Synthesis and tautomeric structure of novel 3,7-bis(arylazo)-2,6-diphenyl-1H-imidazo-[1,2-b]pyrazoles in ground and excited states

Article abstract of DOI:10.1016/S0040-4020(02)00157-6

Two series of novel 3,7-bis(arylazo)-2,6-diphenyl-1H-imidazo[1,2-b]pyrazoles were prepared starting from N-aryl 2-oxo-2-phenylethanehydrazonoyl bromides. The acid dissociation constants in both the ground and excited states, pK and pK^*, respectively, were determined for both series and were correlated by the Hammett equation. The results of such correlations together with spectral data indicated that the title compounds exist predominantly in the 1H-bis-(arylazo) form in both ground and excited states.

Full text of DOI:10.1016/S0040-4020(02)00157-6

TETRAHEDRON
Pergamon
Tetrahedron 58 ,2002) 2875±2880
Synthesis and tautomeric structure of novel
3,7-bisꢀarylazo)-2,6-diphenyl-1H-imidazo-[1,2-b]pyrazoles in
ground and excited states
Ahmad Sami Shawali,p Mahmoud H. Abdelkader and Farag M. A. Eltalbawy
Department of Chemistry, Faculty of Science, University of Cairo, Giza, A.R., Egypt
Received 29 August 2001; revised 4 January 2002; accepted 7 February 2002
AbstractÐTwo series of novel 3,7-bis,arylazo)-2,6-diphenyl-1H-imidazo[1,2-b]pyrazoles were prepared starting from N-aryl 2-oxo-2-
phenylethanehydrazonoyl bromides. The acid dissociation constants in both the ground and excited states, pK and pKp, respectively, were
determined for both series and were correlated by the Hammett equation. The results of such correlations together with spectral data indicated
that the title compounds exist predominantly in the 1H-bis-,arylazo) form in both ground and excited states. q 2002 Published by Elsevier
Science Ltd.
1. Introduction
To the best of our knowledge there have been no reports
hitherto on the synthesis of the latter derivatives. The
interest in such compounds is due to the fact that derivatives
of azapentalene ring system are useful both in the ®eld of
material sciences and theoretical chemistry due to their
unusual electronic features and physical properties.2 For
example, various derivatives of 1H-imidazo[1,2-b]pyrazole
ring system have been recently reported to have potential
utility as magenta photographic couplers3±7 and as
chromogenic redox indicators,8 others have found uses as
herbicides,9 anti-in¯ammatory and anti-allergy10,11 and
anticancer agents against trypanosoma eruzil.4,12 Thus,
from a structure±activity viewpoint, the introduction of
one or two arylazo groups into the azapentalene ring system
may provide derivatives with increased or new utility. The
strategy adopted herein for synthesis of the target
compounds 4 is based on the use of N-aryl 2-oxo-2-phenyl-
ethanehydrazonoyl bromides 1 for the synthesis of arylazo-
pyrazole derivative 3 that preserves a degree of functionality
for building up another heterocyclic ring with another
arylazo group as depicted in Scheme 1. Furthermore, it
was thought interesting to elucidate the actual tautomeric
structure of the compounds in question ,Fig. 1) prior to
investigating their biological activities.
In previous papers from our laboratory, we reported the use
of 2-oxoalkanehydrazonoyl halides as useful precursors for
the synthesis of monoarylazo derivatives of various
nitrogen, sulfur and selenium heterocycles.1 In this context,
we became interested in examining the use of 1 as pre-
cursors for bis,arylazo)azapentalenes, namely 3,7-bis,aryl-
azo)-2,6-diphenyl-1H-imidazo[1,2-b]pyrazoles 4 ,Scheme 1).
