Chapter 16
Synthetic Aspects of Fluorine-Containing Chiral Liquid Crystals 1
2
2
Tamejiro Hiyama , Tetsuo Kusumoto , and Hiroshi Matsutani 1
Department of Material Chemistry, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan Sagami Chemical Research Center, 4-4-1 Nishiohnuma, Sagamihara, Kanagawa 229-0012, Japan
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Synthesis of fluorine-containing chiral molecules useful as a part or a whole of liquid crystalline materials is described with the emphasis on (1) asymmetric hemiacetal synthesis of trifluoroacetaldehyde using a BINOL-titanium catalyst, (2) first stereospecific carbon-carbon bond formation through a nucleophilic substitution at the chiral acetal carbon bearing a trifluoromethyl group using organoaluminium reagents, (3) synthesis of diastereomeric dichiral liquid crystals containing two fluorine atoms at two chiral centers, and (4) unusual transition behaviour of the dichiral liquid crystals.
Since the display using liquid crystals (LCs) is characterized by the features of light weight, space- and energy-saving, low-voltage-drive, and lack of flickering, the L C display is now commonly used for lap-top computers and portable T V and will be used in the near future for most of the office automation (OA) display and the home TV. Although the display using a thin-layer-film transistor (TFT) has been put on the market at present, the ones of large size are less accessible and extremely expensive. In view that high definition T V is growing to be common, the L C display of larger size with high quality and high capacity is yet to be developed. The promising materials are ferroelectric or antiferroelectric liquid crystals (FLCs or AFLCs) which consist of chiral molecules and show chiral smectic C (SmC*) phase or chiral smectic C A (SmCA*) phase. Typical L C molecules that exhibit SmC* and/or SmCA* phase are shown in Figure 1. As readily seen, the L C compounds consist of optically active 1,1,1trifluoro-2-alkanols which contribute to the stabilization of the SmCA phase and thus play key roles of these L C materials. Accordingly methods for the l,l,l-trifluoro-2alkanols have been the target of organic synthesis. Conventional retrosynthetic analysis of the trifluoro alcohols leads to (1) resolution of l,l,l-trifluoro-2-alkanols by chemical or enzymatic methods, (2) asymmetric reduction of l,l,l-trifluoro-2alkanones, or (3) asymmetric carbonyl addition of carbonaceous nucleophiles. Indeed, at present, the enzymatic resolution through the hydrolysis of the carboxylates of l,l,l-trifluoro-2-alkanols prevails. Other methods remain yet to be studied.
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© 2000 American Chemical Society
Ramachandran; Asymmetric Fluoroorganic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
227
, C
8
H
i
0 { > { } ^ ^ 0 ^
ï
Cr 72.5 S m C * 117 SmC* 121.5 SmA 149.6 Iso A
Cr 32 S m C * 97 SmA 104 Iso A
«
g
4
%
e e
Use of Aluminate Reagents Derived from 1-Alkenes. The nucleophilic substitution was achieved using lithium tetraalkylaluminates produced by the T i catalyzed hydroalumination reaction of 1-alkenes with L1AIH4 (5). The whole transformation is illustrated in Scheme 7, and the results are summarized in Table 6. Because the hydroalumination is site-selective, optically active phenyl-substituted or olefinic trifluoro alcohols are now readily available. Synthesis of Dichiral Liquid Crystals Containing Two Fluorines at Two Chiral Centers During the synthetic research of F L C materials for fast switching, we prepared 2-[4[(/?)-2-fluorohexyloxy]phenyl]-5-[4-(S)-2-fluoro-2-methyldecanoyloxy]phenyl]pyrimidine shown in Figure 2 and its diastereomers and homologs.
CH =CHR 2
3
f + Ph^O^SoTs
+
L1AIH4
CP2TiCl catalyst ether 2
LiAl(CH CH R) 2
2
4
b
inversion^ ether, 45 °C, 2.5 h
ÇF3 P h ^ o ^ ^ ^ R
Scheme 7. Alkylation of Hemiacetal Tosylates with Aluminates Derived from 1-Alkenes
Ramachandran; Asymmetric Fluoroorganic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
235
Table 6. Alkylation with the Aluminates Derived from 1-Alkenes Tosylate (Abs. Config., % ee)
Cr
«
_ t-Hj-CHK
Γ
SmX
Η
r
«
H
R
Product ( bs. Config., % yield, % ee) A
IsoX
IsoLiq
Figure 2. A Dichiral Liquid Crystalline Compound and Phase Transtion Temperatures
Ramachandran; Asymmetric Fluoroorganic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
236 The phenolic part was prepared according to the route shown in Scheme 8; the carboxylic moiety by the sequence of reactions shown in Scheme 9. Ee of the chiral center of each substrate was determined to be over 98 % by H P L C . The two parts were combined by conventional esterification.
