Synthesis and tautomeric structure of novel 3-arylazoimidazo[1,2-b] pyrazolo[4,3-d]pyridazines

Article abstract of DOI:10.3184/174751912X13344196041494

Treatment of 3-acetyl-5-amino-4-cyano-1-phenylpyrazole with hydrazine hydrate afforded the hydrazone derivative. Reaction of the latter hydrazone with hydrazonoyl chlorides was found to be site selective as it afforded the corresponding formazan and not t

Full text of DOI:10.3184/174751912X13344196041494

296 RESEARCH PAPER
MAY, 296–299
JOURNAL OF CHEMICAL RESEARCH 2012
Synthesis and tautomeric structure of novel 3-arylazo-
imidazo[1,2-b]pyrazolo[4,3-d]pyridazines
Thoraya A. Farghaly and Ahmad Sami Shawali*
Department of Chemistry, Faculty of Science, University of Cairo, Giza 12613, Egypt
Treatment of 3-acetyl-5-amino-4-cyano-1-phenylpyrazole with hydrazine hydrate afforded the hydrazone derivative.
Reaction of the latter hydrazone with hydrazonoyl chlorides was found to be site selective as it afforded the corre-
sponding formazan and not the amidrazone derivatives. The 1H NMR spectra of the formazan derivatives indicated
that they exist as the bis-hydrazone tautomers. Acid-catalysed cyclisation of these dihydroformazans afforded the
respective 9-amino-3-arylazo-6-methyl-8-phenyl-2-substituted-8H-imidazo[1,2-b]pyrazolo[4,3-d]pyridazines whose
electronic absorption spectra revealed that they exist predominantly in the arylazo tautomeric form. The structures
of all compounds prepared were confirmed by spectral and elemental analyses. Also the mechanism of the reactions
studied are discussed.
Keywords: hydrazonoyl halides, amidrazones, hydrazones, formazans
In recent years, arylazo derivatives of various heterocyclic ring
systems have received much attention from organic chemists
and dye manufacturers.1 In conjunction with our continuing
interest in azo-hydrazone tautomerism of azo colouring
materials2 and the chemistry of hydrazonoyl halides,3–13, we
studied the utility of the latter for the synthesis of arylazo
derivatives of a new triheterocyclic system namely imi-
dazo[1,2-b]pyrazolo[4,3-d]pyridazine which has not been
reported previously. In addition, our objective with such
a study was to shed some light on the site-selectivity in the
reactions to be studied and to investigate the tautomerism of
the target colorants.
When compound 2 was treated with each of the hydrazonoyl
halides 4 in 1,4-dioxane in the presence of triethylamine, it
afforded, in each case, a single product that was identified as
the respective formazan derivative 5 (Scheme 2). In all cases,
the amidrazone derivatives 6 were not formed (Scheme 2).
This finding indicates that the reaction is site specific. The
formation of 5 rather than 6 can be attributed to the lower
nucleophilicity of the 5-amino group that results from being
conjugated with the electron withdrawing cyano group. This
renders the amino-nitrogen of the hydrazone moiety more
nucleophilic and in turn its attack on the carbon atom of the
hydrazonoyl halide is more favourable to give 5 as the final
product. The isolated products 5 can have either the bis-hydra-
zone structure 5 or its tautomeric structure 5A (Scheme 2).
However, their 1H NMR spectra are consistent with the former
tautomeric form 5. For example, their 1H NMR spectra revealed
in each case, in addition to CH3, NH2 and aromatic protons
signals, two common singlets in the regions δ 8.98–9.42 and
10.76–11.08 assignable to the two NH groups of structure 5.
On the basis of this finding, the tautomeric structure 5A was
discarded.
Results and discussion
The required precursors namely 3-acetyl-5-amino-4-cyano-1-
phenyl-pyrazole 1 and hydrazonoyl halides 4a–h were
prepared as previously described from our laboratory.14–16
Treatment of 1 with hydrazine hydrate in refluxing ethanol
yielded a product that was identified as the hydrazone deriva-
tive 2 (Scheme 1). The possible cyclised product 3 was not
formed under the reaction conditions employed (Scheme 1).
