Electrosynthesis of pyrazole-4-carboxylic acids by oxidation of 4-formylpyrazoles on NiO(OH)-electrode in aqueous alkaline solution

Article abstract of DOI:10.1007/s11172-012-0156-9

Electrochemical oxidation of di- and trisubstituted 4-formylpyrazoles on a Ni-anode in aqueous alkali led to the formation of the corresponding pyrazole-4-carboxylic acid in 60-90% yields. The yields of the target products depend on position of substituent in the pyrazole ring and are decreased in the following sequence of substituent at position 1 Me > Et > Ph, as well as when the aqueous medium was replaced with aqueous alcohol (50% Bu ^tOH). Oxidation of 4-formylpyrazoles containing Me groups at the carbon atoms of the pyrazole ring led, to monoacids and also pyrazoledicarboxylic acids in small (1.5-14%) amounts; the latter were the oxidation products of the aldehyde and the Me groups.

Full text of DOI:10.1007/s11172-012-0156-9

Russian Chemical Bulletin, International Edition, Vol. 61, No. 6, pp. 1148—1153, June, 2012
1148
Electrosynthesis of pyrazoleꢀ4ꢀcarboxylic acids by oxidation
of 4ꢀformylpyrazoles on NiO(OH)ꢀelectrode in aqueous alkaline solution
B. V. Lyalin and V. A. Petrosyan
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5328. Eꢀmail: petros@ioc.ac.ru
Electrochemical oxidation of diꢀ and trisubstituted 4ꢀformylpyrazoles on a Niꢀanode in
aqueous alkali led to the formation of the corresponding pyrazoleꢀ4ꢀcarboxylic acid in 60—90%
yields. The yields of the target products depend on position of substituent in the pyrazole ring
and are decreased in the following sequence of substituent at position 1 Me  Et  Ph, as well as
when the aqueous medium was replaced with aqueous alcohol (50% ButOH). Oxidation of
4ꢀformylpyrazoles containing Me groups at the carbon atoms of the pyrazole ring led, to
monoacids and also pyrazoledicarboxylic acids in small (1.5—14%) amounts; the latter were the
oxidation products of the aldehyde and the Me groups.
Key words: electrosynthesis, undivided cell, nickel hydroxide electrode, 1,5ꢀ, 1,3ꢀ, 1,3,5ꢀsubꢀ
stituted 4ꢀformylpyrazoles, 1,5ꢀ, 1,3ꢀ, 1,3,5ꢀsubstituted pyrazoleꢀ4ꢀcarboxylic acids.
The interest to the search of new methods for the
aldehyde used). A suggested mechanism of the process is
given in Scheme 1.
preparation of pyrazoleꢀ4ꢀcarboxylic acids is due to their
wide use as the intermediate products in the synthesis of
medicinal drugs for treatment of the Parkinson decease1
and autoimmune diseases,2 antibacterial agents,3 and funꢀ
gicides.4—6
From the data in Table 1, it follows that position of the
Me group in the pyrazole ring considerably influences the
yields of pyrazoleꢀ4ꢀcarboxylic acid. Thus, in the oxidaꢀ
tion of 4ꢀformylꢀ1,3ꢀdimethylpyrazole the yield of the tarꢀ
get product turned out to be 30% lower than that in the
case of 4ꢀformylꢀ1,5ꢀdimethylpyrazole (cf. entries 1 and 2).
Replacement of the Me groups at the nitrogen atom of the
pyrazole ring with the Et (cf. entries 3 and 4) led to a slight
(by 8%) decrease in the yield of the acid. In order
to verify this effect, we studied EO of 4ꢀformylꢀ3,5ꢀdiꢀ
methylꢀ1ꢀphenylpyrazole, which is poorly soluble in waꢀ
ter, therefore, its electrolysis was carried out in 50% aqueꢀ
ous solution of ButOH. Under the same conditions (for
comparison of the data on oxidation of formylpyrazoles in
aqueous and in aqueousꢀalcoholic media), we studied oxꢀ
idation of 4ꢀformylꢀ1,3,5ꢀtrimethylpyrazole. From the reꢀ
sults of entries 5 and 6, it follows that under comparable
conditions replacement of the Me substituent at position 1
of the pyrazole ring with the Ph one decreases the yield of
pyrazolecarboxylic acid nearly by oneꢀhalf. It could be
suggested that in this case, the increase in the electronꢀ
withdrawing properties of the pyrazole ring decreases the
ability of the carbonyl group to oxidation.
