Synthesis and characterization of a new series of liquid crystal compounds derived from isoxazoles

Article abstract of DOI:10.1080/10587250210533

Two homologous series of phenylisoxazoles derived from 3-[4-(4-trans-n- heptylcyclohexyl)phenyl]-5-(4-alkoxyphenyl)isoxazole and 3-[4-(4-trans-n- heptylcyclohexyl)phenyl]-5-(5-nalkylthiophene)isoxazole have been synthesized. The liquid crystal compounds o

Full text of DOI:10.1080/10587250210533

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Synthesis and Characterization
of a New Series of Liquid
Crystal Compounds Derived
from Isoxazoles
Hugo Gallardo a , César Zucco a & Luciano Da Silva b
a Departamento de Química , Universidade Federal
de Santa Catarina , Florianópolis, Brazil
b Centro de Ensino Superior VII , Universidade do
Vali do Itajaí , São José, Brazil
Published online: 18 Oct 2010.
To cite this article: Hugo Gallardo , César Zucco & Luciano Da Silva (2002)
Synthesis and Characterization of a New Series of Liquid Crystal Compounds Derived
from Isoxazoles, Molecular Crystals and Liquid Crystals, 373:1, 181-190, DOI:
10.1080/10587250210533
To link to this article: http://dx.doi.org/10.1080/10587250210533
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Mol. Cryst. and Liq. Cryst., 2002, Vol. 373, pp. 181-190
# 2002 Taylor and Francis, Inc.
Synthesis and Characterization
of a New Series of Liquid Crystal
Compounds Derived from Isoxazoles
´
HUGOGALLARDOand CE SAR ZUCCO
´
Departamento de Quımica, Universidade Federal de Santa Catarina,
´
Florianopolis, Brazil
LUCIANODA SILVA
´
Centro de Ensino Superior VII, Universidade do Vali do Itajaı,
˜
Sao Jose, Brazil
´
Two homologous series of phenylisoxazoles derived from 3-[4-(4-trans-n-heptylcyclo-
hexyl)phenyl]-5-(4-alkoxyphenyl)isoxazole and 3-[4-(4-trans-n-heptylcyclohexyl)phenyl]-5-(5-n-
alkylthiophene)isoxazole have been synthesized. The liquid crystal compounds obtained were
characterized by NMR, differential scanning calorimetry (DSC), and optical polarized micro-
scopy (POM). The properties of the liquid crystalline phase were investigated as a function of
terminal units and type of core aromatic rings. It was found that (1) the phase transition
temperature decreased with the change of core aromatic rings from phenyl for thienyl, and (2)
the types of mesophase formed by these compounds do not depend on the length of the terminal
chain. Thermotropic nematic and smectic phases were observed.
Keywords Liquid crystals, isoxazole, mesophase
INTRODUCTION
As a continuation of our study of the behavior of liquid crystal molecules
containing five membered heterocyclic ring, we have now prepared and
investigated the mesogenic properties of two homologous series of 3,5-dis-
ubstituted isoxazole. The synthetic intermediates were structurally char-
acterized and the thermal behavior examined [1].
Received 16 April 2001; accepted 18 June 2001.
Address correspondence to Luciano da Silva, Engenharia de Computacao, Universidade do
Vali do Itajai, Sao Jose, SC, Brazil. E-mail:
181

182
H. GALLARDO et al.
In the course of our research program involving the synthesis of novel
liquid crystals, we needed to use trans-1-(4-acetylphenyl)-4-n-heptylcy-
clohexane as a mesogenic subunit and b-diketones as a synthetic inter-
mediate. The subunit phenylcyclohexane is present in large numbers of
commercial liquid crystals, which generally leads to superior thermal,
photo, and electrochemical stability and also to low melting points and
viscosity [2, 3], b-Diketones are classic examples of precursors for the
synthesis of mesogenic heterocycles or metallomesogenic derivatives [4].
Mesogenic isoxazoles and pyrazoles obtained via b-diketones have been
described previously in the literature [5, 6]. In contrast, we describe the
attainment of mesogenic isoxazoles through the use of a,b-unsaturated
ketones. The isoxazole ring incorporates a strong dipole moment, and
this effect could help increase the anisotropy of the polarizability and
consequently favor mesogenic behavior [7]. In this workwe have
attempted to synthesize two families of a,b-unsaturated ketones (inter-
mediates 1, Series I) and their isoxazole derivatives (Series II). In order to
study the influence of the molecular core on mesogenic properties, two
differential rings have been introduced: 4-alkoxyphenyl and 5-alkythio-
phene. The mesogenic behavior of the compounds has been studied, and
a comparative study of properties observed in the different derivatives has
been included.

