Synthesis and mesomorphic studies of a series of liquid crystalline 2-(4-Alkylphenyl)-6-Methoxyquinolines

Article abstract of DOI:10.1080/15421400902841452

This study describes the novel synthesis of a homologous series of quinoline-containing liquid crystalline 2-(4-alkylphenyl)-6-methoxyquinolines (nPQOMe, n=4-8) compounds. Preparation of these compounds was completed in the overall two-step reaction and fair to good yields (47-60%) were obtained. Thermotropic behaviors of these liquid crystalline compounds were investigated using differential scanning calorimetry and polarized optical microscopy. Liquid crystals of these 2-(4-alkylphenyl)-6-methoxyquinolines display only nematic phase. Taylor & Francis Group, LLC.

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Synthesis and Mesomorphic
Studies of a Series of Liquid
Crystalline 2-(4-Alkylphenyl)-6-
Methoxyquinolines
Win-Long Chia a & Chih Hao Chang a
a Department of Chemistry , Fu Jen Catholic
University , Taipei, Taiwan
Published online: 03 Aug 2009.
To cite this article: Win-Long Chia & Chih Hao Chang (2009) Synthesis and
Mesomorphic Studies of a Series of Liquid Crystalline 2-(4-Alkylphenyl)-6-
Methoxyquinolines, Molecular Crystals and Liquid Crystals, 506:1, 47-55, DOI:
10.1080/15421400902841452
To link to this article: http://dx.doi.org/10.1080/15421400902841452
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Mol. Cryst. Liq. Cryst., Vol. 506, pp. 47–55, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400902841452
Synthesis and Mesomorphic Studies of a Series of Liquid
Crystalline 2-(4-Alkylphenyl)-6-Methoxyquinolines
Win-Long Chia and Chih Hao Chang
Department of Chemistry, Fu Jen Catholic University,
Taipei, Taiwan
This study describes the novel synthesis of a homologous series of quinoline-
containing liquid crystalline 2-(4-alkylphenyl)-6-methoxyquinolines (nPQOMe,
n ¼ 4–8) compounds. Preparation of these compounds was completed in the
overall two-step reaction and fair to good yields (47–60%) were obtained.
Thermotropic behaviors of these liquid crystalline compounds were investigated
using differential scanning calorimetry and polarized optical microscopy. Liquid
crystals of these 2-(4-alkylphenyl)-6-methoxyquinolines display only nematic
phase.
Keywords: liquid crystals; quinoline-containing compounds; synthesis; thermal
analysis
1. INTRODUCTION
Advances in the fields such as the applications, the physics, or the the-
ory of liquid crystals depend very critically upon the organic chemist
and his role in synthesizing materials. Since a single liquid crystal
shows a couple of distinguished properties at best, mixtures consisting
of up to twenty or more polar and weakly polar compounds have to be
developed for a given application. Therefore, we believe that there is
still room for improvements in prepararion of new nematic liquid crys-
tals and devising novel molecular synthesizing methodologies [1–2].
Some quinoline-containing liquid crystalline compounds were
synthesized in past years. Some were synthesized by reacting suitable
substituted anilines and benzaldehydes with piruvic acid [3],
whereas some other compounds were prepared by heating aniline with
Address correspondence to Win-Long Chia, Department of Chemistry, Fu Jen
Catholic University, Taipei, Taiwan 242, China. E-mail: 027087@mail.fju.edu.tw
47

48
W.-L. Chia and C. H. Chang
glycerin, or 1,2-glycols or a,b-unsatuaturated aldehyde through Skraup
procedure or Doebner–von Miller variation [4–9]. Another study
reported bis-formulation of acetanilides followed by cyclization with
PPA and then converted into chloroquinoline aldehydes, which were
used as an intermediate for further synthesis of quinoline-containing
liquid crystalline compounds [10–12]. Although these methods are of
great value for constructing important heterocyclic systems, most suffer
from limited scope of the method applied, large number of synthetic
steps and relatively low yields. On the other hand, some recent patents
showed high-speed response of optical switching element was achieved
by quinoline-containing liquid crystalline compounds [13–15]. All these
reasons prompted us to seek a new synthetic method for preparing such
liquid crystalline compounds.
Previously, we prepared various 2- or 4-substituted pyridines and
alkaloids by regioselective addition of organometallic reagents to
l-acylpyridinium salts [16–18]. Recently, we have successfully applied
this methodology to prepare some pyridine-containing liquid crystals
[19,20]. In this article, we extended the same synthetic methodology
to the synthesizing 2-(4-alkylphenyl)-6-methoxyquinolines (nPQOMe),
in which the alky chain length varied from butyl to octyl.
2. RESULTS AND DISCUSSION
2.1. Synthesis of 2-(4-Alkylphenyl)-6-methoxyquinolines
(nPQOMe, n ¼ 4–8)
The 2-(4-Alkylphenyl)-6-methoxyquinolines were synthesized according
to Scheme 1. First, the Grignard reagents 4-alkylphenylmagnesium
SCHEME 1

