Synthesis and tautomerism of 2-[3(5)-aryl(methyl)pyrazol-4-yl]-1- benzimidazoles

Article abstract of DOI:10.1007/s10593-006-0224-x

An efficient method has been developed for the synthesis of 2-[3(5)-aryl(methyl)pyrazol-4-yl]-1H-benzimidazoles by cyclocondensation of 2-acylmethyl-1H-benzimidazoles benzoylhydrazones with DMF dimethylacetal. The tautomerism of the compounds obtained via migrations of a proton between the pyrazole nitrogen atoms has been studied by ¹H NMR. The more stable tautomers have electron acceptor aryl substituents placed at position 3 of the pyrazole ring and electron donor aryl substituents or a methyl at position 5. Springer Science+Business Media, Inc. 2006.

Full text of DOI:10.1007/s10593-006-0224-x

Chemistry of Heterocyclic Compounds, Vol. 42, No. 9, 2006
SYNTHESIS AND TAUTOMERISM
OF 2-[3(5)-ARYL(METHYL)PYRAZOL-
4-YL]-1-BENZIMIDAZOLES
I. B. Dzvinchuk and M. O. Lozinskii
An efficient method has been developed for the synthesis of 2-[3(5)-aryl(methyl)pyrazol-4-yl]-1H-
benzimidazoles by cyclocondensation of 2-acylmethyl-1H-benzimidazoles benzoylhydrazones with DMF
dimethylacetal. The tautomerism of the compounds obtained via migrations of a proton between the
1
pyrazole nitrogen atoms has been studied by H NMR. The more stable tautomers have electron
acceptor aryl substituents placed at position 3 of the pyrazole ring and electron donor aryl substituents
or a methyl at position 5.
Keywords: benzimidazoles, DMF dimethylacetal, hydrazones, pyrazoles, tautomerism.
The tautomerism of pyrazole compounds due to the migrations of a proton between the nitrogen ring
atoms has been systematically studied [1-3] since it depends unpredictable of the nature and position of the ring
substituent. It influences the selectivity of a series of chemical reactions and has to be taken into account when
determining the structure and chemical purity of the corresponding substrates by spectroscopic methods. The
effect on it of the electronic effects of the two aryl substituents at positions 3 (or 5) and 4 of the heterocycle has
not been studied up to this time. It is possible that such a situation is due to the absence of samples and methods
suitable for carrying out a reliable investigation. Thus, starting from 2-phenacylbenzimidazole 1 via its
formylation product 2 and subsequent reaction with hydrazine, we have obtained the pyrazole 3a which,
1
according to H NMR data exists in solution in equilibrium with its tautomeric form 3'a [4]. The signals of the
pyrazole ring aromatic proton for both tautomers are seen separately and would seem suitable and unproblematic
for qualitative and quantitative determination of the composition of the equilibrium. However, the hoped for
assignment of signals to a specific tautomer is complicated by the lack of a sample for comparison. In addition,
the method itself is limited for the preparation of a broad series of compounds for a number of reasons, in
particular the poor selectivity in the final stage reaction. Hence we set ourselves the task of overcoming the
problem by preparing a series of compounds of type 3 and studying their tautomeric behaviour.
Routes for an efficient synthetic method were not obvious and we therefore chose a method of trial and
error in the reactions of 2-phenacylbenzimidazole 1 and its hydrazones. Hence we tried a possible synthetic route
analogous to the known method of preparing 2-(4-pyrazolyl)benzimidazoles [5] based on the cyclocondensation
of compound 1 with aroylhydrazines. However, the corresponding reaction with formylhydrazine occurs to give
a mixture of products, from which we isolated only 1-(3-pyrazolyl)-benzimidazole 4. The result of the reaction is
unexpected. However, the structure of the product is not in doubt because we had obtained it before by refluxing
2-phenacylbenzimidazole hydrazone 5 in DMF [6].
