A new way to access chiral liquid crystals: Organocatalyst-mediated synthesis of chiral rod-like liquid crystals

Article abstract of DOI:10.1080/15421406.2011.570138

In this article, an asymmetric organocatalytic way to prepare chiral liquid crystals from non-chiral starting materials was described. By using L-proline as the organocatalyst, several new chiral rod-like liquid crystals that are elusive with traditional methods were prepared. In addition, a series of novel enone-containing rod-like liquid crystals were also obtained as side-products. Mesomorphic properties of all new compounds were studied by Polarized Optical Microscope and Differential Scanning Calorimetry. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421406.2011.570138

Mol. Cryst. Liq. Cryst., Vol. 541: pp. 53=[291]–59=[297], 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.570138
A New Way to Access Chiral Liquid Crystals:
Organocatalyst-Mediated Synthesis of Chiral
Rod-Like Liquid Crystals
LI WANG, XIAO-JUN LIU, PING HUANG,
QING-PING GONG, YONG-HONG LI,
BI-QIN WANG, AND KE-QING ZHAO
Department of Chemistry and Materials Science, Sichuan Normal
University, Chengdu, Sichuan, China
In this article, an asymmetric organocatalytic way to prepare chiral liquid crystals
from non-chiral starting materials was described. By using L-proline as the organo-
catalyst, several new chiral rod-like liquid crystals that are elusive with traditional
methods were prepared. In addition, a series of novel enone-containing rod-like
liquid crystals were also obtained as side-products. Mesomorphic properties of all
new compounds were studied by Polarized Optical Microscope and Differential
Scanning Calorimetry.
Keywords Chiral rod-like liquid crystals; enone-containing mesogens; L-proline;
organocatalysis
1
. Introduction
Chiral liquid crystal (CLC) is an important family of liquid crystals. This is because
CLCs, compared to non-chiral mesogens, normally exhibit unconventional molecu-
lar stackings and mesomorphic properties [1]. Besides, CLCs could be applied in dis-
plays with ‘‘fast switching, high contrast and large viewing angles’’ [2] due to their
spontaneous polarization (P ) properties.
S
Traditionally, CLCs were prepared by ‘‘chiral-pool’’ [3] methods. However, the
scarcity, and hence high costs of chiral fragments in ‘‘chiral pool’’ largely restricted
the research and development of CLCs. As a result, a new method to prepare CLCs
from readily available, inexpensive starting materials is very indispensable.
The new method to prepare CLCs, as we expected, should satisfy several require-
ments: 1) non-chiral, therefore common and inexpensive starting materials; 2) highly
concise synthetic routes; and 3) the process should be easily handled and environmen-
tally friendly. On our way to search such a method, organocatalytic asymmetric
induction has drawn our attention [4]. Organocatalysts are usually stable, easily
accessible, cheap and non-toxic [4]. Besides, asymmetric organocatalysis has been
Address correspondence to Ke-Qing Zhao, Department of Chemistry and Materials
Science, Sichuan Normal University, Chengdu 610066, Sichuan, China. Tel.:
þ86-2884764743; Fax: þ86-2884764743; E-mail: kqzhao@sicnu.edu.cn
53=[291]

5
4=[292]
L. Wang et al.
studied intensively in recent decades and has already been applied to a wide range of
non-chiral starting materials [4]. Consequently, with all these merits and convenience
of asymmetric organocatalysis, we envisioned that, through this way, chirality could
be introduced into liquid crystals quite efficiently, which hence may largely expand
the knowledge of CLCs.
2
. Experimental Methods
As the starting stage of the research project, we decided to use the most common and
best studied organocatalytic reaction—L-proline-catalyzed asymmetric aldolization
between ketones and aldehydes [5]—to prepare a chiral rod-like liquid crystals 4
and enone-bearing rod-like liquid crystals 5ꢀ10. The synthetic routes are described
in Figures 1 and 2. Very noticeably, the L-proline-catalyzed aldol condensation reac-
tion provided us enones with (E)-configurations exclusively. Though the similar
stereoselective enone formations have been reported by Li et al. [6] aldol condensa-
tions of such bulky aldehydes in our cases have never been reported. Furthermore,
L-proline-catalyzed aldol addition of ketones and eletron-donating group substi-
tuted, highly bulky benzaldehyde derivatives was also reported first time in this arti-
ꢁ
cle even though the high reaction temperature (35 C) and high catalytic loading in
our method provided low enantioselectivity.