Synthesis and Mesomorphic Studies
49
bromides (prepared from commercial readily available 4-alkyl-
bromobezene) 1 were allowed to react with N-phenyloxycarbonyl-
6-methoxyquinolinium chlorides 2 to afford a 1,2-dihydroquinolines
3, which were then oxidized by o-chloranil. This is a valuable approach
to synthesizing liquid crystalline 2-(4-Alkylphenyl)-6-methoxyquino-
lines (nPQOMe) 4, in which the alkyl chain length was varied
from n-butyl to n-octyl. The entire synthetic procedures were
completed in a short two-step process. Fair to good yields (50–63%)
of 2-(4-Alkylphenyl)-6-methoxyquinolines (nPQOMe, n ¼ 4–8) were
obtained and highly pure products were collected by continuous
re-crystallization from n-hexane (Table 1).
It is known nucleophilic attack of a Grignard reagent and its regio-
selectivity on the pyridine ring can be greatly enhanced by phenyl
chloroformate [21,22]. In this article, our study shows synthesis of
quinoline-containing liquid crystalline compounds can also be applied
with the same synthetic methodology. a-Regioselectivity of the quino-
line ring by Grignard reagent attack resulting a substituent in the
a-position of pyridinoid ring was found to be overwhelmingly dominant
by this synthetic methodology. Trace amounts of c-addition product
can be easily separated from the major a-addition product using
simple liquid chromatography (hexane: methylene choloride ¼ 1:2)
due to a high polarity difference in these two compounds.
Direct reaction of 6-methoxyquinoline with 4-pentylphenyllithium
(prepared from 4-pentyliodobenzene with butyllithium) followed by
refluxing under nitrobenzene has been tried by Kosaka and others
[14], but their yield was lower (44.8%) compared to ours (63%). Unfor-
tunately, mesophase of the 2-(4-pentylphenyl)-6-methoxyquinoline is
not found by Kosaka and others, in contrast to what we observed, it
displays enantiotropic nematic mesophase.
TABLE 1 Yields of 2-(4-Alkylphenyl)-6-methoxyquinolines
(nPQOMe, n ¼ 4–8)
Entry
Alkyl
Yielda (%)
4a
4b
4c
4d
4e
Butyl
Pentyl
Hexyl
Heptyl
Octyl
58
60
53
49
47
aIsolated yields are based upon after recrystalization.

50
W.-L. Chia and C. H. Chang
2.2. Thermotropic Behavior of 2-(4-Alkylphenyl)-
6-methoxyquinolines
Thermotropic behaviors of 2-(4-alkylphenyl)-6-methoxyquinoline
(nPQOMe, n ¼ 4–8) were studied by observing optical texture under
a polarized optical microscope (POM) and further verified by differen-
tial scanning calorimetry (DSC) measurements. Phase transition
temperatures and the corresponding transition enthalpies 2-(4-
Alkylphenyl)-6-methoxyquinolines (nPQOMe, n ¼ 4–8) are summar-
ized in Table 2. Only the nematic phase was found in homologues of
2-aryl-6-methoxyquinolines, nPQOMe, in which n ¼ 4, 6, and 8 are
monotropic, and n ¼ 5 and 7 are enantiotropic. It has been found for
many homologous series of liquid crystals that the odd carbon atom
alkyl chain has a terminal CH3 group which extends the long molecu-
lar axis, whereas in an even number carbon chain the terminal CH3
group tends to lie off axis [23]. Therefore, it is quite normal to find
n ¼ 5 and 7 (odd numbered carbon chains) are enantiotropic, while
for n ¼ 4, 6, and 8 (even numbered carbon chains), monotropic. The
odd–even effect of the 2-aryl-6-methoxyquinolines homologues can
also be manifested in the TI-N values shown in Table 2.
The nematic mesophase of our calamitic liquid crystals, nPQOMe,
n ¼ 4–8, generally appears in the moderately high temperature range
between 65ꢀC and 100ꢀC, while the 2-(4-pentylphenyl)-6-methoxyqui-
noline, nPQOMe, n ¼ 5, provides the widest nematic range, 3ꢀC during
heating, and 25ꢀC during cooling. The nematic Schlieren texture of
TABLE 2 Phase Transition Temperatures (ꢀC) and Corresponding
Transition Enthalpies (kJ molꢁ1), in Parentheses, for the Homologous
Series of nPQOMe, n ¼ 4 to 8 were Determined by the Second Scans at
a Heating and Cooling Rate of 5ꢀC minꢁ1 from DSC
a
Entry
TCr-N [TCr-1
]
TN-I
TI-N
TN-Cr
4a
[97.6]
(12.84)
96.9
(21.67)
[92.0]
(21.88)
94.0
86.9
(0.09)
98.4
(0.36)
83.6
79.2
(11.36)
73.5
(20.5)
67.9
(n ¼ 4)
4b
100.0
(0.24)
(n ¼ 5)
4c
(n ¼ 6)’
(0.20)
94.8
(21.61)
79.2
4d
96.2
(n ¼ 7)
4e
(n ¼ 8)
(15.90)
[88.5]
(23.86)
(0.16)
(0.25)
85.8
(0.27)
(15.9)
69.0
(24.50)
aCr ¼ crystalline phase, N ¼ nematic phase, I ¼ isotropic phase.

