Recyclization of 2-(3,6-diaryl-2,5-dihydropyridazin-4-yl)-1H-benzimidazoles to 2-[(3,5-diarylpyrazol-4-yl)methyl]-1H-benzimidazoles

Article abstract of DOI:10.1007/s10593-008-0108-3

2-(3,6-Diaryl-2,5-dihydropyridazin-4-yl)-1H-benzimidazoles undergo a previously unknown type of recyclization of the pyridazine ring to a pyrazole to give 2-[(3,5-diarylpyrazol-4-yl)methyl]-1H-benzimidazoles. It is suggested that the mechanism of this conversion, which includes formation of a secondary enhydrazine group in the diazine ring and its subsequent contraction, occurs after the intramolecular formation and opening of a cyclopropane ring.

Full text of DOI:10.1007/s10593-008-0108-3

Chemistry of Heterocyclic Compounds, Vol. 44, No. 6, 2008
RECYCLIZATION OF 2-(3,6-DIARYL-
2,5-DIHYDROPYRIDAZIN-4-YL)-1H-BENZ-
IMIDAZOLES TO 2-[(3,5-DIARYLPYRAZOL-
4-YL)METHYL]-1H-BENZIMIDAZOLES
I. B. Dzvinchuk1, A. N. Chernega1, M. O. Lozinskii1, and A. A. Tolmachev2
2-(3,6-Diaryl-2,5-dihydropyridazin-4-yl)-1H-benzimidazoles undergo a previously unknown type of
recyclization of the pyridazine ring to a pyrazole to give 2-[(3,5-diarylpyrazol-4-yl)methyl]-1H-benz-
imidazoles. It is suggested that the mechanism of this conversion, which includes formation of a
secondary enhydrazine group in the diazine ring and its subsequent contraction, occurs after the
intramolecular formation and opening of a cyclopropane ring.
Keywords: benzimidazoles, pyrazoles, pyridazines, recyclization.
We have previously synthesized the 2-(3,6-diaryl-2,5-dihydropyridazin-4-yl)-1H-benzimidazoles 1a-g
which showed a fully expected tendency to oxidative aromatization of the dihydrodiazine ring to a diazine to
form the products 2a-g [1]. In this work we report an unusual reaction of compounds 1a-g resulting in
restructuring of the diazine ring residue.
Ar
Ar
H
N
H
N
H
N
N
HNO2
N
N
N
N
Ar1
Ar 1
1a-g
2a-g
HOAc,
120°C, 1 h
H
N
H
N
Ar
Ar
N
N
Ar1
3a-g
Ar1
NH
N
N
N
H
3'a-g
1–3 a Ar = Ar1 = Ph; b Ar = 4-MeOC6H4, Ar1 = Ph; c Ar = 3,4,5-(MeO)3C6H2,
Ar1 = Ph; d Ar = 4-BrC6H4, Ar1 = Ph; e Ar = Ph, Ar1 = 3,4-(MeO)2C6H3;
f Ar = Ph, Ar1 = 4-ClC6H4; g Ar = Ph, Ar1 = 3-O2NC6H4
__________________________________________________________________________________________
1
Institute of Organic Chemistry, National Academy of Sciences of Ukraine, Kiev 02094; e-mail:
2
Rostov@bpci.kiev.ua. Enamine Ltd., Kiev 02042, Ukraine; e-mail: dov@fosfor.kiev.ua. Translated from
Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 853-861, June, 2008. Original article submitted July 9,
2007.
682
0009-3122/08/4406-0682©2008 Springer Science+Business Media, Inc.