2. Results and discussion
The required hydrazonoyl bromides 1 were prepared and
converted into 2-arylhydrazono-3-oxo-3-phenylpropane-
nitriles 2 as previously reported from our laboratory13
,Scheme 1). Treatment of the latter with hydrazine hydrate
in re¯uxing ethanol afforded the respective 5-amino-4-
arylzo-3-phenylpyrazoles 3 in good yields ,Scheme 1).14,15
Scheme 1.
Keywords: acidity; azo compounds; imidazoles; pyrazoles.
Corresponding author; e-mail: haney_sami@mail.com
p
0040±4020/02/$ - see front matter q 2002 Published by Elsevier Science Ltd.
PII: S0040-4020,02)00 157-6

2876
A. S. Shawali et al. / Tetrahedron 58 )2002) 2875±2880
in each case. On the basis of both microanalyses and
spectral data of the isolated products were identi®ed as
3,7-bis,arylazo)-2,6-diphenyl-1H-imidazo[1,2-b]pyrazoles
4 and 5, respectively ,Scheme 1). The other regioisomeric
structures,
namely
2,7-bis,arylazo)-3,6-diphenyl-1H-
imidazo[1,2-b]pyrazoles 6 and 7 were discarded on the
basis that reaction of 5-amino-3-phenylpyrazole with
2-oxohydrazonoyl halides was reported to afford in all
cases examined the respective 3-arylazo-2,6-diaryl-1H-
imidazo[1,2-b]pyrazoles and not the isomeric 2-arylazo-
3,6-diaryl-1H-imidazo[1,2-b]pyrazoles.14,15
The IR spectra of the products 4 and 5 revealed in each case
a weak NH band near 3200 cm21 ,Table 1). Their mass
spectra revealed in each case the respective molecular ion
peak together with ,M111) and ,M112) peaks. Although
such peaks were intense, they were not the base peaks,
however. Their high relative intensities indicate that the
molecular ions of the studied compounds are relatively
stable. In addition the spectra showed characteristic peaks
at m/z values corresponding to C6H5, XC6H4 and C11H6N3
ionic fragments. The latter fragment corresponds to either
2-phenyl- or 6-phenyl-1H-imidazo[1,2-b]pyrazole residue
,Table 2). Their NMR spectra in deuterated chloroform
exhibited a broad NH proton singlet signal centered at d
11.5±11.8 ,Table 1). The shape of the latter signal did not
change with temperature. Although these data are consistent
Figure 1.
The structures of the new derivatives 1e, f, h, 2e±g and
3e±h were evidenced by their microanalyses and spectral
,IR, H NMR, MS) data.
1
When equimolar quantities of the hydrazonoyl bromide 1c
and each of the azo derivatives 3a±g were re¯uxed in
ethanol in the presence of triethylamine, a single product
was obtained in each case. Similar reactions of 3c with each
of the hydrazonoyl bromides 1a±h also yielded one product
1
Table 1. IR and H NMR spectral data of compounds 4 and 5
Compound no.