A general procedure for the L-proline-catalyzed aldolization. To a mixture of an
aldehyde substrate 3 or 11 (1.0 equiv.) in acetone (0.02 mol=L) added L-proline (1.0
ꢁ
equiv.). The reaction was then heated to 35 C, and was monitored by thin-layer
Figure 1. Synthetic routes for the L-proline-catalyzed asymmetric aldolization.
Figure 2. L-proline-catalyzed asymmetric aldolization of aldehydes 11.

Organocatalytic Synthesis of Chiral Liquid Crystals
55=[293]
chromatography. Upon completion, the reaction was concentrated, purified by col-
umn chromatography. The desired aldol products (4, 12ꢀ14)or enone compounds
(
5ꢀ10) were recrystallized from ethanol as white solids.
1
E)-4-(3-Oxobut-1-enyl)phenyl 4-butoxybenzoate, 5 (13% Yield, White Solid.). H
(
NMR (600 MHz, CDCl ) d8.14 (d, J ¼ 8.4 Hz, 2H), 7.61 (d, J ¼ 8.4 Hz, 2H), 7.53
3
(
d, J ¼ 16.2 Hz, 1H), 7.26 (d, J ¼ 8.4 Hz, 2H), 6.98 (d, J ¼ 9.0 Hz, 2H), 6.70 (d,
J ¼ 16.2 Hz, 1H), 4.06 (t, J ¼ 6.6 Hz, 2H), 2.40 (s, 3H), 1.86–1.78 (m, 2H),
1
.55–1.48 (m, 2H), 1.00 (t, J ¼ 7.2 Hz, 3H).
E)-4-(3-Oxobut-1-enyl)phenyl 4-heptoxybenzoate, 6. (28% Yield, White Solid.). H
1
(
NMR (600 MHz, CDCl ) d8.14 (d, J ¼ 9.0 Hz, 2H), 7.61 (d, J ¼ 8.4 Hz, 2H), 7.53 (d,
3
J ¼ 16.2 Hz, 1H), 7.26 (d, J ¼ 7.8 Hz, 2H), 6.98 (d, J ¼ 8.4 Hz, 2H), 6.70 (d,
J ¼ 16.2 Hz, 1H), 4.05 (t, J ¼ 6.6 Hz, 2H), 2.40 (s, 3H), 1.86–1.79 (m, 2H),
1
.50–1.44 (m, 2H),1.42–1.30 (m, 6H), 0.88 (t, J ¼ 7.2 Hz, 3H).
E)-4-(3-Oxobut-1-enyl)phenyl 4-nonyloxybenzoate, 7. (26% Yield, White
(
1
Solid.). H NMR (600 MHz, CDCl ) d8.14 (d, J ¼ 8.4 Hz, 2H), 7.61 (d,
3
J ¼ 8.4 Hz, 2H), 7.53 (d, J ¼ 16.2 Hz, 1H), 7.26 (d, J ¼ 8.4 Hz, 2H), 6.98 (d,
J ¼ 9.0 Hz, 2H), 6.70 (d, J ¼ 16.2 Hz, 1H), 4.05 (t, J ¼ 6.6 Hz, 2H), 2.40 (s, 3H),
1
.85–1.80 (m, 2H), 1.50–1.44 (m, 2H), 1.38–1.26 (m, 10H), 0.88 (t, J ¼ 6.6 Hz, 3H).
