Synthesis of 1-aryl(hetaryl)pyrazol-5-ols and azopyrazoles on their basis

Article abstract of DOI:10.1007/s10593-011-0778-0

Treatment of 3,5-dichloropyrid-2-ylhydrazine, and of mono- and dinitrophenylhydrazines with acetoacetic ester gave hydrazones and the products of their cyclization, namely pyrazolols. It was shown that acetoacetic ester 2,4-dinitrophenylhydrazone cyclizes

Full text of DOI:10.1007/s10593-011-0778-0

Chemistry of Heterocyclic Compounds, Vol. 47, No. 4, July 2011 (Russian original Vol. 47, No. 4, April, 2011)
SYNTHESIS OF 1-ARYL(HETARYL)PYRAZOL-5-OLS
AND AZOPYRAZOLES ON THEIR BASIS
D. N. Kuznetsov1*, A. G. Ruchkina1, and K. I. Kobrakov1
Treatment of 3,5-dichloropyrid-2-ylhydrazine, and of mono- and dinitrophenylhydrazines with acetoace-
tic ester gave hydrazones and the products of their cyclization, namely pyrazolols. It was shown that
acetoacetic ester 2,4-dinitrophenylhydrazone cyclizes to the pyrazolol only by using PPA while other
studied hydrazones undergo tarring under these conditions. The products of azocoupling of pyrazolols
with 3-diazonio-4-hydroxy-5-nitrobenzenesulfonate or 2-hydroxy-5-nitrophenyldiazonium salts are able
to color textile materials. The fungicidal activity of the synthesized compounds has been studied.
Keywords: azopyrazoles, acetoacetic ester, hydrazones, 3,5-dichloropyrid-2-ylhydrazine, mono- and
dinitrophenylhydrazine, pyrazol-5-ols, biological activity, coloration.
Our analysis of literature sources based on CAS data and available through STN has shown that the
interest in the synthesis and studies of the properties of pyrazole derivatives has persisted at a constantly high
level while, in the last 20 years, there has occurred an increasing trend in the number of publications and a jump
has been noted in the number of patents to 30–35 per year [1, 2]. This is explained by the fact that, amongst
pyrazole derivatives, there are known chemical agents for plant protection, chemico-pharmaceutical
preparations, antioxidants, reagents for chemical analysis and for organic synthesis, dyes, and pigments.
In our work we have synthesized a series of pyrazol-5-ol derivatives containing a substituent in the 1
position which can undergo further chemical transformation, and of derived azopyrazoles.
One of the most convenient and widely used methods for preparing pyrazol-5-ols is the heterocycliza-
tion of substituted hydrazines with acetoacetic ester [3] and it is known that the process can occur both with and
without the separation of the intermediate hydrazones 3.
R
HN
Me
Me
O
O
N
O
+
N
R
NHNH2
HO
Me
OEt
OEt
–H2O
N
R
–EtOH
3a–e
1a–e
2
4a–e
1,3,4 а R = Ph, b R = 4-nitrophenyl, c R = 2,4-dinitrophenyl, d R = 2-methoxy-5-nitrophenyl, e R = 3,5-dichloropyrid-2-yl
_____________
*To whom correspondence should be addressed; e-mail: kki@staff.msta.ac.ru.
1 A. N. Kosygin State Textile University, 1 Malaya Kaluzhskaya St., Moscow 119071, Russia.
______________________________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 539–546, April, 2011. Original
article submitted October 16, 2009; revision submitted April 1, 2011.
0009-3122/11/4704-0441©2011 Springer Science+Business Media, Inc.
441

