Synthesis of new liquid-crystalline compounds from the 3-aryl-5- alkylpyrazole series

Article abstract of DOI:10.1134/S1070428006050083

A method was developed for the preparation of new liquid-crystalline substances, pyrazole derivatives, through the corresponding 2-isoxazolines and isoxazoles. The hydrogenolysis of the heterocycle in 3-(4-hydroxyphenyl)-5- amylisoxazole afforded 1-amino-1-(4-hydroxyphenyl)-1-octen-3-one. The acid hydrolysis of the compound followed by treating with hydrazine hydrate resulted in 5-amyl-3-(4-hydroxyphenyl)-1H-pyrazole, whose benzylation and esterification with the corresponding mesogenous benzyl chlorides and benzoyl chlorides provided liquid-crystalline 5-alkyl-3-aryl-1H-pyrazoles. Pleiades Publishing, Inc. 2006.

Full text of DOI:10.1134/S1070428006050083

ISSN 1070-4280 Russian Journal of Organic Chemistry, 2006, Vol. 42, No. 5, pp. 696-700. Ó Pleiades Publishing, Inc. 2006.
Original Russian Text Ó V.N. Kovganko, N.N. Kovganko, 2006, published in Zhurnal Organicheskoi Khimii, 2006, Vol. 42, No. 5, pp. 715-718.
Synthesis of New Liquid-crystalline Compounds
from the 3-Aryl-5-alkylpyrazole Series
V.N. Kovganko and N.N. Kovganko
Sevchenko Institute of Applied Physical Problems, Belarussian State University, Minsk, 220064 Belarus
e-mail: kauhanka@bsu.by
Received March 28, 2005
Abstract—A method was developed for the preparation of new liquid-crystalline substances, pyrazole derivatives,
through the corresponding 2-isoxazolines and isoxazoles. The hydrogenolysis of the heterocycle in 3-(4-hydroxy-
phenyl)-5-amylisoxazole afforded 1-amino-1-(4-hydroxyphenyl)-1-octen-3-one. The acid hydrolysis of the
compound followed by treating with hydrazine hydrate resulted in 5-amyl-3-(4-hydroxyphenyl)-1H-pyrazole, whose
benzylation and esterification with the corresponding mesogenous benzyl chlorides and benzoyl chlorides provided
liquid-crystalline 5-alkyl-3-aryl-1H-pyrazoles.
DOI: 10.1134/S1070428006050083
arylpyrazoles. The synthesis of the compounds is the goal
of the present study.
Nowadays liquid-crystalline compounds containing
five-membered heterocycles are the subject of much
investigation [1–3]. In particular, the pyrazole ring belongs
to such structural fragments of mesomorphous com-
pounds. Quite a number of liquid crystals based on 3,5-di-
substituted pyrazoles was reported [3–10]. The majority
of compounds synthesized belong to 3,5-diarylpyrazoles,
whereas no published data exists on preparation of
mesomorphous compounds from the group of 5-alkyl-3-
The general preparation procedure for liquid crystals
based on 3,5-disubstituted pyrazoles consists in application
as the key intermediate products of the corresponding
b-diketones [4–10]. The substituted b-diketones are
known to be produced by transformation of the cor-
responding isoxazoles [11]. We recently developed
a method for preparation of mesomorphous 5-alkyl-3-
Scheme.
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696

SYNTHESIS OF NEW LIQUID-CRYSTALLINE COMPOUNDS
697
arylisoxazoles proceeding from the transformation of the
corresponding 2-isoxazolines [12]. In this study we planned
to use these 2-isoxazolines and isoxazoles for preparation
of liquid-crystalline 5-alkyl-3-arylpyrazoles.
Phase transition temperature for compounds Vb and Vc, VIa–
VIc
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The formerly performed [12] synthesis of liquid-
crystalline 5-alkyl-3-arylisoxazoles was based on the use
of compound II prepared by the oxidation of the
corresponding 2-isoxazoline I [12–14]. In the further
synthesis of the liquid-crystalline pyrazoles we planned
to carry out the opening of the hetrocycle in isoxazole II.