E)-4-(3-Oxobut-1-enyl)phenyl 4-undecyloxybenzoate, 8. (27% Yield, White
(
1
Solid.). H NMR (600 MHz, CDCl ) d8.13 (d, J ¼ 9.0 Hz, 2H), 7.61 (d,
3
J ¼ 8.4 Hz, 2H), 7.53 (d, J ¼ 16.2 Hz, 1H), 7.26 (d, J ¼ 8.4 Hz, 2H), 6.98 (d,
J ¼ 9.0 Hz, 2H), 6.70 (d, J ¼ 16.2 Hz, 1H), 4.05 (t, J ¼ 6.6 Hz, 2H), 2.40 (s, 3H),
1
.85–1.80 (m, 2H), 1.50–1.44 (m, 2H), 1.39–1.26 (m, 14H), 0.89 (t, J ¼ 6.6 Hz, 3H).
E)-4-(3-Oxobut-1-enyl)phenyl 4-dodecyloxybenzoate, 9. (20% Yield, White
(
1
Solid.). H NMR (600 MHz, CDCl ) d8.13 (d, J ¼ 9.0 Hz, 2H), 7.61 (d,
3
J ¼ 8.4 Hz, 2H), 7.53 (d, J ¼ 16.2 Hz, 1H), 7.26 (d, J ¼ 8.4 Hz, 2H), 6.98 (d,
J ¼ 9.0 Hz, 2H), 6.70 (d, J ¼ 16.2 Hz, 1H), 4.05 (t, J ¼ 6.6 Hz, 2H), 2.40 (s, 3H),
1
.85–1.80 (m, 2H), 1.50–1.45 (m, 2H), 1.38–1.22 (m, 16H), 0.88 (t, J ¼ 6.6 Hz, 3H).
E)-4-(3-Oxobut-1-enyl)phenyl 4-hexadecyloxybenzoate, 10. (22% Yield, White
(
1
Solid.). H NMR (600 MHz, CDCl ) d8.13 (d, J ¼ 9.0 Hz, 2H), 7.61 (d,
3
J ¼ 8.4 Hz, 2H), 7.53 (d, J ¼ 16.2 Hz, 1H), 7.26 (d, J ¼ 9.0 Hz, 2H), 6.98 (d,
J ¼ 9.0 Hz, 2H), 6.70 (d, J ¼ 16.2 Hz, 1H), 4.05 (t, J ¼ 6.6 Hz, 2H), 2.40 (s, 3H),
1
.85–1.79 (m, 2H), 1.51–1.44 (m, 2H), 1.39–1.22 (m, 24H), 0.88 (t, J ¼ 6.6 Hz, 3H).
-(1-Hydroxy-3-oxobutyl)phenyl 4-(heptyloxy)benzoate, 4. (20% Yield, White
4
1
Solid.). H NMR (600 MHz, CDCl ) d8.13 (d, J ¼ 9.0 Hz, 2H), 7.41 (d,
3
J ¼ 8.4 Hz, 2H), 7.19 (d, J ¼ 8.4 Hz, 2H), 6.97(d, J ¼ 8.4 Hz, 2H), 5.19 (dd, J ¼ 3.0,
9
2
1
.0 Hz, 1H), 4.04 (t, J ¼ 6.6 Hz, 2H), 3.01 (br, 1H), 2.90 (dd, J ¼ 9.0, 17.4 Hz, 1H),
.84 (dd, J ¼ 3.6, 18.0 Hz, 1H), 2.22 (s, 3H), 1.85–1.78 (m, 2H), 1.51–1.44 (m, 2H),
.40–1.27 (m, 6H), 0.90 (t, J ¼ 6.6 Hz, 3H).