One of the efficient approaches to the preparation of
organic acids consists in the oxidation of the correspondꢀ
ing alcohols on a Niꢀanode by galvanostatic electrolysis in
aqueous alkaline medium. Under these conditions, the
oxidant is NiO(OH), which is formed on the surface of the
Niꢀanode and selfꢀregenerated in the course of the elecꢀ
trolysis. Earlier,7 we have shown a principal possibility of
the electrosynthesis of pyrazoleꢀ4ꢀcarboxylic acids in oxiꢀ
dation of 4ꢀformylꢀ1,3,5ꢀtrimethylpyrazole to 1,3,5ꢀtriꢀ
methylpyrazoleꢀ4ꢀcarboxylic acid as an example.
Taking these data into account, in the present work we
have studied the influence of the nature of substituents
(Me, Et, Ph), their amount and position in the pyrazole
ring on the efficiency of electrooxidation (EO) of 4ꢀformylꢀ
pyrazoles.
Results and Discussion
The process of EO of substituted 4ꢀformylpyrazoles on
the NiO(OH) electrode was performed in aqueous soluꢀ
tion of NaOH under conditions similar to those described
by us earlier.7 Already in the first experiment, the electrolꢀ
ysis of 4ꢀformylꢀ1,3ꢀdimethylpyrazole (Table 1, entry 1)
led to the synthesis of 1,3ꢀdimethylpyrazoleꢀ4ꢀcarboxylic
acid in 66% yield (calculated based on the amount of the
The use of 50% aqueous solution of ButOH decreases
both the conversion of 4ꢀformylꢀ1,3,5ꢀtrimethylpyrazole
and the yield of the target product (cf. entries 3 and 5),
which was apparently due to the lower reaction temperaꢀ
tures (because of volatility of ButOH) and lower conꢀ
centrations of alkali (because of its lower solubility in
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1139—1143, June, 2012.
1066ꢀ5285/12/6106ꢀ1148 © 2012 Springer Science+Business Media, Inc.

Electrosynthesis of pyrasoleꢀ4ꢀcarboxylic acids
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 6, June, 2012
1149
Scheme 1
Table 1. Dependence of yields of pyrazolecarboxylic acids from the nature of formylpyrazoles in their anodic oxidationa
Entry
Starting
Conversion of
Formed acid
Yieldb of acid (%)
formylpyrazole
formylpyrazole (%)
I
II
1
97.0
66.2
68.2
4.8
97.5
1.5
4.9
97.5
1.5
2
100.0
3
82.4
77.4
1.7
93.9
2.1
(to be continued)

1150
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 6, June, 2012
Lyalin and Petrosyan
Table 1 (continued)
Entry
Starting
formylpyrazole
Conversion of
formylpyrazole (%)
Formed
acid
Yieldb of acid (%)
I
II
4
74.4
60.0
80.9
14.1
22.0
19.0
80.0
5 c
27.5
6 c
14.2
14.0
97.8
98.6
97.8
7
100.0
a Conditions: Niꢀanode, Tiꢀcathode, Cpyrazole = 0.2 mol L–1, CNaOH = 1.0 mol L–1, ja = 6 mA cm–2, T = 70 C,
Q = 2 F (mol of the starting formylpyrazole)–1
.
1
b Calculated based on the reacted formylpyrazole and the starting formylpyrazole, using the H NMR spectroscopic data
for the isolated mixture of the electrolysis products.
c Electrolysis was performed in 0.3 M KOH in 50% aqueous ButOH, Cpyrazole = 0.05 mol L–1, ja = 2.4 mA cm–2, T = 50 C.