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In the first stage the hydrogenolysis of the isoxazole
ring in compound II on Raney nickel afforded in a 92%
yield enaminoketone III whose structure was confirmed
isotropic liquid–nematic phase are given in parentheses.
IV at the phenol group with mesogenous benzyl chlorides
and acyl chlorides. The reaction of pyrazole IV with
benzyl chlorides in the presence of potassium carbonate
and sodium iodide furnished the corresponding benzyl
ethers Va–Vc in 84–90% yields. The benzoylation of
compound IV with 4-alkoxybenzoyl chlorides in
tetrahydrofuran in the presence of triethyl amine
afforded the corresponding esters VIa–VIc in 39–52%
yields.
1
by the data of UV, IR, and H NMR spectra. The UV
spectrum of compound III contained two characteristic
maxima at 268 and 329 nm. In the IR spectrum of
enaminoketone III the vibrations of the system of
conjugated bonds C=O and C=C appear as strong bands
–1
in the region 1485–1600 cm . In the 1H NMR spectrum
of compound III the vinyl proton H2 gave rise to a singlet
at d 5.44 ppm, whereas the analogous proton in the
1H NMR spectrum of the initial isoxazole II appeared
as a singlet at d 6.23 ppm [12]. The presence of an amino
group in the enaminoketone III is confirmed by its
Note that the preparation of compounds Va–Vc and
VIa–VIc may be accompanied by benzylation and
benzoylation not only of hydroxy group but also of the
nitrogen of the pyrazole ring. However in the 1H NMR
spectra of compounds Va–Vc and VIa–VIc appears
a single signal of the benzyl or benzoyl group respectively,
and in their IR spectra absorption bands are observed in
the region 3445–3475 cm belonging to the vibrations of
the N-H bond. Besides in the IR spectra of esters VIa–
VIc the vibrations of the C=O bond give rise to the band
at 1720–1725 cm characteristic of substituted aryl
–1
stretching vibrations band at 3480 cm . In the spectrum
of the compound in question an absorption band is also
–1
observed at 3580 cm corresponding to the stretching
vibrations of the phenol hydroxy group that is also presnt
in the IR spectrum of initial isoxazole II [12]. In the
1H NMR spectrum of compound III the protons of OH
and NH2 groups appeared as a one-proton multiplet at
6.90–7.02 ppm and two broadened one-proton singlets
at 5.25 and 10.04 ppm respectively.
–1
–1
benzoates [12, 14]. These findings support the structures
of benzyl ethers Va–Vc and benzoates VIa–VIc.
In the subsequent stage of the synthesis the enamino-
ketone III was subjected to hydrolysis by boiling its tetra-
hydrofuran solution in the presence of diluted hydrochloric
acid. The reaction of the arising b-diketone with hydrazine
hydrate afforded pyrazole IV in a quantitative yield.
The study of phase transitions in benzyl ethers Va–
Vc and benzoates VIa–VIc revealed that five among
them are liquid-crystalline (see the table), and compound
Va did not form a mesophase. Compounds VIa and VIb
possess monotropic nematic phases in a range of 12 and
11°C respectively. The other compounds are char-
acterized by enantiotropic mesophases. It should be noted
that the characteristic feature of 3,5-diarylpyrazoles
described in the literature is in general the formation of
a smectic phase A [4–10]. In the case of compounds we
synthesized the wide range of the smectic phase C for
benzyl ether Vc should be noted, and also the presence
of nematic phases for benzoates VIa–VIc.
The structure of compound IV was also confirmed
1
by the data of UV, IR, and H NMR spectra. The UV
spectrum of compound IV is characterized by the absorp-
tion maximum corresponding to the conjugation system
between the phenol and pyrazole rings at 258.5 nm. In
the 1H NMR spectrum the proton of the heterocycle H4
appeared as a singlet at 6.32 ppm, in a characteristic
position for 3,5-disubstituted pyrazoles [4–10].
To obtain the target liquid-crystalline compounds we
carried out benzylation and esterification of compound
Hence the results of our study demonstrate that the
great synthetic potential of 2-isoxazolines and isoxazoles
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 42 No. 5 2006

V. KOVGANKO, N. KOVGANKO
698
makes it possible to apply them as intermediate substances
for preparation of liquid crystals.