0
4
-(1-Hydroxy-3-oxobutyl)phenyl 4 -(octyloxy)biphenyl-4-carboxylate, 12. (18%
1
Yield, White Solid.). H NMR (600 MHz, CDCl ) d8.23 (d, J ¼ 8.4 Hz, 2H), 7.69
3
(
d, J ¼ 8.4 Hz, 2H), 7.60 (d, J ¼ 8.4 Hz, 2H), 7.44 (d, J ¼ 8.4 Hz, 2H), 7.22 (d,
J ¼ 8.4 Hz, 2H), 7.01 (d, J ¼ 9.0 Hz, 2H), 5.21 (dd, J ¼ 3.0, 9.0 Hz, 1H), 4.02 (t,
J ¼ 6.0 Hz, 2H), 3.33 (br, 1H), 2.91 (dd, J ¼ 9.0, 17.4 Hz, 1H), 2.86 (dd, J ¼ 3.0,

5
6=[294]
L. Wang et al.
1
0
7.4 Hz, 1H), 2.22 (s, 3H), 1.85–1.78 (m, 2H), 1.51–1.44 (m, 2H), 1.40–1.24 (m, 8H),
.90 (t, J ¼ 6.6 Hz, 3H).
-(1-Hydroxy-3-oxobutyl)phenyl 4 -(undecyloxy)biphenyl-4-carboxylate, 13. (22%
0
4
1
Yield, White Solid.). H NMR (600 MHz, CDCl ) d8.23 (d, J ¼ 8.4 Hz, 2H), 7.69
3
(
d, J ¼ 7.8 Hz, 2H), 7.60 (d, J ¼ 8.4 Hz, 2H), 7.43 (d, J ¼ 7.8 Hz, 2H), 7.22 (d,
J ¼ 7.8 Hz, 2H), 7.01 (d, J ¼ 8.4 Hz, 2H), 5.21 (dd, J ¼ 6.6, 6.6 Hz, 1H), 4.02 (t,
J ¼ 6.0 Hz, 2H), 3.33 (br, 1H), 2.91 (dd, J ¼ 9.0, 17.4 Hz, 1H),2.86 (dd, J ¼ 2.4,
1
1
7.4 Hz, 1H), 2.23 (s, 3H), 1.84–1.78 (m, 2H), 1.51–1.45 (m, 2H), 1.39–1.24 (m,
4H), 0.89 (t, J ¼ 6.6 Hz, 3H).
-(1-Hydroxy-3-oxobutyl)phenyl 4 -(hexadecyloxy)biphenyl-4-carboxylate, 14. (19%
0
4
1
Yield, White Solid.). H NMR (600 MHz, CDCl ) d8.23 (d, J ¼ 8.4 Hz, 2H), 7.69
3
(
d, J ¼ 8.4 Hz, 2H), 7.59 (d, J ¼ 8.4 Hz, 2H), 7.44 (d, J ¼ 8.4 Hz, 2H), 7.22 (d,
J ¼ 8.4 Hz, 2H), 7.01 (d, J ¼ 9.0 Hz, 2H), 5.21 (dd, J ¼ 3.0, 8.4 Hz, 1H), 4.02 (t,
J ¼ 6.0 Hz, 2H), 3.35 (br, 1H), 2.91(dd, J ¼ 9.0, 17.4 Hz, 1H),2.86 (dd, J ¼ 3.6,
1
2
7.4 Hz, 1H),2.23 (s, 3H), 1.83–1.78 (m, 2H), 1.56–1.47 (m, 2H), 1.38–1.22 (m,
4H), 0.88 (t, J ¼ 6.6 Hz, 3H).
3
. Results and Discussion
3
.1. Mesomorphic Studies of the Chiral Compound 4 and Enone-containing Rod-
Like Liquid Crystals (5ꢀ10)
POM and DSC investigations of the chiral compound 4 did not show any meso-
morphic properties, even though its starting aldehyde was reported to exhibit liquid
crystal behaviors [8]. This was not surprising, since the introduction of chirality
would cause uneven molecular plane, and hence, not-well-organized molecular
stackings, which is harmful to liquid crystal phases formation [1].
In sharp contrast, however, the new enone-containing compounds (5ꢀ10)
showed excellent liquid crystal properties (phase transition temperatures were
included in Table 1). These enone-containing liquid crystals (ELCs) exhibited higher
melting points and wider mesomorphic ranges than their aldehyde precursors [8].
This might be due to the strengthened intermolecular p-p interactions, and hence
elevated level of molecular stackings that were brought by enone functionalities.
