Synthesis and mesomorphic properties of cholesteryl p-polyfluoroalkoxy-m-nitrobenzoate

Article abstract of DOI:10.1016/S0022-1139(01)00386-4

Homologous cholesteryl p-polyfluoroalkoxy-m-nitrobenzoate were synthesized and their phase transition behaviours were studied. Due to the activation of chlorine atom in aromatic ring by introducing o-nitro and p-trifluoromethyl groups, the fluoroalkoxylat

Full text of DOI:10.1016/S0022-1139(01)00386-4

Journal of Fluorine Chemistry 109 42001) 205±208
Synthesis and mesomorphic properties of cholesteryl
p-poly¯uoroalkoxy-m-nitrobenzoate
Kan Wang, Hengfeng Li, Jianxun Wen*
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Fenglin Road 354, Shanghai 200032, PR China
Received 1 July 2000; received in revised form 6 March 2001; accepted 21 March 2001
Abstract
Homologous cholesteryl p-poly¯uoroalkoxy-m-nitrobenzoate were synthesized and their phase transition behaviours were studied. Due
to the activation of chlorine atom in aromatic ring by introducing o-nitro and p-tri¯uoromethyl groups, the ¯uoroalkoxylation can directly
be processed using potassium carbonate as base. # 2001 Elsevier Science B.V. All rights reserved.
Keywords: Fluoroalkylation; Liquid crystals
1. Introduction
heteroaryl halides [8±10]. Tri¯uoromethyl and nitro groups
were considered as the activating substituents in aromatics
and tri¯uromethyl benzene can be converted into benzoic
acid easily. The direct aromatic nucleophilic ¯uoroalkox-
ylation can be carried out under the condition of weak base
such as potassium carbonate. Strong base such as sodium
hydride or potassium hydroxide is not needed.
In this paper, we describe a convenient method to prepare
cholesteryl p-poly¯uoroxy-m-nitrobenzoic acid using com-
mercially available reagents and discuss the mesomorphic
properties of these liquid crystals.
Owing to their excellent switching characteristics, ferro-
electric liquid crystals have been intensely studied in
recent years. These ferroelectric liquid crystalline 4FLC)
compounds consist of one or more chiral centres. Steriod,
as we know, is a cheap chiral resource. Vill et al. reported
that steroidal liquid crystals containing a long alkyl chain,
e.g. cholesteryl p-hexdecyl benzoate and cholesteryl p-
hexdecycloxyphenyl carbonate display a monotropic
ferroelectric phase [1]. We also know liquid crystals
containing highly ¯uorinated alkyl or alkoxy chain as
terminal groups are intended to form smectic C phase
and increase its temperature range [2±4]. So cholesteryl
p-per¯uoroalkyl benzoate and cholesteryl p-per¯uoroalkyl
phenyl carbonate were synthesized by our group [5], but
no ferroelectric phase was observed. Janulis et al. have
shown the in¯uence of 4CH2)n as a spacer between the
¯uorinated tail and the rigid core on the nature of the
mesomorphic phase notably for obtaining a smectic C
phase [6]. So the synthesis of cholesteryl p-poly¯uoroalk-
oxy benzoate was desired.
2. Results and discussion
In general, it is dif®cult to obtain 1H,1H-per¯uoroalk-
oxybenzene by the direct etheri®cation of the alcohols due
to the activation of chlorine atom in aromatic ring. The
¯uoroalkoxylation can easily be carried out by introducing
o-nitro and p-tri¯uoromethyl groups, using potassium car-
bonate as base.
Transition temperatures of liquid crystals were measured
by DSC and phase identi®cation was made by comparing the
observed textures with those in the literature [11,12]. The
results were summarized in Table 1.
From the observation of the mesomorphic properties of
these compounds, some interesting results have been
obtained. First, the clearing points were increased with
increasing length of ¯uorocarbon chains, but the melting
points were decreased with increasing length of ¯uoro-
carbon carbons. As a result of the two opposite trends,
There are various methods which have been developed to
synthesize ¯uoroalkyl aryl ethers [7]. Direct aromatic
nucleophilic ¯uoroalkoxylation was used in some methods.
These reactions can be carried out for activated aryl and
* Corresponding author. Tel.: ‡86-21-641-63300;
fax: ‡86-21-641-66128.
E-mail address: wenjianxun@pub.sioc.ac.cn 4J. Wen).