We have found that refluxing compounds 1a-g in acetic acid (method A) causes ready recyclization to the
previously unknown 2-[(3,5-diarylpyrazol-4-yl)methyl]-1H-benzimidazoles 3a-g which exist in solution in
equilibrium with their tautomeric forms 3'a-g.
The reaction is complete after 1 h. The process can be controlled by the decolorization of the reaction
mixture since the starting compounds have a yellow color and the final products are colorless. However, freshly
prepared reagents 1a-f should be used since they are of low stability due to the increased tendency of the
dihydropyridazine ring to undergo oxidative aromatization.
In the majority of experiments the reaction occurs highly selectively with quite a broad range of variation
of the Ar and Ar1 substituents. The products 3a-f are obtained in 83-96% yields. However, the reaction is limited by
the presence of a nitro group in the starting reagents. Thus the reaction of the nitro-substituted compound 1g gives a
mixture of products according to TLC and the corresponding product 3g can only be separated in 24% yield
together with a 37% yield of the oxidative aromatization product 2g. In this case the lack of selectivity is evidently
due to the known tendency of a nitro group to take part in oxidative-reductive processes.
The recyclization can also occur with catalysis by mineral acid or base. Hence heating compound 1a
with hydrochloric acid gives the bis hydrochloride 4 from which the free base 3a can be separated after
treatment with ammonia (method B). Compound 1a also isomerises to product 3a upon refluxing in an alcoholic
solution of potassium hydroxide (method C). The product 3a may also be prepared by treating 2-(benzimidazol-
2-yl)-1,4-diphenylbutane-1,4-dione 5 (obtained by method [2]) with hydrazine in a refluxing mixture of pyridine
and acetic acid (method D). This method is particularly attractive since the reaction occurs via intermediate
formation of the low stability compound 1a without needing its isolation.
Ph
H
H
N
N
Ph
O
O
_
HCl – HOAc, 100°C, 4 h
74 %
2 Cl
+
N
H
N
+
NH
Ph
N
H
Ph
5
4
1a
89%
– 2 HCl
Py–HOAc,
120°C, 2 h
N2H4
H
N
KOH, EtOH, 80-85°C, 5 h
83 %
Ph
[1a]
73 %
N
NH
Ph
N
3a
Compound 3a shows chemical properties in agreement with its structure. It reacts with acetic anhydride
or with DMF dimethylacetal at the nitrogen atoms of the benzimidazole and pyrazole rings to give the
derivatives 6 and 7 but, in contrast to the starting compound 1a, is not changed by treatment with nitrous acid in
the conditions reported in the study [1].
HNO2
Me
COMe
N
N
N
Ac2O
Me2NCH(OMe)2
Ph
Ph
3a
Py, 100°C, 3 h
N
Py, 100-105°C, 4.5 h
N
N
Ph
Me
Ph
COMe
N
N
7
6
683