With the aim of studying the effect of the structure of a substituent in the arylhydrazine on the course of
the cyclization and the structure of the radical at position 1 of the pyrazolol on the properties of the target azo-
pyrazoles as potential azo dyes, we selected the hydrazines 1a–e as model compounds for reaction with ester 2.
The synthesis of compounds 4a,b has been well studied and occurs in a single stage without separation
of the intermediate hydrazones 3a or 3b. The reaction occurs upon heating the reagents in water or aqueous
alcoholic medium at pH 5.5–6.8 over 3 h [3, 4].
Similarly, refluxing equimolar amounts of hydrazine 1e and acetoacetic ester (2) in ethanol in the pre-
sence of conc. HCl over 8 h gave the 1-(3,5-dichloropyrid-2-yl)-3-methyl-1H-pyrazol-5-ol (4e). In this case the
marked increase in reaction time points to a relatively low reactivity for the hydrazine 1e in the studied
condensation [5].
There is opposing information in the literature regarding the preparation of the 1-(2,4-dinitrophenyl)-
3-methyl-1H-pyrazol-5-ol (4c). Thus, by refluxing the acetoacetic ester 2 and hydrazine 1c in methanol in the
presence of hydrochloric acid a compound with mp 93–93.5ºC was obtained [6, 7], for which the authors
assigned the pyrazolol structure 4c. Using the method reported in these studies, we also obtained a substance
with mp 93ºC in 95% yield. However, 1H NMR spectroscopy and elemental analysis allowed us to conclude that
the compound produced is not a pyrazolol but the hydrazone 3c which has been described in the study [8]. The
spectroscopic and elemental analytical data for the hydrazone 3c are given in the Experimental.
Using the method proposed by Khromov-Borisov, we have heated the acetoacetic ester 2,4-dinit-
rophenylhydrazone 3c in conc. H2SO4 and it does not form the pyrazolol 4c, as proposed by the authors of [8],
but gives compound 5 in 40% yield with mp 121–122ºC as the product of acid decomposition of hydrazone 3c.
Besides the use of spectroscopic and elemental analytical data, we have additionally proved the structure
of the compound obtained by a counter synthesis. Hence, we have confirmed the conclusions of the authors of
[9] regarding the impossibility of obtaining pyrazolol 4c through the Khromov-Borisov method.
4c
conc.
H2SO4
Me
3c
H
N
55–60°C
MeCOMe
N
Me
1c
EtOH, HCl
O2N
NO2

5
We were also unable to cyclize hydrazone 3c in alcoholic medium with the addition of catalytic amounts
of aqueous solution of NaOH, HCl, or AcOH. In all cases the starting material was returned in the reactions
studied.
Cyclization of the hydrazone 3c to pyrazolol 4c could be achieved in PPA at 100ºC over 40 min in 76%
yield. The characteristics of the pyrazolol 4c were fully in agreement with those for the 1-(2,4-dinitrophenyl)-
3-methyl-1H-pyrazol-5-ol obtained before by an alternative method [10].
Exchange of one of the nitro groups for a methoxy in the phenyl radical (hydrazine 1d) allowed us to
prepare successfully the pyrazolol 4d both in one and two stages. Thus, a short time (5 min) reflux of equimolar
amounts of hydrazine 1d with the acetoacetic ester 2 in ethanol in the presence of traces of hydrochloric acid
gave a virtually quantitative yield of the hydrazone 3d. When using its hydrochloride salt in place of hydrazine
1d, the reaction proceeds without the addition of hydrochloric acid. The cyclization of hydrazone 3d to pyrazo-
lol 4d takes place by refluxing in alcohol for 3 h but only in the presence of hydrochloric acid. The use of PPA
as condensing agent leads to the tarring of the starting hydrazone 3d. Pyrazolol 4b can be prepared in a single
stage from hydrazine 1d and the acetoacetic ester 2 by refluxing for 3 h in the presence of hydrochloric acid.
The synthesized pyrazolols 4a,c–e gave azopyrazoles 7 and 9, the structure of which allowed an assess-
ment of their potential as acidic (compounds 7a,c–e) or dispersive (compounds 9a,c–e) azo dyes for coloring
wool and polycaproamide, respectively.
442

