Facile synthesis of novel monodisperse liquid-crystalline oligomers based on Michael adducts from millipede cyanoacetates

Article abstract of DOI:10.1246/cl.2007.452

Facile synthesis of novel monodisperse liquid-crystalline (LC) oligomers was performed. The functionalization of the end group of 4 (and 6) millipede cyanoacetates with 8 (and 12) mesogenic units by Michael addition reaction produced a monodisperse LC oct

Full text of DOI:10.1246/cl.2007.452

452
Chemistry Letters Vol.36, No.3 (2007)
Facile Synthesis of Novel Monodisperse Liquid-crystalline Oligomers
Based on Michael Adducts from Millipede Cyanoacetates
Sadahiro Nakanishiꢀ1;2 and Mitsuru Ueda3
1Japan Chemical Innovation Institute, Department of Organic and Polymeric Materials,
Graduate School of Science and Engineering, Tokyo Institute of Technology,
2-12-1-H120 O-okayama, Meguro-ku, Tokyo 152-8552
2Core Technology Center, Nitto Denko Corp., 1-1-2 Shimohozumi, Ibaraki, Osaka 567-8680
3Department of Organic and Polymeric Materials, Graduate School of Science and Engineering,
Tokyo Institute of Technology, 2-12-1-H120 O-okayama, Meguro-ku, Tokyo 152-8552
(Received December 8, 2006; CL-061444; E-mail: snakanishi@polymer.titech.ac.jp)
Facile synthesis of novel monodisperse liquid-crystalline
pentaerythritol with excess cyanoacetic acid in the presence
(LC) oligomers was performed. The functionalization of the
end group of 4 (and 6) millipede cyanoacetates with 8 (and
12) mesogenic units by Michael addition reaction produced
a monodisperse LC octamer (and dodecamer) in high yields. Dif-
ferential scanning calorimetry and optical polarizing microscopy
revealed LC properties of the synthesized LC monodisperse
oligomers. The monodisperse LC oligomers were morphologi-
cally stable glass-forming LC materials and exhibited a large
variation in nematic phase temperatures in the range of approx-
imately 80 to 300 ꢁC.
of p-toluenesulfonic acid monohydrate (Scheme 1). Further,
tetra-cyanoacetate 1 was coupled with mesogenic acrylate 2,
which is responsible for LC property, by the Michael addition.
The Michael addition reaction of 1 equiv. of tetra-cyanoacetate
1 and a slightly excess amount (8.1 equiv.) of mesogenic acrylate
29 using an organic base catalyst 1,8-diazabicyclo[5.4.0]undec-
7-ene (DBU)10 proceeded smoothly at 50 ꢁC in 30 min. Repreci-
pitation was carried out twice in methanol and it produced the
Michael adduct LC octamer 3 as a white powder with a high
yield (93%). Although some decomposed products were pro-
duced in this basic reaction, they were removed along with the
unconverted acrylate by reprecipitation. Further purifications
such as column chromatography were not required. The synthet-
ic procedures are described in detail in Supporting Information.
The formation of LC octamer 3, which was the desired prod-
uct, was confirmed by 1H NMR (Figure 1), 13C NMR, elemental
analysis, and matrix-assister laser desorption ionization with
time of flight mass spectrometer (MALDI-TOF-MS). The
1H NMR spectrum of LC octamer 3 (Figure 1) showed the
characteristic peaks of two methylene protons (indicated as b
and c) derived from the acryl moiety for ꢀ values in the range
of 3.0–2.3. Because no peaks corresponding to the methylene
(NCCH2CO2-) or methyn (NCCHRCO2-) protons were ob-
served, the methylene carbons (NCCH2CO2-) of the substrate
cyanoacetate were completely converted into the quaternary
carbon by the addition of the two acrylates. Thus, the octa-
adduct was obtained from four-pedal alcohol through four-pedal
cyanoacetate.