0022-1139/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 4 0 1 ) 0 0 3 8 6 - 4

206
K. Wang et al. / Journal of Fluorine Chemistry 109 -2001) 205±208
Table 1
The transition temperatures of the synthesized compoundsa
3. Experimental details
3.1. General techniques
Compounds
n
Transition temperature 48C)
4a
4b
4c
2
4
8
Cr 179.0 Ch 183.8 I 179.9 Ch 121.9 Recr
Cr 178.0 Ch 184.4 I 176.5 Ch 155.8 SmA 137.3 Recr
Cr 164.3 SmA 209.0 I 194.1 SmA 131.9 Recr
The structures of the ®nal products and intermediates
were elucidated by a variety of spectral methods. IR spectra
were recorded on a PE-983G spectrophotometer using solid
a Cr: crystal; Ch: cholesteric phase; SmA: smectic A phase; I: isotropic
liquid; Recr: recrystallization.
1
KBr pellets or liquid ®lms. H NMR spectra with TMS as
internal standard and 19F NMR spectra with tri¯uoroacetic
acid 4TFA) as external standard were recorded on a Bru-
ker300 spectrometer 4300 MHz), a Varian EM360L spectro-
meter 460 MHz) or a FX-90Q 490 MHz). For 19F NMR
spectra, the high ®eld was positive. MS spectra were mea-
sured with a Finnigan-4021 spectroscope. The phase transi-
tion temperatures of the target compounds were measured
visually by optical polarizing microscope 4Olympus PM-6)
®tted with a heating stage 4Metter FP-80) and a control unit
4FP-82), and by differential scanning calorimetry 4DSC,
Shimadzu DSC-50 calorimeter with a data system, heating
and cooling rate 58C minÀ1). The transition temperatures
reported in this paper were the peak values of the transition
on DSC traces.
mesomorphic range becomes wider when the length of
poly¯uoroalkoxy chain increases. Secondly, the sample with
n ˆ 2 shows only a enantiotropic Ch phase, while the
sample with n ˆ 4 exhibits enantiotropic Ch phase and
monotropic SmA phase. For the sample with n ˆ 8, no
Ch phase but enantiotropic SmA phase was observed. So
it is worth noting that in these cases, the introduction of
¯uorine atoms induces the suppression of cholesteric phase
and increases the thermal stability of the mesophases and
temperature range of the smectic phase [13,14]. It is ascribed
to the in¯uence of introduction of per¯uoroalkyl chain as
there is a strong attractive interaction between per¯uoroalkyl
chains. When the length of per¯uoroakyl chain is short, the
lateral attractive interaction is weaker than that between
terminal chains, which favours the formation of the Ch
phase. With increasing length of per¯uoroalkyl chain, the
lateral attractive interaction which favours the stacking of
molecules in the layer increases. Thus it is obvious that the
smectic phase will appear.
Some high ordered smectic phases, such as SmB and SmE
phases, which are observed in the cholesteryl p-per¯uor-
oalkyl benzoate, were not observed in these compounds.
Due to the per¯uoroalkyl chains being more rigid than the
corresponding alkyl chain, cholesteryl p-per¯uoroalkyl
benzoate are more rigid than the compounds with a soft ±
OCH2± spacer which exhibit only Ch and SmA phases.
But it is regrettable that the SmCÃ phase which was
desired was not observed in these compounds.
3.2. Synthesis
The mesogens are synthesized in four steps according to
the following 4Scheme 1).
3.2.1. Preparation of p-2,2,3,3-tetrafluoropropoxy-
m-nitro-trifluoromethylbenzene -2a)
To 4.554 g 40.033 mol) of potassium carbonate in a dried
¯ask purged with nitrogen, a solution of 4.356 g 40.033 mol)
2,2,3,3-tetra¯uoropropanol 41a) in dried DMF 430 ml) was
added slowly at room temperature. Then 6.765 g 40.03 mol)
p-chloro-m-nitro-tri¯uoromethylbenzene was added at room
temperature. The reaction mixture was stirred for 10 h at
1208C, poured into 100 ml of diluted HCl after cooling and
extracted by diethyl ether. The ether extracts are washed
Scheme 1.