Azopyrazoles were synthesized by a standard azocoupling method using the pyrazolols 4a,c–e with the
diazonium salts 6 and 8.
OH
+
N2
O2N
Me
N
N
OH
–
SO3
N
R
O2N
N
6
OH
KOH, Na2CO3, EtOH
SO3Na
OH
+
7a, c–e
–
N2
Cl
4a, c–e
Me
N
OH
NO2
N
R
N
8
N
OH
KOH, Na2CO3, EtOH
NO2
9a, c–e
4,7,9 а R = Ph, c R = 2,4-dinitrophenyl, d R = 2-methoxy-5-nitrophenyl, e R = 3,5-dichloropyrid-2-yl
TABLE 1. Characteristics of the Azopyrazoles 7a,c–e and 9a,c–e
UV spectrum,
Com-
pound
IR spectrum,
mp, °C
Rf*

max, nm
Yield*3, %
ν, cm–1
(log )*2
7a

0.76
3600–3300,
1536, 1324, 1264,
1188, 1044, 640
496.0 (4.21)
90
7c
7d
7e



0.79
0.71
0.63
1608, 1540, 1348,
1208, 1052
504.0 (4.24)
498.0 (2.84)
500.0 (3.86)
83
76
80
1504, 1344, 1252,
1208, 1180
3600–3400, 1516,
1324, 1260, 1220,
1188, 1044, 608
9a
9c
330
170
0.69
0.66
3300–3000, 1540,
1488, 1344, 1280,
1232, 1160, 1044
409.8 (3.78)
419.4 (4.18)
85
89
1604, 1536, 1344,
1304, 1160, 832
9d
9e
185
350
0.54
0.63

417.2 (3.97)
426.6 (3.75)
80
80
1532, 1504,
1448, 1344, 1308,
1084, 744
_______
* Solvent systems: water–25% NH4OH solution–ethanol, 1:1:1; (7a,c–e),
ethyl acetate (9a,c–e).
*2 Spectra of the azo compounds 7a,c–e recorded in water and 9a,c–e in
ethanol.
*3 Yield of unpurified dyes.
443