IR n ,cm21
)
1H NMR d ,multiplicity, number of protons)a
4a
4b
4c
4d
4e
4f
4g
5a
5b
5d
5e
5f
31203.8 ,s, 3H), 6.95 ,d, 2H), 7.92 ,d, 2H), 7.1±7.4 and 8.2±8.47 ,m, 15H), 11.8 ,s, 1H)
31702.4 ,s, 3H), 7.24 ,d, 2H), 7.37 ,d, 2H), 7.4±8.6 ,m, 15H), 11.7 ,s, 1H)
31507.2±H8.)5, ,1m1.,62,0s, 1H)
31108.19 ,d, 2H), 8.43 ,d, 2H), 7.19±8.14 ,m, 15H), 11.5 ,s, 1H)
31607.4±8.8 ,m, 19H), 11.9 ,s, 1H)
3155
31707.74 ,d, 2H), 7.94 ,d, 2H), 7.2±7.4 and 8.1±8.5,m, 15H), 11.7 ,s, 1H)
3122 3.8 ,s, 3H), 6.95 ,d, 2H), 7.92 ,d, 2H), 7.1±7.4 and 8.2±8.47 ,m, 15H), 11.6 ,s, 1H)
31302.4 ,s, 3H), 7.24 ,d, 2H), 7.37 ,d, 2H), 7.4±8.6 ,m, 15H), 11.6 ,s, 1H)
3115 8.19 ,d, 2H), 8.43 ,d, 2H), 7.19±8.14 ,m, 15H), 11.7 ,s, 1H)
6.91±8.45 ,m, 19H), 11.55 ,s, 1H)
31307.26±8.5 ,m, 19H), 11.6 ,s, 1H)
31507.26±8.5 ,m, 19H), 11.55 ,s, 1H)
5g
5h
31707.52 ,d, 2H), 8.33 ,d, 2H), 7.6±8.2 and 8.5±8.6 ,m, 15H), 11.6 ,s, 1H)
1.7 ,t, Jˆ7 Hz, 3H), 4.7 ,q, Jˆ7 Hz, 2H), 7.4±8.7 ,m, 19H), 11.66 ,s, 1H)
3170, 1705
a
CDCl3.
Table 2. Mass spectral data of the compounds 4 and 5
Compound
no.
m/z ,%)
4a
4b
4c
4d
4e
4f
4g
5a
5b
5d
5e
5f
498 ,94), 497 ,100), 469 ,11), 390 ,14), 381 ,15), 115 ,16), 180 ,10), 104 ,11), 91 ,10), 77 ,95)
483 ,13), 482 ,48), 481 ,10), 391 ,21), 377 ,11), 272 ,13), 194 ,33), 180 ,5), 105 ,33), 91 ,54), 77 ,100)
469 ,17), 468 ,53), 467 ,32), 390 ,28), 363 ,13), 351 ,16), 180 ,23), 91 ,15), 77 ,100)
503 ,25), 502 ,74), 501 ,41), 391 ,43), 384 ,32), 214 ,31), 180 ,3), 115 ,19), 111 ,27), 77 ,100)
503 ,11), 502 ,11), 501 ,23), 390 ,23), 384 ,24), 272 ,16), 216 ,17), 180 ,16), 111 ,35), 91 ,19), 77 ,100)
514 ,6), 513 ,8), 512 ,15), 435 ,15), 395 ,20), 349 ,63), 322 ,14), 292 ,14), 246 ,27), 205 ,14), 180 ,100), 116 ,18), 77 ,24)
514 ,26), 513 ,52), 512 ,29), 390 ,39), 361 ,10), 349 ,22), 273 ,10), 179 ,18), 115 ,21), 103 ,16), 91 ,16), 77 ,100)
498 ,67), 497 ,96), 470 ,15), 421 ,19), 350 ,32), 246 ,13), 180 ,35), 122 ,10), 107 ,36), 104 ,16), 91 ,14), 77 ,100)
483 ,15), 482 ,41), 481 ,15), 480 ,10), 404 ,24), 377 ,15), 180 ,46), 114 ,15), 104 ,19), 91 ,80), 77 ,100)
503 ,25), 502 ,66), 501 ,66), 424 ,24), 396 ,10), 351 ,29), 180 ,39), 118 ,39), 111 ,34), 104 ,18), 91 ,20), 77 ,100)
503 ,25), 502 ,26), 501 ,59), 424 ,41), 362 ,16), 350 ,66), 322 ,18), 271 ,23), 242 ,24), 180 ,88), 111 ,57), 89 ,22),77 ,100)
514 ,36), 513 ,30), 512 ,77), 435 ,55), 407 ,26), 369 ,59), 300 ,39), 293 ,44), 290 ,100), 280 ,37), 244 ,56), 180 ,55), 181 ,81), 117 ,85), 77 ,85)
514 ,19), 513 ,55), 512 ,12), 437 ,13), 326 ,35), 350 ,13), 242 ,16), 180 ,28), 115 ,19), 105 ,22), 91 ,20), 77 ,100)
541 ,24), 540 ,65), 512 ,24), 351 ,35), 247 ,22), 180 ,50), 115 ,22), 103 ,32), 91 ,22), 77 ,100)
5g
5h

A. S. Shawali et al. / Tetrahedron 58 )2002) 2875±2880
2877
Table 4. Acid dissociation constants pK and pKp of compounds 4 and 5
with the assigned structures 4 and 5, they cannot distinguish
between the four possible tautomeric forms A±D ,Fig. 1).