TABLE 1. Characteristics of Compounds Synthesized
Found, %
——————
Calculated, %
Com-
pound
Empirical
formulan
Yield, %
(method)
mp, °С
С
Н
N
3a
C23H18N4
78.69
78.83
5.22
5.18
15.87
15.99
294.5-296
95 (A), 66 (B),
83 (C), 73 (D)
3b
3c
3d
3e
3f
3g
4
C24H20N4O
C26H24N4O3
C23H17BrN4
C25H22N4O2
C23H17ClN4
C23H17N5O2
C23H18N4·2HCl*
C27H22N4O2
C25H22N4
75.63
75.77
70.64
70.89
64.23
64.35
73.03
73.15
71.64
71.78
69.68
69.86
65.12
65.26
74.52
74.64
5.37
5.30
5.55
5.49
4.06
3.99
5.34
5.40
4.56
4.45
4.25
4.33
4.83
4.76
5.12
5.10
14.59
14.73
12.64
12.72
12.94
13.05
13.53
13.65
14.41
14.56
17.59
17.71
13.17
13.23
12.74
12.89
272-273.5
211.5-213
263-265
94 (A)
94 (A)
95 (А)
83 (А)
96 (А)
24 (A)
74
255-257
313.5-315
240-241.5
292-293
6
182.5-184
191.5-193
89
7
79.18
79.34
5.94
5.86
14.67
14.80
88
_______
* Found, %: Cl 16.78. Calculated, %: Cl 16.75.
TABLE 2. 1H NMR Spectra of the Compounds Synthesized
Com-
pound
Chemical shifts, δ, ppm (J, Hz)
3a
3b
4.17 (2H, s, CH2); 7.11 (2H, m, H-5,6); 7.33 (1H, m, H-7); 7.39 (6H, m, 6HPh-m,-p);
7.51 (1H, m, H-4); 7.55 (4H, d, J = 7.0, 4HPh-o); 12.19 (1H, s, H-1*); 13.33 (1H, s, H-1'*)
3.72 (3H, s, H3CO); 4.14 (2H, s, CH2); 6.95 (2H, m, 2HAr-m); 7.11 (2H, m, H-5,6);
7.34 (3H, m, 3HPh-m,-p); 7.40 (1H, m, H-7); 7.47 (2H, d, J = 8.0, 2HAr-o);
7.51 (1H, m, H-4); 7.54 (2H, d, J = 7.0, 2HPh-o); 12.18 (1H, s, H-1); 13.20 (1H, s, H-1')
3c
3.52 (6H, s, 2H3CO-m); 3.62 (3H, s, H3CO-o); 4.14 (2H, s, CH2); 6.84 (2H, s, Ar);
7.12 (2H, m, H-5,6); 7.34 (1H, m, H-7); 7.42 (3H, m, 3HPh-m,-p); 7.52 (1H, m, H-4);
7.59 (2H, d, J = 7.5, 2HPh-o); 12.29 (1H, s, H-1); 13.25 (1H, s, H-1')
3d
3e
4.16 (2H, s, CH2); 7.10 (2H, m, H-5,6); 7.35 (1H, m, H-7); 7.38 (3H, m, 3HPh-m,-p);
7.52 (6H, m, 2 HPh-o + 4 HAr); 7.60 (1H, m, H-4); 12.20 (1H, s, H-1); 13.39 (1H, s, H-1')
3.31 and 3.72 (3H and 3Н, s, 2H3CO); 4.13 (2Н, s, СН2); 6.97 (1H, m, HAr-5);
7.06 (1H, d, J = 7.5, HAr-6); 7.11-7.15 (3H, m, H-5,6 + HAr-2); 7.28 (1H, m, H-7);
7.35 (1H, m, H-7); 7.40–7.42 (3H, m, 3HPh-m,-p); 7.52 (1H, m, H-4);
7.57 (2H, d, J = 7.0, 2HPh-o); 12.23 (1H, s, H-1); 13.20 (1H, s, H-1')
3f
4.17 (2H, s, CH2); 7.11 (2H, m, H-5,6); 7.26-7.53 (9H, m, 5HPh + H-4,7 + 2 HAr-m);
7.59 (2H, d, J = 7.0, 2HAr-o); 12.21 (1H, s, H-1); 13.40 (1H, s, H-1')
3g
4.22 (2H, s, CH2); 7.10 (2H, m, H-5,6); 7.29–7.50 (4H, m, 3HPh-m,-p + H-7);
7.57 (3H, d, J = 7.0, 2HPh-o + H-4); 7. 62 and 7.72 (0.8 + 0.2Н, two t, J = 7.0, 1HAr-5);
8.02 and 8.04 (0.2 + 0.8H, two d, J = 7.0, HAr-6); 8.10 and 8.18 (0.8 + 0.2H, two d,
J = 7.0, HAr-4); 8.43 and 8.47 (0.8 + 0.2H, two s, HAr-2); 12.27 (1H, s, H-1);
13.56 and 13.60 (0.8 + 0.2H, two s, H-1')
4
6
4.70 (2H, s, CH2); 7.28 (2H, t, J = 7.5, 2HPh-p); 7.36 (4H, t, J = 7.5, 4HPh-m);
7.40 (2H, m, H-5,6); 7.55 (4H, d, J = 7.5, 4HPh-o); 7.57 (2H, m, H-4,7)
2.72 and 2.73 (3H and 3H, two s, 2H3CCO); 4.22 (2H, s, CH2);
7.29-7.33 (7H, 5H5-Ph + + H-5,6); 7.34-7.39 (3H, m, 3H3-Ph-m,-p);
7.60-7.64 (3H, m, 2H3-Ph-o + H-4); 7.81 (1H, d, J = 8.0, H-7)
7
3.58 and 3.79 (3H and 3H, two s, 2H3C); 3.40 (2H, s, CH2); 7.12 (1H, t, J = 7.0, H-5);
7.18 (2H, t, J = 7.0, H-6); 7.25 (1H, m, H3-Ph-p); 7.31 (2H, m, 2H3-Ph-m);
7.39-7.43 (6H, m, 5H5-Ph + H-7); 7.52 (1H, d, J = 7.0, H-4); 7.55 (2H, d, J = 7.0, 2H3-Ph-o)
_______
* Undergoes deuterium exchange.
684