ButOH).* It cannot be excluded either that EO of aldeꢀ
hyde hindered the formation of associates of the aldehyde
with the added organic solvent.9 As a result, the major
amount of the electricity passed was spent on the disꢀ
charge of OH– ions.
which are the oxidation products of both the aldehyde
and the Me groups. For example, the EO of 4ꢀformylꢀ
1,3ꢀdimethylpyrazole, probably, proceeds according to
Scheme 2.
As to the position of the aldehyde group in the pyrazole
ring, this factor, apparently, did not essentially influence
the efficiency of EO of formylpyrazole (cf. entries 2 and 7).
It is extremely interesting that the EO of 4ꢀformylꢀ
pyrazoles with Me group at the carbon atom of the pyrꢀ
azole ring leads, besides the monoacids, to small amounts
(2—14%) of pyrazoledicarboxylic** acids (see entries 1—4),
Scheme 2
* Earlier,8 a decrease in the yields of the target acids when aqueꢀ
ous media were replaced with 50% aqueous ButOH was observed
by us in the electrooxidation of arylalkanols.
** According to the revised data, the formation of small amounts
of pyrazoledicarboxylic acid (yield 2%) was observed earlier
during the EO studies7 of 4ꢀformylꢀ1,3,5ꢀtrimethylpyrazole.
These results are given in Table 1 for comparison (entry 3).
However, it should be underscored that under these
conditions the EO of 3,5ꢀdimethylpyrazole did not lead to

Electrosynthesis of pyrasoleꢀ4ꢀcarboxylic acids
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 6, June, 2012
1151
1H NMR spectra were recorded on a Bruker ACꢀ300 spectroꢀ
meter using DMSOꢀd6 as a solvent.
the formation of even trace amounts of the corresponding
acids, whereas the similar electrolysis of 3(5)ꢀmethylpyrꢀ
azoleꢀ5(3)ꢀcarboxylic acid was accompanied by the forꢀ
mation of pyrazoleꢀ3,5ꢀdicarboxylic acid (the yield was
16% calculated based on the starting product).
Comparing these results with the data of entries 1—4
(see Table 1), we suppose that the oxidation of the Me
group at position 3(5) of the ring occurs only in such
a case, when a COOH group is formed in the first step of
the oxidation of 4ꢀformylpyrazole. This was a nontrivial
observation, since until now the oxidation of organic
compounds under such conditions was believed10 to take
place only at the groups capable of being adsorbed on
the NiO(OH)ꢀelectrode (of the type —OH, —CHO, or
—NH2, but not a Me group at all).
In conclusion, a convenient method was suggested
for the electrochemical preparation of pyrazoleꢀ4ꢀcarbꢀ
oxylic acids starting from 4ꢀformylpyrazoles, which alꢀ
lows one to obtain the target products with higher yields
than under chemical usual oxidation procedure (for exꢀ
ample, the use of KMnO4 gave 1,3ꢀdimethylꢀ and 1,5ꢀdiꢀ
methylpyrazoleꢀ4ꢀcarboxylic acids11 in 52 and 62% yields,
respectively).
1. Electrooxidation of 4ꢀformylꢀ1,3ꢀdimethylpyrazole.
4ꢀFormylꢀ1,3ꢀdimethylpyrazole (2.48 g, 0.02 mol) and 1 M aq.
