Diazonium salts, amine reactions preparation

Diazonium salts can be prepared directly by replacement of an aromatic hydrogen without the necessity of going through the amino group.136 The reaction is essentially limited to active substrates (amines and phenols), since otherwise poor yields are obtained. Since the reagents and the substrate are the same as in reaction 1-3, the first species formed is the nitroso compound. In the presence of excess nitrous acid, this is converted to the diazonium... 

Incidentally, 34 contributes more to the hybrid than 35, as shown by bond-distance measurements. In benzenediazonium chloride, the C—N distance is 1.42 A, and the N—N distance 1.08 A, which values fit more closely to a single and a triple bond than to two double bonds (see Table 1.5). Even aromatic diazonium salts are stable only at low temperatures, usually only < 5°C, although more stable ones, such as the diazonium salt obtained from sulfanilic acid, are stable up to 10 or 15°C. Diazonium salts are usually prepared in aqueous solution and used without iso-lation, although it is possible to prepare solid diazonium salts if desired (see 13-23). The stability of aryl diazonium salts can be increased by crown ether complexion. For aromatic amines, the reaction is very general. Halogen, nitro, alkyl, aldehyde, sulfonic acid, and so on, groups do not interfere. Since aliphatic amines do not react with... 

The preparation and reactions of diazonium salts. The reaction of aromatic amines with the nitrosonium ion (+NO), generated from nitrous acid (HN02), yields aryl diazonium salts, which can be converted (on loss of nitrogen) to a range of functional groups. 

They are prepared by the action of HNO2 on aromatic amines. The amine is dissolved in excess of mineral acid and sodium nitrite is added slowly until a slight excess of HNO2 is present. The reaction is usually carried out in ice-cold solution. The solution then contains the diazonium salt of the mineral acid used, anhydrous diazonium salts of unpredictable stability may be precipitated with complex anions like PF , SnCl6 BF4 . 

The most noteworthy reaction of azo-compounds is their behaviour on reduction. Prolonged reduction first saturates the azo group, giving the hydrazo derivative (C NH-NH C), and then breaks the NH NH linkage, with the formation of two primary amine molecules. If method (1) has been employed to prepare the azo-compound, these two primary amines will therefore be respectively (a) the original amine from which the diazonium salt was prepared, and (6) the amino derivative of the amine or phenol with which the diazonium salt was coupled. For example, amino-azobenzene on complete reduction gives one equivalent of aniline, and one of p-phenylene diamine, NHaCeH NH benzene-azo-2-naphthoI similarly gives one equivalent of aniline and one of... 

The experimental conditions necessary for the preparation of a solution of a diazonium salt, diazotisation of a primary amine, are as follows. The amine is dissolved in a suitable volume of water containing 2 5-3 equivalents of hydrochloric acid (or of sulphuric acid) by the application of heat if necessary, and the solution is cooled in ice when the amine hydrochloride (or sulphate) usually crystallises. The temperature is maintained at 0-5°, an aqueous solution of sodium nitrite is added portion-wise until, after allowing 3-4 minutes for reaction, the solution gives an immediate positive test for excess of nitrous acid with an external indicator—moist potassium iodide - starch paper f ... 

The reaction of an aryl diazonium salt with potassium iodide is the standard method for the preparation of aryl iodides The diazonium salt is prepared from a primary aro matic amine m the usual way a solution of potassium iodide is then added and the reac tion mixture is brought to room temperature or heated to accelerate the reaction... 

Aminoisothiazoles have all been prepared by nitration and subsequent reduction, usually in good yield, although the reduction of 4-nitroisothiazole to 4-aminoisothiazole has only been achieved in 35% yield. 4-Aminoisothiazoles behave as normal aromatic amines and the diazonium salts undergo the Sandmeyer reaction and reductive deamination. ... 

Inclusion of a para acetyl group requires a somewhat different approach to the preparation of these compounds. Reaction of the diazonium salt from p-aminoacetophenone with sulfur dioxide affords the sulfonyl chloride, 203 this is then converted to the sulfonamide, 204, Elaboration via the carbamate with cyclohexyl-amine affords acetohexamide (205). ... 

