Diazonium-compounds, general preparation reactions

Diazonium compounds are usually prepared in mineral acid solution, and the nitrous acid generated from sodium nitrite. Sufficient acid must be used to generate nitrous acid and to form the salt of the base, and still leave the solution acid. In practice 2J—2f mols. of hydrochloric acid are generally employed. In most cases it is essential that the reaction be carried out at about 0°, as many diazonium solutions decompose above this temperature. The reaction goes very readily in some cases but in others, and especially where an acid group is present, e.g., naphthylamine sul-phonic acids, the reaction is only carried out with difficulty. It is possible to diazotise a solid in suspension, but the reaction is usually very slow. If the solid is dissolved and reprecipitated in a fine state of division the action goes much more quickly. 

The widespread use of aryl boronic acids or aryl boronates in various metal-catalysed C-C bond-forming reactions has created a substantial demand for these versatile nncleophiles. A general procedure for the preparation of these compounds, based on a NHC/Pd catalysed coupling, has been reported by Fiirtsner and co-workers nsing aryl chlorides and the tetraalkoxy diboron derivative 27 as conpling partners. Very good yields were obtained in several cases especially when electron poor aryls were employed [169]. Milder reaction conditions can be achieved when diazonium salts are used instead of chlorides [170] (Scheme 6.51). 

This diazotization reaction is compatible with the presence of a wide variety of substituents on the benzene ring. Arenediazonium salts are extremely important in synthetic chemistry, because the diazonio group (N=N) can be replaced by a nucleophile in a radical substitution reaction, e.g. preparation of phenol, chlorobenzene and bromobenzene. Under proper conditions, arenediazonium salts react with certain aromatic compounds to yield products of the general formula Ar-N=N-Ar, called azo compounds. In this coupling reaction, the nitrogen of the diazonium group is retained in the product. 

Perhaps the best-known method of preparing aromatic azo compounds involves the coupling of diazonium salts with sufficiently reactive aromatic compounds such as phenols, aromatic amines, phenyl ethers, the related naphthalene compounds, and even sufficiently reactive aromatic hydrocarbons. Generally, the coupling must be carried out in media which are neutral or slightly basic or which are buffered in the appropriate pH range. The reaction may also be carried out in nonaqueous media. While some primary and secondary aromatic amines initially form an A-azoamine, which may rearrange to the more usual amino-C-azo compound, tertiary amines couple in a normal manner. 

Aromatic fluorination A new method of aromatic fluorination involves treatment of aryltriazenes, readily prepared from aryldiazonium ions and dialkyl-amines, with 70% hydrogen fluoride in pyridine. The yields of product from this reaction are usually higher than those obtained by the reaction of HF-pyridine with a diazonium ion (6, 285) o-methoxy, iodo-, bromo- and nitro-substituted aryltriazenes generally give unsatisfactory yields. This method may be useful for the synthesis of " F-labeled compounds. 

Azo compounds have the general formula R -N=N-R, where R and R are alkyl or aryl groups. Aromatic azo compounds are generally strongly colored and many are easily prepared via a so-called diazo coupling reaction, in which a diazonium cation is coupled with a relatively active (i.e., highly nucleophilic) arene such as a phenol or aniline. The dye called aniline yellow, for example, is prepared as follows ... 

The most widely used approach to the preparation of PESs in both academic research and technical production is a polycondensation process involving a nucleophilic substitution of an aromatic chloro- or fluorosulfone by a phenoxide ion (Eq. (3)). Prior to the review of new PESs prepared by nucleophilic substitution publications should be mentioned which were concerned with the evaluation and comparison of the electrophilic reactivity of various mono- and difunctional fluoro-aromats [7-10]. The nucleophilic substitution of aromatic compounds may in general proceed via four different mechanism. Firstly, the Sni mechanism which is, for instance, characteristic for most diazonium salts. Secondly, the elimination-addition mechanism involving arines as intermediates which is typical for the treatment of haloaromats with strong bases at high temperature. Thirdly, the addition-elimination mechanism which is typical for fluorosulfones as illustrated in equations (3) and (4). Fourthly, the Snar mechanism which may occur when poorly electrophilic chloroaromats are used as reaction partners will be discussed below in connection with polycondensations of chlorobenzophenones. 

As regards the former, whilst overall yields are usually not high Table 5.32), the reactions, particularly those using diazonium salts as the sources of radicals, provide easy routes to many compounds not otherwise readily availablei 27. Early preparative experiments created the impression that 2- and 4-substitution occurred, and because of the ease of isolation of the 4-substituted pyridine its importance was generally overestimated. This... 

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