__________________________________________________________________________________________
Institute of Organic Chemistry, National Academy of Sciences of Ukraine, Kiev 02660; e-mail:
iochkiev@ukrpack.net. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1370-1377,
September, 2006. Original article submitted February 21, 2005.
1190
0009-3122/06/4209-1190©2006 Springer Science+Business Media, Inc.

N
Ph
NH2NHCH=O
N
N
N
H
4
O
Ph
H
H
Ph
H
O
N
N
N
N
N2H4
HCOOCOMe
O
H
1
2
Ph
Ph
H
N
N
N
N
N
N
N
H
N
H
H
3a
3'a
Ph
Ph
CH(OEt)3 or
N
N
N
N
Me2NCH(OMe)2
NH2
N CHX
N
N
H
H
5
6a,b
HCOOCOMe
3a + 4
6 a X = OEt, b X = NMe2
We have also investigated the report in [7] of a method of synthesis of pyrazoles based on the reaction of
2-phenacylazaheterocycle hydrazones with carboxylic anhydrides. However, the corresponding reaction of
hydrazone 5 with acetoformic anhydride is complicated by the formation of compound 4 (by TLC analysis) and
the target product 3a could only be separated as the hydrochloride salt in 65% yield.
The intention of preparing compound 3a by the reaction of hydrazone 5 with ethyl orthoformate or with
DMF dimethylacetal was not realized. The reaction takes place at the hydrazone amino group to give compounds
6a,b. In all attempts to carry out a selective subsequent intramolecular cyclization at the methylene group we did
not achieve the hoped for results.
The synthetic problem was resolved via cyclocondensation of the 2-acylmethyl-1H-benzimidazoles 7a-i
using DMF dimethylacetal. The reaction possibly occurs via intermediate formation of a compound of type 8
and then to the target pyrazoles 3a-i through loss of N,N-dimethylbenzamide. The process was carried out in
dioxane at 80°C and is complete after 40 min without significant complication by side reactions.
Dimethylbenzamide was observed in the reaction mixture by TLC but does not hinder the separation of the
pyrazoles in high yields (75-99%) because of its increased solubility.
The result and selectivity of the given reaction is difficult to predict since we previously found that such
an electrophilic reagent as trifluoroacetic anhydride reacts with benzoylhydrazones of type 7 by a different route.
Thus compound 7a undergoes recyclization to form a pyrazole ring by opening of the benzimidazole [8] but
compound 7i undergoes polycyclocondensation to give [1,3,4]oxadiazolo[2',3':2,3]pyrimido[1,6-a]-
1191

R
H
R
N
N
N
N
N
Me2NCH(OMe)2
– 2MeOH
N
NH
CO
N
COPh
NMe2
H
Ph
H
7a–i
8a–i
R
R
H
N
N
N
N
N
N
N
N
H
–
PhCONMe2
H
H
3'a–i
3a–i
Ph
Ph
Me2NCH(OMe)2
N
N
N
N
N
NH
Me
N
– 2MeOH,
– HNMe2
Me
N
H
H
9
10
3, 3', 7 a R = Ph, b R = 4-MeOC6H4, c R = 4-MeC6H4, d R = 3,4,5-(MeO)3C6H2,
e R = 3-ClC6H4, f R = 3-O2NC6H4, g R = 4-O2NC6H4 (g), h R = 2-furyl, i R = Me
benzimidazole [9]. None the less the synthetic method we have developed for type 3 pyrazoles is quite efficient
and likely to have a broader applicability. In particular it was found that 2-phenacylbenzimidazole
methylhydrazone 9 also readily gives the 1-methyl-3-phenyl-substituted pyrazole 10 which has an obvious
structural similarity to tautomer 3a and was used by us as a model compound for studying the tautomerism of
type 3 compounds in DMSO-d6.
1
The physicochemical parameters for the novel compounds are given in Table 1 and H NMR spectral
data in Table 2.
