Acid-base characteristics of 5-diazoimidazole derivatives

Abstract

The transformation of 5-diazoimidazoles containing a carboxamide, ethoxycarbonyl, or nitro group at position 4 into diazonium salts was studied by UV and IR spectroscopy. It was shown by means of model compounds and chemical transformations that nitrosamines are involved in the transformation.Study of the acid-base transformations of certain diazo compounds of the azole series into the corresponding diazonium salts has made it possible to obtain the pK a values of these compounds [1]. For 5-diazoimidazole-4-carboxamide (Ia) it was shown that at pH 1, 2.5, and 7 it exists in the form of the diazonium salt, diazo compound, and diazohydroxide respectively [2].The present communication is devoted to a study of the specific characteristics of the acid-base equilibrium in 5-diazoimidazole derivatives at acidities H 0 -3.38 to -0.02 and in the range of pH 0-10.We studied the IR and UV spectra of the amide (Ia), ethyl 5-diazoimidazole-4-carboxylate (Ib), and 5-diazo-4-nitroimidazole (Ic) under various conditions. 4-Ethoxycarbonylimidazole-5-diazonium tetrafluoroborate (II) was synthesized from the imidazole (Ib) as model compound, and its spectra were recorded. The data are given in Table 1.As seen from Table 1, in strongly acidic media (H o -1.9-0) the diazoimidazoles (Ia, b) change into the corresponding diazonium salts (IIIa, b). This is demonstrated by the shift of the frequency for the stretching vibrations of the diazo group in the IR spectra by 40-60 era-1 toward larger values. [They approximate to the frequency for the vibrations of the diazonium group in the model compound (II).] In the UV spectra, recorded under the same conditions, a hypsochromic shift of the longwave absorption maximum by 30 nm is observed. At pH 1 the IR spectrum of the ester (Ib) contains bands for the vibrations of both the diazo and the diazonium groups.Thus, the presented data indicate that compound (Ib) in 1-5 N hydrochloric acid exists in the form of the diazonium salt (IIIb). At pH 1 there is an equilibrium between the salt (IIIb) and the diazo compound (Ib). At pH 5-9.2 only compound (Ib) is present in the solution.The existence of ranges of pH in which 5--diazoimidazoles and the corresponding diazonium salts exist in the solution makes it possible to determine the pK a values of the diazonium salts as NH acids. For this purpose the UV spectra of compounds (Ia, b) in 5-0.01 N hydrochloric acid and also of 5-diazo-4-nitroimidazole (Ic) in 50, 20, 10, 5, and 2.5% sulfuric acid (H 0 -3.38, -1.01, -0.31, -0.02, and 0 respectively) and in water were studied. Here it was unexpectedly found that the spectral curves of solutions of compounds (Ia, b) did not pass through an isobestic point (Figs. 1, 2). A similar pattern was observed for solutions of diazonitroimidazole (Ic) (Fig. 3). This could be explained by the stow establishment of an equilibrium between the two tautomeric forms (A and B) of the diazonium salts (III) and also by the participation of the diazo compound (I) in an equilibrium. However, only one band for the stretching vibra...

Isoxazolyl-Derived 1,4-Dihydroazolo[5,1-c][1,2,4]Triazines: Synthesis and Photochemical Properties

“…The starting diazopyrazole 6 [ 61 ], diazoimidazoles 7a , b [ 62 , 63 ], and enamines 3a – d [ 47 ] were obtained as previously described.…”

Non-aromatic azolo[5,1-c][1,2,4]triazines: a pot, atom and step economic (PASE) synthesis, mechanistic insight and fluorescence properties

“…Ethyl ester 5-diazopyrazole-4-carboxylic acid 24 5a , ethyl ester 5-diazoimidazole-4-carboxylic acid 25 5b , methyl ester 5-diazoimidazole-4-carboxylic acid 26 5c , 5-diazoimidazole-4-carboxamide 27 5d and 5-diazoimidazole-4- N -methylcarboxamide 1 5e were synthesized using known procedures.…”

“…The reaction of the 4-substituted 5-diazoazoles 5a-e (n N2 = 2170-2200 cm À1 ), obtained by diazotization 1,[24][25][26][27] of the corresponding 5-aminoazoles, with thiadiazol-5-yl enamines [28][29][30][31] 3a-d was carried out in polar solvents at room temperature (Scheme 3). In all cases, 1,2,3-thiadiazolyl-1,4-dihydroazolo[5,1c][1,2,4]triazines (TDATs) 6a-i were obtained in 65-79% yields.…”