The composition and structure of the synthesized compounds were confirmed by their elemental
analytical data (Table 1) and 1H NMR spectra (Table 2). The structure of compounds 3a and 3g also agree with
chromato-mass analytical data and the diacetyl derivative 6 with the IR spectrum. Moreover the structure of
compound 3a was unambiguously established by an X-ray structural study (see Fig. 1). In the molecule the
Figure 1. General view of the compound 3a molecule from X-ray analysis.
system N(1)N(2)C(1-7) is almost planar, the deviation from the plane not exceeding 0.027 Å. The benzene rings
are twisted from the N(3)N(4)C(9-11) plane by 78.4 and 40.1º. All of the bond lengths and angles in the
molecule are close to the corresponding parameters in a series of related compounds [5, 6]. In the solid state the
molecules of compound 3a are associated by N(2)–H···N(4) intermolecular hydrogen bonds as centrosymmetric
dimers which, in turn are linked in a 2D network by N(3)–H···N(1) hydrogen bonds (Fig. 2).
1
The H NMR spectra point to an inhibition of migration of protons between the ring nitrogen atoms in
the benzimidazole fragment of compounds 3a-g. As a result the H-1 signal is a rather broad singlet at
12.18-12.29 ppm and the signals for H-4 and H-7 appear separately at 7.51-7.60 and 7.28-7.40 ppm.
Tautomerism arising from migration of a proton between the pyrazole ring nitrogen atoms in the series of
compounds 3a-f causes a broadening of the aromatic substituent protons signals and also H-1' (singlet at
685

13.20-13.40 ppm). It should be noted that both phenyl groups in compound 2a are chemically equivalent and
their signals coincide: the o-protons appear as a doublet at 7.55 ppm and the m and p-protons as a broadened
multiplet at 7.39 ppm. On the other hand, in compound 3g the electronic nature of the aromatic substituents
differ more significantly (Ar = Ph, Ar1 = 3-O2NC6H4), broadening is not observed, and doubling of the signals
for the nitrophenyl group and H-1' is observed which corresponds to the appearance of the individual tautomers.
According to the integrated intensities of these signals the tautomer form 3g content is ~ 80%. Its increased
stability is likely due to the fact that the electron donor effect of the nitrogen atom in the pyrazole ring is
transmitted to the 3-nitrophenyl substituent by the shortest conjugated chain. In form 3'g the conjugated chain is
two bonds longer and hence it is energetically less favored. We have previously observed similar prototropic
1
effects in the H NMR spectra of structural analogs of compounds 3a-g – 2-(pyrazol-4-yl)-1H-benzi-midazoles
which are substituted in the pyrazole ring by one or two aryl groups [3, 4].
Figure 2. Crystal packing of compound 3a.
686