They indicate, however, that each of the studied compounds
exists predominantly in one tautomeric form and that any
other tautomer form, if it were present, it will be at a level of
ca. 1% or less so that it would be unlikely detected by any of
the spectroscopic methods employed.
a
b
Compound no.
pK
lmax
lmax
Dn ,cm21
)
pKp
4a
4b
4c
4d
4e
4f
4g
5a
5b
5d
5e
5f
10.37
10.19
10.02
467
465
460
536
532
531
2758
2756
2906
4.63
4.48
3.98
9.50460 535
51 30
3.15
9.31
8.68
8.54
10.22
10.12
9.65
9.55
8.81
8.64
9.29
455
532
533
600
533
531
538
538
3181
3274
3342
2744
2813
3243
3254
2.68
1.74
1.59
4.51
4.27
2.91
2.78
454
500
465
462
458
458
458
498
To elucidate the actual tautomeric form of compounds 4 and
5, their electronic absorption spectra and correlations of
their acid dissociation constants with the Hammett equation
were examined. Their electronic spectra in dioxane revealed
in each case two characteristic intense absorption bands in
the regions 600±400 and 350±290 nm. The data are
summarized in Table 3. Such an absorption pattern is simi-
lar to that reported for the azo chromophore.16 Furthermore,
as compounds 4c and 5c are the same compound obtained
from different precursors, the electronic spectra of 4c, taken
as typical example of the two series investigated, were
recorded in solvents of different polarities. The spectra
obtained showed little, if any, shift ,Table 3). This result
indicates that the compounds exist in one tautomeric form,
namely the bis,arylazo) form, either A or D ,Fig. 1).
5403322
598
1.90
3358
5g
5h
1.66
470555
3258
2.50
a
In acid medium.
In alkaline medium; sˆ^0.04.
b
deviation. Such excellent linear correlations indicate that
the compounds studied exist in the 1H-structure, namely
the bis,arylazo) form A ,Fig. 1). This is because if they
exist in one of the mixed azo-hydrazone tautomeric forms
B or C, their pK values will be better correlated with the
enhanced Hammett substituent constant s2 rather than s
as the substituent will be in a direct interaction with the
negative charge on the nitrogen atom in the conjugate
base. The assignment of form A is further substantiated by
the similarity of the r values 1.75 and 1.45 found for the two
series studied 4 and 5, respectively and the similarity of such
Table 3. Electronic absorption spectra of compounds 4 and 5
Compound no.