Table 1. Thermotropic properties of the ELCs
CH COCH=CHC H OOCC H OC H2n þ 1, [n ¼ 4, 7, 9, 11, 12, 16]
3
6
4
6
4
n
Mesophases and transition temperatures and enthalpy changes
ꢁ
ꢁ
Compd.
n
Second heating= C (DH, KJ=mol)
First cooling= C (DH, KJ=mol)
5
6
7
8
9
4
K119(84.5)N139(1.3)I
K112(62.2)S 125(2.9)N133(1.34)I
I139(2.0)N93(75.8)K
I132(1.5)N125(2.6)S 75(51.8)K
7
9
A
A
K57(17.4)S 108(68.1)S 134(9.5)I
B
I134(9.7)S 88(62.7)S 38(20.6)K
A
A
B
11
12
16
K52(34.0)S 106(64.3)S 136(10.5)I
B
I136(10.1)S 74(54.9)S 44(33.1)K
A B
A
K54(42.4)S 104(66.8)S 136(10.6)I
B
I136(10.8)S 80(62.7)S 47(41.9)K
A B
A
1
0
K76(49.6)S 102(67.3)S 134(12.0)I
A
I135(11.9)S 87(68.3)S 70(46.9)K
A B
B
K: Crystal phase; S: Smectic phase; N: Nematic phase; I: Isotropic phase.

Organocatalytic Synthesis of Chiral Liquid Crystals
Table 2. Thermotropic properties of CLCs
57=[295]
CH COCH CHOHC H OOCC H C H OC H2m þ 1, [n ¼ 8, 11, 16]
3
2
6
4
6
4
6
4
m
Mesophases and transition temperatures
ꢁ
ꢁ
Compd.
n
Second heating= C (DH, KJ=mol)
First cooling= C (DH, KJ=mol)
ꢂ
ꢂ
12
13
14
8 K79(36.0)S 205(1.1)S (1.8)223I
11 K128(98.8)S209(13.0)I
I223(1.8)SC 206(1.0)S 76(32.44)K
A
A
C
I215(8.9)S81(78.1)K
ꢂ
ꢂ
16 K123(95.3)SC 208(12.7)I
I213(9.9)SC 91(84.3)K
Cry: Crystal phase; S: Smectic Phase; N: Nematic Phase; I: Isotropic phase.
Besides, by varying the length of the terminal alkyl chain, the ELCs’ melting points
and the width of mesomorphic ranges were first increased (n ¼ 4ꢀ12), then started to
fall (n ¼ 12 and 16), which is typical to most rod-like liquid crystals.
3
.2. Revision of Our Design and Mesomorphic Studies of Chiral Rod-Like Liquid
Crystals (12ꢀ14)
The absence of liquid crystal properties of 4 could be ascribed to its presumed
uneven molecular plane, and hence disordered molecular stackings [1]. This hypoth-
esis provided us a clue that other functional groups which were able to enhance and
further stabilize molecular stackings should be added into 4.
After deliberate considerations, we decided to introduce one more phenyl moiety
into the chiral molecule 4, and then three bisphenyl-derived benzaldehydes 11 were
synthesized (Fig. 2), which were then utilized for the preparation of chiral aldol
products 12ꢀ14.
Very gratifyingly, our revised design has been successful: POM and DSC studies
showed that all new chiral compounds 12ꢀ14 beared with excellent mesomorphic
Figure 3. DSC traces of the CLCs.