III in 20 ml of THF was added 5.5 ml of diluted hydro-
chloric acid, 1:10. The resulting mixture was boiled at
reflux for 1 h, then 20 ml of chloroform and 30 ml of
a saturated sodium chloride solution was added. The
chloroform layer was separated, the water layer was
additionally extracted with chloroform (2´10 ml). The
combined chloroform extracts were washed with 15 ml
of the saturated sodium chloride solution, then the solvent
was distilled off under a reduced pressure. The residue
was diluted with 10 ml of methanol, and 0.3 ml
(6.18 mmol) of hydrazine hydrate was added. The solution
obtained was boiled for 1 h, then diluted with 50 ml of
water, and cooled to 4°C. The separated precipitate of
pyrazole IV was filtered off, washed on the filter in
succession with 100 ml of water and 5 ml of cooled
petroleum ether. After drying in a vacuum we obtained
0.688 g (96%) of compound IV, mp 150–151°C (from
toluene). UV spectrum, lmax, nm: 258.5. IR spectrum
EXPERIMENTAL
IR spectra were recorded on a spectrophotometer
Specord 75-IR in KBr cell from chloroform solutions (if
not otherwise indicated). UV spectra were taken on
a spectrophotometer Specord M40 from methanol
solutions (if not otherwise indicated). 1H NMR spectra
of solutions in deuterochloroform (if not otherwise
indicated) were registered on a spectrometer Bruker
Avance 400 (400 MHz) using HMDS as an internal
reference. The reaction progress was monitored and the
purity of compounds obtained was checked by TLC on
Kieselgel 60 F254 (Merck) plates. The melting points and
phase transition temperatures were measured on a heating
block coupled with a polarization microscope. The
mesophase type was estimated by comparison of the
observed texture with the corresponding standards given
in a monograph [15].
–1
(THF), cm : 3650–3050 (OH, NH), 1610, 1525, 1500
1
(C=Car). H NMR spectrum [(CD3)2CO], d, ppm: 0.88 t
(3H, CH3, J 7 Hz), 1.28–1.40 m (4H), 1.67 quintet (2H,
J 7.5 Hz), 2.64 t (2H, J 7.5 Hz) (CH2alk), 6.32 s (1H, H4),
6.84 d (2H, ArH, J 8.5 Hz), 7.63 d (2H, ArH, J 8.5 Hz).
1-Amino-1-(4-hydroxyphenyl)-1-octen-3-one
(III). A dispersion of 2 g of Raney nickel in 15 ml of
methanol was saturated with hydrogen at stirring for
30 min. Then a solution was added of 2.30 g (9.96 mmol)
of 5-amyl-3-(4-hydroxyphenyl)isoxazole (II) (prepared
by procedure [12]) in 40 ml of methanol. The reaction
mixture thus obtained was stirred under hydrogen
atmosphere for 9.5 h. The catalyst was filtered off and
washed on the filter with 30 ml of methanol. The solvent
from the filtrate was distilled off under a reduced pressure,
and the residue was applied to a column packed with
silica gel and subjected to gradient elution with a mixture
of ethyl acetete with petroleum ether, from 1:3 to 2:1. On
distilling off of the eluent under the reduced pressure we
obtained 2.13 g (92%) of enaminoketone III, mp 120–
121°C (from toluene–petroleum ether). UV spectrum,
5-Amyl-3-[4-(4-cyanophenyl)methoxy]phenyl-
1H-pyrazole (Va). To a solution of 0.060 g (0.261 mmol)
of phenol IV and 0.039 g (0.257 mmol) of cyano-
benzyl chlorode in 7 ml of acetone was added 0.077 g
(0.513 mmol) of sodium iodide and 0.500 g (3.623 mmol)
of potassium carbonate. The reaction mixture obtained
was boiled at stirring for 5 h, then 30 ml of water was
added. The separated precipitate was filtered off and
washed with water. Yield 0.078 g (88%). The analytically
pure sample was obtained by double recrystallization from
on 2-propanol. mp 136°C (from 2-propanol). UV spec-
–1
trum, lmax, nm: 234, 259. IR spectrum, cm : 3445 (NH),
2950, 2920, 2850 (C–Halk), 2225 (CºN), 1605, 1570, 1550,
1515, 1495 (C=Car). 1H NMR spectrum, d, ppm: 0.89 t
(3H, CH3, J 7 Hz), 1.29–1.40 m (4H), 1.67 quintet (2H,
CH2, J 7.5 Hz), 2.64 t (2H, CH2, J 7.5 Hz), 5.14 s (2H,
Ar'–CH2–O–Ar), 6.29 s (1H, H4), 6.96 d (2H, ArH, J 9
Hz), 7.55 d (2H, ArH, J 9 Hz), 7.64–7.70 m (4H,ArH).