This finding is consistent with other literature reports on the
hydrazinolysis of 1,5-disubstituted-3-acetyl-4-cyanopyrazole
derivatives.17,18 The structure of the product 2 was assigned
When each of the formazan derivatives 5 was heated in
glacial acetic acid, they underwent dehydrative cyclisation and
afforded the respective 9-amino-3-arylazo-6-methyl-8-phenyl-
2-substituted-8H-imidazo[1,2-b]pyrazolo[4,3-d]pyridazines 8
(Scheme 3). The structures of the isolated products 8 were
1
on the basis of its spectral data (IR, MS and H NMR) and
elemental analyses (see Experimental). For example, its IR
1
confirmed by their spectra (IR, MS and H NMR) and by
spectrum exhibited characteristic bands at ν 3415, 3334, 3237,
1
3191 (2 NH2) and 2211 (CN) cm−1. The H NMR spectrum
elemental analyses (see Experimental). For example, their IR
spectra, while they revealed the absence of both the nitrile
and carbonyl absorption bands present in the spectra of their
precursors 5, they showed instead two common bands in the
regions 3410–3354 and 3390–3241 cm−1 due to the symmetric
and asymmetric stretching vibrations of the –NH2 group.
As the latter products 8 can have either the amino-azo
structure 8, or less likely, the imino-hydrazone structure 9
(Scheme 4), their electronic absorption spectra were also
studied to shed some light on their actual tautomeric structure.
Their visible spectra in dioxane revealed, in each case,
two absorption bands in regions 430–488 and 325–375 nm
(Table 1). Such an absorption pattern is similar to that reported
for arylazo compounds.18 Furthermore, the spectra of 8f, taken
as an example of the series prepared, in solvents of different
polarities, revealed little, if any, change indicating that such
compounds exist only in the depicted azo tautomeric form 8
(Scheme 3).19
of 2 revealed, in addition to aromatic proton signals, character-
istic signals at δ 1.97 (s, 3H, CH3), 6.46 (s, 2H, NH2), 6.62 (s,
2H, NH2).
Scheme 1 (a) m-CPMA/p-TsOH; (b) p-TsOH.
Conclusions
A series of new formazan derivatives 5 were synthesised and
cyclised into aryazoheterocycles 8. The reactions leading to 5
* Correspondent. E-mail: as_shawali@mail.com

JOURNAL OF CHEMICAL RESEARCH 2012 297
Scheme 2
Scheme 3
proved to be site-selective. Compounds 5 have a bis-hydrazone
structure whereas compounds 8 exist predominantly as the azo
tautomeric form.
Experimental
All melting points were determined on an Electrothermal apparatus
and are uncorrected. Solvents were generally distilled and dried prior
to their use. The IR spectra were measured on a Pye-Unicam SP300
1
instrument in potassium bromide discs. The H NMR spectra were
recorded on a Varian Mercury VXR-300 spectrometer (300 MHz for
1H NMR) and the chemical shifts were related to that of the solvent
DMSO-d6. The mass spectra were recorded on a GCMS-Q1000-EX
Shimadzu and GCMS 5988-A HP spectrometers, the ionising voltage
was 70 eV. Elemental analyses were carried out at the Microanalytical
Centre of Cairo University, Giza, Egypt. 3-Acetyl-5-amino-4-cyano-
1-phenylpyrazole 1 and the hydrazonoyl halides 2 were prepared as
previously described in Literature.14–16
5-Amino-3-acetyl-4-cyano-1-phenylpyrazole hydrazone (2):A mixture
of 3-acetyl-5-amino-4-cyano-1-phenylpyrazole (1) (2.5 g, 0.01 mol)
and an excess of hydrazine hydrate (16 g, 0.5 mol) was refluxed for
25 h. The reaction mixture was cooled and the solid that precipitated
was filtered off and crystallised from ethanol to give compound 2 as
Table 1 UV spectra of compounds 8 in 1,4-dioxane
Compd. No.
λmax (log ε)
8a
8c
8d
8e
8fa
8g
430 (4.86), 351 (4.45)
453 (3.88), 350 (3.54)
487 (4.70), 375 (4.24)
434 (4.71), 340 (3.59)
479 (4.62), 370 (3.78)
488 (4.36), 325 (3.08)
a Solvent: λmax (log ε): acetic acid 478 (4.39), 369 (3.59); chloro-
form 479 (4.51), 371 (3.78); DMF 480 (4.19), 372 (3.46); ethanol
479 (4.29), 368 (4.00).