Over the past decade, dendrimer synthesis, which is one of
the core technologies in macromolecular chemistry, has under-
gone intense development.1 Many reactive end groups that exist
at the periphery of dendrimers easily react with various function-
alities. The synthesis and characterization of liquid crystals
with multiple mesogenic moieties in the periphery, such as
cyclosiloxane-based liquid-crystalline (LC) materials2 and LC
dendrimers3 have attracted considerable attention because of
their similarity to side chain LC polymers.4 Goodby and Saez
have reported the synthesis of millipede oligomers in which
mesogens (cyanobiphenyl and phenyl benzoate types) are
connected to the pentaerythritol base core by ester bonds.5 It is
reported that LC materials in which mesogenic units are intro-
duced via alkyl chains show thermotropic nematic, and smectic
LC phases. Their low viscosity derived from their structural low
chain entanglement is advantageous for fast switching optical
materials.6 However, the preparation of high-quality monodis-
perse LC materials is in a tedious process on a practical scale.
This is because the reaction conditions require efficient removal
of water and/or oxygen and tedious purification process. Ueda et
al. have reported the preparation of poly(propyleneimine)-base
LC dendrimers7 by the Michael addition reaction of poly(propyl-
eneimine) with mesogenic acrylates that are known as the pre-
cursors for LC polyacrylates. However, these amine-containing
compounds are not desirable because of their color tends to
change due to oxidation etc.
NC
O
O
O
O
O
O
O
O
CN
O
NC
OH
HO
HO
OH
OH
p-TsOH, toluene
NC
CN
O
1 (92%)
O
O
O
DBU, NMP
O
CN
2
NC
O
O
O
O
CN
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
In this study, we perform facile synthesis of monodisperse
LC oligomers by the Michael reaction8 of millipede cyanoace-
tates with mesogenic acrylate 2. We utilize the mesogenic
acrylate 2 as a Michael acceptor, and not as the precursor of
polyacrylates, whose synthetic process has been established,
and millipede cyanoacetates as a Michael donor.
O
NC
NC
O
CN
CN
NC
NC
CN
CN
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
NC
CN
3 (93%)
The millipede cyanoacetate 1 was synthesized by reacting
Scheme 1. Synthesis of LC octamer 3.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.3 (2007)
453
f
g
O
O
O
O
d
a
b
h
i
j
k
O
j
C
O
O
i, k
5 (g 79 N 305 I )
3 (g 78 N 299 I )
CN
c
e
C
N
2
h
4
3
g
f
e
d
LC oligomer
(g 56 N 271 I )
a
b, c
O
O
O
O
O
CN
n
8
7
6
5
4
3
2
0
50
100
150
200
250
300
350
Figure 1. 1H NMR spectrum of LC octamer 3.
Temperature (°C)
Figure 2. DSC thermograms of LC octamer 3, dodecamer 5,
and oligomer (Mw ¼ 4500, Mw=Mn ¼ 1:26).
The methodology for obtaining multi-adducts in which the
number of mesogenic moieties is twice the number of hydroxy
groups of millipede alcohol through millipede cyanoacetate is
further applicable to the synthesis of LC dodecamer 5 as in the
case of the synthesis of LC octamer 3. Six-pedal dipentaerythri-
tol was selected as the precursor of the LC dodecamer. Dipen-
taerythritol was esterized with cyanoacetic acid to obtain six-
pedal cyanoacetate 4 in 90% yield (Scheme 2). The Michael re-
action of 4 (1 equiv.) and mesogenic acrylate 2 (12.5 equiv.) was
carried out using DBU catalyst at 50 ꢁC for 3 h and reprecipita-
tion was performed twice in methanol. LC dodecamer 5 was ob-
tained in 90% yield and it was characterized by NMR, elemental
analysis, and MALDI-TOF-MS (see Supporting Information).
LC octamer 3 and dodecamer 5 were soluble in THF,
cyclohexanone, dichloromethane, and amide solvents such as
1-methyl-2-pyrrolidinone (NMP) and N,N-dimethylacetamide
(DMAc). The transition temperatures of these monodisperse
LC oligomers were compared with those of the LC oligomer
(Mn ¼ 4500, Mw=Mn ¼ 1:26, polystyrene as standard) that
was obtained by the conventional radical polymerization of
mesogenic acrylate 2 with an AIBN initiator (see Supporting
Information). The transition temperatures of the LC materials
were determined by DSC analysis (Seiko Instruments Inc.