lmax ,dioxane) ,log 1)
4a
4b
4ca
4d
4e
4f
4g
5a
5b
5d
5e
5f
514 ,4.33), 289 ,4.15)
506 ,4.30), 293 ,4.14)
502 ,4.37), 287 ,4.24)
508 ,4.30), 289 ,4.15)
490,4.26), 291 ,4.14)
495 ,4.19), 295 ,4.11)
508 ,4.55), 323 ,4.17)
487 ,4.38), 300 ,4.28)
510,4.38), 287 ,4.26)
500 ,4.39), 288 ,4.25)
501 ,4.25), 277 ,4.15)
503 ,4.47), 280 ,4.36)
501 ,4.37), 324 ,4.10)
505 ,4.52), 280 ,4.48)
r
values with that reported for 2-arylazophenols
,rˆ1.22).17 For typical hydrazones, the r value was
reported to be 2.57.16 Finally, if the compounds had the
5H-structure, i.e. form D ,Fig. 1), the r value for series 5
will be much less than that of series 4; i.e. less than 1.0. This
is because, in structure D, the bridge A0 ±NvN±
C,3)vC,2)±N,1)vC,7a)±C,7)vC,6)± between the acidic
site ±N,5)H and the substituent in Ar0 group is longer than
the bridge Ar±NvN±C,7)vC,6)± between the same
acidic site ±N,5)H and the substituent in Ar group. Con-
sidering the transmissive factors ,p 0 0.58 and 0.45) of the
two extra ±NvC± and CvC double bonds, respectively,21
the value of r,5) is expected to be much less than that of
r,4). This was not found experimentally, however, and
therefore the tautomeric form D was discarded.
5g
5h
a
Solvent: lmax nm ,log 1) ethanol: 514 ,4.38), 271 ,4.28); chloroform: 494
,4.37), 274 ,4.24); acetic acid: 511 ,4.33), 288 ,4.21); cyclohexane: 494
,4.28), 274 ,4.20); pyridine: 498 ,4.30), 290 ,sh) ,4.21); ether: 499 ,4.34),
271 ,4.28).
Next, the acid dissociation constants ,pK), of the two series
4a±g and 5a±h were determined by the spectrophotometric
method in 80% ,v/v) dioxane±water mixture at 278C and
ionic strength of 0.1. From the pH-absorbance data, the pK
values were calculated ,see Section 3) and the results are
summarized in Table 4. As shown, the data reveal that the
acidity constants of both series 4 and 5 are in¯uenced by the
ring substituent, being increased by electron-withdrawing
substituents and decreased by electron-donating sub-
stituents. Correlation of these pK data with Hammett
substituent constant sx using the least-squares method
resulted in the following equations:
Finally, we examined the correlation of the acidity constants
pKp of 4 and 5 in their corresponding singlet excited states
each with the Hammett substituent constant. The values of
pKp were calculated by utilizing the so-called Forster
energy-cycle.18 According to this cycle
pKp ˆ pK 1 0:625ꢀDn†=T
where pK and pKp are the acid dissociation constants of the
compound under study in the ground and the singlet excited
states, respectively, and Dn represents the frequency
difference in cm21 between the values of absorption lmax
of the compound in acid and alkaline media.19 The results of
such calculations are summarized in Table 4. Plots of pKp
for the two series 4 and 5 each vs sx were linear. The
equations of the regression lines are
pKꢀ4† ˆ 9:93 2 1:75sx;
r ˆ 0:995; s ˆ ^0:03
pKꢀ5† ˆ 9:94 2 1:46sx; r ˆ 0:987; s ˆ ^0:04
where r and s are the correlation coef®cient and standard
pKpꢀ4† ˆ 3:90 2 3:04sx;
r ˆ 0:90; s ˆ ^0:05

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A. S. Shawali et al. / Tetrahedron 58 )2002) 2875±2880
1
pKpꢀ5† ˆ 3:84 2 2:70sx;
r ˆ 0:99; s ˆ ^0:03
1654; H NMR ,CDCl3) d 1.34 ,t, Jˆ7 Hz, 3H), 4.31 ,q,
Jˆ7 Hz, 2H), 7.1±8.2 ,m, 9H), 9.2 ,s, 1H). Anal. calcd
,Found) for C17H15BrN2O3 ,375.2): C, 54.42 ,54.4); H,
4.03 ,4.3); N, 7.47 ,7.6)%.
As shown, the rp values obtained are larger than those of
the ground state. Assuming that the atom connectivity is the
same in ground and excited states, such results
emphasize the importance of the electronic interactions in
the excited state.19 Furthermore, such linear correlations
together with the similarity of the rp values collectively
indicate that the studied compounds 4 and 5 exist pre-
dominantly in the 1H-structure, namely form A ,Fig. 1) in
their excited states too. This is only to be expected if the
lifetime of the excited state before electronic relaxation is
too short to allow equilibration with other tautomeric
structures.