183
RESULTS AND DISCUSSION
Synthesis
Synthesis of a,b-unsaturated ketones
The most common procedure for the synthesis of a,b-unsaturated ketones is
the classic Claisen-Schimidt reaction. The synthesis involves the condensa-
tion of the aromatic aldehydes with 4-[trans-4-n-heptylcyclohexyl] acet-
ophenone in the presence of an aqueous alkali. There is a pronounced
preference for the formation of trans-double bonds. This stereoselectivity is
1
confirmed by H NMR spectroscopy. The vinyl system AB shows two sets
of doublets with JAB ¼ 15–17 Hz, which are typical values for the trans-vinyl
system. In this reaction, all compounds formed under the conditions
described are obtained in good yields. 4-[Trans-4-n-heptylcyclohex-
yl]acetophenone, 2-formyl-5-alkylthiophene and 4-alkoxybenzaldehyde
were prepared according to the methods described in the literature [8–10].
The synthetic pathway to the target compounds 1 is depicted in Scheme 1.
Synthesis of 3,5-diarylisoxazoles
This synthesis first involves the preparation of the a,b-unsaturated oximes
from the reaction of a,b-unsaturated ketones with hydroxylamine hydro-
chloride [11]. In the subsequent reaction, the oxime is oxidized with tetra-
kis(pyridine)cobalt(II) dichromate (TPCD). This is a convenient procedure
for the preparation of the 3,5-diarylisoxazoles in high yields. The synthetic
pathway to the target compounds 2 is depicted in Scheme 2.
The structure of all target compounds and intermediates was confirmed
1
by a combination of H NMR, 13C NMR (Bruker HX-200), and infrared
spectroscopy (Perking-Elmer 783 IR spectrophotometer). The purity of the
compound was evaluated by thin layer chromatography and elemental
analysis. All of the data, summarized in the experimental section of this
article, confirm the proposed structures.
Mesogenic Characterization
All of the compounds obtained were characterized after the synthetic and
purification processes. All mesophases were identified according to their
textures observed by optical microscopy, using the classification system
reported by Sackmann and Demus, and Gray and Goodby [13, 14]. The
optical and thermal data of a,b-unsaturated ketone intermediaries are given
in Table 1. None of the a,b-unsaturated ketone intermediaries are

184
H. GALLARDO et al.
SCHEME 1 1A1 R ¼ CH3; 1A2 R ¼ C7H15; 1B1 R ¼ H; 1B2 R ¼ C7H15; 1A3 R ¼ C8H17
1A4 R ¼ C9H18; 1B3 R ¼ C10H21; 1B4 R ¼ C12H25
;
.
mesogenic. The replacement of alkoxyphenyl, thiophene, and alkylthiophene
as linking groups with the intention of increasing the linearity of molecules
did not result in the production of mesomorphism.
The behavior observed in these compounds can, in principle, be explained
in terms of two effects:
1. The deviation from linearity of the linking group a,b-unsaturated ketone
is in the order of 60 degrees, and the introduction of five membered
heteroaromatic rings would only reduce the deviation to an angle in the
order of 50 degrees.
2. Free rotation about the C-C single bond of the linking group a,b-
unsaturated ketone, which can also permit the existence of conforma-
tional isomers that might destroy the anisotropy necessary for mesophase
formation.
The separation of these two effects is necessary to better understand the re-
lation between molecular structure and mesomorphic behavior. To study the