ό
^
_
r ^ J^JJ
C
N
n-Pr CuMgBr 2
XT NH»HC1
OH
MeONa, MeOH
7
9
%
F
98% ee
Scheme 8. Synthesis of a Phenolic Part
Scheme 9. Synthesis of a Carboxylic Acid Part
Ramachandran; Asymmetric Fluoroorganic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
237 Unusual Endothermic Transition The ester in Figure 2 showed an endothermic transition from SmC* to IsoX, where the apparently optically isotropic phase is denoted as IsoX. Unusual behaviors of IsoX are following: (1) A l l properties suggest isotropic liquid except for X-ray analysis which indicates layer ordering. (2) The sequence of the phase transitions on cooling differs from those on heating. (3) Phase transition of SmC* -» IsoX taking place on cooling is endothermic! In contrast, the diastereomer of the compound in Figure 2 showed a direct exothermic transition from the isotropic liquid to the IsoX phase. We have studied the transition behavior and the structure of the IsoX phase by means of differential scanning calorimetry (DSC), optical microscopy, solid-state N M R and X-ray diffraction measurements. On cooling the compound, the Iso Liq phase changed at 117 °C into an SmC* phase, which slipped into an optically isotropic phase (IsoX) with a sharp endothermic peak at 107 °C. The transition was observed by optical microscopy to occur immediately. The texture of the IsoX phase was apparently different from that of a homeotropic smectic A (SmA) or SmC phase. On further cooling, an unidentified ferroelectric smectic modification (SmX) appeared gradually from the IsoX phase with a broad exothermic peak. Spontaneous polarization in the SmC* and SmX phases were -249 nC/cm at 115 °C and -341 nC/cm at 95 °C, respectively; spontaneous polarization and switching behavior were not observed in the IsoX phase. Viscosity of the IsoX phase was higher than that of the SmC* and SmX phases. A marked supercooling was observed for the appearance of the IsoX phase. Changing the cooling rate from 10 K/min to 0.1 K/min reproduced the endothermic transition behavior. Although the endothermic transition enthalpy can be explained in terms of a transition of a metastable SmC* phase to a stable IsoX phase, such a well reproducible endothermic transition is unprecedented in thermotropic liquid crystals (61. 2
2
1 3
Structural Elucidation of IsoX Phase. Spectral analysis by C N M R and X-ray diffraction measurements suggests that upon the SmC* -» IsoX transition, an interlayer correlation may be broken, whereas dimerization or tetramerization via a stereospecific intermolecular interaction through fluorines at chiral centers within each layer is assumed to be operating. The dimerization or tetramerization reduces the anisotropy of molecular motion to increase rotation around the short axis. This change in molecular dynamics may release entropy of the system to induce the endothermic transition. The IsoX phase is not a result of the competition between helical structure and mesophase ordering but a result of the chirality-dependent stereospecific interaction or chiral molecular recognition (7.
Acknowledgments The authors sincerely thank co-workers listed in the following references for their sincere devotion.
Ramachandran; Asymmetric Fluoroorganic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
238 References 1. Poras, H.; Matsutani, H.; Yaruva, J.; Kusumoto, T.; Hiyama, T. Chem. Lett. 1998, 665. 2. Matsutani, H.; Ichikawa, S.; Yaruva, J.; Kusumoto, T.; Hiyama, T. J. Am. Chem. Soc. 1997, 119, 4541. 3. Matsutani, H.; Poras, H.; Kusumoto, T.; Hiyama, T. Chem. Commun. 1998, 1259. 4. Yonezawa, T.; Sakamoto, Y.; Nogawa, K.; Yamazaki, T.; Kitazume, T. Chem. Lett., 1996, 855. 5. Matsutani, H.; Poras, H.; Kusumoto, T.; Hiyama, T. Synlett, in press. 6. Yoshizawa, Α.; Umezawa, J.; Ise, N . ; Sato, R.; Soeda, Y . ; Kusumoto, T.; Sato, K.; Hiyama, T.; Takanishi, Y.; Takezoe, H. Jpn. J. Appl. Phys., 1998, 37, L942. 7. Kusumoto, T.; Sato, K . ; Katoh, M.; Matsutani, H . ; Yoshizawa, Α.; Ise, N.; Umezawa, J.; Takanishi, Y.; Takezoe, H.; Hiyama, T. Mol. Cryst. Liq. Cryst., in press.
Ramachandran; Asymmetric Fluoroorganic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
