Amines diazotization, review

The reaction is carried out analogously to the diazotization of primary amines. A review by Lieber and Minnis270 deals with carbamoyl azides. 

Butler recently reviewed the diazotization of heterocyclic amines (317). Reactions with nitrous acid yield in most cases N-exocyclic compounds. Since tertiary amines are usually regarded as inen to nitrosation, this... 

Some recent general reviews deal with the mechanism of N-nitrosation in aqueous solution (345), the nitrosation of secondary amines (346). the effect of solvent acidity On diazotization (347) and the reactivity of diazonium salts (1691). Therefore, a complete rationalization of the reactivity of amino azaaromatics would be timelv. 

Aromatic diazonium compounds became industrially very important after Griess (1866a) discovered in 1861/62 the azo coupling reaction, by which the first azo dye was made by C. A. Martius in 1865 (see review by Smith, 1907). This is still the most important industrial reaction of diazo compounds. Hantzsch and Traumann (1888) discovered that a heteroaromatic amine, namely 2-aminothiazole, can also be diazotized. Heteroaromatic diazonium compounds were, however, only used for azo dyes much later, to a small extent in the 1930 s, but intensively since the 1950 s (see Zollinger, 1991, Ch. 7). 

An increased hydrogen ion concentration, that is a considerably greater amount of acid than the theoretical two equivalents of Scheme 2-1, is necessary in the diazotization of weakly basic amines. The classic example of this is the preparation of 4-nitrobenzenediazonium ions 4-nitroaniline is dissolved in hot 5-10 m HC1 to convert it into the anilinium ion and the solution is either cooled quickly or poured onto ice. In this way the anilinium chloride is precipitated before hydrolysis to the base can occur. On immediate addition of nitrite, smooth diazotization can be obtained. The diazonium salt solution formed should be practically clear and should not become cloudy on standing in the dark. Some practice is necessary, and details can be found in the books emphasizing preparative aspects (Fierz-David and Blangey, 1952 Saunders and Allen, 1985 in Houben-Weyl, Vol. E 16a, Part II, the chapter written by Engel, 1990). These books give a series of detailed prescriptions for specific examples and a useful review of the principal variations of the methods of diazotization. Such reviews have also been written by Putter (1965) and Schank (1975). 

The C-nitrosation of aromatic compounds is characterized by similar reaction conditions and mechanisms to those discussed earlier in this section. The reaction is normally carried out in a strongly acidic solution, and in most cases it is the nitrosyl ion which attacks the aromatic ring in the manner of an electrophilic aromatic substitution, i. e., via a a-complex as steady-state intermediate (see review by Williams, 1988, p. 58). We mention C-nitrosation here because it may interfere with diazotization of strongly basic aromatic amines if the reaction is carried out in concentrated sulfuric acid. Little information on such unwanted C-nitrosations of aromatic amines has been published (Blangey, 1938 see Sec. 2.2). 

Most diazotized heteroaromatic amines undergo normal coupling reactions with common aromatic coupling components, as well as with CH acidic compounds (review Butler, 1975). 

For reviews of rearrangements arising from diazotization of aliphatic amines, see, in Patai The Chemistry of the Amino Group Wiley NY, 1968, the articles by White, E.H. ... 

The diazotization of heteroaromatic amines is a ticklish procedure. In spite of the great increase in interest for disperse dyes based on heterocyclic diazo components, little systematic knowledge is available. In a review of such diazo components13 practically nothing is mentioned on suitable methods of diazotization and on yields (which are in part low). The somewhat older review of Butler14 is, in this respect, more informative. So too is the section on synthesis in the general review on diazoazoles by Cirrincione and coworkers15. 

The methods used for preparing cyanobenzo[6]thiophenes are merely summarized in the present section they are discussed in more detail elsewhere in this review. Cyanobenzo[6]thiophenes are most conveniently prepared by heating a halobenzo[6]thiophene with cuprous cyanide in a suitable solvent (Section VI, D, 2), or by means of a Sandmeyer reaction of the appropriate diazotized amine (Section VI, F, 4 and 5). Less commonly they are prepared from aldehydes or aldoximes, by methods described in Section VI, L, 1, or from carbox-... 

The foimation of aromatic diazonium salts from aromatic primary amines is one of the oldest synthetic procedures in organic chemistry. Methods based on nitrosation of amine with nitrous acid in aqueous solution are die best known, but diere are variants which are of particular use widi weakly basic amines and for the isolation of diazonium salts fiom nonaqueous media. General reviews include a book by Saunders and AUen and a survey of preparative methods by Schank. There ate also reviews on the diazotization of heteroaromatic primary amines and on the diazotization of weakly basic amines in strongly acidic media. The diazotization process (Scheme 11) goes by way of a primary nitrosamine. 

The conversion of an acid to an amine of one less carbon may be conveniently accomplished by way of the azide and rearrangement to the isocyanate. The azide may be obtained either from the acyl chloride and sodium azide or from an ester by treatment with hydrazine and subsequent diazotization. An excellent review including scope and limitations of the reactions, selection of experimental conditions and procedures, and a tabulation of compounds prepared thereby has been presented. ... 

Thus the acceptance of the Bloomstrand-Strecker-Erlenmeyer formula for the diazonium base and salts, and of the Hantzsch modification of the Kekule formula for the isomeric diazotates and the acid diazo hydroxide, together with the tautomeric transformations which occur, makes possible the explanation of all the facts which we have considered in the preceding discussion. These may be stated again briefly for the sake of emphasis and review, (i) The reaction of diazotization (2) the diazonium base and salts, (3) the diazotates and acid diazo hydroxide, (4) the isomerism of the diazotates, (5) the diazonium sulphites and diazo sulphonic acidsj (6) the relationship of diazo compounds to nitroso amines (7) to hydrazines (8) the coupling with amino and hydroxy compounds to form amino azo and hydroxy azo compounds, (9) the reactions of decomposition. Most of these last reactions we have not considered, but will do so presently and we shall find that they all may be likewise satisfactorily explained. The tautomeric constitution of the diazo compounds, therefore, meets every test and is generally accepted. 

Diazotization in a nonaqueous medium is useful in synthesis (review [2980]), as is demonstrated by the cyclization of the aminopyrazole (79.8) with nitrosyl chloride amines which lacked the methoxy group failed to cyclize on to the carbon atom [3201]. 2-Diazoazoles [such as (79.9)] undergo cycloadditions with... 

Regitz diazo transfer reactions have been reviewed previously.1-3 The following two main routes have been known for the synthesis of diazo compounds (1) diazotization of amines, oximes, nitrosoamines, and hydrazones (2) transfer of the diazo function from tosyl or mesyl azides to active methylene compounds. 

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