The electron absorption spectra of the dyes showed a shift of max to the long wavelength end of the
spectrum relative to the starting components. Thus, the absorption maximum for the synthesized dyes lie in the
region from 390 to 550 nm whereas the starting diazo- and azo components have absorption maxima in the
range 230–370 nm.
Samples of wool and polycaproamide fabrics colored respectively with the acidic 7a,c–e or dispersive
9a,c–e dyes have a red-brown color and are well stabilized (4–5 point score) to dry and wet wear.
Studies of samples colored by the synthesized azopyrazoles showed results on the 2–4 point scale to-
wards the action of light.
The series of compounds synthesized in this work (3c,d, 4c–e, 7c,d, and 9a,c–e) were studied for their
activity towards the micromycetes most often encountered on textile materials and causing both mechanical and
chemical damage to the fibre [11].
The method of carrying out the investigation is given in the Experimental.
The insufficient number of samples does not allow us to make unambiguous conclusions regarding the
effect of the structure of the studied compounds on their fungicidal activity. Amongst the studied materials only
the hydrazone 3c shows zero activity towards all of the test subjects at concentrations of 0.1 and 1.0%. The
remaining compounds showed modest activity (40–60%) towards the fungal growth of Aspergillus niger
(v. Teigh). The activity of the studied compounds towards the other test subjects was low (0–20%) with the
exception of the pyrazolol 4e, which was active towards the fungi Aspergillus flavus (Link Fr.) and Penicillium
chrysogenum (Thom). The change from hydrazone 3c to pyrazolol 4c and azopyrazole 9c is accompanied by an
increase in activity (from 0 to 40–60%). Amongst the azo dyes 9 the most active is compound 9c (60%) which
contains a 2,4-dinitrophenyl substituent in the 1 position. Exchange of one of the nitro groups for a methoxy
group leads to loss in activity (9d, 0–20%). Decreasing activity is also noted when exchanging a
2,4-dinitrophenyl radical for a 3,5-dichloropyrid-2-yl (9e, 20–40%) or phenyl radical (9a, 20–40%).
EXPERIMENTAL
IR spectra were recorded on a Specord M80-2 spectrometer for KBr tablets using a KBr prism in the
frequency range 600–3800 cm–1. Electronic absorption spectra in water and in ethanol (compounds 3c, 4c–e)
were measured at a concentration of 1×10–4 molar on a Specord M-40 instrument at a thickness of 0.5–1.0 cm.
1H NMR spectra were taken at 22–24ºC on a Bruker AC-200 spectrometer (200 MHz) using DMSO-d6
(compounds 3c,d 4c,d, and 5) or CDCl3 (compound 4e) with TMS as internal standard. Elemental analysis was
performed on a Carlo Erba-110 instrument and monitoring of the reaction course and purity of the compounds
obtained was carried out by TLC on Silufol UV-254 plates.
Hydrazine hydrochlorides 1a–c from Acros Organics were of chemically pure grade. Hydrazines 1d,e
were synthesized by methods [12, 13].
3-Methyl-1-phenyl-1H-pyrazol-5-ol (4a) and 3-Methyl-1-(4-nitrophenyl)-1H-pyrazol-5-ol (4b) were
prepared by methods [3, 4].
3-Diazonio-4-hydroxy-5-nitrobenzenesulfonate (6) and 2-Hydroxy-5-nitrophenyldiazonium chlo-
ride (8) were prepared by method [14].
Acetoacetic Ester 2,4-Dinitrophenylhydrazone (3c) [5]. A mixture of the hydrazine 1c (1.98 g,
10 mmol) and acetoacetate ester (1.3 g, 10 mmol) in methanol with the addition of conc. HCl was refluxed for
3 h. Cooling the reaction mixture gave a bright-yellow solid. Yield of hydrazone 3.0 g (95%); mp 93ºC (alcohol)
and Rf 0.15 (chloroform–benzene, 1:1). UV spectrum, max, nm (log ): 354.1 (4.29). 1H NMR spectrum, , ppm
(J, Hz): 1.35 (3H, t, J = 6.9, CH3); 2.20 (3H, s, CH3); 3.55 (2H, s, CH2); 4.15 (2H, q, J = 6.9, CH2); 7.80 (1H, d,
J = 5.8, H-6); 8.40 (1H, d, J = 5.8, H-5); 8.91 (1H, s, H-3); 10.80 (1H, s, NH). Found, %: C 45.95; H 4.37;
N 17.93. C12H14N4O6. Calculated, %: C 46.45; H 4.55; N 18.06.
Acetone 2,4-Dinitrophenylhydrazone (5) [7, 8]. A solution of hydrazone 3c (1.55 g, 5 mmol) and
conc. H2SO4 (22.8 g, 224 mmol) was heated for 1 h at 55–60ºC, cooled, poured into water (300 ml), and
444

