Synthesis of new chiral dopants derived from 2-phenylpropanoic acid derivatives for nematic liquid crystals

Article abstract of DOI:10.1246/bcsj.74.2219

New chiral dopants for nematic liquid crystals were synthesized using optically active 2-phenylpropanoic acid derivatives. The magnitude of the helical twisting power (HTP) was largely influenced by the terminal group of the asymmetric frame and the core

Full text of DOI:10.1246/bcsj.74.2219

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© 2001 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 74, 2219–2222 (2001) 2219
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Synthesis of New Chiral Dopants Derived from 2-Phenylpropanoic Acid
Derivatives for Nematic Liquid Crystals
Synthesis of New Chiral Dopants
Y. Aoki et al.
Yoshio Aoki, Hiroaki Shitara, Takuji Hirose, and Hiroyuki Nohira*
01
19
22
Department of Applied Chemistry, Faculty of Engineering, Saitama University,
Shimo-ohkubo 255, Saitama, Saitama 338-8570
SJA8
09-2673
177
(Received May 28, 2001)
New chiral dopants for nematic liquid crystals were synthesized using optically active 2-phenylpropanoic acid de-
rivatives. The magnitude of the helical twisting power (HTP) was largely influenced by the terminal group of the asym-
metric frame and the core structures. (S)-1-[4-(trans-4-Butylcyclohexyl)phenyl]-2-phenylpropane-1-one showed a large
HTP value (20.4 µm−1). The relationship between the HTP and the molecular structures of the chiral dopants is dis-
cussed.
01
01
.4
.3
Chiral nematic liquid crystals, which are applicable to super
twisted nematic (STN) devices, are attractive materials. Re-
cently, liquid crystals are being widely applied to the STN
mode of flat panel display devices of portable computers and
Fig. 1. The structural formula of the chiral dopant.
TV sets, because a liquid-crystal display (LCD) is of greater
advantage than, or over, a cathode-ray tube (CRT) concerning
size, thickness, weight, and power consumption.1 Generally,
the chiral nematic materials used for a LDC consist of achiral
host liquid crytal mixtures, which have low viscosities and a
wide temperature range of the nematic phase, and a chiral
dopant having a large helical twisting power (HTP). This
chiral dopant induces chirality of the whole material of the liq-
uid crystal mixture. Chirality is one of the most interesting
subjects concerning liquid crystals.2,3
The HTP can be calculated by (Eq. 1),4 where p is the pitch
Fig. 2. The structural formula of new chiral dopants.
of the chiral nematic phase in µm and c is the mass fraction of
logues of the new chiral dopants, 4*–7*, derived from optical-
ly active 2-phenylpropanoic acid derivatives, and discuss the
relationship between the HTP and the molecular structures of
the dopants (Fig. 2).
the dopant. On the other hand, in order to discuss a HTP by a
molecule, we have proposed molar helical twisting power
(MHTP),5 which is defined as follows (Eq. 2). Here, m is the
moles of the dopant per 1 kg of the mixture. Mw is the molec-
ular weight of the chiral dopant. In both expressions, chiral
dopants are required to have the ability to shorten the helical
pitch of the chiral nematic phase. To satisfy this requirement,
many optically active compounds have been synthesized.1 We
have also synthesized a variety of chiral compounds for the
same purpose.5,6 However, the relationship between the HTP
values and the molecular structures of chiral dopants is not
clarified.
Results and Discussion
Synthesis of Optically Active 2-(4-Butylphenyl)propano-
ic Acid. (S)-2-(4-Butylphenyl)propanoic acid (11*) was pre-
pared from (S)-2-chloropropanoic acid and butylbenzene using
an analogous method, which was employed in the synthesis of
ibuprofen (Scheme 1).7
Synthesis of Chiral Dopants. A new chiral dopant 4*
was synthesized by the Friedel-Crafts reaction of (S)-2-(4-bu-
tylphenyl)propanoic acid (11*) with pentyloxybenzene. The
reduction of 4* with lithium aluminium hydride (lithium tet-
rahydridoaluminate) gave 5* (Scheme 2).
HTP = (pc)−1
(1)
MHTP = (pm)−1 =HTP•Mw/1000 (µm−1 mol−1 kg) (2)
In order to study the effect of a core structure, a chiral
dopant having a cyclohexylbenzen structure was synthesized.