3.1.4. 2-N-ꢀ3-Chlorphenylhydrazono)-3-oxo-3-phenyl-
propanenitrile ꢀ2e). Yellow needles, yield 90%, mp
1888C ,EtOH), IR n ,KBr) cm21 3220, 2218, 1654; H
1
NMR ,CDCl3) d 7.2±8.2 ,m, 9H), 9.6 ,s, 1H). Anal. calcd
,Found) for C15H10ClN3O ,283.7): C, 63.51 ,63.6); H, 3.55
,4.0); N, 14.81 ,14.5)%.
3.1.5. 2-N-ꢀ3-Nitrophenylhydrazono)-3-oxo-3-phenyl-
propanonitrile ꢀ2f). Yellow needles, yield 85%, mp
1948C ,AcOH), IR n ,KBr) cm21 3210, 2215, 1635; H
1
In conclusion, the results of the present study demonstrate
collectively that N-aryl 2-oxoethanehydrazonoyl bromides
1 provide useful entry to synthesis of the title bis,arylazo)
compounds 4 and 5 and that both series of compounds exist
in solution almost exclusively in the 1H-bis,arylazo)
tautomeric form A in both the ground and excited states.
NMR ,CDCl3) d 7.20±8.10,m, 9H), 12.0,s, 1H). Anal.
calcd ,Found) for C15H10N4O3 ,294.3): C, 61.22 ,61.5); H,
3.42 ,3.6); N, 19.04 ,18.8)%.
3.1.6. 2-N-ꢀ4-Nitrophenylhydrazono)-3-oxo-3-phenyl-
propanonitrile ꢀ2g). Yellow needles, yield 90%, mp
1
2308C ,AcOH), IR n ,KBr) cm21 3209, 2214, 1650; H
NMR ,CDCl3) d 7.26±8.15 ,m, 9H), 12.0,s, 1H). Anal.
calcd ,Found) for C15H10N4O3 ,294.3): C, 61.22 ,61.2); H,
3.42 ,3.7); N, 19.04 ,18.9)%.
3. Experimental
3.1. General
3.1.7. 5-Amino-4-ꢀ3-chlorophenylazo)-3-phenylpyrazole
ꢀ3e). Dark yellow needles, yield 80%, mp 1988C ,AcOH),
Melting points were determined on a Gallenkamp apparatus
and are uncorrected. IR spectra were recorded in potassium
bromide using Perkin±Elmer FTIR 1650and Pye-Unicam
1
IR n ,KBr) cm21 3417, 3385; H NMR ,CDCl3) d 6.2 ,s,
2H), 7.25±8.05 ,m, 10H). MS m/z ,%) 299 ,26), 298 ,75),
297 ,56), 186 ,100), 130 ,70), 128 ,43), 111 ,15), 103 ,25),
77 ,40). Anal. calcd ,Found) for C15H12ClN5 ,298): C, 60.51
,60.5); H, 4.06 ,4.2); N, 23.50 ,23.1)%.
1
SP300 infrared spectrophotometers. H NMR spectra were
recorded in deuterated chloroform using a Varian Gemini
200 NMR spectrometer. Mass spectra were recorded on a
GCMS-QP 1000 EX Shimadzu and GCMS 5988-A HP
spectrometers. Electronic absorption spectra were recorded
on Perkin±Elmer Lambada 40spectrophotometer.
Elemental analyses were carried out at the Microanalytical
Laboratory of Cairo University, Giza, Egypt.