185

186
H. GALLARDO et al.
TABLE 1 Optical and thermal data a,b-unsaturated ketones 1-[4-(trans-4-n-heptycyclohex-
yl)phenyl)]-3-R-2-propen-1-one 1
Compound
Transition
Temperature (ꢀC)
1A1
1A2
1A3
1A4
1B1
1B2
1B3
1B4
C-I
C-I
C-I
C-I
C-I
C-I
C-I
C-I
95.0–97.0
110.0–112.0
109.0–111.0
98.0–100.0
92.0–94.0
85.0–87.0
84.0–86.0
83.0–85.0
interrelation between the two effects, compounds 2 were designed. These
compounds minimize the effect of conformational isomerism but with line-
arity that in principle is equal to the a,b-unsaturated ketone intermediary.
The optical, thermal, and corresponding enthalpy values of compounds 2 are
given in Table 2. Transition enthalpies were determined by differential
scanning calorimetry using a Perkin-Elmer DSC-2. The thermograms were
recorded upon cooling at a rate of 5ꢀC minÀ1. All mesophases were identified
according to their textures observed by optical microscopy. All compounds 2
are mesogenic. Nematic and smectic phases were observed. All compounds in
this series exhibit enantiotropic phases. Once the isotropic liquid is cooled, a
Schlieren texture allows us to identify a nematic phase. Once it is cooled
further, a focal-conic texture appears in the SA phase. This latter texture is
easily transformed into a homeotropic texture by the application of
mechanical stress. In compound 2A1 only a nematic phase was observed, and
in compound 2B4 only a smectic A (SA) phase was observed.
The principal characteristic observed in compounds 2 is the large thermal
stability. This phenomenon is very common in mesogenic compounds that
incorporate a strong transverse dipole moment. In this series, this behavior
is attributed to the presence of isoxazole heterocyclic rings, which enhance
the polarization of the molecules. These facts would explain the high
clearing points observed.
In spite of the low melting point and molecular geometry of a,b-un-
saturated ketones, none of the compounds of the intermediary series exhibit

187
TABLE 2 Optical, thermal, and thermodynamic data for 3-[4-(trans-40-n-heptylcyclohex-
ylphenyl)]-5-R-isoxazole 2
Compound
Transition
Temperature (ꢀC)
DH (KJ=mol)
2A1
C-N
N-I
C-SA
SA-N
N-I
C-SA
SA-N
N-I
C-SA
SA-N
N-I
C-SA
SA-N
N-I
C-SA
SA-N
N-I
C-SA
SA-N
N-I
C-SA
SA-I
135.0
278.0
112.3
225.3
234.0
107.0
221.9
229.1
98.7
223.0
229.7
107.0
130.0
186.0
82.6
25.04
0.98
26.02
1.20
1.12
30.98
1.58
1.00
24.47
1.87
1.23
27.05
1.32
1.14
24.09
1.81
1.87
22.25
1.74
1.54
30.12
4.95
2A2
2A3
2A4
2B1
2B2
2B3
2B4
165.2
178.4
74.6
168.0
171.0
79.4
166.8
mesogenic behavior. In these compounds, the length-to-breadth ratio is
clearly low and doesn’t favor the parallel molecular organization typical of
the liquid crystal state. The deviation is from linearity to 27.4–25.7ꢀ. In
contrast, the corresponding isoxazole derivatives have a more linear shape,
with angle of only 10.7ꢀ.
The mesogenic behavior observed can be explained through the geo-
metric and electronic factors associated with the molecular structure. The
length-to-breadth ratio is greater in compounds of series 2 than those in
series 1, and consequently mesomorphism is favored. The enone linking
groups disrupt the electronic conjugation between the aromatic group joined
to the mesogenic core. In contrast, when the linking group is an isoxazole,
the aromatic conjugation can be extended to the mesogenic core. This fact
favors molecular planarity and also increases the anisotropy of electronic