Synthesis and Mesomorphic Studies
51
FIGURE 1 Polarized optical micrograph of nematic Schlieren texture of
2-(4-butylphenyl)-6-methoxyquinoline (4a) nPQOMe, n ¼ 4 arises from
isotropic phase on cooling to 86ꢀC with magnification of ꢂ100.
2-(4-butylphenyl)-6-methoxyquinoline (4a) nPQOMe, n ¼ 4 arises from
isotropic phase on cooling to 86ꢀC is shown in Fig. 1, and its corre-
sponding second scan thermogram is shown in Fig. 2. Furthermore,
FIGURE 2 Thermogram of 2-(4-butylphenyl)-6-methoxyquinoline, nPQOMe,
n ¼ 4, were determined by the second scans at a heating and cooling rate of
5ꢀC minꢁ1 from DSC.

52
W.-L. Chia and C. H. Chang
in a thick sample under pom we could not observe the broken focal
conic structure, which is a characteristic phenomenon of smectic
C phase.
Although several 2-arylquinolines liquid crystalline analogues were
examined previously [3], disappointingly (except the one reported by
Kosaka et al. [14]), those 2-arylquinolines showed no methoxy substi-
tuents at the 6-position of quinoline, and therefore, are different from
those currently being examined. For example, 2-(4-butylphenyl)-
6-butoxyquinoline (4-PQ-O4) was found to have a nematic mesophase
between 77ꢀC to 112ꢀC; 2-(p-ethylphenyl)-6-ethoxyquinoline (2-PQ-O2)
melted to be isotropic liquid at 139ꢀC and has a monotropic nematic
phase transition observed at 109ꢀC [3]. Similar quinoline-containing
liquid crystalline homologues of 6-Alkyl-2-(4-alkyloxyphenyl)quinoline
(nO-PQ-m, n ¼ 5–10, m ¼ 5–8) has been reported, in which with longer
tails on both ends of the phenylquinoline core exhibit to be a smectic C
base material [4]. Mesomorphic properties of 2-phenylnaphthalenes may
also be compared with those of 2-phenylquinolines. For example,
6-(4-hexylphenyl)-2-methoxynaphthalene (6-PNP-OMe) has a nematic
phase between 113ꢀC to 122ꢀC [24], which indicates that the
nematic stability of phenylnaphthalene core (PNP) is higher than
that of phenylquinoline core (PQ). With longer alkyl chains on both
ends of 2,6-disubstituted phenylnaphthalene core, 2-(4-octylphenyl)-
6-butylnaphthalene (8-PNP-4) displays only smectic phases (Cryst.
ꢁ3.5 SmE 75.2 SmB 88.8 SmA 93.0 Iso. Unit:ꢀC) [25]. In order to
reach nematic mesophase in ambient temperature as well as with
an appropriate and extended range, we believe that more researches
have to be conducted for either short chain ends and=or unsymmetric
chain ends on both sides of the suitable liquid crystalline core
structures.
3. EXPERIMENTAL
All chemicals and solvents were of reagent grade from Aldrich
Chemical Co. Anhydrous solvents and chemicals were freshly distilled
before use. The 1H and 13C-NMR spectrums were recorded on a
Brucker AC 300 spectrometer. Infrared spectrums were recorded
using a Perkin-Elmer 1600 Series spectrometer.
Thermographs were carried out on a Perkin-Elmer DSC 7 Series,
calibrated with pure indium. Polarizing optical microscopy was carried
out on an Olympus BH-2 research microscope equipped with a Mettler
FP90=FP82HT hot stꢁa1ge. All phase transitions were measured with a
scan rate of 5ꢀC min
.