solution of NaOH (100 mL) were placed into a cell, the elecꢀ
trolysis was carried out using a 270 mA current at 70 C. After
2 F (mol of the starting compound)–1 (3860 C) of electricity was
passed, the electrolysis was stopped, the reaction mixture was
stirred for 0.5 h, then cooled to 20 C, and acidified with conꢀ
centrated HCl (to pH 1). A precipitate formed was filtered off,
washed with water (2×15 mL), and dried at 100 C to obtain
1,3ꢀdimethylpyrazoleꢀ4ꢀcarboxylic acid (1.72 g, 61.4%), which
was identified by the m.p. 188 C (cf. Ref. 11: m.p. 188—189 C)
and the 1H NMR spectral characteristics. The mother liquor left
after filtration off the precipitate was combined with the washꢀ
ings of the precipitate, water was evaporated under reduced presꢀ
sure. The residue was extracted with Me2CO (4×25 mL) and the
solvent was evaporated to obtain a white powder (0.37 g), which,
1
according to the H NMR spectroscopic data was a mixture of
4ꢀformylꢀ1,3ꢀdimethylꢀ, 1,3ꢀdimethylpyrazoleꢀ4ꢀcarboxylic acid
and 1ꢀmethylpyrazoleꢀ3,4ꢀdicarboxylic acid. Their molar ratio,
1 : 1.56 : 1.61, was determined from the integral intensities of
the signals in the 1H NMR spectrum for 4ꢀformylꢀ1,3ꢀdimethylꢀ
pyrazole at  8.10 (s, 1 H, H(5)), 1,3ꢀdimethylpyrazoleꢀ4ꢀcarbꢀ
oxylic acid at  7.90 (s, 1 H, H(5)), and 1ꢀmethylpyrazoleꢀ3,4ꢀ
dicarboxylic acid at  8.44 (s, 1 H, H(5)). Based on these data,
the yields of 1,3ꢀdimethylpyrazoleꢀ4ꢀcarboxylic and 1ꢀmethylꢀ
pyrazoleꢀ3,4ꢀdicarboxylic acids were found to be 66.2 and 4.8%,
respectively, the conversion of 4ꢀformylꢀ1,3ꢀdimethylpyrazole
was 97%.
2. Electrooxidation of 4ꢀformylꢀ1,5ꢀdimethylpyrazole.
4ꢀFormylꢀ1,5ꢀdimethylpyrazole (2.48 g, 0.02 mol) and 1 M
aq. solution of NaOH (100 mL) were placed into a cell and the
electrolysis was carried out as indicated above. After the elecꢀ
trolysis and the product isolation were complete (see above),
1,5ꢀdimethylpyrazoleꢀ4ꢀcarboxylic acid (0.20 g, 7.1%) was obꢀ
tained. The acid was identified based on the m.p. 185 C (cf.
Ref. 11: m.p. 184—185 C) and the 1H NMR spectral characterꢀ
istics. From the mother liquor (after separation of the main prodꢀ
uct), additional 2.70 g of the product was isolated (see above),
which was (the 1H NMR spectroscopic data) a mixture of
1,5ꢀdimethylpyrazoleꢀ4ꢀcarboxylic acid and 1ꢀmethylpyrazoleꢀ
4,5ꢀdicarboxylic acid. Their molar ratio, 60 : 1, was determined
based on the integral intensities of the signals for 1.5ꢀdimethylꢀ
pyrazoleꢀ4ꢀcarboxylic acid at  3.70 (s, 3 H, Me) and 1ꢀmethylꢀ
pyrazoleꢀ4,5ꢀdicarboxylic acid at  4.08 (s, 3 H, Me). Based on
these data, the yields of 1.5ꢀdimethylpyrazoleꢀ4ꢀcarboxylic and
1ꢀmethylpyrazoleꢀ4,5ꢀdicarboxylic acid were 97.5 and 1.5%, reꢀ
spectively, with the conversion of 4ꢀformylꢀ1,5ꢀdimethylpyrazole
being quantitative.
Experimental
Electrolysis was performed under galvanostatic conditions,
using a B5ꢀ8 source of the direct current, in an undivided cell
with a coating for thermostating, Niꢀanode (S = 45 cm2) and
Tiꢀcathode (S = 20 cm2). A coulometer constructed in the Deꢀ
sign Office of the N. D. Zelinsky Institute of Organic Chemistry
of the Russian Academy of Sciences was included into the elecꢀ
tric circuit. In the course of the electrolysis, the reaction mixture
was stirred with a magnetic stirrer, the temperature was mainꢀ
tained by a Uꢀ1 thermostat. Before starting the experiments,
Niꢀanode was activated according to the known procedure;12
a preliminary electrolysis was performed in the solution conꢀ
taining 0.1 M NiSO4, 0.1 M NaOAc, and 0.005 M NaOH, at
ja = 1 mA cm–2, with a periodical alternation of polarization of
the electrodes. This procedure is required for the formation on
the surface of the Niꢀanode of multiꢀlayered coating containꢀ
ing NiO(OH).