DSC6200) and the thermal behavior of the texture was observed
using a polarizing optical microscope (Olympus BH2 polarizing
microscope with a Mettler Toledo FP82 hot stage). Figure 2
shows the DSC thermograms of 3, 5, and the LC oligomer for
the second heating scan. The scans exhibit no crystallization,
thereby indicating the morphological stability of these glass-
forming LC materials. The nematic phase was assigned by
observing schlieren textures. The glass-transition temperature
(Tg) of LC octamer 3 (78 ꢁC) and dodecamer 5 (79 ꢁC) were
higher than those of the LC oligomer (56 ꢁC). In contrast to a
broad peak that appeared as a nematic to isotropic transition
(Tc) for the LC oligomer (271 ꢁC), sharp peaks were observed
in LC octamer 3 (299 ꢁC) and dodecamer 5 (305 ꢁC).
In summary, the monodisperse LC oligomers were synthe-
sized by the Michael addition reaction of millipede cyanoace-
tates (1 and 4) and mesogenic acrylate 2. These oligomers are
morphologically stable and have a wide nematic temperature
range. This method used in this study will serve as an efficient
technique for producing thin film optical components.
This work was supported by the New Energy and Develop-
ment Organization (NEDO) through a grant for ‘‘Project on
Nanostructured Polymeric Materials’’ under the Nanotechnology
Program.
References and Notes
1
J. M. Frechet, Science 1994, 263, 1710; D. A. Tomalia,
P. R. Dvornic, Nature 1994, 372, 617.
2
F. H. Kreuzer, D. Andrejewski, W. Haas, N. Haberle,
G. Riepl, P. Spes, Mol. Cryst. Liq. Cryst. 1991, 199, 345;
K. D. Gresham, C. M. McHugh, T. J. Bunning, R. J. Crane,
H. E. Klei, E. T. Samulski, J. Polym. Sci., Part A: Polym.
Chem. 1994, 32, 2039.
3
4
V. Percec, M. Holerca, Biomacromol. 2000, 1, 6; V. Percec,
W. D. Cho, G. Ungar, J. Am. Chem. Soc. 2000, 122, 10273;
T. Kato, Science 2002, 295, 2414; S. Ponomarenko, N.
Boiko, V. Shibaev, Polym. Sci., Ser. C 2001, 43, 1; I. M.
Saez, J. W. Goodby, R. M. Richardson, Chem.—Eur. J.
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V. P. Shibaev, S. G. Kosromin, N. A. Plate, Eur. Polym. J.
1982, 18, 651.
5
6
I. M. Saez, J. W. Goodby, Liq. Cryst. Today 2004, 13, 1.
M. Irie, Chem. Rev. 2000, 100, 1685; S. H. Chen, H. M. P.
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Adv. Mater. 2003, 15, 1061.
O
O
O
NC
O
CN
O
NC
HO
HO
OH
OH
OH
O
O
O
O
OH
NC
CN
O
O
O
O
O
O
p-TsOH, toluene
7
8
9
K. Yonetake, T. Masuko, T. Morishita, K. Suzuki, M. Ueda,
R. Nagahata, Macromolecules 1999, 32, 6578.
M. E. Jung, Comprehensive Organic Synthesis, Pergamon,
Oxford, 1991, Vol. 4, p. 1.
Synthetic method and characterization of the mesogenic
acrylate used in this Michael reaction are described in the
Supporting Information.
OH
NC
CN
4 (90%)
2
DBU, NMP
O
O
O
O
O
O
C
O
O
CN
C
N
2
3
2
5 (90%)
10 W. Ye, J. Xu, C. Tan, C. Tan, Tetrahedron Lett. 2005, 46,
6875.
Scheme 2. Synthesis of LC dodecamer 5.
Published on the web (Advance View) February 17, 2007; doi:10.1246/cl.2007.452