298 JOURNAL OF CHEMICAL RESEARCH 2012
1
yellow crystals, yield (1.97 g, 82%), m.p. 255 °C; IR (KBr) ν = 3415,
(NH2), 3250, 3049 (2NH), 2210 (CN), 1635 (CO) cm−1; H NMR
1
3334, 3237, 3191 (2NH2), 2211 (CN) cm−1; H NMR (DMSO-d6)
(DMSO-d6) δ 2.36 (s, 3H, CH3), 6.82 (s, 2H, NH2), 7.16–8.13 (m,
16H, ArH), 8.69 (s, 1H, naphthoyl-H), 9.37 (s, 1H, NH), 11.08 (s, 1H,
NH) ppm; MS, m/z (%) 512 (M+, 2), 494 (39), 405 (59), 153 (39), 149
(10), 143 (4), 126 (28), 82 (100), 76 (96). Anal. Calcd for C30H24N8O
(512.56): C, 70.30; H, 4.72; N, 21.86. Found: C, 70.11; H, 4.66;
N,21.57%.
δ 1.97 (s, 3H, CH3), 6.46 (s, 2H, NH2), 6.62 (s, 2H, NH2), 7.39–7.53
(m, 5H, ArH) ppm; MS, m/z (%) 240 (M+, 100), 210 (25), 184 (4), 155
(14), 134 (4), 103 (13), 77(46). Anal. Calcd for C12H12N6 (240.26):
C,n59.99; H, 5.03; N, 34.98. Found: C, 60.32; H, 5.23, N, 35.06%.
Reaction of compound 2 with hydrazonoyl halides 4
3-(2-Naphthoyl)-5-(p-chlorophenyl)-1-[5-amino-4-cyano-1-phenyl-
pyrazol-3-yl)ethylidene]-1,2-dihydroformazan (5h): Brown crystals,
yield (0.96 g, 70%), m.p. 188 °C; (ethanol); IR (KBr) ν 3426, 3321
To a mixture of 2 (0.6 g, 2.5 mmol) and the appropriate hydrazonoyl
halide 4 (2.5 mmol) in 1,4-dioxane (20 mL) was added triethylamine
(0.35 mL) and the mixture was refluxed for 15 h, then cooled. The
solution was poured onto ice and concentrated hydrochloric acid. The
solid that produced was filtered off and crystallised from the appropri-
ate solvent to give the corresponding compound 5. The products 5a–h
prepared together with their physical constants are listed below.
3-Acetyl-5-(m-tolyl)-1-[5-amino-4-cyano-1-phenylpyrazol-3-yl)-
ethylidene]-1,2-dihydroformazan (5a): Yellow solid, yield (0.77 g,
74%), m.p. 250 °C; (ethanol); IR (KBr) ν 3411, 3330 (NH2), 3232,
3156 (2NH), 2207 (CN), 1668 (CO) cm−1; 1H NMR (DMSO-d6) δ 2.27
(s, 3H, CH3), 2.28 (s, 3H, CH3), 2.49 (s, 3H, COCH3), 6.83 (s, 2H,
NH2), 7.15–7.57 (m, 9H, ArH), 8.99 (s, 1H, NH), 10.77 (s, 1H, NH)
ppm; MS, m/z (%) 414 (M+, 87), 399 (68), 367 (11), 295 (15), 240
(13), 226 (76), 209 (28), 118 (48), 105 (78), 91 (100). Anal. Calcd for
C22H22N8O (414.46): C,63.75; H, 5.35; N, 27.04. Found: C, 63.54; H,
5.39; N, 27.08%.