K. Wang et al. / Journal of Fluorine Chemistry 109 -2001) 205±208
207
neutral with brine, dried over anhydrous MgSO4 and the
solvent distilled off in vacuo. Puri®cation was possible by
column chromatography on silica gel using petroleum
ether±ethyl acetate 420:1) as the eluent to give 42a) as a
liquid 8.215 g, yield 85%.1H NMR 4CDCl3) d 4ppm): 4.95
4t, 2H, J ˆ 14 Hz, OCH2CF2), 6.45 4tt, J1 ˆ 52 Hz,
J2 ˆ 6 Hz, 1H, HCF2), 7.63 4d, 1H, J ˆ 9 Hz, ArH), 8.33
4d, 1H, J ˆ 9 Hz, ArH), 8.52 4s, 1H, ArH). 19F NMR
4CDCl3) d 4ppm): À13.7 4s, 3F, CF3), 49 4m, 2F, CF2),
63.9 4dtt, J1 ˆ 56 Hz, J2 ˆ 28 Hz, J3 ˆ 4 Hz, 2F, HCF2).
p-2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-Hexadec¯uorononoxy-
m-nitrobenzoic acid 43c): Yield 70%. 1H NMR
4CD3COCD3) d 4ppm): 4.75 4t, 2H, J ˆ 14 Hz, OCH2CF2),
6.40 4tt, J1 ˆ 52 Hz, J2 ˆ 6 Hz, 1H, HCF2), 7.30 4d, 1H,
J ˆ 9 Hz, ArH), 8.00 4d, 1H, J ˆ 9 Hz, ArH), 8.12 4s, 1H,
ArH). 19F NMR 4CD3COCD3) d 4ppm): 42.1 4m, 2F, CF2),
45.0 4m, 8F, 4CF2)4), 46.2 4m, 2F, CF2), 52.3 4m, 2F, CF2),
‡
61.0 4d, J1 ˆ 56 Hz, 2F, HCF2). MS 4m/z): 597 4M , 31.23),
‡
‡
167 4m-NO2C6H3COOH , 89.06), 150 4m-NO2C6H3CO ,
100.00).
‡
‡
,
MS 4m/z): 321 4M , 44.87), 191 4m-NO2C6H3CF3
100.00).
3.2.3. Preparation of cholesteryl p-2,2,3,3-
tetrafluoropropoxy-m-nitrobenzoate -4a)
p-2,2,3,3,4,4,5,5-Oct¯uoropentoxy-m-nitro-tri¯uoro-
methylbenzene 42b): Yield 83%. 1H NMR 4CDCl3) d 4ppm):
4.90 4t, 2H, Jˆ14Hz, OCH2CF2), 6.30 4tt, J1 ˆ 52 Hz,
J2 ˆ 6 Hz, 1H, HCF2), 7.48 4d, 1H, J ˆ 9 Hz, ArH), 8.12
4d, 1H, J ˆ 9 Hz, ArH), 8.40 4s, 1H, ArH). 19F NMR
4CDCl3) d 4ppm): À14.7 4s, 3F, CF3), 42.8 4m, 2F, CF2),
48.5 4m, 2F, CF2), 53.3 4m, 2F, CF2), 60.8 4d, J1 ˆ 56 Hz,
p-2,2,3,3-Tetra¯uoropropoxy benzoic acid 43a) 4500 mg,
1.68 mmol), dicyclohexylcarbodiimide 4DCC) 4347 mg,
1.68 mmol), cholesterol 4650 mg, 1.68 mmol), N,N-
dimethylaminopyridine 4DMAP) as catalyst, in dry tetra-
hydrofuran 4THF) 410 ml), stirred for 48 h at room tem-
perature. Then added water and ®ltered off, extracted by
diethyl ether, and the ®ltrate washed with water, dried over
anhydrous sodium sulfate. The solvent was removed in
vacuo and the residue was puri®ed by column chromato-
graphy on silica gel using petroleum ether±ethyl acetate
48:1) as eluent to give a white crystal which was recrystal-
lized from acetone±methanol to give 44a) as white ¯aky
‡
‡
,
HCF2). MS 4m/z): 421 4M , 30.78), 191 4m-NO2C6H3CF3
100.00).
p-2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-Hexadec¯uorononoxy-
1
m-nitro-tri¯uoromethylbenzene 42c): Yield 75%. H NMR
4CDCl3) d 4ppm): 4.76 4t, 2H, J ˆ 14 Hz, OCH2CF2), 6.26
4tt, J1 ˆ 52 Hz, J2 ˆ 6 Hz, 1H, HCF2), 7.38 4d, 1H,
J ˆ 9 Hz, ArH), 7.95 4d, 1H, J ˆ 9 Hz, ArH), 8.25 4s,
1H, ArH). 19F NMR 4CDCl3) d 4ppm): À15.5 4s, 3F,
CF3), 41.4 4m, 2F, CF2), 44.2 4m, 8F, 4CF2)4), 45.5 4m,
2F, CF2), 51.5 4m, 2F, CF2), 59.0 4d, J1 ˆ 56 Hz, 2F, HCF2).