Optically active 2-phenylpropanoic acid was converted to the
corresponding acid chloride, which was treated with trans-1-
butyl-4-phenylcyclohexane in the presence of anhydrous alu-
minium chloride to give 6*. The reduction of 6* with triethyl-
We recently reported new chiral dopants, 1*–3*, prepared
from optically active 2-phenylpropanoic acid (Fig. 1), and
showed that their HTPs were largely influenced by the linkage
structures between the asymmetric frame and the core moi-
eties.5 In this paper, we report on the synthesis of some ana-

2220 Bull. Chem. Soc. Jpn., 74, No. 11 (2001)
Synthesis of New Chiral Dopants
Scheme 1. Synthetic route of (S)-2-(4-Butylphenyl)propanoic acid (11*).
than 2* and 3*, respectively.
The chiral dopant 5*, having a methylene linkage at the
chiral center, showed a larger HTP value than 4* having the
ketone carbonyl one. This tendency agrees well with the previ-
ously reported results.5 A carbonyl group is more rigid than a
methylene group and has a relatively large polarization. The
steric effect of a dopant molecule seems to be more important
than its rigidity or polarity to induce the helical microstructure
of the whole matrix. However, the details of the steric effect
are not still clear, as demonstrated for 6* and 7* in the next
paragraph.
The HTP value of the dopant 6*, which has a cyclohexyl-
benzen core and a carbonyl linkage, was larger than of 7*,
which has the same core, but a methylene linkage. This is
completely opposite to the precious results. The chiral dopant
6* exhibited the largest HTP value (20.4 µm−1), probably due
to the flexible and nonpolar cyclohexane structure. In particu-
lar, the MHTP value of 6* (7.09 µm−1 mol−1 kg) increased
more than twice that of 2* (3.05 µm−1 mol−1 kg). On the other
hand, MHTP of 7*, which had the same core, but a methylene
linkage, showed almost the same MHTP of 3* and two thirds
of 6*. The structural change from 3* to 7*, the introduction of
a flexible cyclohexyl group into the flexible asymmetric core,
did not work for higher HTP.
Scheme 2. Syntheisof(R)-2-(4-butylphenyl)-1-(4-pentyloxy-
phenyl)propane (5*).
silane gave 7* (Scheme 3).
Relationship between Structure and Helical Twisting
Power of 2-Phenylpropanoic Acid Derivatives. The HTP
of each chiral dopant was measured as a one weight percent
mixture with the host liquid crystal (ZLI-1132); the results are
summarized in Table 1. The introduction of a butyl group at
the benzene ring of the 2-phenylpropanoyl group, i.e. the chiral
center, significantly reduced the HTP value. The introduction
of an alkyl group on both sides make a chiral dopant more
symmetrical, and seems to be the reason for the reduction of
HTP. Thus, dopants 4* and 5* exhibited smaller HTP values
In conclusion, it was shown that optically active 2-phenyl-
Scheme 3. Syntheis of (R)-1-[4-(trans-4-butylcyclohexyl)phenyl]-2-phenylpropane (7*).

Y. Aoki et al.
Bull. Chem. Soc. Jpn., 74, No. 11 (2001) 2221
Table 1. HTPa) and MHTPa) of Chiral Dopants
Compound
R
X
Y
n
HTP/µm−1 MHTP/µm−1 mol−1 kg
1*b)
2*b)
3*b)
4*
H
H
H
CH2
CO
CH2
O
O
O
O
O
2
5
5
5
5
19.1
10.3
17.0
2.5
4.58
3.05
4.79
0.91
3.23
n-C4H9 CO
n-C4H9 CH2
5*
9.2
6*
7*
H
H
CO
4
4
20.4
12.9
7.09
4.30
CH2
a) Host liquid crystal (ZLI-1132):dopant = 99:1 (by weight). b) Reported in the
Ref. 6.
under nitrogen for 24 h, and then neutralized with sodium methox-
ide, cooled to room temperature, and diluted with dichlo-
romethane (40 mL) and water (40 mL). The organic layer was
dried with anhydrous sodiun sulfate, and concentrated under a
vacuum to give a light-yellow liquid (0.638 g, 86%). 1H NMR
(CDCl3) δ 0.92 (3H, t, J = 7.2 Hz), 1.31 (3H, d, J = 6.7 Hz),
1.33–1.38 (2H, m), 1.59–1.63 (2H, m), 2.63 (2H, t, J = 7.8 Hz),
3.21 (3H, s), 3.36 (3H, s), 4.38 (1H, q, J = 6.7 Hz), 7.17 (2H, d, J
propanoic acid derivatives can be a good chiral center for
dopants, and that the cyclohexylbenzen core is effective to in-
duce a helical structure for a nematic liquid crystal. As a re-
sult, a good chiral dopant 6* having a cyclohexylbenzen group
at the chiral center was developed for chiral nematic liquid
crystal applications.
