1-(Pyrimidin-4-yl)pyrazol-5(4H)-one derivatives: II.1 electrophilic substitution in 5-hydroxy-3-methyl- 1-(6-methyl-2- methylsulfanylpyrimidin-4-yl)-1H-pyrazole

Article abstract of DOI:10.1134/S1070363211080202

Electrophilic substitution of hydrogen in bicyclic 5-hydroxy-3-methyl-1-(6- methyl-2-methylsulfanylpyrimidin- 4-yl)-¹H-pyrazole occurs exclusively at the C⁴ atom of the five-membered heteroring. Unstable electrophilic substitution pr

Full text of DOI:10.1134/S1070363211080202

ISSN 1070-3632, Russian Journal of General Chemistry, 2011, Vol. 81, No. 8, pp. 1705–1710. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © A.V. Erkin, V.I. Krutikov, 2011, published in Zhurnal Obshchei Khimii, 2011, Vol. 81, No. 8, pp. 1360–1366.
1-(Pyrimidin-4-yl)pyrazol-5(4H)-one Derivatives:
II.1 Electrophilic Substitution in 5-Hydroxy-3-methyl-
1-(6-methyl-2-methylsulfanylpyrimidin-4-yl)-1H-pyrazole
A. V. Erkin and V. I. Krutikov
St. Petersburg State Institute of Technology, Moskovskii pr. 26, St. Petersburg, 190013 Russia
e-mail: kruerk@yandex.ru
Received October 28, 2010
Abstract—Electrophilic substitution of hydrogen in bicyclic 5-hydroxy-3-methyl-1-(6-methyl-2-methylsul-
fanylpyrimidin-4-yl)-1H-pyrazole occurs exclusively at the C4 atom of the five-membered heteroring. Unstable
electrophilic substitution products are stabilized via addition of the second substrate molecule with formation of
bridged adducts.
DOI: 10.1134/S1070363211080202
Me
0.079
The most studied chemical reaction of activated α-
methylene ketones of the 1-pyrimidinylpyrazol-5(4H)-
one series (I) is their transformation into ylidene
derivatives by the action of carbonyl compounds [2–5].
Other electrophilic substitution reactions of com-
pounds I were poorly studied.
0.039
H
N
−0.147
O
−0.356
N
−0.177 N
0.026
Me
S
−0.120
N
−0.196
Me
The goal of the present work was to examine
reactions of 5-hydroxy-3-methyl-1-(6-methyl-2-methyl-
sulfanylpyrimidin-4-yl)pyrazole (II) with some elec-
trophilic reagents or reagents generating electrophilic
species during the process and determine the structure
of products thus formed. A common feature of the
reactions considered below is that electrophilic
substitution in bicyclic compound II involves
exclusively the C4 atom in the pyrazole ring. This is
favored by specific delocalization of π-electron
density, leading to compensation of partial positive
charge on the C4 atom and hence to increase in its
nucleophilicity.
The UV spectrum of II contains a long-wave
absorption band at about λ 310 nm (see figure, curve
1), which corresponds to π→π*-electron transitions
mainly in the hydroxypyrazole fragment. No such band
is observed in the spectra of pyrimidinylpyrazolones I,
e.g., in the spectrum of 4-(4-dimethylaminobenzyl-
idene)-3-methyl-1-(6-methyl-2-methylsulfanylpyrimidin-
4-yl)pyrazol-5(4H)-one (III) [1], for which analogous
resonance is impossible. The lack of activating sub-
stituents like hydroxy group hampers increase of
nucleophilicity and hinders electrophilic attack on
pseudoaromatic position in the pyrimidine ring of II.
Me
Me
N−
Me
Transformations of compound II by the action of
electrophilic reagents can be arbitrarily divided into
two groups. The first of these includes reactions
leading to monosubstituted products, in particular
halogenation, azo coupling, and aminomethylenation.
The bromination of II with molecular bromine in
carbon tetrachloride at 25°C gave a mixture of 4-bromo-
5-hydroxy-3-methyl-1-(6-methyl-2-methylsulfanyl-
pyrimidin-4-yl)pyrazole (IV) and unidentified com-
HC−
+
HO
+
HO
N
N
HO
N
N
R
N
R
R
R = 6-methyl-2-methylsulfanylpyrimidin-4-yl.