3-Acetyl-5-phenyl-1-[5-amino-4-cyano-1-phenylpyrazol-3-yl)-
ethylidene]-1,2-dihydroformazan (5b): Yellow crystals, yield (0.75 g,
75%), m.p. 230 °C; (ethanol); IR (KBr) ν 3403, 3323 (NH2), 3230
(NH), 3047 (NH), 2212 (CN), 1667 (CO) cm−1; 1H NMR (DMSO-d6)
δ 2.27 (s, 3H, CH3), 2.42 (s, 3H, COCH3), 6.84 (s, 2H, NH2), 7.23–
7.57 (m, 10H, ArH), 9.01 (s, 1H, NH), 10.83 (s, 1H, NH) ppm; MS,
m/z (%) 400 (M+, 95), 384 (53), 175 (17), 141 (15), 119 (28), 104 (39),
77 (100). Anal. Calcd for C21H20N8O (400.44): C, 62.99; H, 5.03; N,
27.98. Found: C, 63.04; H, 5.25; N, 27.88%.
3-Acetyl-5-(p-chlorophenyl)-1-[5-amino-4-cyano-1-phenyl-pyrazol-
3-yl)ethylidene]-1,2-dihydroformazan (5c): Pale brown crystals, yield
(0.87 g, 80%), m.p. 220 °C; (ethanol); IR (KBr) ν 3403 (NH2), 3327,
3232 (2NH), 2211 (CN), 1668 (CO) cm−1; 1H NMR (DMSO-d6) δ 2.27
(s, 3H, CH3), 2.49 (s, 3H, COCH3), 6.86 (s, 2H, NH2), 7.26 (d, J =
9 Hz, 2H, ArH), 7.36 (d, J = 9 Hz, 2H, ArH), 7.47–7.56 (m, 5H, ArH),
9.03 (s, 1H, NH), 10.83 (s, 1H, NH) ppm; MS, m/z (%) 436 (M++2,
29), 434 (M+, 64), 419 (53), 240 (16), 226 (100), 210 (62), 184 (33),
141 (40), 111 (42), 77 (77). Anal. Calcd for C21H19ClN8O (434.88): C,
58.00; H, 4.40; N, 25.77. Found: C, 58.21; H, 4.30; N, 25.89%.
3-Acetyl-5-(m-nitrophenyl)-1-[5-amino-4-cyano-1-phenyl-pyrazol-
3-yl)ethylidene]-1,2-dihydroformazan (5d): Green crystals, yield
(0.87 g, 78%), m.p 236 °C; (ethanol); IR (KBr) ν 3420, 3331 (NH2),
3210, 3150 (2NH), 2211 (CN), 1674 (CO) cm−1; 1H NMR (DMSO-d6)
δ 2.29 (s, 3H, CH3), 2.49 (s, 3H, COCH3), 6.89 (s, 2H, NH2), 7.46–
8.16 (m, 9H, ArH), 9.08 (s, 1H, NH), 10.98 (s, 1H, NH) ppm; MS,
m/z (%) 445 (M+, 73), 427 (47), 295 (14), 225 (100), 209 (65), 184
(66), 156 (33), 118 (60), 91 (64), 76 (91). Anal. Calcd for C21H19N9O3
(445.43): C, 56.62; H, 4.30; N, 28.30. Found: C, 56.45; H, 4.22; N,
28.24%.
3-Acetyl-5-(p-tolyl)-1-[5-amino-4-cyano-1-phenylpyrazol-3-yl)-
ethylidene]-1,2-dihydroformazan (5e): Golden yellow crystals, yield
(0.79 g, 76%), m.p. 226-228 °C; (ethanol); IR (KBr) ν 3412, 3381
(NH2), 3329 (NH), 3231 (NH), 2212 (CN), 1620 (CO) cm−1; 1H NMR
(DMSO-d6) δ 2.24 (s, 3H, CH3), 2.26 (s, 3H, CH3), 2.46 (s, 3H,
COCH3), 6.84 (s, 2H, NH2), 7.05 (d, J = 9 Hz, 2H, ArH), 7.26 (d,
J = 9 Hz, 2H, ArH), 7.47–7.56 (m, 5H, ArH), 8.98 (s, 1H, NH), 10.76
(s, 1H, NH) ppm; MS, m/z (%) 414 (M+, 80), 399 (55), 240 (14), 226
(74), 190 (20), 119 (56), 90 (100), 77 (59). Anal. Calcd for C22H22N8O
(414.46): C,63.75; H, 5.35; N, 27.04. Found: C, 63.84, H, 5.21; N,
27.00%.
