs at the para position, except when this position is already occupied, in which case ortho position is favoured. The pH of solution is quite important it must be mildly acidic or neutral, since no reaction takes place if the pH is too low. 

As shown in other sections of this chapter, overall attention has shifted from diazonium salts as aryl radical sources to bromo- or iodobenzenes. One of the few recent attempts to improve the classical Pschorr cyclization using diazonium ions as starting materials led to the discovery of new catalysts [119]. Results from a first samarium-mediated Pschorr type show the variety of products that can be expected from intramolecular biaryl syntheses under reductive conditions (Scheme 22). Depending on the substitution pattern of the target aromatic core and the reaction conditions, either the spirocycle 60, the biphenyl 61, or the dearomatized biphenyl 62 were formed as major product from 63 [120]. 

Although aromatic halides are inert to both SN1 and SN2 reactions (see Chapter 8), aromatic diazonium ions can act as the electrophilic partner in a nucleophilic substitution reaction. These ions are highly reactive because the leaving group, N2, is an extremely weak base ... 

Diazonium Salts as Electrophiles Diazo Coupling Arenediazonium ions act as weak electrophiles in electrophilic aromatic substitutions. The products have the structure Ar—N=N—Ar, containing the —N=N— azo linkage. For this reason, the products are called azo compounds, and the reaction is called diazo coupling. Because they are weak electrophiles, diazonium salts react only with strongly activated rings (such as derivatives of aniline and phenol). 

In this paper reactions of aromatic, heteroaromatic and related diazonium ions with nucleophiles are dia ussed. In such reactions substitution by the diazonium ion of an electrofugic atom or group bonded to carbon takes place. Occasionally reference is made to N- and P-coupling. In Section 4 the respective substitution at nitrogen (formation of diazoamino compounds) is included for comparative purposes. 

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