188
H. GALLARDO et al.
polarizability, thus increasing molecular interaction (van der Waals forces)
and consequently promoting liquid crystal behavior.
CONCLUSION
Two new series of the compounds were synthesized. The liquid crystal
properties of these compounds were investigated. In general, the a,b-
unsaturated ketones groups significantly increase the molecular width,
making the parallel alignment of the molecules necessary for the achieve-
ment of mesomorphism difficult. Consequently, mesogenic behavior is only
observed in the isoxazoles. This group allows for extended conjugation
through the molecule and emphasizes the importance of electronic factors in
mesophase formation.
EXPERIMENTAL SECTION
The transition temperatures for all compounds were determined by optical
microscopy using a Leitz Ortholux polarizing microscope in conjuntion with
a Mettler FP 52 heating stage. The transition temperatures and enthalpies
were determined using a Perkin-Elmer DSC-2 differential scanning calori-
meter, which was calibrated by measuring the known melting point
(429.6 K) and heat of fusion (28.46 KJ molÀ1) of indium. The heating and
cooling rate was 10ꢀC minÀ1. The purity of the compounds was evaluated
by thin layer chromatography and elemental analysis (Perkin-Elmer 2400).
The IR spectra were recorded using KBr discs with a Perkin-Elmer 283
1
spectrometer, and the H NMR and 13C-NMR spectra were recorded at
200 MHz (Bruker HX-200).
General Procedure for Preparation of the
a,b-Unsatured Ketones
Sodium hydroxide (8.0 mmol) was dissolved in 10 ml of ethanol and 25 ml
of water and through stirring 4-[trans-4-n-heptycyclohexyl]acetophenone
(7.0 mmol) was added to the solution, heated to 40ꢀC in a steam bath for 15
min, and then 7.5 mmol of aromatic aldehyde was added. The temperature
of the reaction mixture was kept at about 40–50ꢀC with continued stirring
for 2 h. After cooling to room temperature, the crude product was filtered
under suction and recrystallized from EtOH giving pale yellow crystals.

189
1-[4-(trans-4-n-heptylcyclohexyl)phenyl]-3[4-n-octoxyphenyl]-2-
propen-1-one- 1A3
mp 111,0ꢀC, yield 60%. IR(KBr), n[cmÀ1] 2900, 2840, 1680, 1590, 840, 729;
1H NMR (200 MHz, CDCl3, TMS) d [ppm]: 7.94 (d, 2H, J ¼ 8.3 Hz), (d,
1H, J ¼ 15.6 Hz), 7.56 (d, 2H, J ¼ 8.7 Hz), 7.40 (d, 1H, J ¼ 15.6 Hz), 7.30
(d, 2H, J ¼ 8.3 Hz), 6.92 (d, 2H, J ¼ 8.7 Hz), 3.97 (t, 2H, J ¼ 6.5, OCH2À),
2.53 (tt, 1H, CHÀ), 1.82 (m, 4H, 2CH2À), 1.28 (m, 29H, 1CHÀ, 14CH2À),
0.81–0.87 (m, 6H, 2CH3); 13C NMR (62.5 MHz, CDCl3, TMS) d [ppm]:
189.97, 161.20, 153.05, 144.27, 136.30, 130.14, 128.61, 127.49, 127.04,
119.58, 114.86, 68.15, 44.75, 37.39, 37.26, 34.05, 33.45, 31.94, 31.89, 29.97,
29.53, 29.40, 29.27, 29.17, 26.99, 26.01, 22.70, 14.13.
1-[4-(trans-4-n-heptylcyclohexyl)phenyl]-3[5-n-dodecylthyenyl]-2-
propen-1-one- 1B4
mp 83–85ꢀC, yield 66%. IR(KBr), n[cmÀ1] 2922, 2856, 1648, 1578, 850,
722; 1H NMR (200 MHz, CDCl3, TMS) d [ppm]: 7.88 (d, 2H,
J ¼ 8.1 Hz), 7.82(d, 1H, J ¼ 15 Hz), 7.27 (d, 2H, J ¼ 8.1 Hz), 7.17 (d, 1H,
J ¼ 15 Hz), 7.12 (d, 1H, J ¼ 3,5 Hz), 6.71 (d, 1H, J ¼ 3,5 Hz), 2,87 (t,2H,
J ¼ 7,3 Hz), 2.53 (tt, 1H, CHÀ), 1.04–1.87 (m, 41H), 0.80–0.86 (m, 6H,
2CH3); 13C NMR (62.5 MHz, CDCl3, TMS) d [ppm]: 190.10, 153.81,
151.22, 138.86, 137.98, 136.75, 133.25, 129.19, 127.72, 126.26, 120.14,
45.41, 37.89, 34.68, 34.09, 32.57, 31.26, 30.60, 30.29, 30.17, 30.00, 29.71,
27.62, 23.35, 14.78.
General Procedure for Preparation of the 3,5-Disubstituted
Isoxazoles
A mixture of 0.02 mol of enone, 0.03 mol of hydroxylamine hydrochloride,
50 ml of ethanol and three drops of pyridine was refluxed in a water bath
for 2 h. The ethanol was removed by evaporation of the solution. To the
cooled residue 10 ml of water was added, and the solution was cooled in an
ice bath and stirred until the enone-oxime crystallized. The solid was fil-
tered, washed with a little water and dissolved in 60% AcOH (20 ml) at
60ꢀC. To this solution 0.02 mol of tetrakis(pyridine) cobalt(II) was added.
After 1 min at 60ꢀC, the mixture was allowed to cool to room temperature.
The precipitated 3,5-diarylisoxazole or 3,5-arylthienilisoxazole was filtered
with a Buchner funnel, washed with a small amount of dilute AcOH, and
dried in air. The crude product was purified by two recrystallizations from
hexane.