Experimental
= 8.1 Hz), 7.39 (2H, d, J = 8.1 Hz). IR (neat) 3030, 2957, 2933,
2858, 2832, 1693,1607, 1336, 1180, 1090, 1055 cm−1
+9.6° (c 0.9, CHCl3).
. [α]D
24
The structures of all intermediates and products were confirmed
by 1H NMR spectroscopy (Bruker ARX-400, 400 MHz) and infra-
red spectroscopy (Perkin-Elmer FT-1640). The optical purity was
determined by high-performance liquid chromatography using a
set of JASCO LC 900 series (detector: JASCO CD-1595, chiral
column, “CHIRALCEL OJ” (Daicel Chem. Ind., Ltd., 4.6 mm ×
250 mm)). Specific rotations were measured with a JASCO DIP-
370 polarimeter. The helical pitch of a chiral nematic phase was
measured using wedge-shaped samples (Cano-wedge cell, tan θ =
0.0083, 0.0140, 0.0194, 0.0288, E. H. C. Ind., Ltd.,) by means of
the resulting Cano lines.8 The liquid crystal (LC) sample was pre-
pared by adding one weight percent of a chiral dopant into the
achiral host LC mixture, ZLI-1132 (Merck).
Synthesis of (S)-1-(4-Butylphenyl)-2-chloropropane-1-one
(8*): (S)-2-Chloropropanoic acid (3.186 g, 29.3 mmol) was dis-
tilled with benzoyl chloride (8.120 g, 57.7 mmol) to give an acid
chloride as a colorless liquid (1.938 g, 52%). To a mixture of an-
hydrous aluminium chloride (0.589 g, 4.41 mmol) in dry dichlo-
romethane (5 mL) were added the acid chloride (0.460 g, 3.62
mmol) and butylbenzene at ice-water temperature (0.484 g, 3.61
mmol). The mixture was stirred for 5 h at that temperature, and
hydrolyzed with 4 M (= mol/dm−3) hydrochloric acid, and ex-
tracted with dichloromethane. The organic layer was washed with
a 5% sodium hydrogencarbonate solution, and dried with anhy-
drous sodium sulfate. The solvent was evaporated under reduced
pressure to give a light-yellow liquid (0.678 g, 83%). 1H NMR
(CDCl3) δ 0.93 (3H, t, J = 7.2 Hz), 1.33–1.39 (2H, m), 1.58–1.66
(2H, m), 1.74 (3H, d, J = 6.8 Hz), 2.67 (2H, t, J = 7.8 Hz), 5.24
(1H, q, J = 6.6 Hz), 7.29 (2H, d, J = 8.2 Hz), 7.93 (2H, d, J = 8.2
Hz). IR (neat) 3030, 2957, 2931, 2860, 1689, 1606, 1181, 955
cm−1. [α]D25 +28.0° (c 0.9, CHCl3).
Synthesis of (S)-Methyl 2-(4-butylphenyl)propanoate (10*):
The mixture of 9* (0.638 g, 2.36 mmol) and anhydrous zinc chlo-
ride (0.063 g, 0.46 mmol) in toluene (5 mL) was refluxed for 23 h,
then cooled to room temperature, diluted with toluene (10 mL),
and filtered. After the solvent was evaporated under reduced pres-
sure, the remaining mixture was purified by column chromatogra-
phy (silica gel: 27 g, 10% ethyl acetate in hexane) to give a light-
1
yellow liquid (0.369 g, 71%). H NMR (CDCl3) δ 0.91 (3H, t, J =
7.3 Hz), 1.34–1.37 (2H, m), 1.48 (3H, d, J = 7.2 Hz), 1.58–1.60
(2H, m), 2.58 (2H, t, J = 7.8 Hz), 3.65 (3H, s), 3.69 (1H, q, J =
7.2 Hz), 7.13 (2H, d, J = 8.0 Hz), 7.19 (2H, d, J = 8.0 Hz). IR
(neat) 2955, 2931, 2858, 1738, 1693 cm−1. [α]D23 +38.2° (c 1.0,
CHCl3).