5
8=[296]
L. Wang et al.
properties. Besides, with the lengthening of the terminal alkyl chain, these CLCs
showed higher melting points but narrower mesomorphic ranges. Phase transition
temperatures and enthalpy changes of the CLCs were generalized in Table 2; their
DSC traces were shown in Figure 3; and selected POM photographs were shown
in Figure 4. It should be noted that the two peaks observed during the cooling pro-
cess of 12 (n ¼ 8, Fig. 3) could be associated with a monotropic phase transition and
a crystallization, respectively. Vice versa, the two peaks observed during the heating
process of 13 (n ¼ 11, Fig. 3) could be explained as a crystal to S phase transition
A
followed by a monotropic phase transition. Moreover, it is very noteworthy to men-
tion that the newly prepared CLCs are elusive with traditional ‘‘chiral-pool’’ meth-
ods. Most importantly, this new methodology can generate CLCs that have a
substituent with high polarity (e.g., hydroxy) on the stereogenic center. This is of
interest, because the dipole moment of 12 and 14 (Table 2) may result into high spon-
ꢂ
taneous polarization (Ps) as presumed SC phases were observed (Fig. 4) [1].
ꢁ
Figure 4. Selected POM photographs of the CLCs. (a) POM graph of 13 at 148 C; (b) POM
ꢁ
graph of 14 at 157 C. (Figure appears in color online.)

Organocatalytic Synthesis of Chiral Liquid Crystals
. Conclusions and Perspectives
59=[297]
4
In summary, we have successfully developed an unprecedented, organocatalytic
method to synthesize chiral rod-like liquid crystals, starting from inexpensive, readily
available and environmentally benign materials. Besides, a series of novel
enone-containing rod-like mesogens were also prepared using this method. Improve-
ments of the methodology and preparations of other categories of chiral liquid crys-
tals are currently under investigations. Our future work will be focused on the study
of spontaneous polarization properties of the newly synthesized chiral liquid crystals.
Furthermore, our ultimate goal is to synthesize chiral liquid crystals from non-chiral
and non-mesogenic starting materials.
Acknowledgment
We would like to thank the National Natural Science Foundation of China (No.
2
0872104 and No. 50973076) for generous financial support.
References
[
[
1] Demus, D., et al. (1998). In: Handbook of Liquid Crystals, Gooby, J. W., Gray, G. W., &
Spiess, H. W. (Eds.), Wiley-VCH: Weinheim, Chapter 5, 115.
2] (a) Dierking, I. (2003). Textures of Liquid Crystals, Wiley-VCH: Weinheim, Chapter 7,
1
00; (b) Meyer, R. B., Li e´ bert, L., Strzelecki, L., & Keller, P. (1975). J. Phys. Lett., 36,
L69; (c) Durand, G., & Martinot-Lagarde, Ph. (1980). Ferroelectrics, 24, 89.
3] For a detailed explanation of ‘‘chiral-pool’’ method: Clayden, J., et al. (2001). Organic
Chemistry, Oxford University Press: New York, Chapter 45, 1222.
4] For a recent review of organocatalysis in asymmetric synthesis: (a) Pellisier, H. (2007).
Tetrahedron, 63, 9267; (b) Hayashi, Y. (2005). J. Syn. Org. Chem. Jpn., 63, 464; (c) Seayad,
J., & List, B. (2005). Org. Biomol. Chem., 3, 719.
[
[
[
5] For examples of L-proline-catalyzed aldolization between ketones, and aldehydes see: List,
B., Lerner, R. A., & Barbas, C. F. (2000). J. Am. Chem. Soc., 122, 2395; List, B. (2002).
Tetrahedron, 58, 5573.
[
[
6] Wang, J.-F., Lei, M., Li, Q., Ge, Z.-M., & Li, R.-T. (2009). Tetrahedron, 65, 4826.
7] (a) Alkylation of ethyl-4-hydroxybenzoate then hydrolysis was performed following the
procedure reported by Shoichi, K., Hiroyuki, M., Machiko, F., Shigeharu, N., Koichi,
S., & Kazuya, S. (2008). Chem. Mater., 20, 3675; (b) Esterification of 4-alkoxybenzoic
acid, and 4-hydroxybenzaldehyde was performed following the procedure reported by
Christina, K., Amaranatha, R. R., Anton, H., Ute, B., & Carsten, T. (2006). J. Am. Chem.
Soc., 128, 3051.
[8] Malthete, J., Canceill, J., Gabard, J., & Jacques, J. (1981). Tetrahedron, 37, 2815.

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