190
H. GALLARDO et al.
3-[4-(trans-4 0n-heptylcyclohexyl)phenyl]-5[2-(5-n-octoxyphenyl]-
isoxazole- 2A3
mp 107.0ꢀC, yield 78%. IR(KBr), n[cmÀ1] 3124, 2954, 2920, 2848, 1616, 1568,
1506, 1390, 1256, 838, 720; 1H NMR (200 MHz, CDCl3, TMS) d [ppm]: 7.70
(d, 2H, J ¼ 8.0 Hz), 7.68 (d, 2H, J ¼ 8.8 Hz), 7.23 (d, 2H, J ¼ 8.0 Hz), 6.90 (d,
2H, J ¼ 8.8 Hz), 6.59 (s, 1H), 3.93 (t, 2H, J ¼ 6.5 Hz OCH2À), 2.53 (tt, 1H,
CHÀ), 1.90 (m, 33H, 16CH2À, 1CHÀ), 0.81–0.87 (m, 6H, 2CH3). 13C NMR
(62.5 MHZ, CDCl3, TMS) d [ppm]: 170.92, 163.55, 161.35, 150.63, 128.05,
127.42, 120.91, 115.53, 96.63, 68.15, 45.24, 38.10, 37.97, 34.88, 34.21, 32.62,
30.66, 30.21, 30.08, 29.95, 29.86, 27.68, 26.69, 23.31, 23.39, 14.28.
3-[4-(trans-4 0n-heptylcyclohexyl)phenyl]-5[2-(5-n-dodecylthienyl]-
isoxazole- 2B4
mp 85–87ꢀC, yield 58%. IR(KBr), n[cmÀ1] 2956, 2920, 2848, 1614, 1569,
1
1504, 1390, 1256, 838, 720; H NMR (200 MHz, CDCl3, TMS) d [ppm]:
7.70 (d, 2H, J ¼ 8.2 Hz), 7.30 (d, 1H, J ¼ 3.6 Hz), 7.20 (d, 2H, J ¼ 8.2 Hz),
6.70 (s, 1H), 2.78 (t, 2H, J ¼ 7.0 Hz), 2.45 (tt, 1H, CHÀ), 0.90 (m, 39H), 0.81
(m, 6H, 2CH3); 13C NMR (62.5 MHZ, CDCl3, TMS) d [ppm]: 166.04,
163.38, 150.64, 149.98, 127.28, 127.47, 127.38, 127.13, 125.78, 96.96, 45.18,
38.07, 37.92, 34.81, 34.15, 32.59, 32.18, 30.78, 30.64, 30.30, 30.21, 30.02,
29.72, 27.66, 23.36, 14.78.
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