1
(NH2), 3219, 3055 (2NH), 2213 (CN), 1626 (CO) cm−1; H NMR
(DMSO-d6) δ 2.37 (s, 3H, CH3), 6.75 (s, 2H, NH2), 6.87 (d, J = 9 Hz,
2H, ArH), 7.08 (d, J = 9 Hz, 2H, ArH), 7.45–7.79 (m, 11H, ArH), 8.04
(s, 1H, naphthoyl-H), 9.42 (s, 1H, NH), 11.00 (s, 1H, NH) ppm; MS,
m/z (%) 547 (M+, 1), 527 (32), 447 (50), 433 (79), 238 (12, 209 (18),
126 (18), 91 (19), 77 (100). Anal. Calcd for C30H23N8OCl (547.01): C,
65.87; H, 4.24; N, 20.48. Found: C, 65.90; H, 4.33; N, 20.40%.
Synthesis of 2,6,8-trisubstituted-9-amino-3-arylazo-imidazo[1,2-b]
pyrazolo-[4,3-d]pyridazines (8)
A solution of the appropriate formazan 5 (1 mmol) in glacial acetic
acid (10 mL) and two drops of HCl was refluxed for 50–60 h (moni-
tored by TLC) then cooled. The cold reaction mixture was poured
onto ice cold solution of sodium acetate. The precipitate formed
was filtered off and crystallised from the appropriate solvent to give
the respective imidazo[1,2-b]pyrazolo[4,3-d]pyridazine 8. The
compounds 8 together with physical constants are listed below.
2,6-Dimethyl-3-[m-tolylazo]-8-phenyl-9-amino-imidazo[1,2-b]
pyrazolo[4,3-d]pyridazine (8a): Dark red solid, yield (0.19 g, 47%),
m.p. 294–298 °C; (ethanol/1,4-dioxane); IR (KBr) ν 3354, 3241 (NH2)
cm−1; H NMR (CDCl3) δ 2.32 (s, 3H, CH3), 2.35 (s, 3H, CH3), 2.58
1
(s, 3H, CH3), 6.65 (s, 2H, NH2), 7.22–7.58 (m, 9H, ArH) ppm; MS,
m/z (%) 396 (M+, 23), 367 (29), 226 (35), 190 (46), 142 (19), 132 (20),
119 (55), 91 (100), 77 (97). Anal. Calcd for C22H20N8 (396.45):
C,66.65; H, 5.08; N, 28.26. Found: C, 66.43, H, 5.19; N, 28.02%.
2,6-Dimethyl-3-[p-chlorophenylazo]-8-phenyl-9-amino-imidazo
[1,2-b]pyrazolo[4,3-d]pyridazine (8c): Pale brown solid, yield (0.16 g,
38%), m.p. > 300 °C; (1,4-dioxane); IR (KBr) ν 3384, 3310 (NH2)
cm−1; H NMR (DMSO-d6) δ 2.27 (s, 3H, CH3), 2.49 (s, 3H, CH3),
1
6.80 (s, 2H, NH2), 7.27 (d, J = 9 Hz, 2H, ArH), 7.30 (d, J = 9 Hz, 2H,
ArH), 7.36–7.56 (m, 5H, ArH) ppm; MS, m/z (%) 418 (M++2, 19), 416
(M+, 27), 337 (25), 335 (42), 320 (46), 226 (33), 209 (22), 152 (25),
127 (44), 111 (61), 91 (28), 77 (100). Anal. Calcd for C21H17ClN8
(416.87): C, 60.50; H, 4.11; N, 26.88. Found: C, 60.45; H, 4.23; N,
26.78%.
2,6-Dimethyl-3-[m-nitrophenylazo]-8-phenyl-9-amino-imidazo
[1,2-b]pyrazolo[4,3-d]pyridazine (8d): Brown solid, yield (0.15 g,
36%), m.p 280–282 °C; (ethanol); IR (KBr) ν 3400, 3390 (NH2) cm−1;
1H NMR (DMSO-d6) δ 2.29 (s, 3H, CH3), 2.54 (s, 3H, CH3), 6.92
(s, 2H, NH2), 7.36–8.15 (m, 9H, ArH) ppm; MS, m/z (%) 427 (M+,
34), 346 (54), 331 (66), 226 (23), 224 (31), 163 (29), 119 (37), 92
(48), 85 (39), 77 (100). Anal. Calcd for C21H17N9O2 (427.42): C, 59.01;
H, 4.01; N, 29.49. Found: C, 59.27; H, 4.05; N, 29.34%.