A new effective metal-ffee radical approach by Murphy et al generates the free radical by treatment with tetrathiafulvalene (TTF).1491 As depicted in scheme 16 the aromatic amine 79 is transformed into the diazonium salt 81 which on treatment with TTF leads to the radical 82. The following stereoselective cyclization gives the hexahydrocarba-zole scaffold 80, a substructure of alkaloids like aspi-dospermidin, strychnin and vinblastin. Also the non-toxic tris(trimethylsilyl)silane was employed for domino reactions, eg. for the preparation of the alkaloid aspidospermidin. 

Benzenedlazonlum chloride is prepared by the reaction of anUme with nitrous acid at 273-278K. Nitrous acid is produced In the reaction mixture by the reaction of sodium nitrite with hydrochloric acid. The conversion of prlmaiy aromatic amines into diazonium salts is known as diazotisation. Due to its Instability, the diazonium salt Is not generally stored and is used immediately after its preparation. 

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. 

Triazenes have been prepared by the treatment of resin-bound aromatic diazonium salts with secondary amines (Figure 3.27). Regeneration of the amine can be effected by mild acidolysis (Entry 1, Table 3.23). Triazenes have been shown to be stable towards bases such as TBAF, potassium hydroxide, or potassium tert-butoxide [454], and under the conditions of the Heck reaction [455]. Primary amines cannot be linked to supports as triazenes because treatment of triazenes such as R-HN-N=N-Ar-Pol with acid leads to the release of aliphatic diazonium salts into solution [373]. Triazenes derived from primary amines can, however, be used for the preparation of amides and ureas (see Section 3.3.4),... 

Support-bound triazenes, which can be prepared from resin-bound secondary aliphatic amines and aromatic diazonium salts [455], undergo cleavage upon treatment with acids, leading to regeneration of the aromatic diazonium salts. In cross-linked polystyrene, these decompose to yield nitrogen and, preferentially, radical-derived products. If the acidolysis of polystyrene-bound triazenes is conducted in the presence of hydrogen-atom donors (e.g. THF), unsubstituted arenes can be obtained (Entries 8 and 9, Table 3.47). In the presence of alkenes or alkynes and Pd(OAc)2, the initially formed diazonium salts undergo Heck reaction to yield vinylated or alkynylated arenes (Entry 10, Table 3.47). Similarly, unsubstituted arenes can be obtained by oxida-... 

The triazene linker can be used for the immobilization of aromatic diazonium salts, and therefore for aromatic amines, but not for aliphatic amines due to the instability of their diazonium salts. Cleavage of the linker can be achieved under mild acidic conditions to yield the benzylamine resin and the corresponding diazonium salt [136,145,146]. The main difference between the preparation of triazenes in solution and triazenes on solid support is the respective amine, namely bisben-zylamine and polymer-supported benzylamine 114. In solution, it was used in excess to quench unstable diazonium salts and force the reaction to completion. In the solid-phase approach it was immobilized and cannot be used in excess with respect to low loadings. A simple three-step procedure (Scheme 36) starting from benzylamine resin 114 via carboxylate 115 led to the successful preparation of ester resins 116 in essentially higher loadings. Treatment of resin 114 with 4-carboxy-benzene diazonium tetrafluoroborate yielded benzoic acid resin 115. 

The diazonium sulphate is used in preference to the diazonium chloride, since the presence of chloride ions gives rise to small quantities of the aryl chloride as a by-product. The solution must be acidic in order to avoid the coupling reaction between unreacted diazonium salt and the phenol (see Section 6.7.2, p. 946). For the preparation of phenols and cresols, the aqueous solution of the diazonium compound is warmed to about 50 °C at higher temperatures the reaction may become unduly vigorous and lead to appreciable quantities of tarry compounds. For certain substituted amines, a higher temperature (e.g. boiling 40 60% sulphuric acid) is necessary to decompose the diazonium salt... 

Azo compounds are prepared by the interaction of a diazonium salt with a phenol in the presence of sodium hydroxide or with an amine in the presence of sodium acetate. The coupling reaction is an electrophilic substitution involving the diazonium ion which reacts at the position of greatest electron availability, i.e. the position ortho or para to the electron releasing phenoxy or amino groups. 

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