1
For communication I, see [1].
1705

1706
ERKIN, KRUTIKOV
60000
50000
40000
30000
20000
10000
0
λ, nm
UV spectra of compounds (1) II, (2) IV, (3) VI, (4) VII, (5) VIII, (6) IX, and (7) X.
pound [Rf 0.19 (A)]. Chromatographically pure bromo-
pyrazole IV was isolated from that mixture in a poor
yield by double recrystallization.
indicating monobromination of bicyclic compound II.
The same conclusion followed from analysis of the UV
spectrum of bromopyrazole IV, where absorption bands
with their maxima at λ 250 and 307 nm had only
different intensities as compared to the spectrum of II
(see figure, curve 2). These findings allowed us (1) to
rule out exhaustive bromination of compound II to 4,4-
dibromo-3-methyl-1-(6-methyl-2-methylsulfanylpyri-
midin-4-yl)-1H-pyrazol-5(4H)-one (V) which is less
aromatic than monobromo derivative IV and (2) to
believe that lactam structure IVa is unlikely to be formed.
1
The H NMR spectrum of compound IV lacked
signal from proton on C4 in the pyrazole ring, which is
typical of initial compound II (δ 5.4 ppm), but
contained a broadened singlet at about δ 12 ppm due to
proton in the hydroxy group or NH proton in
tautomeric 4-bromo-5-methyl-1-(6-methyl-2-methylsul-
fanylpyrimidin-4-yl)-1,2-dihydropyrazol-3-one (IVa),
Me
Me
Me
NH
Br
HO
Br
N
N
HO
S
O
Br2
N
N
N
N
N
N
N
Me
Me
Me
S
S
N
N
Me
Me
Me
II
IV
IVа
Treatment of bicyclic compound II with benzene-
diazonium chloride in aqueous sodium hydroxide at a
temperature not exceeding 10°C led to the formation
of 3-methyl-1-(6-methyl-2-methylsulfanylpyrimidin-4-
yl)-4-phenylhydrazonopyrazol-5(4H)-one (VI). Com-
pound VI displayed in the 1H NMR spectrum a
multiplet from aromatic protons in the region δ 7.1–
7.7 ppm and a signal at δ 12.5 ppm from the NH
proton or OH group in tautomeric 5-hydroxy-3-
methyl-1-(6-methyl-2-methylsulfanylpyrimidin-4-yl)-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 8 2011

1-(PYRIMIDIN-4-YL)PYRAZOL-5(4H)-ONE DERIVATIVES: II.
1707
4-phenylazopyrazole (VIa). Replacement of the 4-H
proton in the pyrazole ring by phenylazo group follows
from the electronic absorption spectrum of VI, in
which the absorption band with its maximum at λ 402 nm
is displaced to the red region relative to the long-wave
absorption band in the spectrum of initial compound
II. Taking into account the presence of a shoulder at
about λ 300 nm due to n→π*-electron transitions (see
figure, curve 3), the existence of phenylhydrazone VI
as a mixture with some amount of tautomeric phenyl-
azopyrazole VIa cannot be excluded completely. In
fact, in the IR spectrum of VI we observed a weak
(Irel ≈ 30%) absorption band at 1682 cm–1, which obviously
belongs to stretching vibrations of the carbonyl group
conjugated with the azomethine fragment and involved
in intramolecular hydrogen bond with the NH group.
Hydrogen bonding of this type is characterized by a
weak absorption band (Irel ≈ 10%) at 3433 cm–1, which
corresponds to stretching vibrations of secondary
amino group.