3.1.8. 5-Amino-4-ꢀ3-nitrophenylazo)-3-phenylpyrazole
ꢀ3f). Dark yellow needles, yield 75%, mp 2108C ,AcOH),
1
IR n ,KBr) cm21 3418, 3300; H NMR ,CDCl3) d 6.7 ,s,
2H), 7.42±8.52 ,m, 10H). MS m/z ,%) 309 ,19), 308 ,100),
186 ,96), 130 ,65), 128 ,41), 103 ,30), 77 ,29). Anal. calcd
,Found) for C15H12N6O2 ,308.3): C, 58.44 ,58.5); H, 3.92
,4.2); N, 27.26 ,27.0)%.
N-Aryl 2-oxo-2-phenyethanehydrazonoyl bromides 1,13
2-arylhydrazono-3-oxo-3-phenylpropanenitriles 213 and
5-amino-4-arylazo-3-phenylpyrazoles 314 were prepared as
previously reported from our laboratory. The physical
constants of the new derivatives 1e, f, h, 2e±g and 3e±g
are given below.
3.1.9. 5-Amino-4-ꢀ4-nitrophenylazo)-3-phenylpyrazole
ꢀ3g). Dark brown needles, yield 80%, mp 2788C ,EtOH),
1
IR n ,KBr) cm21 3417, 3385; H NMR ,CDCl3) d 6.81 ,s,
2H), 7.40±8.30 ,m, 10H). MS m/z ,%) 309 ,80), 308 ,25),
186 ,95), 130 ,100), 77 ,58). Anal. calcd ,Found) for
C15H12N6O2 ,308.3): C, 58.44 ,58.4); H, 3.92 ,4.0); N,
27.26 ,27.1)%.
3.1.1. N-ꢀ3-Chlorophenyl) 2-oxo-2-phenylethanehydra-
zonoyl bromide ꢀ1e). Yellow needles, yield 70%, mp
1
1708C ,EtOH), IR n ,KBr) cm21 3225, 1654; H NMR
,CDCl3) d 7.3±8.3 ,m, 9H), 8.91 ,s, 1H). Anal. calcd
,Found) for C14H10BrClN2O ,337.6): C, 49.81 ,49.6); H,
2.99 ,3.2); N, 8.30,8.0)%.
3.1.10.
3,7-Bisꢀarylazo)-2,6-diphenyl-1H-imidazo[1,2-
b]pyrazoles ꢀ4 and 5). General procedure. To a mixture
of hydrazonoyl bromide 1c and the appropriate pyrazole
derivative 3 ,5 mmol each) in dioxane ,30mL) was added
triethylamine ,0.7 mL, 5 mmol). The reaction mixture was
re¯uxed for 3 h, cooled and poured onto cold water while
stirring. The solid that precipitated was ®ltered off, dried
and crystallized from dioxane or acetic acid to give the
respective 4.
3.1.2. N-ꢀ3-Nitrophenyl) 2-oxo-2-phenylethanehydra-
zonoyl bromide ꢀ1f). Yellow needles, yield 75%, mp
1
2158C ,Dioxane), IR n ,KBr) cm21 3240, 1653; H NMR
,CDCl3) d 7.4±8.2 ,m, 9H), 10.5 ,s, 1H). Anal. calcd
,Found) for C14H10BrN3O3 ,348.2): C, 48.29 ,48.5); H,
2.90,3.2); N, 12.07 ,12.3)%.
Repetition of the same procedure using the pyrazole deriva-
tive 3c with the hydrazonoyl bromides 1a±h yielded the
respective 5a±h, respectively.
3.1.3. N-ꢀ4-Ethoxycarbonylphenyl) 2-oxo-2-phenyl-
ethanehydrazonoyl bromide ꢀ1h). Yellow needles, yield
80%, mp 1808C ,Dioxane), IR n ,KBr) cm21 3224, 1705,

A. S. Shawali et al. / Tetrahedron 58 )2002) 2875±2880
2879
Table 5. Physical constants of 2,6-diphenyl-3,7-bis,arylazo)-1H-imidazo-[1,2-b]pyrazoles 4
Compound no.