1
The H NMR spectrum of salt 4 shows a phenylene fragment typical of benzimidazole salts (a
symmetrical picture for the H-5,6 resonance signals and for H-4,7 at lower field, see [2]). The appearance of the
phenyl ring is similar to those above mentioned for the base 3a but with separately observed o-, m-, and p-
protons in successive order from low to higher field.
1
The H NMR spectrum of the diacetyl derivative has a number of features. The acetyl group on the
benzimidazole fragment nitrogen atom has a deshielding effect on the adjacent phenyl fragment H-7 proton
which thus appears to low field at 7.81 ppm (in compounds 3a-f the H-7 signal occurs at 7.28-7.40 ppm). The
second acetyl group (on the pyrazole fragment nitrogen atom) is sterically hindered by the nearby phenyl
substituent and it appears to be placed orthogonally to the plane of the heterocycle, thus appearing as a narrow
multiplet at 7.29 ppm. In the spectrum of the dimethyl derivative 7 this effect appears in part (the multiplet for
the 5-phenyl group is somewhat broadened at 7.39-7.43 ppm) because the methyl group (in contrast to an acetyl)
does not possess magnetic anisotropic properties and shows less steric hindrance.
The resonances of the CH2 groups of the synthesized compound are in the order 4.13-4.22 (3a-g), 4.70
(4), 4.22 (5), and 3.40 ppm (7) and are shifted to low field with the occurrence of structural factors which lower
the electron density on the heterocycles.
Based on the structure of the starting and formed compounds the recyclization occurs as the result of a
restructuring of the carbon-carbon bonds. Its mechanism likely includes an initial 1,3-proton shift step in the
diazine ring from the methylene group to the nitrogen atom. The secondary enhydrazine group formed in this
way can react intramolecularly with the already present carbon-carbon bond to give closure of a cyclopropane
ring and contraction of the six to a five membered heterocycle. This increases the electrophilicity of the
pyridazine ring C-3 atom, promoted because of the electron acceptor effect of the benzimidazole fragment.
Subsequent opening of the three membered ring at another carbon-carbon bond with a 1,2-proton shift possibly
occurs spontaneously. This assists the electronic effect of both nitrogen atoms of the 4,5-dihydropyrazole ring
(donor for one and acceptor for the other) and also simultaneously the aromatization of the pyrazole ring to give
the final products 3a-g.
Ar
Ar
H
N
H
H
N
H
N
N
1a–g
+ N
H
_
N
NH
Ar1
N
H
Ar1
H
Ar
H
N
H
N
N
Ar
+
3a–g
N
N
N
Ar1
N
H
Ar1
H
N
_
The recyclization of certain pyridazine compounds to pyrazoles has been reported previously [7-11] but
they differ from that given above by one overall feature which is a result of the opening of an existing and
formation of a new nitrogen-carbon bond. We have not found any evidence for the restructuring of a pyridazine
ring to a pyrazole via formation of a new and fission of an existing carbon-carbon bond. It should also be noted
that there have also been reported four representatives of 2-(4-pyrazolyl)-1H-benzimidazoles substituted in the
pyrazole ring by an aryl (position 1) and two methyl groups (positions 3 and 5) which were synthesized by a
classical method (cyclocondensation of arylhydrazines with 3-[(1H-benzimidazol-2-yl)methyl]pentane-
2,4-dione) and they possess clear bactericidal and fungicidal activity [12].
Hence in the case of the reaction of 2-(3,6-diaryl-2,5-dihydropyridazin-4-yl)-1H-benzimidazoles to
2-[(3,5-diarylpyrazol-4-yl)methyl]-1H-benzimidazoles we have revealed a novel type of recyclization of a
687