β-hydroxy-α,β-unsaturated ketone fragment [6]. In
addition, a broad absorption band with several maxima
was present in the high-frequency region; it was
assigned to stretching vibrations of free hydroxy group
(3628 cm–1) and that involved in hydrogen bonding
with the carbonyl group (3385 and 3211 cm–1). The
lower intensity (~22%) of the band at 3628 cm–1 as
compared to the absorption band originating from H-
bonded OH group (~39%) suggests low probability for
tautomerization of compound VII into 5-hydroxy-3-
methyl-1-(6-methyl-2-methylsulfanylpyrimidin-4-yl)-
1H-pyrazole-4-carbaldehyde (VIIa). This suggests
some similarity between hydroxymethylidene-
pyrazolone VII and hydroxymethylenation product of
isoxazol-5(4H)-one, which has the structure of β-
hydroxy-α,β-unsaturated ketone [7]. Formylpyrazole
structure VIIa contradicts the UV spectrum where
strong absorption bands with their maxima at λ 260
and 305 nm were present (see figure, 4). Taking into
account red shift of the short-wave absorption band
assigned mainly to π→π*-electron transitions in the
C=N bonds and small blue shift of the long-wave
absorption band belonging to π→π*-electron
transitions in multiple carbon–carbon bonds [8], we
can state that the contribution of electron transitions in
the C4=C5 bond in the pyrazole ring to the overall
absorption pattern of hydroxymethylenepyrazolone
VII is smaller than the corresponding contribution for
initial compound II.
Ph
Ph
Me
Me
N
N
N
N
H
N
N
PhN2Cl
O
HO
N
N
N
II
N
N
Me
Me
S
S
N
Me
Me
VI
VIа
Me
Me
HO
O
Aminomethylenation of compound II with a
N
N
O
S
HO
mixture of triethyl orthoformate and aniline in ethanol
at 50°C followed an anomalous pattern. After
concentration of the reaction mixture under reduced
pressure, we obtained an oily residue which did not
crystallize on treatment with some solvents (diethyl
ether, benzene, or cyclohexane). We succeeded in
crystallizing the oily material by treatment with
aqueous ammonia. Analysis of the spectral data of the
product purified by additional recrystallization allowed
us to identify it as 4-hydroxymethylidene-3-methyl-1-
(6-methyl-2-methylsulfanylpyrimidin-4-yl)-1H-pyrazol-
5(4H)-one (VII). The 1H NMR spectrum of VII
contained a doublet signal from the exocyclic CH=
proton at δ 7.9 ppm and a poorly resolved doublet from
the hydroxy proton at δ 9.4 ppm, the intensity ratio of
these signals being 1:1. In the IR spectrum of
compound VII we observed absorption bands at 1701
and 1639 cm–1, the first of which belongs to carbonyl
stretching vibrations, and the second, to the conjugated
(1) (EtO)3CH
N
N
II
(2) NH4OH
N
N
Me
Me
N
S
N
Me
Me
VII
VIIа
When the reaction of compound II with triethyl
orthoformate and aniline was carried out under
solvent-free conditions at 80°C, the expected
aminomethylenation product, 3-methyl-1-(6-methyl-2-
methylsulfanylpyrimidin-4-yl)-4-phenylaminomethyl-
idene-1H-pyrazol-5(4H)-one (VIII) was obtained. The
1H NMR spectrum of VIII contained doublets from
protons in the exocyclic CH= group at δ ~8.5 ppm and
NH group at δ ~11 ppm with an intensity ratio of 1:1.
On the other hand, the UV and IR spectra of the
product indicate that it has the structure of isomeric
Schiff base VIIIa. In particular, the UV absorption
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 8 2011

1708
ERKIN, KRUTIKOV
pattern in the region λ 300–400 nm differed from that
observed for compounds VI and VII (see figure, curve
5), and the IR spectrum contained absorption bands at
1637 and 1572 cm–1 due to stretching vibrations of
conjugated C=N and C4=C5 bonds. Compound VIIIa
also displayed in the IR spectrum a broad absorption
band with low intensity (<40%) in the region 3500–
2000 cm–1, which may be assigned to intramolecular
hydrogen bond between the 5-OH group in the
pyrazole ring and exocyclic nitrogen atom.