l
max, nm: 268, 329 (CH3OH); 256, 320 (dioxane). IR
–1
spectrum, cm : 3580, 3480, 3425–3015 (OH, NH), 2995
(C–Har), 2950, 2925, 2855 (C–Halk), 1600, 1570, 1530,
1485 (C=O, C=C, C=Car). IR spectrum (THF), cm :
–1
3500–3030 (OH, NH), 1600, 1575, 1530, 1485 (C=O,
1
Similarly were prepared compounds Vb and Vc.
C=C, C=Car). H NMR spectrum, d, ppm: 0.87 t (3H,
CH3, J 7 Hz), 1.26–1.38 m (4H), 1.58–1.72 m (2H)
(CH2), 2.37 t (2H, C4H2, J 7.5 Hz), 5.44 s (1H, H2),
6.91 d (2H, ArH, J 8.5 Hz), 7.45 d (2H, ArH, J 8.5 Hz),
5.25 br.s (1H), 6.90–7.02 m (1H), 10.04 br.s (1H) (OH,
NH2).
5-Amyl-3-[4-(4'-propyloxy-4-biphenyl)methoxy]-
phenyl-1H-pyrazole (Vb). Yield 90%. UV spectrum,
–1
l
max, nm: 266. IR spectrum, cm : 3450 (NH), 3000 (C–
Har), 2955, 2925, 2855 (C–Halk), 1605, 1570, 1555, 1515,
1
1490 (C=Car). H NMR spectrum, d, ppm: 0.89 t (3H,
CH3, J 7 Hz), 1.05 t (3H, CH3, J 7 Hz), 1.30–1.40 m
(4H), 1.67 quintet (2H, J 7.5 Hz), 1.82 sextet (2H,
5-Amyl-3-(4-hydroxyphenyl)-1H-pyrazole (IV).
To a solution of 0.726 g (3.116 mmol) of enaminoketone
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 42 No. 5 2006

SYNTHESIS OF NEW LIQUID-CRYSTALLINE COMPOUNDS
699
J 7 Hz), 2.65 t (2H, J 7.5 Hz) (CH2), 3.96 t (2H, Ar'–
OCH2, J 7 Hz), 5.11 s (2H, Ar'–CH2–O-–Ar), 6.29 s
(1H, C4H), 6.96 d (2H, ArH, J 9 Hz), 7.01 d (2H, ArH,
J 8.5 Hz), 7.47 d (2H, ArH, J 8 Hz), 7.51 d (2H, ArH,
J 9 Hz), 7.56 d (2H, ArH, J 8 Hz), 7.64 d (2H, ArH,
J 8.5 Hz).
Compounds VIb and VIc were prepared in the same
way.
4-(5-Amyl-1H-pyrazol-3-yl)phenyl 4-hexyloxy-
benzoate (VIb). Yield 52%. UV spectrum, lmax, nm:
–1
267. IR spectrum, cm : 3450 (NH), 2950, 2925, 2855
(C–Halk), 1720 (C=O), 1600, 1570, 1555, 1500 (C=Car),
1
3-[4-(4'-Amyl-4-biphenyl)methoxy]phenyl-5-
1245, 1200, 1160, 1070 (C–O). H NMR spectrum, d,
amyl-1H-pyrazole (Vc). Yield 84%. UV spectrum, lmax
nm: 263. IR spectrum, cm : 3445 (NH), 3000 (C–Har),
,
ppm: 0.90 t (3H, CH3, J 7 Hz), 0.91 t (3H, CH3, J 7 Hz),
1.28–1.40 m (8H), 1.47 quintet (2H, J7 Hz), 1.68 quintet
(2H, J 7.5 Hz), 1.81 quintet (2H, J 7 Hz), 2.66 t (2H,
J 7.5 Hz) (CH2), 4.03 t (2H, ArOCH2, J 7 Hz), 6.35 s
(1H, H4), 6.96 d (2H, ArH, J 9 Hz), 7.23 d (2H, ArH,
J 9 Hz), 7.77 d (2H, ArH, J 9 Hz), 8.13 d (2H, ArH,
J 9 Hz).
