Diazonium-compounds, general preparation

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. 

A deamination procedure for which general application is claimed has been described by Hodgson and Madden.6 An aqueous solution of the diazotized amine is treated with the molar quantity of naphthalene-1,5-, disulfonic acid or 2-naphthol-l-sulfonic acid, whereupon the stabilized diazonium compound is precipitated. In most cases the stabilized diazo salts require slight variations in technique for their preparation in optimum yield. "The dry salt is reduced at room temperature in ethanol containing zinc or copper. Overall yields of the order of 90% are reported however, the purity of the product is not specified. 

Derived diazonium compounds, in general, undergo the usual interconversions although yields are often rather low. Reference has already been made (see p. 404) to an unsuccessful attempt to prepare 3-hydroxy-6-methylphenanthridine from the amine.11 1-Amino-phenanthridone gives only impure 1-hydroxyphenanthridone on diazotization in sulfuric acid followed by heating,127 but the method is more satisfactory in the case of 2-amino-iV-methylphenanthri-done.126 Phenanthridones are usually more readily available than the corresponding 6-amino compounds, but as a structural proof phenan-thridone itself has been obtained by the action of nitrous acid on 6-aminophenanthridine.150... 

A generally applicable procedure for conversion of aromatic amines into phenols or naphthols consists of boiling aromatic diazonium compounds (for their preparation see page 581). This method will be illustrated here by some examples. 

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). 

As with resorcinol, the 1,3-diamines of the benzene series are useful in the preparation of disazo dyes that can form the basis of brown shades. Generally the 2,4-disazo compound is the product of coupling with two equivalents of diazonium salt, but at higher pH values formation of the 4,6-isomer can be significant [31]. Again, the 1,2- and 1,4-diamines reduce diazonium salts on attempted coupling. 1,3,5-Triaminobenzene has been found to couple three times [32]. 

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. 

A fairly general procedure for the synthesis of o- and p-quinones consists in coupling a phenol with a diazonium salt and reducing the resulting azo compound to an aminophenol with sodium dithionite. Mild oxidation with, for example, iron(m) chloride results in the formation of the corresponding quinone (e.g. the preparation of 1,2-naphthoquinone described and formulated in Expt 6.131). 

In general, the Balz-Schiemann process involves the preparation of an insoluble arenediazonium ictrafluoroborate, either by aqueous diazotization of the corresponding arylamine with tetrafluoroboric add (Route A), or by addition of tetrafluoroboric acid or alkaline tetrafluoroborates to an aqueous solution of another previously formed arenediazonium salt (Route B). After filtration and drying, the diazonium tetrafluoroborate is decomposed to form the corresponding organo-fluorine compound. 

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. 

A few examples of the reduction of aliphatic diazo compounds to hydrazines exist," but this is not a generally applicable method for the synthesis of alkylhydrazines. On the other hand, arylhydrazines can be prepared by reduction of aromatic diazonium salts.The most commonly used reagents for this conversion are sulfur dioxide (or sodium sulfite) and tin(II) chloride, these being used to reduce arenediazo-nium chlorides in aqueous solution. Several other reagents, including sodium amalgam and triphenylphosphine, have been used for specific reductions of this type. Arenediazonium tetrafluoro-borates have been reduced to the correspwnding hydrazinium salts (3) by benzeneselenol in dichloro-... 

Reduction of nitro compounds to amines is an essential step in what is probably the most important synthetic route in aromatic chemistry. Nitro compounds are readily prepared by direct nitration when a mixture of o- and p-isomers is obtained, it can generally be separated to yield the pure isomers. The primary aromatic amines obtained by the reduction of these nitro compounds are readily converted into diazonium salts the diazonium group, in turn, can be replaced by a large number of other groups (Sec. 23.11). In most cases this sequence is th best method of introducing these other groups into the aromatic ring. In addition, diazonium salts can be used to prepare the extremely important class of compounds, the azo dyes. 

The advantages of the synthesis of aryl halides from diazonium salts will be discussed in detail in Sec. 25.3. Aryl fluorides and iodides cannot generally be prepared by direct halogenation. Aryl chlorides and bromides can be prepared by direct halogenation, but, when a mixture of 0- and p isomers is obtained, it is difficult to isolate the pure compounds because of their similarity in boiling point. Diazonium salts ultimately go back to nitro compounds, which are usually obtainable in pure form. 

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