4ꢀFormylꢀ1,3ꢀdimethylꢀ and 4ꢀformylꢀ1,5ꢀdimethylpyrꢀ
azoles,13 4ꢀformylꢀ1,3,5ꢀtrimethylꢀ and 4ꢀformylꢀ1ꢀethylꢀ3,5ꢀdiꢀ
methylpyrazoles,14 4ꢀformylꢀ3,5ꢀdimethylꢀ1ꢀphenylpyrazole,15
5ꢀformylꢀ1ꢀmethylpyrazole,16 and 3(5)ꢀmethylpyrazoleꢀ5(3)ꢀ
carboxylic acid were synthesized according to the procedures
described earlier.17 The acids obtained were identified using
1H NMR spectroscopy by comparison their spectra with the
spectra of the authentic samples: the described in the literature
1ꢀmethylpyrazoleꢀ3,4ꢀdicarboxylic acid and 1ꢀmethylpyrazoleꢀ
4,5ꢀdicarboxylic acid,18 1ꢀethylꢀ5ꢀmethylpyrazolꢀ and 1,5ꢀdiꢀ
methylpyrazoleꢀ3,4ꢀdicarboxylic acids19 or obtained according
to the known procedures 1,3ꢀdimethylpyrazoleꢀ and 1,5ꢀdiꢀ
methylpyrazoleꢀ4ꢀcarboxylic acids,11 1,3,5ꢀtrimethylpyrazolꢀ,
1ꢀethylꢀ3,5ꢀdimethylpyrazolꢀ, and 3,5ꢀdimethylꢀ1ꢀphenylpyrꢀ
azoleꢀ4ꢀcarboxylic acids,20 1ꢀmethylpyrazoleꢀ5ꢀcarboxylic acid,21
and pyrazoleꢀ3,5ꢀdicarboxylic acid.22
3. Electrooxidation of 4ꢀformylꢀ1,3,5ꢀtrimethylpyrazole.
4ꢀFormylꢀ1,3,5ꢀtrimethylpyrazole (2.76 g, 0.02 mol) and 1 M
aq. solution of NaOH (100 mL) were placed into a cell and the
electrolysis was carried out as indicated above. After the elecꢀ
trolysis and the product isolation were complete (see above),
1,3,5ꢀtrimethylpyrazoleꢀ4ꢀcarboxylic acid (2.17 g, 70.4%) was
obtained. The acid was identified based on the m.p. 219 C (cf.
Ref. 20: m.p. 217.5 C) and the 1H NMR spectral characterisꢀ
tics. From the mother liquor (after isolation of the main prodꢀ
uct), additional 0.69 g of a white powder was isolated as indicatꢀ
ed above, which according to the 1H NMR spectroscopic data

1152
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 6, June, 2012
Lyalin and Petrosyan
was a mixture of 4ꢀformylꢀ1,3,5ꢀtrimethylpyrazole, 1,3,5ꢀtriꢀ
methylpyrazoleꢀ4ꢀcarboxylic acid and 1,5ꢀdimethylpyrazoleꢀ
3,4ꢀdicarboxylic acid. Their molar ratio, 7.8 : 4.6 : 1.0, was deꢀ
termined from the integral intensities of the signals for the startꢀ
ing aldehyde at  9.70 (s, 1 H, CHO), the overlapped signals of
the aldehyde and 1,3,5ꢀtrimethylpyrazoleꢀ4ꢀcarboxylic acid at
 3.65 (m, 3 H, Me), and the signal for 1,5ꢀdimethylpyrazoleꢀ
3,4ꢀdicarboxylic acid at  3.84 (s, 3 H, Me). Based on these data,
the yields of 1,3,5ꢀtrimethylpyrazoleꢀ4ꢀcarboxylic and 1,5ꢀdiꢀ
methylpyrazoleꢀ3,4ꢀdicarboxylic acids were 77.4 and 1.7%, reꢀ
spectively, the conversion of 4ꢀformylꢀ1,3,5ꢀtrimethylpyrazole
was 82.4%.
