Phosphoric acid, diazotization with

Phenyl lithium. 429 Phenylmagnesium bromide, 428 with 2-azidothiazoles, 113 Phosphate, chlorodiethyl, 401 chlorodiethylthio, 401 Phosphoric acid, diazotization with, 66 Phosphorous compounds, with 2-aminothia-zoles, 68, 69... 

The phthalide 25, obtainable by condensation of 4,4 -bisdimethyl-aminobenzophenone-2-carboxylic acid with 3-dimethylaminoacetanilide and subsequent hydrolysis, was diazotized in sulfuric acid and the resultant diazonium salt treated with copper powder to yield 26. However, better yields are reportedly obtained by carrying out ring closure of the diazonium salt in phosphoric acid.103 A further synthetic route has also been described in which phthalides undergo intramolecular cyclization in the presence of aluminum chloride and urea.104,105 Thus, Crystal Violet lactone (2) has been directly converted into phthalide 26.106... 

Amino-l,2,4-thiadiazoles (129) are diazotised at — 10°C in phosphoric acid, and couple in the usual way with reactive aromatic compounds such as naphthol, giving moderate yields of diazo dyes <82AHC(32)285>. Diazotization of (129) (R = H and Ph) may be accomplished using aqueous sodium nitrite and hydrochloric acid. Treatment of the resulting salt with sodium azide yields the 3-azido derivative (130) (Equation (17)) <86CC800>. 

The major drawback of the usual diazotization with alkaline nitrite lies in the production of two moles of water per mole of aniline. As in such conditions water is responsible for phenolic byproducts and rapid corrosion of vessels, several modifications have been recommended. The first is the addition, before sodium nitrite, of a dehydrating compound, such as concentrated sulfuric acid (used in an equimolar amount with respect to aniline),47 phosphoric oxide,48 or water-sensitive halides (PC15, SbCl5, BF3, etc., with MX /ArNH2 1 2).48 The second, which is more expensive, involves diazotization agents which deliver only one mole of water per mole of aniline. 

Diazonium Salts from 3-Amino-1,2,4-thiadiazoles The 3-amino group in 1,2,4-thiadiazoles (e.g. in the 5-phenyl homolog) is also capable of being diazotized, preferably in concentrated phosphoric acid. The resulting diazonium salt may be coupled in the usual way, but with sufficiently reactive partners only (e.g. phenol and 0-naphthol).126... 

Frequently, weakly basic amines, such as negatively substituted 4-nitroandines or aminoheterocycles, must be diazotized. These reactions require concentrated sulfuric acid, phosphoric acid (85 wt %), glacial acetic acid, propionic acid, or mixtures of thereof as the reaction medium, with nitrosylsulfuric acid as the reagent. On an industrial scale, this reaction is conducted preferably in enameled, stirred vessels. Whenever the stability of the amine or the diazonium salt permits, diazotization is carried out in concentrated sulfuric acid at slightly elevated temperature (10-40 °C). With regard to safety, factors such as temperature and concentration must be controlled carefully to avoid explosion [42],... 

If the amine is basic enough to form a salt with dilute mineral acids in aqueous solution, the normal diazotization method of adding sodium nitrite in aqueous solution to a solution or suspension of the amine salt is satisfactory. Variations in die order of addition of die reagents are sometimes used, for example, when there is another functional group present which is sensitive to nitrous acid, or if die amine salt is very insoluble. Weakly basic amines can often be diazotized successfully either in concentrated sulfuric acid or in a mixture of sulfuric acid with acetic or phosphoric acid. It is likely that nitrosylsul fiiTic acid is formed on addition of sodium nitrite to concentrated sulfuric acid, and that this is the nitro sating agent when the amine is subsequently added. 

A successful procedure involves the treatment of the weakly basic amine in concentrated acid, sulfuric, phosphoric, or glacial acetic, with nittosyl-sulfuric acid. In this manner, the more intractable amines having two or more meta-directing or halogen substituents are subjected to diazotization, e.g., 2,4,6-trinitroaniline (picramide) and 2,6-diiodo-4-nitroaniIine. Pyridine has been used as a solvent in diazotizations with nitrosylsul-furic acid. ... 

Aminothiazole is easily diazotized in a concentrated solution of nitric and phosphoric acid. A large-scale procedure for the diazotization of 2-aminothiazole and its conversion to 2-bromothiazole has been reported . The synthesis of 2-halothiazoles can be easily achieved from 2-aminothiazole by (i) diazotization and (ii) treatment with CuCl, CuBr, or Nal <85JHC1621>. When the diazonium salt derived from 2-aminothiazole is treated with sodium azide the 2-azido derivative in equilibrium with its tetrazole cyclization product are obtained <92BMC1603>. 

Amino-l,2,4-thiadiazoles are diazotized at — 10°C in phosphoric acid, and couple with naphthol to give moderate yields of diazo dyes.182,388 In their limited stability, the 3-diazonium salts resemble those of 3-aminoiso-thiazole, but differ from the more stable 1,2,4-thiadiazole- and isothiazole-5-diazonium salts a consideration of the 7t-electron densities leads to the opposite conclusion.182... 

Diazotization of 2-aminothiazoles (71) in borofluoric-phosphoric acids affords high yields of thiazolyl-2-diazonium tetrafluoroborates (72) as yellow crystalline fairly stable solids. These afford 2-fluorothiazoles (73 X = F) on being carefully heated under reduced pressure in admixture with sand and potassium fluoride, or on being warmed to 60—80 °C in... 

The diazonium cation prepared from 4-aminopyridine by Witt s method couples with phenols 448 and with primary aromatic amines3 3, but the value of this method in the coupling with dimethylaniline is doubt-ful389. 2-Aminopyridine fails also, and indeed no 2-pyridyl azo compounds have been obtained directly by diazotization and coupling. Successful coupling of dimethylaniline to pyridine-4-diazonium cation prepared in a mixture of phosphoric and nitric acids has been reported oec ... 

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