1
In the H NMR spectrum of compound 10 the pyrazole ring 5 proton resonates at 8.25 ppm. Hence the
singlets in compound 3a at 8.31 and 8.08 ppm correspond to the tautomers 3a and 3a'. The signal assigned to
tautomer 3'a appears at higher field, apparently quite consistently. In fact, in this structure the nitrogen atom at
position 1 of the pyrazole ring has a significant electron donor effect on position 3 of the ring. In tautomer 3a
such an effect is possible but is less efficient since the chain of conjugation is one multiple bond longer. In the
spectra of the remaining compounds 3a-i the signals for the pyrazole aromatic ring proton appear, respectively,
in the range 8.27-8.46 and 8.03-8.19 ppm. Some shift of both signals occurs to low field with strengthening of
the electron acceptor properties of the R substituent and is fully as expected. The quantitative composition of the
tautomeric mixture can be determined from the ratio of integrated intensities of the signals mentioned. In some
examples the signal for the pyrazole 3 proton is obscured by the signals for the R substituent (compounds 3e-h).
However, this situation does not hinder a study of the tautomerism since many other signals are doubled and
have the same integrated intensity ratios. Thus the proton on the pyrazole ring nitrogen atom, in some cases the
proton at position 1 of the benzimidazole ring, or the separate protons of the aromatic R substituent, and also
methyl group of compound 3i resonate as two signals which correspond to the two tautomers and are well
separated with no other signal overlap in the majority of cases.
1192

TABLE 1. Parameters for the Compounds Synthesized
Found, %
——————
Calculated, %
Com-
pound
Empirical
formula
mp, °С
Yield, %
С
Н
N
3b
3c
3d
3e
3f
C17H14N4O
70.27
70.33
74.36
74.43
65.06
65.13
65.11
65.20
62.89
62.95
62.97
62.95
67.15
67.19
66.59
66.65
4.72
4.86
5.03
5.14
5.12
5.18
3.93
3.76
3.78
3.63
3.71
3.63
3.98
4.03
5.02
5.08
19.24
19.30
20.28
20.42
15.81
15.99
18.87
19.01
22.85
22.94
22.95
22.94
22.33
22.39
28.17
28.26
220.5-222
163.5-165
155-158
91
96
75
77
75
99
98
82
89
79
98
C17H14N4
C19H18N4O3
C16H11ClN4
C16H11N5O2
C16H11N5O2
C14H10N4O
C11H10N4
152.5-155
158-162
3g
3h
3i
273.5-275
218.5-220
248.5-250
180-182
6a
6b
10
C18H18N4O
C18H19N5
70.42
70.57
70.74
70.80
74.39
74.43
5.86
5.92
6.32
6.27
5.09
5.14
18.15
18.29
22.87
22.93
20.35
20.42
234-235.5
247-248.5
C17H14N4
We have found that, in equilibrium solution, compounds 3a-i contain the tautomer of type 3 in 47, 40,
43, 45, 65, 76, 76, 42, and 32% respectively. It is apparent that the content of tautomers 3a-g increases with
growth of the electron acceptor properties of the aryl fragment R (according to the σ-constant for the m- an
p-substituent in the phenyl ring). The substituents R in compounds 3h-i (2-furyl, Me) have a different steric
effect on the close environment but have a clearly marked electron donor character and are able to stabilize
tautomers of type 3'.
We rationalize the observed tendency in the change of the tautomeric properties of compounds 3a-i in
the following way. The type 3 molecular system can be considered as a combination of three conjugated systems
which have three interrelated and interacting points. One of these is the pyrrole type nitrogen atom which can
arise from either of the two pyrazole ring nitrogen atoms (see mechanism in [5]) and have a marked electron
donor effect. The second is the 2-benzimidazolyl fragment which has an electron acceptor effect. The third is the
R substituent which has a changing nature and hence electronic properties. The pyrrole type nitrogen atom can
have an electron donor effect on the benzimidazolyl fragment and electron acceptor substituent R in both
tautomeric forms 3 and 3' but in the latter the effect is less efficient because the chain of conjugation with
substituent R is lengthened by one double bond. Hence an electron acceptor R substituent can stabilize tautomer
form 3. The benzimidazolyl fragment can have an electron acceptor effect on substituent R only in tautomer 3'.