2,6-Dimethyl-3-[p-tolylazo]-8-phenyl-9-amino-imidazo[1,2-b]-
pyrazolo[4,3-d]pyridazine (8e): Dark red solid, yield (0.17 g, 42%),
m.p. > 300 °C; (ethanol / 1,4-dioxane); IR (KBr) ν 3410, 3336 (NH2)
cm−1; H NMR (CDCl3) δ 2.27 (s, 3H, CH3), 2.29 (s, 3H, CH3), 2.57
1
(s, 3H, CH3), 6.65 (s, 2H, NH2), 7.08 (d, J = 8 Hz, 2H, ArH), 7.27 (d,
J = 8 Hz, 2H, ArH), 7.35–7.85 (m, 5H, ArH) ppm; MS, m/z (%) 396
(M+, 10), 226 (30), 121 (22), 119 (34), 106 (22), 91 (100), 77 (70).
Anal. Calcd for C22H20N8 (396.45): C,66.65; H, 5.08; N, 28.26. Found:
C, 66.51, H, 5.30; N, 28.14%.
2,8-Diphenyl-6-methyl-3-phenylazo-9-amino-imidazo[1,2-b]
pyrazolo[4,3-d]pyridazine (8f): Dark red solid, yield (0.14 g, 31%),
m.p. > 300 °C; (DMF); IR (KBr) ν 3380, 3356 (NH2) cm−1; 1H NMR
(DMSO-d6) δ 2.25 (s, 3H, CH3), 6.65 (s, 2H, NH2), 7.34–8.02 (m,
15H, ArH) ppm; MS, m/z (%) 444 (M+, 9), 105 (54), 91 (84), 77 (100).
Anal. Calcd for C26H20N8 (444.49): C, 70.26; H, 4.54; N, 25.21. Found:
C, 70.39; H, 4.44; N, 25.08%.
3-Benzoyl-5-phenyl-1-[5-amino-4-cyano-1-phenylpyrazol-3-yl)-
ethylidene]-1,2-dihydroformazan (5f): Yellow crystals, yield (0.92 g,
80%), m.p. 175 °C; (ethanol); IR (KBr) ν 3436, 3340 (NH2), 3302,
3215 (2NH), 2213 (CN), 1659 (CO) cm−1; 1H NMR (DMSO-d6) δ 2.34
(s, 3H, CH3), 6.82 (s, 2H, NH2), 7.17–8.42 (m, 15H, ArH), 9.29 (s, 1H,
NH), 11.04 (s, 1H, NH) ppm; MS, m/z (%) 462 (M+, 1), 301 (15),131
(10), 105 (29), 77 (100). Anal. Calcd for C26H22N8O (462.51): C,
67.52; H, 4.79; N, 24.23. Found: C, 67.29; H, 4.67; N, 24.18%.
3-(2-Naphthoyl)-5-phenyl-1-[5-amino-4-cyano-1-phenylpyrazol-
3-yl)ethylidene]-1,2-dihydroformazan (5g): Green crystals, yield
(0.97 g, 76%), m.p. 180 °C; (ethanol/dioxan); IR (KBr) ν 3429, 3312
2-(2-Naphthyl)-6-methyl-3-[phenylazo]-8-phenyl-9-amino-imidazo
[1,2-b]pyrazolo[4,3-d]pyridazine (8g): Brown solid, yield (0.19 g,
39%), m.p. 276–278 °C; (ethanol/1,4-dioxane); IR (KBr) ν 3405,
3335 (NH2) cm−1; H NMR (DMSO-d6) δ 2.29 (s, 3H, CH3), 6.72 (s,
1
2H, NH2), 7.29–8.04 (m, 16H, ArH), 8.58 (s, 1H, naphthyl-H) ppm;
MS, m/z (%) 494 (M+, 62), 127 (39), 105 (29), 91 (64), 77 (100). Anal.
Calcd for C30H22N8 (494.55): C, 72.86; H, 4.48; N, 22.66. Found: C,
72.95; H, 4.68; N,22.42%.

JOURNAL OF CHEMICAL RESEARCH 2012 299
9
A.S. Shawali and M.A. Abdalla, Adv. Heterocycl. Chem., 1995, 63, 277.
Received 5 December 2011; accepted 14 March 2012
Paper 1101025 doi: 10.3184/174751912X13344196041494
Published online: 10 May 2012
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