Ph
Ph
Me
Me
N
H
N
CHO(OEt)3, PhNH2
N
N
O
HO
N
N
N
II
N
N
Me
Me
S
N
S
Me
Me
VIIIa
VIII
The second group of electrophilic substitution
reactions of compound II includes those leading to
bridged products, namely hydroxymethylation and
nitrosation. Pyrimidinyl-substituted pyrazole II reacted
with an equimolar amount of formaldehyde in aqueous
sodium hydroxide at 25°C to produce bis[5-hydroxy-3-
methyl-1-(6-methyl-2-methylsulfanylpyrimidin-4-yl)-
1H-pyrazol-4-yl]methane (IX) rather than 5-hydroxy-
4-hydroxymethyl-3-methyl-1-(6-methyl-2-methylsul-
fanylpyrimidin-4-yl)pyrazole.
roxy-3-methyl-1-(6-methyl-2-methylsulfanylpyrimidin-
4-yl)-1H-pyrazol-4-ylimino]-1H-pyrazol-5(4H)-one
(X). Protons on C5 in the pyrimidine rings of X were
1
nonequivalent, and they resonated in the H NMR
spectrum as two singlets at δ 7.37 and 7.40 ppm.
Methyl protons gave rise to a complex multiplet in the
region δ 2.2–2.6 ppm, and the intensity ratio of the
methyl proton signal and broadened singlet belonging
to the OH proton was equal to 18:1. The presence of a
conjugated fragment in molecule X is illustrated by the
UV and IR spectra. The UV spectrum of X (see figure,
curve 7), contained absorption bands with their maxima
at λ 255 and 310 nm and a shoulder at λ 350 nm due to
n→π*-electron transitions in the O=C−C=N bond
sequence. In the IR spectrum of X we observed a very
strong absorption band at 1570 cm–1, which may be
assigned to stretching vibrations of the exocyclic C=N
group conjugated with the carbonyl group. The re-
duced frequency of O–H stretching vibrations (3138 cm–1)
and low intensity of the corresponding absorption band
(~32%) are likely to result from intermolecular
hydrogen bonding (O···HO); however, judging by the
carbonyl stretching vibration frequency (1741 cm–1)
and intensity of the νC=O band (~55%), the above
hydrogen bond is weak.
Me
Me
N
N
OH
Me
HO
Me
CH2O
N
N
N
N
II
N
N
Me
Me
S
S
IX
1
In the H NMR spectrum of IX, protons in the
bridging methylene group resonated at δ ~3.0 ppm,
and the intensity ratio of that signal and the signal from
5-H in the pyrimidine rings was 1:1. The UV spectrum
of IX (λmax 250, 305 nm) resembled the spectrum of
initial compound II, indicating similar electron
transitions, and increased intensity of the short-wave
absorption band (Δlogε ≈ 0.5) implied low probability
for prototropic transformations of the hydroxypyrazole
fragment in dipyrazolylmethane IX (see figure, cutve 6).
N
Me
Me
N
N
N
O
HO
Me
NaNO2
AcOH
N
N
N
N
II
Treatment of compound II with an aqueous
solution of sodium nitrite at 25°C in dilute acetic acid
resulted in the formation of poorly soluble 3-methyl-1-
(6-methyl-2-methylsulfanylpyrimidin-4-yl)-4-[5-hyd-
N
Me
Me
S
Me
S
X
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 8 2011

1-(PYRIMIDIN-4-YL)PYRAZOL-5(4H)-ONE DERIVATIVES: II.
1709
Presumably, the formation of bridged products like
dipyrazolylmethane IX is determined by delocalization
of partial positive charge on the α-carbon atom in
intermediate zwitterion A via addition of the second
molecule of ionized compound II, which acts as
nucleophile. Indirect evidence in favor of the proposed
scheme is poor yield of compound IX. This means that
zwitterionic structure A can be stabilized not only
as described above but also via reversible addition
of hydroxide ion with formation of a hybrid of
anions B and C, which may be stabilized in turn by
solvation.
Me
Me
Me
H
H
O−
−
CH2=O
O−
O
O
N
N
N
N
R
N
N
R
R
B
C
+CH2
Me
Me
Me
CH2
O
OH
−
O−
O
N
N
N
−
−
HO
N
N
R
N
R
R
А
R = 6-methyl-2-methylsulfanylpyrimidin-4-yl.