Hence it is actually this tautomeric form which can stabilize electron donor substituents R.
Thus we have developed a preparatively convenient method for the synthesis of 2-(4-pyrazolyl)-1H-
benzimidazoles containing a substituent at one of the carbon atoms of the pyrazole ring and studied their
tautomerism. It was found that the more stable tautomer has electron acceptor aryl substituents at position 3 of
the pyrazole ring but electron donor aryl or a methyl group at position 5.
1193

TABLE 2. 1H NMR Spectra of the Compounds Synthesized
Com-
pound
Chemical shifts, δ, ppm. (J, Hz)
3a
3b
7.11-7.19 (2H, m, H-5,6); 7.33-7.46 (4H, m, Ph: H-3,4,5 + H-7); 7.55-7.57 (1H, m, H-4);
7.83-7.86 (2H, d, J = 6.0, Ph: H-2–6); 8.08 and 8.31 (0.53 + 0.47H, two s, H-3'(5'));
12.38 and 12.43 (0.47 + 0.53H, br. s, H-1); 13.39 and 13.55 (0.47 + 0.53H, two s, H-1')
3.77 and 3.81 (3H, two overlapping s, OCH3); 6.94-6.96 and 7.03-7.06 (0.4 + 0.6H, two d,
J = 7.2 and J = 7.8, 4-CH3OC6H4: H-3,5); 7.14-7.17 (2H, m, H-5,6); 7.50-7.53 (2H, m,
H-4,7); 7.63-7.86 (2H, m, 4-CH3OC6H4: H-2,6); 8.07 and 8.31 (0.6 + 0.4H, two s,
H-3'(5')); 12.48 (1H, br. s, H-1); 13.30 and 13.43 (0.4 + 0.6H, two s, H-1')
3c
3d
3е
2.35 (3H, s, CH3); 7.14-7.17 (2H, m, H-5,6); 7.14-7.29 (2H, m, 4-CH3C6H4: H-3,5);
7.51 (2H, m, H-4,7); 7.75 (2H, m, 4-CH3C6H4: H-2,6); 8.08 and 8.31 (0.57 + 0.43H,
two s, H-3'(5')); 12.39 (1H, br. s, H-1); 13.34 and 13.49 (0.43 + 0.57H, two s, H-1')
3.72, 3.77 and 3.85 (9H, tree s, 3CH3O); 7.16-7.19 (2H, m, H-5,6); 7.52-7.55 (2H, m,
H-4,7); 7.59 and 7.67 (0.45 + 0.55H, two s, (CH3O)3C6H2); 8.13 and 8.36 (0.55 + 0.45H,
two s, H-3'(5')); 12.47 (1H, br. s, H-1); 13.34 and 13.56 (0.45 + 0.55H, two s, H-1')
7.11-7.20 (2H, m, H-5,6); 7.41-7.57 (4H, m, H-4,7 + 3-ClC6H4: H-4,5); 7.87 (1H, m,
3-ClC6H4: H-6); 8.11 and 8.13 (1 + 0.35H, two overlapping s, 3-ClC6H4: H-2 + H-3');
8.37 (0.65H, s, H-5'); 12.43 and 12.52 (0.65 + 0.35H, two s, H-1);
13.49 and 13.66 (0.65 + 0.35H, two s, H-1')
3f
7.14-7.22 (2H, m, H-5,6); 7.47-7.49 (1H, m, H-7); 7.57-7.59 (1H, m, H-4);
7.71-7.77 (1H, m, 3-O2NC6H4: H-5); 8.21-8.28 (1 + 0.24H, m, 3-O2NC6H4: H-4 + H-3');
8.39-8.49 (1 + 0.76H, m, 3-O2NC6H4: H-6 + H-5'); 9.13 (1H, s, 3-O2NC6H4: H-2);