pyridazine ring to a pyrazole, differing in the fact that it involves restructuring of carbon–carbon bonds. The
reaction likely has a wider general character and can find its place in organic synthetic practice.
EXPERIMENTAL
Monitoring of the reaction course and purity of the synthesized compounds was carried out by TLC on
Silufol UV-254 plates in the solvent system benzene – ethanol (9: 1) and revealed using UV light. The 1H NMR
spectra of the compounds were recorded on a Bruker Avance DRX 500 (500 MHz) spectrometer using DMSO-
d6 with TMS as standard. The IR spectrum of compound 6 was recorded on a UR-20 instrument for a KBr tablet.
Chromato-mass analysis for compounds 3a and 3g was carried out on an Agilent 1100 Series high resolution
liquid chromatograph fitted with an Agilent LC/MSD SL mass detector. Compounds were dried for 4 h at 125ºC
before elemental analysis and spectroscopic investigation.
X-ray Investigation of a Single Crystal of Compound 3a was performed at room temperature on a
Bruker Apex II automatic diffractometer (MoKα radiation, λ = 0.71073 Å, θmax = 26.5º, -26 ≤ h ≤ 30, 12 ≤k ≤ 13,
-16 ≤ l≤ 18). Unit cell parameters for the crystal: a = 24.694(2), b = 10.5061(9), c = 15.1393(12) Å, β = 112.228(6)º,
space group C2/c. 11706 reflections were collected, the structure was refined in full matrix anisotropic
approximation, and all of the H atoms were revealed in electron density difference synthesis with R = 0.037 and
Rw = 0.039, GOF = 1.195 (3716 independent reflections with Rint = 0.019). The full set of X-ray structural data
for compound 3a has been placed in the Cambridge structural database (reference CCDC 651222).
2-[(3,5-Diphenylpyrazol-4-yl)methyl]-1H-benzimidazole (3a). A. A mixture of compound 1a (0.25 g)
and glacial acetic acid (1.25 ml) was held for 1 h at 120ºC. The reaction mixture was diluted with acetone
(1.25 ml), water (2 ml) and 20% aqueous ammonia solution (2 ml) and refluxed with stirring to full
crystallization of the oil produced. After cooling, the precipitate was filtered off, washed with water, and dried to
give analytically pure product (0.237 g) (100% pure by chromato mass analysis). Found, M+1 = 351. C23H18N4.
Calculated, M = 350.
Products 3b-f were prepared similarly from compounds 1b-f (the 3d-f products were recrystallized
from a mixture of pyridine and ethanol (1:3)).
B. A mixture of the bis hydrochloride 4 (0.2 g), pyridine (1.5 ml), and 20% aqueous ammonia (0.5 ml)
was heated to reflux with stirring and diluted dropwise with water (4 ml). After cooling, the precipitate was
filtered off, washed with water, and dried to give the pure compound 3a (0.149 g).
C. A mixture of compound 1a (0.175 g, 0.5 mmol), potassium hydroxide (0.084 g, 1.5 mmol), and
ethanol (1.5 ml) was refluxed for 6 h. The hot reaction solution was diluted with water (1.5 ml), heated with
stirring to reflux, and left to cool slowly. The precipitate formed was filtered off, washed with 2-propanol, dried,
and recrystallized from chlorobenzene to give the pure compound 3a (0.145 g).
D. A mixture of compound 5 (0.172 g, 0.5 mmol), 80% hydrazine hydrate (0.2 ml), pyridine (2 ml), and
glacial acetic acid (0.5 ml) was refluxed for 3 h. Water (1 ml) was added with stirring to the hot reaction solution.
After cooling, the precipitate was filtered off and washed with water to give the pure compound 3a (0.128 g).
Samples of the compound 3a prepared by the methods A to D were identical by TLC and mixed sample
melting points.
2-{[(3-Nitrophenyl)-5-phenyl-1H-pyrazol-4-yl]methyl}-1H-benzimidazole (3g).
A
mixture of
compound 1g (0.395 g, 1 mmol) and glacial acetic acid (2 ml) was held for 1 h at 120ºC. After cooling, the
precipitate was filtered off (after it was crystallized from a mixture of acetic acid and water (1:1) to give
compound 2g (0.148 g), identical in TLC data and mixed sample melting point to a sample prepared according
the method [1]). The filtrate was basified with 20% aqueous ammonia solution (4 ml) and heated to reflux. After
cooling, the aqueous layer was poured off and the residue refluxed with acetonitrile (2 ml) to complete
crystallization. After cooling, the precipitate was filtered off, recrystallized from a mixture of pyridine and
688

water (2: 1) and then ethanol and water (1:1) to give the product 3g (0.096 g) which was 99.77% pure from
chromato-mass analysis. Found, M+1 = 396. C23H17N5O2. Calculated, M = 395.
2-[(3,5-Diphenyl-1H-pyrazol-2-io-4-yl)methyl]-3H-benzimidazol-1-ium Dichloride (4). A mixture of
compound 1a (0.35 g, 1 mmol), glacial acetic acid (1.5 ml), water (1 ml), and concentrated hydrochloric acid
(0.3 ml, 3 mmol) was heated for 4 h at 100-105ºC. After cooling, the precipitate was filtered off and washed
with 2-propanol to give analytically pure product (0.314 g).
1-Acetyl-2-[(1-acetyl-3,5-diphenyl-1H-pyrazol-4-yl)methyl]-1H-benzimidazole (6). A mixture of
compound 3a (0.35 g, 1 mmol), acetic anhydride (0.284 ml, 3 mmol), and anhydrous pyridine (1 ml) was held for 3
h at 100-105ºC. 2-Propanol (1 ml) and water (1 ml) were added and stirred. After cooling, the precipitate was
filtered off and washed with 2-propanol to give analytically pure product (0.387 g). IR spectrum: 1270 cm-1 (C=O).
1-Methyl-2-[(1-methyl-3,5-diphenyl-1H-pyrazol-4-yl)methyl]-1H-benzimidazole (7). A mixture of
compound 3a (0.175 g, 0.5 mmol), DMF dimethylacetal (0.5 ml), and anhydrous pyridine (0.5 ml) was held for
4.5 h at 100-105ºC. 2-Propanol (1 ml) and water (1 ml) were added and the product was refluxed with stirring to
the beginning of crystallization. After cooling, the precipitate was filtered off, washed with a mixture of
2-propanol and water (1:1), and dried to give analytically pure product (0.168 g).
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