6. Electrooxidation of 5ꢀformylꢀ1ꢀmethylpyrazole. 5ꢀFormylꢀ
1ꢀmethylpyrazole (2.2 g, 0.02 mol) and 1 M aq. solution of NaOH
(
100 mL) were placed into a cell and the electrolysis was carried
out as indicated above. After the electrolysis and the product
isolation were complete (see entry 1), 1ꢀmethylpyrazoleꢀ5ꢀcarbꢀ
oxylic acid (2.0 g, 79%) was obtained. The acid was identified
based on the m.p. 221 C (cf. Ref. 21: m.p. 221—222 C) and the
1H NMR spectral characteristics. Water was evaporated from
the mother liquor (after separation of the main product), the
residue formed was treated with Me2CO (4×25 mL) to additionally
isolate 0.45 g of 1ꢀmethylpyrazoleꢀ5ꢀcarboxylic acid (identification
based on the m.p. and the 1H NMR spectral characteristics).
The yield of 1ꢀmethylpyrazoleꢀ5ꢀcarboxylic acid was 97.8%, with
the conversion of the starting aldehyde being quantitative.
7. Electrooxidation of 3(5)ꢀmethylꢀ5(3)ꢀcarboxylic acid.
3(5)ꢀMethylꢀ5(3)ꢀcarboxylic (0.64 g, 0.005 mol) acid and 1 M
aq. solution of NaOH (100 mL) were placed into a cell and the
electrolysis was carried out as indicated above (see entry 1),
passing 6 F (mol of the starting compound)–1 (2895 C) of elecꢀ
tricity. After the electrolysis was stopped, the reaction mixture
was stirred for 30 min, then cooled to 20 C, acidified with
conc. HCl (to pH 1), and water was evaporated at reduced presꢀ
sure. The residue was extracted with Me2CO (2×25 mL) and
EtOH (2×25 mL), the solvent was evaporated to obtain a white
powder (0.76 g), which, according to the 1H NMR spectroscopic
data, was a mixture of 3(5)ꢀmethylꢀ5(3)ꢀcarboxylic acid and pyrꢀ
azoleꢀ3,5ꢀdicarboxylic acid. Their molar ratio, 5.4 : 1.0, was deꢀ
termined based on the integral intensities of the signals for
3(5)ꢀmethylꢀ5(3)ꢀcarboxylic acid at  6.45 (s, 1 H, H(4)) and
pyrazoleꢀ3,5ꢀdicarboxylic acid at  7.05 (s, 1 H, H(4)). Based on
these data, the yield of pyrazoleꢀ3,5ꢀdicarboxylic acid was 16%,
the conversion of 3(5)ꢀmethylꢀ5(3)ꢀcarboxylic acid was 17%.