12.52 and 12.61 (0.76 + 0.24H, two s, H-1); 13.62 and 13.87 (0.76 + 0.24H, two s, H-1')
3g
3h
7.18-7.20 (2H, m, H-5,6); 7.54-7.56 (2H, m, H-4,7); 8.26 (4 + 0.24H, m,
4-O2NC6H4 + H-3'); 8.46 (0.76H, s, H-5'); 12.56 (1H, br.. s, H-1);
13.72 and 13.91 (0.24 + 0.76H, two s, H-1')
6.60 and 6.73 (0.42 + 0.58H, two narrow m, 2-furyl: H-3); 7.19 (2H, m, H-5,6);
7.53-7.73 (3H, m, H-4,7 + 2-furyl: H-4); 7.88 (0.42H, narrow m, 2-furyl: H-2);
8.19 (0.58 + 0.58H, narrow m, 2-furyl: H-2 + H-3'); 8.40 (0.42H, s, H-5');
12.42 and 12.58 (0.42 + 0.58H, two s, H-1); 13.44 and 13.75 (0.42 + 0.58H, two s, H-1')
3i
2.57 and 2.65 (0.32 + 0.68H, two s, CH3); 7.10-7.17 (2H, m, H-5,6); 7.44-7.47 (1H, m,
H-7); 7.57-7.59 (1H, m, H-4); 8.03 and 8.27 (0.68 + 0.32H, two s, H-3'(5'));
12.30 and 12.40 (0.32 + 0.68H, two s, H-1); 12.84 and 12.98 (0.32 + 0.68H, two s, H-1')
6a
1.23 (3H, t, J = 6.6, CH3); 4.24 (2H, q, J = 6.6, OCH2); 4.59 (2H, s, CCH2);
7.04-7.13 (2H, m, H-5,6); 7.39-7.47 (5H, m, Ph: H-3,4,5 + H-4,7);
7.91 (2H, m, Ph: H-2,6); 8.49 (1H, s, OCH); 12.08 (1H, s, H-1)
6b
10
2.78 (6H, s, 2CH3); 7.08-7.11 (2H, m, H-5,6); 6.17-7.43 (6H, m, С6Н5 + H-7);
7.53 (1H, m, H-4); 7.59 (1H, s, NCH); 12.15 (1H, s, H-1)
3.98 (3H, s, CH3); 7.11-7.19 (2H, m, H-5,6); 7.32-7.40 (3H, m, Ph: H-3,4,5);
7.43-7.46 (1H, m, H-7); 7.56-7.59 (1H, m, H-4); 7.78-7.81 (2H, m, Ph: H-2–6);
8.25 (1H, s, H-5'); 12.35 (1H, s, H-1)
EXPERIMENTAL
Monitoring of the course of the reactions and purity of the compounds prepared 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 Varian VXR-300 instrument (300 MHz) using DMSO-d6
and TMS as internal standard. For investigation of the pyrazole tautomerism a 0.1 M solution of the type 3
pyrazole in DMSO-d6 was recorded twice at 3 and 24 h from the moment of preparation as a monitor of the
achievement of equilibrium. The IR spectra of compounds 6a,b were recorded on a UR-20 instrument for KBr
tablets. The starting materials were prepared by known methods: 1 [10], 5 [6], 7a-i [5, 11[, and 9 [12]. Before
determination of elemental composition and spectroscopic properties the synthesized compounds were dried for
7 h in a water pump vacuum desiccator at 115°C.
