Aminoquinolines, diazotization

Triazolo[4,5-/i]quinoline 143 and triazolo[4,5-/i]quinoline-5-arsonic acid 144 were isolated from mother liquors after reduction and bis-diazotization of 5,7-dinitro-8-(4-toluenesulfone)aminoquinoline in the presence of cupric sulfate with trisodium arsenite (32JCS2196). 

Aminopyridines, aminopyridine oxides, and 3-aminoquinoline are obviously diazotized by analogous mechanisms. Kalatzis (1967 b) studied the diazotization of 4-aminopyridine over a very large range of acid concentrations (0.0025-5.0 m HC104). This compound is comparable to 2-aminothiazole in its acid-base properties the heterocyclic nitrogen is easily protonated at pH 10, whereas the amino group is a very weak base (pKa = -6.5). Therefore, the kinetics indicate that the (mono-protonated) 4-aminopyridinium ion reacts with the nitrosyl ion. The... 

Aminopyridines90 91, aminopyridine-1 -oxides25, 3-aminoquinoline92 and 2-aminothi-azole93 are obviously diazotized by analogous processes. For 4-aminopyridine, 3-aminoquinoline and 2-aminothiazole it was shown that the monocation, protonated at the heterocyclic nitrogen, is nitrosated preferentially. 

Deamination of 4,5-dimethyl-8-aminoquinoline by hypophosphorous add gives only a 6% yield of 4,5-dimethylquinoline, There is some evidence, however, that the diazotization takes an abnormal course.84... 

In a search for new spasmolytics, Heilbron and Hey and their co-workers synthesized a number of pyridylquinolines by coupling diazotized aminoquinolines with pyridine.59-60 Pyridine has also been substituted by the 3-pyridyl radical in the Gomberg reaction, the 2-and 3-substituted products being obtained in 55 and 5% yield, respectively, together with a third product obtained in 20% yield which was tentatively formulated as the 4-isomer.61 The same radical was coupled with 4-methyl- and 4-ethyl-pyridine and gave in each case mixtures of the two possible substitution products in which the radical had coupled predominantly with the carbon atom of pyridine adjacent to the alkyl substituent, as shown in Eq. (19).61... 

Aminothieno 2,3-b pyridine (81) closely resembled the analogous 3-aminoquinoline, behaving as a typical aromatic primary amine.86 It could be diazotized, and the diazonium salt yielded the 5-bromo- (48%), -chloro- (40%), -hydroxy- (65%), and -cyanothieno[2,3- >]pyridine (13%) under the usual conditions. 

The chemical properties of 2- afid 4-aminopyridines and related compounds are different from those of aniline for example, they are not easily diazotized and can sometimes be hydrolyzed with concomitant loss of ammonia. These facts have been taken to indicate that 2- and 4-aminopyridinc, 2- and 4-aminoquinoline, 9-aminophenanthridine, and 9-aminoacridine- exist in the imino form. However, it gradually became apparent that these differences in chemical properties could be explained by the changed electronic environment of the groups concerned without recourse to tautomerism ... 

---