EXPERIMENTAL
off, washed with carbon tetrachloride, dried in air,
recrystallized twice from acetonitrile, and dried under
reduced pressure. Yield 52 mg (7.7%), mp 164°C, Rf
0.43 (A). 1H NMR spectrum, δ, ppm: 2.23 s (3H, Me),
2.44 s (3H, Me), 2.55 s (3H, Me), 7.85 s (1H, CH), 12.16
s (1H, OH). Found, %: C 37.89; H 3.56; N 25.02.
C10H11BrN4OS. Calculated, %: C 38.11; H 3.52; N 25.35.
1
The H NMR spectra were recorded on a Bruker
WM-400 spectrometer (400.13 MHz) from solutions in
DMSO-d6 using the residual solvent signal as
reference. If signal from protons in one methyl group
was overlapped by the residual solvent signal, the
intensity of the former is not given. The IR spectra
were measured in KBr on an FSM-1201 spectrometer
with Fourier transform. The UV spectra were
measured on an SF-26 spectrophotometer from
solutions in ethanol with a concentration of (0.3–
0.5)×10–4 M. The purity of the products was checked
by TLC on Silufol UV-254 plates using acetone–
heptane (2:1, A) and butan-1-ol–acetic acid–water
(1:1:1, B) as eluents. Spots were developed by UV
irradiation or treatment with iodine vapor. The
3-Methyl-1-(6-methyl-2-methylsulfanylpyrimidin-
4-yl)-4-phenylhydrazono-1H-pyrazol-5(4H)-one (VI).
A suspension of 0.5 g of compound II in 5% aqueous
sodium hydroxide was cooled to 0–5°C, and a solution
of benzenediazonium chloride prepared from 0.21 g of
aniline, 0.15 g of sodium nitrite, and 2 ml of dilute
(1:1) hydrochloric acid was added dropwise under
vigorous stirring at such a rate that the temperature did
not exceed 10°C. The mixture was stirred until it
warmed up to room temperature, and the precipitate
was filtered off, thoroughly washed with water, and
dried at a temperature not exceeding 60°C. The
product was recrystallized from benzene–cyclohexane
(1:2), washed with a small amount of cyclohexane,
and dried in a high vacuum. Yield 0.25 g (35%), mp
elemental compositions were determined on
Hewlett–Packard B-185 CHN analyzer.
a
Charges on atoms in molecule II were calculated
using HyperChem software package.
4-Bromo-3-methyl-1-(6-methyl-2-methylsulfanyl-
pyrimidin-4-yl)-1H-pyrazol-5-ol (IV). Triethylamine,
0.21 g, was added to a solution of 0.5 g of compound
II in 15 ml of carbon tetrachloride, a solution of 0.34 g
of bromine in 3 ml of carbon tetrachloride was then
added dropwise under vigorous stirring, and the mix-
ture was stirred for 30 min. The precipitate was filtered
1
196°C (decomp.), Rf 0.66 (A). H NMR spectrum, δ,
ppm: 2.31 s (3H, Me), 2.38 s (3H, Me), 2.52 s (3H,
Me), 7.14–7.74 m (6H, CH, Harom), 12.48 br.s (1H, NH).
Found, %: C 56.29; H 4.56; N 24.44. C16H16N6OS.
Calculated, %: C 56.46; H 4.74; N 24.69.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 8 2011

1710
ERKIN, KRUTIKOV
4-Hydroxymethylidene-1-(6-methyl-2-methyl-
Me), 2.40 s (3H, Me), 2.53 s (3H, Me), 3.03 s (2H,
CH2), 7.94 s (2H, CH), 11.09 s (2H, OH). Found, %: C
51.83; H 4.67; N 23.28. C21H24N8O2S2. Calculated, %:
C 52.05; H 4.99; N 23.12. After prolonged storage, an
additional portion of compound IX, 29 mg, separated
from the mother liquor, mp 236°C.
sulfanylpyrimidin-4-yl)-1H-pyrazol-5(4H)-one (VII).