Synthesis and Mesomorphic Studies
53
3.1. General Procedure for the Synthesis of 2-(4-AIkyIphenyl)-
6-methoxyquinolines (nPQOMe, n ¼ 4–8)
For 3a: To a (Grignard) solution of 1-bromo-4-butylbenzene (10 mmol)
in THF (20 ml) was added freshly dried magnesium granules
(11 mmol) under an inert atmosphere. The Grignard solution 1 was
then slowly added by syringe into a preformed solution of 6-
methoxyquinolinium chloride 2, which was prepared from phenyl
chloroformate (10 mmol), 6-methoxyquinoline (10 mmol) in dry THF
(20 ml) under ꢁ20ꢀC for half an hour. The resulting solution was
heated slowly to room temperature and stirred for another 8 hours.
After evaporating the THF, the residue was extracted with Et2O.
The organic layer was further washed once with 20% NH4Cl solution
and twice with distilled water and brine, and dried with magnesium
sulfate. The intermediate 3a was then used for the next oxidation step
without further purification. For 4a: To a solution of dry toluene
(20 ml) and crude 3a was added about 1.5eq. o-chloranil. The reaction
mixture was heated to reflux for about three hours under inert
atmosphere and then quenched by adding 1N NaOH (25 ml) and
Et2O (25 ml) and filtered through Celite. Normal aqueous work up
and isolation with column chromatography (hexane: methylene
choloride ¼ 1:2) afford 2-(4-butylphenyl)-6-methoxyquinoline (4a).
The fairly good yield (58%) of product 4a was obtained by
re-crystallization several times from n-hexane. The yield was based
upon the overall two-step reaction. All compounds gave satisfactory
data from 1H-NMR, 13C-NMR, IR, and elemental analysis as
illustrated below.
3.1.1. 2-(4-Butylphenyl)-6-methoxyquinoline
(nPQOMe, n ¼ 4) (4a)
1H-NMR (CDCl3): d 8.00–8.11 (m, 4H), 7.80 (d, 1H, J ¼ 8.4 Hz), 7.36
(dd, 1 H, J1 ¼ 9.3 Hz, J2 ¼ 3 Hz), 7.31 (d, 2 H, J ¼ 8.1 Hz), 7.06 (d, 1 H,
J ¼ 2.7 Hz), 3.93 (s, 3 H), 2.67 (t, 2 H, J ¼ 7.5 Hz), 1.72–1.58 (m, 2 H),
1.47–1.30 (m, 2 H), 0.94 (t, 3 H, J ¼ 7.5 Hz). 13C-NMR (CDCl3): ppm
157.6, 155.2, 144.4, 144.0, 137.3, 135.4, 131.1, 129.0, 128.0, 127.2,
122.2, 119.2, 119.2, 105.1, 55.6, 35.5, 22.4, 14.0. IR (KBr): cmꢁ1
3061, 2959, 2928, 2855, 1599, 1495, 1252, 1022, 835. Anal. Calcd for
C20H21NO: C, 82.44; H 7.26; N 4.81, Found: C, 82.00; H, 7.25; N, 4.72.
3.1.2. 2-(4-Pentylphenyl)-6-methoxyquinoline
(nPQOMe, n ¼ 5) (4b)
1H-NMR (CDCl3): d 8.29–8 00 (m, 4 H), 7.80 (d, 1 H, J ¼ 8.4 Hz), 7.36
(dd, 1 H, J1 ¼ 9 Hz, J2 ¼ 2.7 Hz), 7.31 (d, 2 H, J ¼ 8.4 Hz), 7.05 (d, 1 H,