Yield ,%)
Mp ,8C) ,solvent)a
Molecular formula
Anal. calcd/found ,%)
C
H
N
4a
4b
4c
4d
4e
4f
,a) dark80b2r3o0wn needles
C
30H23N7O ,497.5)
C30H23N7 ,481.6)
29H21N7 ,467.5)
72.42
72.6
74.83
74.6
74.504.53 .97
74.5
69.39
69.2
69.39
69.2
67.96
67.8
67.96
67.9
4.66
4.6
4.81
4.8
19.70
19.5
20.36
20.1
85
262 ,a) dark green needles
90265 ,a) dark green needles
C
20
4.8
4.01
4.4
4.01
4.1
3.93
4.021.8
3.93
4.1
21.0
19.53
19.5
19.53
19.5
21.86
,a) dark90r2e6d0plates
C
29H20ClN7 ,501.9)
C29H20ClN7 ,501.9)
C29H20N8O2 ,512.5)
29H20N8O2 ,512.5)
85
85
268 ,a) dark green needles
254 ,a) dark red needles
4g
90261 ,b) dark brown plates
C
21.86
21.7
a
Solvent of crystallization: ,a) acetic acid; ,b) dioxane.
Table 6. Physical constants of 2,6-diphenyl-3,7-bis,arylazo)-1H-imidazo-[1,2-b]pyrazoles 5
Compound no.
Yield ,%)
Mp ,8C) ,solvent)a
Molecular formula
Anal. calcd/found ,%)
C
H
N
5a
5b
5d
5e
5f
85
230,a) dark brown needles
C
30H23N7O ,497.5)
30H23N7 ,481.6)
72.42
72.6
74.83
74.8
69.39
69.1
69.39
69.2
67.96
67.6
67.96
67.7
71.23
70.9
4.66
4.7
4.81
5.020
4.01
4.1
4.01
4.019.4
3.93
4.1
19.70
19.8
20.36
70256 ,a) dark green needles
90254 ,a) dark brown needles
C
C
.2
29H20ClN7 ,501.9)
C29H20ClN7 ,501.9)
29H20N8O2 ,512.5)
29H20N8O2 ,512.5)
32H25N7O2 ,539.6)
19.53
19.5
19.53
75
184 ,a) dark red needles
70195 ,a) dark red needles
90252 ,b) dark brown needles
70222 ,a) dark red needles
C
C
C
21.86
22.2
21.86
21.7
18.17
18.1
5g
5h
3.93
4.1
4.67
4.5
a
Solvent of crystallization: ,a) acetic acid; ,b) dioxane.
The physical constants of the products 4a±g and 5a±h
isolated are given in Tables 5 and 6, respectively.
The experimental procedure followed in the determination
of pK constants and their calculations from the absorbance-
pH data are as previously described.21 The pK values were
reproducible to ^0.02 pK unit. The results are given in
Table 4.
3.2. pK determination
The acid dissociation constants pK values of the compounds
4 and 5 were determined spectrophotometrically in 80%
,v/v) dioxane±water mixture at 278C and ionic strength
0.01. An Orion 420A pH meter ®tted with combined glass
electrode type 518635 was employed for measurement of
pH values. The instrument was accurate to 0.01 pH unit. It
was calibrated using two standard Beckman buffer solutions
of pH 4.01 and 7.00. The pH meter readings ,B) recorded in
dioxane±water solutions were converted to hydrogen ion
concentration [H1] by means of the widely used relation
of van Uitert and Haas20 namely: 2log[H1]ˆB1log UH,
where log UH is the correction factor for the solvent compo-
sition and ionic strength used for which B is read. The value
was determined by recording the pH values for a series of
hydrochloric acid and sodium chloride such that the ionic
strength is 0.1 in 80% ,v/v) dioxane±water at 278C. The
value of log UH was found to be 20.48.
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