–1
2950, 2925, 2850 (C–Halk), 1610, 1570, 1550, 1515, 1490
1
(C=Car). H NMR spectrum, d, ppm: 0.89 t (6H, CH3,
J 7 Hz), 1.30–1.40 m (8H), 1.60–1.72 m (4H), 2.60–
2.68 m (4H) (CH2), 5.11 s (2H, Ar'–CH2–O–Ar), 6.29 s
(1H, H4), 7.01 d (2H, ArH, J 9 Hz), 7.24 d (2H, ArH,
J 8 Hz), 7.48 d (2H, ArH, J 8 Hz), 7.50 d (2H, ArH,
J 8 Hz), 7.59 d (2H, ArH, J 8 Hz), 7.64 d (2H, ArH,
J 9 Hz).
4-(5-Amyl-1H-pyrazol-3-yl)phenyl 4-nonyloxy-
benzoate (VIc). Yield 39%. UV spectrum, lmax, nm:
–1
268. IR spectrum, cm : 3455 (NH), 2925, 2855 (C–Halk),
4-(5-Amyl-1H-pyrazol-3-yl)phenyl 4-amyloxy-
benzoate (VIa). To a solution of 0.055 g (0.264 mmol)
of 4-amyloxybenzoic acid in 5 ml of dichloromethane
was added 1 drop of dimethylformamide and 0.2 ml
(2.78 mmol) of thionyl chloride. The reaction mixture
obtained was boiled for 1 h, the solvent and excess thionyl
chloride were distilled off at the atmospheric pressure.
The residue was dissolved in 5 ml of dichloromethane,
and the solvent was distilled off at a reduced pressure to
remove completely the thionyl chloride. The acyl chloride
thus obtained was dissolved in 5 ml of THF, 0.35 ml
(2.52 mmol) of triethylamine, and 0.060 g (0.261 mmol)
of phenol IV was added. The reaction mixture was stirred
for 2.5 h at 20°C, then 15 ml of water and 15 ml of chloro-
form was added. The organic layer was separated, the
water layer was additionally extracted with chloroform
(2´10 ml). The combined organic solutions were washed
with 20 ml of water.After drying with sodium sulfate the
solvent was distilled off under a reduced pressure, and
the residue was recrystallized from 2-propanol. Yield
0.055 g (50%). UV spectrum, lmax, nm: 263. IR spectrum,
cm 1: 3475 (NH), 3005 (C–Har), 2955, 2930, 2870, 2855
(C–Halk), 1725 (C=O), 1600, 1570, 1555, 1515, 1500
(C=Car), 1250, 1200, 1165, 1070 (C–O). 1H NMR spec-
1720 (C=O), 1600, 1570, 1500 (C=Car), 1250, 1200,
1165, 1070 (C–O). H NMR spectrum, d, ppm: 0.89 t
1
(3H, CH3, J 7 Hz), 0.90 t (3H, CH3, J 7 Hz), 1.22–
1.41 m (14H), 1.47 quintet (2H, J 7 Hz), 1.69 quintet
(2H, J 7.5 Hz), 1.81 quintet (2H, J 7 Hz), 2.67 t (2H,
J 7.5 Hz) (CH2), 4.03 t (2H, ArOCH2, J 7 Hz), 6.36 s
(1H, H4), 6.96 d (2H, ArH, J 9 Hz), 7.23 d (2H, ArH,
J 9 Hz), 7.78 d (2H, ArH, J 9 Hz), 8.14 d (2H, ArH,
J 9 Hz).
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RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 42 No. 5 2006