54
W.-L. Chia and C. H. Chang
J ¼ 3 Hz), 3.93 (s, 3 H), 2.67 (t, 2 H, J ¼ 7.2 Hz), 1.76–1.57 (m, 2 H),
1.38 ꢃ 1.29 (m, 4 H), 0.89 (t, 3 H, J ¼ 6.6 Hz). 13C-NMR (CDCl3): ppm
157.5, 155.2, 144.4, 144.0, 137.2, 135.4, 131.1, 128.9, 128.0, 127.2,
122.2, 119.2, 105.1, 55.5, 35.7, 31.5, 31.1, 22.6, 14.1. IR (KBr): cmꢁ1
3062, 2957, 2920, 2855, 1600, 1495, 1250, 1027, 833. Anal. Calcd for
C21H23NO: C, 82.58; H 7.59; N 4.59, Found: C, 82.36; H, 7.55, N, 4.52.
3.1.3. 2-(4-Hexylphenyl)-6-methoxyquinoline
(nPQOMe, n ¼ 6) (4c)
1H-NMR (CDCl3): d 8.13–8.00 (m, 4 H), 7.81 (d, 1 H, J ¼ 8.7 Hz), 7.37
(dd, 1 H, J1 ¼ 9.3 Hz, J2 ¼ 3 Hz), 7.32 (d, 2 H, J ¼ 8.1 Hz), 7.08 (d, 1 H,
J ¼ 2.7 Hz), 3.93 (s, 3 H), 2.67 (t, 2 H, J ¼ 7.5 Hz), 1.72–1.58 (m, 2 H),
1.45–1.19 (m, 6 H), 0.98–0.82 (m, 3 H). 13C-NMR (CDCl3): ppm 157.6,
155.2, 144.4, 144.0, 137.3, 135.4, 131.2, 129.0, 128.0, 127.2, 122.2,
119.2, 105.1, 55.6, 35.8, 31.8, 31.4, 29.0, 22.7, 14.1. IR (KBr): cmꢁ1
3061, 2957, 2920, 2853, 1599, 1495, 1252, 1028, 831. Anal. Calcd for
C22H25NO: C, 82.72; H 7.89; N 4.38, Found; C, 82.41; H, 7.88; N, 4.32.
3.1.4. 2-(4-Heptylphenyl)-6-methoxyquinoline
(nPQOMe, n ¼ 7) (4d)
1H-NMR (CDCl3): d 8.13 –; 8.00 (m, 4 H), 7.81 (d, 1 H, J ¼ 8.4 Hz),
7.37 (dd, 1 H, J1 ¼ 9.3 Hz, J2 ¼ 3 Hz), 7.31 (d, 2 H, J ¼ 8.1 Hz), 7.08 (d,
1 H, J ¼ 3 Hz), 3.94 (s, 3 H), 2.67 (t, 2 H, J ¼ 7.5 Hz), 1.74 – 1.57 (m,
2 H), 1.40 – 1.20 (m, 8 H), 0.88 (t, 3 H, J ¼ 7.2 Hz). 13C-NMR (CDCl3):
ppm 157.6, 155.2, 144.4, 144.1, 137.3, 135.4, 131.2, 128 9, 128.0,
127.2, 122.2, 119.2, 105.1, 55.6, 35.8, 31.9, 31.4, 29.3, 29.2, 22.7,
14.1. IR (KBr): cmꢁ1 3061, 2958, 2922, 2853, 1599, 1495, 1252, 1022,
833. Anal. Calcd for C23H27NO: C, 82.84; H 8.16; N 4.20, Found: C,
82.52; H, 8.08; N, 4.18.
3.1.5. 2-(4-Octylphenyl)-6-methoxyquinoline
(nPQOMe, n ¼ 8) (4e)
1H-NMR (CDCl3): d 8.13 – 8.00 (m, 4 H), 7.81 (d, 1 H, J ¼ 8.7 Hz),
7.36 (dd, 1 H, J1 ¼ 9.3 Hz, J2 ¼ 3 Hz), 7.31 (d, 2 H, J ¼ 8.1 Hz), 7.08 (d,
1 H, J ¼ 3.0 Hz), 3.94 (s, 3 H), 2.67 (t, 2 H, J ¼ 7.5 Hz), 1.75–1.58 (m,
2 H), 1.42–1.19 (m, 10 H), 0.88 (t, 3 H, J ¼ 7.2 Hz). 13C-NMR (CDCl3):
ppm 157.6, 155.2, 144.4, 144.1, 137.3, 135.4, 131.2, 128.9, 128.0,
127.2, 122.2, 119.2, 105.1, 55.6, 35.8, 31.9, 31.5, 29.6, 29.3, 22.7,
14.1. IR (KBr): cmꢁ1 3063, 2957, 2919, 2849, 1599, 1497, 1250, 1028,
833. Anal. Calcd for C24H29NO: C, 82.95; H 8.41; N 4.03, Found: C,
82.82; H, 8.33; N, 3.84.

Synthesis and Mesomorphic Studies
55
ACKNOWLEDGMENTS
Financial support of our work by the office of research and develop-
ment of Fu Jen Catholic University is gratefully acknowledged.
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