neutralized to pH 6–7, to give a yellow solid. Yield 0.5 g (40%); mp 121–122ºC (alcohol) and Rf 0.23
1
(chloroform–benzene, 1:1). H NMR spectrum, , ppm (J, Hz): 2.10 (6H, s, 2CH3); 7.80 (1H, d, J = 5.8, H-6);
8.40 (1H, d, J = 5.8, H-5); 8.91 (1H, s, H-3); 10.80 (1H, s, NH). Found, %: C 45.29; H 4.13; N 23.24.
C9H10N4O4. Calculated, %: C 45.38; H 4.23; N 23.52.
1-(2,4-Dinitrophenyl)-3-methyl-1H-pyrazol-5-ol (4c). A mixture of hydrazone 3c (3.48 g, 11 mmol)
and PPA (31.6 g) was heated for 40 min at 100ºC. The hot mixture was poured into iced water (40 ml) with
vigorous stirring to give a light-yellow precipitate. Yield 2.2 g (76%); mp 160ºC (alcohol) and Rf 0.43
1
(chloroform). UV spectrum, max, nm (log ): 311.7 (3.86). H NMR spectrum, , ppm (J, Hz): 2.18 (3H, s,
CH3); 5.40 (1H, s, H-4 pyrazolol); 7.80 (1H, d, J = 5.8, H-6 Ar); 8.40 (1H, d, J = 5.8, H-5 Ar); 8.91 (1H, s, H-3
Ar); 12.20 (1H, s, OH). Found, %: C 45.18; H 3.01; N 21.0. C10H8N4O5. Calculated, %: C 45.56; H 3.05; N 21.2.
Acetoacetic Ester 2-Methoxy-5-nitrophenylhydrazone (3d). A mixture of hydrazine 1d (0.91 g,
5 mmol) and acetoacetic ester (0.64 g, 5 mmol) was refluxed in ethanol (10 ml) for 10 min with the addition of
conc. HCl. Cooling the reaction mixture gave a yellow precipitate. Yield 1.32 g (90%); mp 125ºC (alcohol) and
Rf 0.31 (chloroform). 1H NMR spectrum, , ppm (J, Hz): 1.35 (3H, t, J = 6.9, CH3); 2.20 (3H, s, CH3); 3.40 (2H,
s, CH2); 3.95 (3H, s, OCH3); 4.15 (2H, q, J = 6.9, CH2); 7.20 (1H, d, J = 9.1, H-3); 7.70 (1H, d, J = 9.1, H-4);
8.20 (1H, s, H-6). Found, %: C 52.54; H 5.57; N 14.10. C10H8N4O5. Calculated, %: C 52.88; H 5.80; N 14.23.
3-Methyl-1-(2-methoxy-5-nitrophenyl)-1H-pyrazol-5-ol (4d). A solution of hydrazone 3d (0.5 g,
2 mmol) was heated in ethanol (20 ml) for 3 h with the addition of conc. HCl. Removal of the solvent gave
pyrazolol 4d as a yellowish oil. Yield 0.42 g (86%); Rf 0.16 (hexane–benzene–methanol, 5:1:1). UV spectrum,