A solution of 0.31 g of triethyl orthoformate in 2 ml of
ethanol was added to a solution of 0.5 g of compound
II and 0.20 g of aniline in 10 ml of ethanol, and the
mixture was stirred for 30 min at room temperature
and heated for 2 h at 50°C. The solvent was distilled
off under reduced pressure, the oily residue was treated
with 6 ml of 25% aqueous ammonia, the resulting
suspension was kept for 1 h at room temperature, and
the precipitate was filtered off, washed with a small
amount of cold water, and dried under reduced
pressure over phosphorus(V) oxide. The dried product
was recrystallized from benzene. To complete pre-
cipitation, the suspension in benzene was diluted with
an equal volume of cyclohexane, and the precipitate
was filtered off, washed with cyclohexane, and dried in
a high vacuum. Yield 82 mg (14%), mp 196°C, Rf 0.14
(A). 1H NMR spectrum, δ, ppm: 2.14 s (3H, Me), 2.37
s (3H, Me), 2.50 s (Me), 7.62 s (1H, CH), 7.92 d (1H,
CH), 9.44 d (1H, OH). Found, %: C 49.71; H 4.59;
N 19.92. C11H12N4O2S. Calculated, %: C 49.99; H
4.58; N 21.20.
4-[5-Hydroxy-3-methyl-1-(6-methyl-2-methylsul-
fanylpyrimidin-4-yl)-1H-pyrazol-4-ylimino]-3-
methyl-1-(6-methyl-2-methylsulfanylpyrimidin-4-
yl)-1H-pyrazol-5(4H)-one (X). A solution of 0.15 g of
sodium nitrite in 3 ml of water was added dropwise
under vigorous stirring to a solution of 0.5 g of
compound II in 50% aqueous acetic acid. The result-
ing suspension was stirred for 30 min at room tem-
perature, and the precipitate was filtered off, washed
with water, and dried in air. The product was
recrystallized first from acetonitrile and then from
acetic acid, thoroughly washed with water, and dried at
80°C over a period of 8 h. Yield 0.19 g (18%), mp
1
232°C, Rf 0.91 (B). H NMR spectrum, δ, ppm: 2.24 s
(6H, Me), 2.41 s (6H, Me), 2.51 s (6H, Me), 7.37/7.40
d (2H, CH), 14.70 br.s (1H, OH). Found, %: C 49.11;
H 4.14; N 25.79. C20H21N9O2S2. Calculated, %: C
49.68; H 4.38; N 26.07.
3-Methyl-1-(6-methyl-2-methylsulfanylpyrimidin-
4-yl)-4-(phenylaminomethylidene)-1H-pyrazol-5(4H)-
one (VIII). A mixture of 0.5 g of compound II, 0.31 g
of triethyl orthoformate, and 0.20 g of aniline was
heated to 80°C and kept for 1 h at that temperature.
The semisolid product was ground, crystallized from
ethanol, and dried under reduced pressure over
phosphorus(V) oxide. Yield 0.34 g (47%), mp 186°C,
The authors thank Novbytkhim Ltd. (St. Petersburg)
for full support of the study and I.V. Klaptyuk (All-
Russian Research Fire-Prevention Institute, Ministry of
the Russian Federation for Civil Defense,
Emergencies, and Elimination of Consequences of
Natural Disasters) for recording the IR spectra.
1
Rf 0.43 (A). H NMR spectrum, δ, ppm: 2.28 s (3H,
REFERENCES
Me), 2.40 s (3H, Me), 2.53 s (3H, Me), 7.20–7.57 m
(5H, Ph), 7.61 s (1H, CH), 8.55 d (1H, CH), 11.20 d
(1H, NH). Found, %: C 59.69; H 5.13; N 20.73.
C17H17N5OS. Calculated, %: C 60.16; H 5.05; N 20.63.
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(IX). A formaldehyde solution, 0.18 g, was added to a
solution of 0.5 g of compound II in 5% aqueous
sodium hydroxide. The mixture was stirred for 2 h at
room temperature and was left to stand until the
formalin odor disappeared. The mixture was acidified
with acetic acid, and the precipitate was filtered off,
washed with water, and dried at a temperature not
exceeding 60°C. The product was recrystallized from
ethanol and dried under reduced pressure over
phosphorus(V) oxide. Yield 0.19 g (18%), mp 238°C,
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1
Rf 0.87 (B). H NMR spectrum, δ, ppm: 2.31 s (3H,
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 8 2011