4. Electrooxidation of 4ꢀformylꢀ1ꢀethylꢀ3,5ꢀdimethylpyrazole.
4ꢀFormylꢀ1ꢀethylꢀ3,5ꢀdimethylpyrazole (3.36 g, 0.02 mol) and
1 M aq. solution of NaOH (100 mL) were placed into a cell and
the electrolysis was carried out as indicated above. After elecꢀ
trolysis was complete, the reaction mixture was acidified with
conc. HCl (see above), water was evaporated under reduced
pressure, the residue formed was extracted with Me2CO (2×25 mL)
and EtOH (2×25 mL) to obtain a white powder (3.9 g) containꢀ
1
ing (the H NMR spectroscopic data) a mixture of 4ꢀformylꢀ1ꢀ
ethylꢀ3,5ꢀdimethylpyrazole, 1ꢀethylꢀ3,5ꢀdimethylpyrazoleꢀ4ꢀ
carboxylic acid, and 1ꢀethylꢀ5ꢀmethylpyrazoleꢀ3,4ꢀdicarboxylꢀ
ic acid. Their molar ratio, 1.83 : 4.35 : 1.0, was determined
from the integral intensities of the signals for 4ꢀformylꢀ1ꢀethylꢀ
3,5ꢀdimethylpyrazole at  4.00 (q, 2 H, CH2), 1ꢀethylꢀ
3,5ꢀdimethylpyrazoleꢀ4ꢀcarboxylic acid at  4.10 (q, 2 H, CH2),
and 1ꢀethylꢀ5ꢀmethylpyrazoleꢀ3,4ꢀdicarboxylic acid at  4.25
(q, 2 H, CH2). Based on these data, the yields of 1ꢀethylꢀ
3,5ꢀdimethylpyrazoleꢀ4ꢀcarboxylic acid and 1ꢀethylꢀ5ꢀmethylꢀ
pyrazoleꢀ3,4ꢀdicarboxylic acid were 60 and 14.1%, respectiveꢀ
ly, the conversion of 4ꢀformylꢀ1ꢀethylꢀ3,5ꢀdimethylpyrazole
was 74.4%.
This work was financially supported by the Division of
Chemistry and Materials Science of the Russian Academy
of Sciences (Program for Basic Research No. 01).
5. Electrooxidation of 4ꢀformylꢀ3,5ꢀdimethylꢀ1ꢀphenylpyrꢀ
azole. 4ꢀFormylꢀ3,5ꢀdimethylꢀ1ꢀphenylpyrazole (1.0 g, 0.005 mol),
50% aq. solution of ButOH (100 mL), and KOH (1.68 g,
0.03 mol) were placed into a cell, and the electrolysis was carꢀ
ried out using a 108 mA current at 50 C. After 2 F (mol aldeꢀ
hyde)–1 of electricity was passed, the electrolysis was stopped,
ButOH was salted off the reaction solution by addition of solid
NaCl with subsequent separation of the aqueous and organic
fractions. The alcohol was evaporated from the organic fraction
under reduced pressure. The residue obtained after the evaporaꢀ
tion (unreacted starting aldehyde with NaCl impurities) was
mixed with water (5 mL) and treated with CHCl3 (2×10 mL).
The organic extract was dried with Na2SO4 and the solvent was
evaporated to obtain 4ꢀformylꢀ3,5ꢀdimethylꢀ1ꢀphenylpyrazole
(0.81 g) (identified by 1H NMR spectroscopy). The aqueous
fractions (after isolation of the aldehyde) were combined, acidiꢀ
fied with conc. HCl (to pH 1), and extracted with CHCl3 (3×25 mL).
The extract was dried with Na2SO4 and the solvent was evapoꢀ
rated to obtain the product (0.20 g), which was (the 1H NMR
spectroscopic data) a mixture of 3,5ꢀdimethylꢀ1ꢀphenylpyrazoleꢀ
4ꢀcarboxylic acid and 4ꢀformylꢀ3,5ꢀdimethylꢀ1ꢀphenylpyrazole.
Their molar ratio, 3.24 : 1.0, was determined based on the inteꢀ
gral intensities of the signals for 3,5ꢀdimethylꢀ1ꢀphenylpyrazoleꢀ
4ꢀcarboxylic acid at  2.35 (s, 3 H, Me) and 4ꢀformylꢀ3,5ꢀdimeꢀ
thylꢀ1ꢀphenylpyrazole at  2.43 (s, 3 H, Me). Based on these
data, the yield of 3,5ꢀdimethylꢀ1ꢀphenylpyrazoleꢀ4ꢀcarboxylic
acid was 14%, the conversion of 4ꢀformylꢀ3,5ꢀdimethylꢀ1ꢀ
phenylpyrazole was 14.2%.
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Received March 6, 2012