2-[3(5)-Phenylpyrazol-4-yl]-1H-benzimidazole (3a). A. A solution prepared by mixing 90% formic
acid (0.17 ml, 4 mmol) and acetic anhydride (0.8 ml, 8 mmol) was added to compound 1 (0.236 g, 1 mmol). The
reaction mixture was stirred for 2-3 min and then held for 5 h at 20°C. Water (3 ml) was added and the product
1194

was heated with stirring to reflux, and then evaporated to dryness using a water pump. The residue was treated
with acetic acid (0.5 ml), conc. HCl (0.5 ml), and water (3 ml) and heated with stirring to reflux. After cooling,
the precipitate was filtered off and washed with water and acetone. The 3a hydrochloride obtained was mixed
with acetone (1 ml) and 20% ammonia (0.5 ml) and then water (2 ml) and refluxed with stirring with evaporation
of the acetone until crystallization of the product was complete. The precipitate was filtered, washed with water,
and dried at 80°C. Yield 0.17 g (65%). According to TLC, mp, and 1H NMR spectrum the product obtained was
identical to a sample prepared by method [4].
B. A mixture of compound 7a (0.354 g, 1 mmol), DMF dimethyl acetal (0.178 g, 1.5 mmol), and
anhydrous dioxane (1 ml) was heated at 80°C for 40 min, stirring for the first 3-5 min for formation of
homogeneous solution. Water was added and the product was refluxed, distilling off the solvent to a volume of
1-1.5 ml. After cooling, the precipitate was triturated to a powder, filtered, washed with water, and dried at 80°C.
The product was formed in a pure state, its crystallization from ethanol-water (1: 1) occurring slowly with
appreciable losses.
Compounds 3b-i were prepared similarly from the corresponding benzoylhydrazones 7b-i. Compound
3f was isolated as the hydrochloride salt, as in procedure A. Compound 3g was crystallized from a ethanol–
pyridine (5:1) and 3i from pyridine–water (1:1).
1-(5-Phenylpyrazol-3-yl)-1H-benzimidazole (4). A mixture of compound 1 (0.236 g, 1 mmol),
formylhydrazine (0.132 g, 2.2 mmol), triethylamine hydrochloride (0.028 mg, 0.2 mmol), and DMF (2.0 ml) was
held for 4 h at 150-155°C. Water (3 ml) was added and the product was heated with stirring to reflux. After
cooling, the precipitate was filtered off and crystallized from ethanol. Yield 0.18 g (69%). According to TLC,
mp. and 1H NMR spectrum the product obtained was identical to a sample prepared by method [6].
2-(5-Ethoxy-2-phenyl-3,4-diazapenta-2,4-dienyl)-1H-benzimidazole (6a). A mixture of compound 5
(0.25 g, 1 mmol), ethyl orthoformate (0.222 g, 1.5 mmol), and anhydrous pyridine (1.0 ml) was heated at 120°C
for 2 h. Water (4 ml) was added and the product was refluxed, evaporating the solvent to 1-1.5 ml volume. The
oil separated crystallized on trituration. The precipitate was filtered off, dried at 80°C, and crystallized from
toluene. IR spectrum, ν, cm-1: 1630 (C=N).
2-(5-Dimethylamino-2-phenyl-3,4-diazapenta-2,4-dienyl)-1H-benzimidazole (6b). A mixture of
compound 5 (0.25 g, 1 mmol), anhydrous dioxane (1 ml), and DMF dimethylacetal (0.178 g, 1.5 mmol) was
heated with stirring to formation of a homogeneous solution and stirred for 1 h at 95-100°C. After cooling,
2-propanol (1 ml) was added and stirred. The precipitate was filtered off, washed with 2-propanol, and dried in a
water pump vacuum at 115°C. IR Spectrum, ν, cm-1: 1640 (C=N).
2-[1-Methyl-3-phenylpyrazol-4-yl]-1H-benzimidazole (10). A mixture of compound 9 (0.264 g,
1 mmol), DMF dimethylacetal (0.143 g, 1.2 mmol) and anhydrous dioxane (1 ml) was heated at 80°C for
40 min. Water (1 ml) was added and the product was heated with stirring to reflux. After cooling, the precipitate
was filtered off, washed with a mixture of 2-propanol and water (1:1), and crystallized from a mixture of
pyridine and water (1:2).
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1196