max, nm (log ): 221.6 (4.00), 246.5 (3.93), 295.5 (3.86). 1H NMR spectrum, , ppm (J, Hz): 2.20 (3H, s, CH3);
3.90 (3H, s, OCH3); 5.60 (1H, s, H-4 pyrazolol); 7.20 (1H, d, J = 9.1, H-3 Ar); 7.70 (1H, d, J = 9.1, H-4 Ar);
8.00 (1H, s, H-6 Ar); 12.20 (1H, s, OH). Found, %: C 52.94; H 4.57; N 16.97. C11H11N3O4. Calculated, %: C
53.01; H 4.45; N 16.86.
1-(3,5-Dichloropyridin-2-yl)-3-methyl-1H-pyrazol-5-ol (4e). A mixture of the hydrazine 1e (1.79 g,
10 mmol), acetoacetic ester (1.28 g, 10 mmol), and several drops of conc. HCl was refluxed in ethanol (15 ml)
for 15 h and evaporated to dryness in vacuo. The residue was treated with NaOH solution (5%, 15 ml), extracted
with ether (2×20 ml), and the extract was dried over MgSO4. Solvent was removed in vacuo. The material
obtained is a dark-yellow, viscous oil which crystallized upon trituration and prolonged storage at –15ºC,
soluble in ethanol. Yield 2.0 g (85%); mp 40–42ºC, Rf 0.74 (benzene–methanol, 20:1). UV spectrum, max, nm
(log ): 322.8 (3.81). 1H NMR spectrum, , ppm (J, Hz): 1.97 (3H, s, 3-CH3); 5.88 (1H, s, H-4 pyrazolol); 7.74
(1H, d, J = 2.4, H-4 pyridine); 7.90 (1H, d, J = 2.4, H-6 pyridine). Found, %: C 44.37; H 2.91; N 17.60.
C9H7Cl2N3O. Calculated, %: C 44.26; H 2.87; N 17.21.
Azopyrazoles 7a,c–e. The azo component was prepared by dissolving the pyrazolol 4a (3.24 g,
18.6 mmol) in water (20 ml) or compounds 4c–e in ethanol (15 ml) at 80ºC in the presence of NaOH (0.74 g,
18.6 mmol) and Na2CO3 (1.97 g, 18.6 mmol). The azocoupling was carried out at a temperature of 5–10ºC with
stepwise addition over 25 min of a solution of the diazo component 6 to a solution of the azo component and
with continuous stirring. After addition of all of the solution of the diazo component to the reaction mass it was
stirred for 1–1.5 h at 20–25ºC. The obtained dye was either filtered off or salted out with solid NaCl.
Azopyrazoles 9a,c–e. The azo component was prepared by dissolving the pyrazolol 4a (1.83 g,
10 mmol) in water (20 ml) or compounds 4c–e in ethanol (15 ml) at 80ºC. The solution obtained was treated
with NaOH (0.43 g, 10 mmol) and AcONa (2 g, 24 mmol) and cooled to 5ºC. A suspension of the diazo
compound 8 was added at 5–10ºC over 15–20 min to this reaction product. During the addition of the diazo
component it was necessary constantly to maintain a basic medium using NaOH solution (10%). The reaction
product was stirred for 1.5–2 h at 20–25ºC. At the end of the azocoupling the solution obtained was acidified to
pH 5–6 and the precipitate formed was filtered off.
Coloring of Samples of Wool and Polycaproamide Fibres with the Azopyrazoles 7a,c–e and 9a,c–e.
Coloring of the wool and polycaproamide was carried out by the methods for acidic and dispersive dyes
respectively [15].
445

TABLE 2. Dependence of the Growth of the Test Culture on Fungicidal
Activity
Fungicidal activity, %
Characteristics of the growth of the test culture, point scale
0
20
40
60
80
100
Abundant growth of mycelium, sporulation occurs, 5
Limited growth of mycelium, suppressed sporulation, 4
Suppression of mycelial growth, 3
Cobweb type mycelium, 2
Complete inhibition of growth, 1
Complete inhibition of growth, formation of zones of growth inhibition, 0
Stability of colored samples towards dry and wet wear, wet treatment was assessed on the five point
system and the effect of light by the eight point system according to the GOST’s (GOST 9733.27-83, GOST
9733.4-83, and GOST 9733.0-83) [16].
Experiments for fungicidal activity were performed in the biology laboratory of State Research Insti-
tute of Restoration using GOST 9.048-75 [11].
The test culture micromycetes used were: Aspergillus niger, Aspergillus flavus, Penicillium chry-
sogenum, and Ulocladium ilicis (previously known as Stemphylium ilicis).
The fungal forms were selected because members of Aspergillus niger amongst imperfect fungi are in
the first position regarding biochemical damage to textiles. The Aspergillus niger conidia can cause the death of
micromycetes competing for the substrate. It is also known that Aspergillus niger mycelial spores can survive in
conditions where micelles of other fungal forms prove vulnerable and die. Moreover, they are very convenient
for microscopic examination. Changes observed in the growth of other test cultures in the presence of the
synthesized dyes were considered as extra information regarding the properties of the compounds studied and of
the biochemical features of the growth of the micromycetes.
The biological study of the compounds was carried out using the “disc” method. The control was the
test culture grown under the same conditions but without the compound studied. On the third day the nature of
the fungal growth was assessed on a six point scale according to the percentage indicators of growth inhibition
(Table 2).
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