Synthesis of (S)-2-(4-Butylphenyl)propanoic Acid (11*):
10* (0.369 g, 1.67 mmol) was hydrolyzed by refluxing for 3 h in
the presence of conc. hydrochloric acid (5 mL) and acetone (5
mL). The resulting solution was evaporated under reduced pres-
sure and the remaining mixture was basified by dilute sodium hy-
droxide solution, and washed with diethyl ether. The aqueous lay-
er was acidified by conc. hydrochloric acid (1 mL) and the aque-
ous layer was extracted with diethyl ether, and dried with anhy-
drous sodium sulfate. The solvent was evaporated under reduced
pressure to give a light-yellow liquid (0.187 g, 54%). 1H NMR
(CDCl3) δ 0.91 (3H, t, J = 7.2 Hz), 1.31–1.37 (2H, m), 1.49 (3H,
d, J = 7.1 Hz), 1.54–1.61 (2H, m), 2.58 (2H, t, J = 7.8 Hz), 3.70
(1H, q, J = 7.2 Hz), 7.13 (2H, d, J = 8.0 Hz), 7.22 (2H, d, J = 8.0
Hz). IR (neat) 3095, 2932, 2855, 2725, 2637, 1707, 1514 cm−1
.
[α]D22 +43.0° (c 1.0, CHCl3).
Synthesis of (S)-2-(4-Butylphenyl)-1-(4-pentyloxyphenyl)
propane-1-one (4*): After (S)-2-(4-butylphenyl)propanoic acid
11* (0.187 g, 0.85 mmol) was stirred in thionyl chloride (2 mL)
for 1 h at 70 °C, the mixture was concentrated. To the remaining
mixture were added a solution of pentyloxybenzene (0.406 g, 2.47
Synthesis of (S)-1-(4-Butylphenyl)-2-chloro-1,1-dimethox-
ypropane (9*): To a methanol solution (5 mL) of 8* (0.610 g,
2.27 mmol) and trimethyl orthoformate (0.758 g, 7.14 mmol) was
added 5 drops of conc. sulfuric acid. The mixture was refluxed

2222 Bull. Chem. Soc. Jpn., 74, No. 11 (2001)
Synthesis of New Chiral Dopants
mmol) in carbon disulfide (4 mL) and anhydrous aluminium chlo-
ride (0.190 g, 1.42 mmol). The mixture was stirred for 3 h at room
temperature, hydrolyzed by ice-water (20 mL) and conc. hydro-
chloric acid (0.5 mL), and extracted with diethyl ether. The organ-
ic layer was dried with anhydrous sodium sulfate. The solvent
was evaporated under reduced pressure and the remaining mixture
was purified by column chromatography (silica gel: 31 g, 10%
ethyl acetate in hexane) to give a light-yellow liquid (0.291 g,
93%). 1H NMR (CDCl3) δ 0.89 (3H, t, J =7.3 Hz), 0.91 (3H, t, J
= 6.8 Hz), 1.31–1.39 (6H, m), 1.49 (3H, d, J = 6.8 Hz), 1.53–
1.56 (2H, m), 1,75–1.77 (2H, m), 2.53 (2H, t, J = 7.8 Hz), 3.95
(2H, t, J = 6.5 Hz), 4.61 (1H, q, J = 6.8 Hz), 6.83 (2H, d, J = 8.8
Hz), 7.08 (2H, d, J = 8.0 Hz), 7.18 (2H, d, J = 8.0 Hz), 7.93 (2H,
d, J = 8.8 Hz). IR (neat) 3051, 2956, 2870, 1674, 1601, 1509
cm−1. [α]D23 +33.0° (c 1.1, CHCl3).
Synthesis of (R)-2-(4-Butylphenyl)-1-(4-pentyloxyphenyl)
propane (5*): To a mixture of lithium aluminium hydride
(0.037 g, 0.98 mmol) and anhydrous aluminium chloride (0.156 g,
1.17 mmol) in dry diethyl ether (4 mL) was added a solution of 4*
(0.291 g, 0.79 mmol) in dry diethyl ether (2 mL). The reaction
mixture was refluxed for 0.5 h, hydrolyzed by 4 M hydrochloric
acid (20 mL), and extracted with diethyl ether. The organic layer
was dried with anhydrous sodium sulfate. The solvent was evapo-
rated under reduced pressure and the remaining mixture was puri-
fied by column chromatography (silica gel: 12 g, 10% ethyl ace-
tate in hexane) to give a colorless liquid (0.239 g, 85%). 1H NMR
(CDCl3) δ 0.92 (6H, t, J = 7.2 Hz), 1.91 (3H, d, J = 6.7 Hz),
1.34–1.43 (6H, m), 1.55–1.58 (2H, m), 1.73–1.80 (2H, m), 2.57
(3H, t, J = 7.8 Hz), 2.62–2.68 (1H, m), 2.85–2.95 (2H, m), 6.76
(2H, d, J = 8.4 Hz), 6.97 (2H, d, J = 8.4 Hz). IR (neat) 3008,
2956, 2870, 1613, 1511 cm−1. [α]D23 −46.3° (c 0.9, CHCl3).
Synthesis of (S)-1-[4-(trans-4-Butylcyclohexyl)phenyl]-2-
phenylpropane-1-one (6*): After (S)-2-phenylpropionic acid
(0.235 g, 1.56 mmol) was stirred in thionyl chloride (2 mL) for 1 h
at 70 °C, the mixture was concentrated. To the remaining mixture
were added a solution of trans-1-butyl-4-phenylcyclohexane
(0.512 g, 2.37 mmol) in dry dichloromethane (8 mL) and anhy-
drous aluminium chloride (0.319 g, 2.39 mmol). The mixture was
stirred for 5 h at room temperature, hydrolyzed by ice and 4 M hy-
drochloric acid (10 mL), and extracted with dichloromethane.
The organic layer was dried with anhydrous sodium sulfate. The
solvent was evaporated under reduced pressure and the remaining
mixture was purified by column chromatography (silica gel: 30 g,
10% ethyl acetate in hexane) to give a white solid (0.448 g, 82%).
The optical purity of 6* (94%ee) was determined by HPLC analy-
sis; carrier solvent, 5% 2-propanol/hexane; flow rate, 0.5 mL/min;
detection wavelength, 254 nm; retention time, (+)-form, 11 min,
CD: positive, (−)-form, 14 min, CD: negative. 1H NMR (CDCl3)
δ 0.90 (3H, t, J = 3 Hz), 1.00–1.15 (2H, m), 1.23–1.48 (9H, m),
1.51 (3H, d, J = 6.7 Hz), 1.80–1.90 (4H, m), 2.42–2.56 (1H, m),
4.67 (1H, q, J = 6.7 Hz), 7.18–7.33 (7H, m), 7.88 (2H, d, J = 8.3
Hz). IR (KBr) 3084, 3027, 2920, 2850, 1677, 1604, 1457 cm−1
.
[α]D24 +63.4° (c 0.6, CHCl3). Found: C, 85.15; H, 9.36%. Calcd
for C25H32O: C, 86.15; H, 9.25%.
Synthesis of (R)-1-[4-(trans-4-Butylcyclohexyl)phenyl]-2-
phenylpropane (7*): To a solution of 6* (0.106 g, 0.30 mmol)
in trifluoroacetic acid (2 mL) was added the triethylsilane (0.105
g, 0.90 mmol). The mixture was stirred for 4 h at room tempera-
ture, hydrolyzed by 3 M sodium hydroxide solution (10 mL), and
extracted with diethyl ether. The organic layer was dried with an-
hydrous sodium sulfate. The solvent was evaporated under re-
duced pressure and the remaining mixture was purified by thin-
layer chromatography (silica gel, hexane) to give a white solid
(0.080 g, 80%). 1H NMR (CDCl3) δ 0.89 (3H, t, J = 6.9 Hz),
0.95–1.08 (2H, m), 1.21 (3H, d, J = 6.6 Hz), 1.25–1.45 (9H, m),
1.80–1.93 (4H, m), 2.40–2.49 (1H, m), 2.65–2.75 (1H, m), 2.90–
3.08 (2H, m), 7.01 (2H, d, J = 8.0 Hz), 7.08 (2H, d, J = 8.0 Hz),
7.15–7.33 (5H, m). IR (KBr) 3084, 3028, 2923, 2844, 1602, 1452
cm−1. [α]D22 −37.6° (c 0.7, CHCl3).
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