1
crystals 980 mg, yield 88%. H NMR 4CDCl3) d 4ppm):
0.65±0.80 4s, 6H), 0.82±2.10 4m, 35H), 2.42±2.60 4m, 2H),
4.50±4.55 4m, 1H), 4.70 4t, 2H, J ˆ 14 Hz, OCH2CF2), 5.45
4m, 1H), 6.35 4tt, J1 ˆ 52 Hz, J2 ˆ 6 Hz, 1H, HCF2), 7.27
4d, 1H, J ˆ 9 Hz, ArH), 8.40 4d, 1H, J ˆ 9 Hz, ArH), 8.70
4s, 1H, ArH). 19F NMR 4CDCl3) d 4ppm): 47.8 4m, 2F, CF2),
‡
‡
,
MS 4m/z): 621 4M , 27.79), 191 4m-NO2C6H3CF3
100.00).
‡
62.9 4d, J1 ˆ 56 Hz, 2F, HCF2). MS 4m/z): 368 4OCh ,
‡
3.2.2. Preparation of p-2,2,3,3-tetrafluoropropoxy-
m-nitrobenzoic acid -3a)
100.00), 280 4p-HC2F4CH2O-m-NO2C6H3COOH , 27.02).
IR 4KBr) cmÀ1: 2934, 2866, 1715, 1616, 1534, 1355, 1322,
1295, 1248, 1131, 1051, 980, 953, 924, 839, 761, 723, 549.
Anal. for C37H51F4NO5, calc. C: 66.75, H: 7.72, N: 2.10, F:
11.41%; found C: 66.60, H: 7.77, N: 2.24, F: 11.23%.
Cholesteryl p-2,2,3,3,4,4,5,5-oct¯uoropentoxy-m-nitro-
benzoate 44b): Yield 85%. 1H NMR 4CDCl3) d 4ppm):
0.65±0.75 4s, 6H), 0.85±2.08 4m, 35H), 2.45±2.55 4m,
2H), 4.55±4.65 4m, 1H), 4.75 4t, 2H, J ˆ 14 Hz, OCH2CF2),
5.45 4m, 1H), 6.30 4tt, J1 ˆ 52 Hz, J2 ˆ 6 Hz, 1H, HCF2),
7.35 4d, 1H, J ˆ 9 Hz, ArH), 8.40 4d, 1H, J ˆ 9 Hz, ArH),
8.60 4s, 1H, ArH). 19F NMR 4CDCl3) d 4ppm): 42.1 4m, 2F,
CF2), 47.4 4m, 2F, CF2), 52.8 4m, 2F, CF2), 60.3 4d,
p-2,2,3,3-Tetra¯uoropropoxy-m-nitro-tri¯uoromethyl-
benzene 42a) 46.42 g, 0.02 mol) was added to fuming sul-
furic acid 45 ml), then heated to 1208C, stirred for 2 h,
poured into 100 ml of ice water and ®ltered off. Then the
pale yellow crystal was recrystallized from ethanol to yield
1
43a) as white ¯aky crystals 5.346 g. Yield 90%. H NMR
4CD3COCD3) d 4ppm): 4.95 4t, 2H, J ˆ 14 Hz, OCH2CF2),
6.45 4tt, J1 ˆ 52 Hz, J2 ˆ 6 Hz, 1H, HCF2), 7.33 4d, 1H,
J ˆ 9 Hz, ArH), 8.03 4d, 1H, J ˆ 9 Hz, ArH), 8.22 4s, 1H,
ArH). 19F NMR 4CD3COCD3) d 4ppm): 49 4m, 2F, CF2),
64.1 4dtt, J1 ˆ 56 Hz, J2 ˆ 28 Hz, J3 ˆ 4 Hz, 2F, HCF2).
‡
‡
‡
MS 4m/z): 297 4M , 57.42), 167 4m-NO2C6H3COOH ,
J1 ˆ 56 Hz, 2F, HCF2). MS 4m/z): 765 4M , 1.43), 368
‡
‡
96.76), 150 4m-NO2C6H3CO , 100.00).
p-2,2,3,3,4,4,5,5-Oct¯uoropentoxy-m-nitrobenzoic acid
43b): Yield 84%. H NMR 4CD3COCD3) d 4ppm): 4.74
4OCh , 100.00). IR 4KBr) cmÀ1: 2954, 2866, 1710, 1616,
1533, 1358, 1323, 1292, 1253, 1177, 1133, 1090, 998, 923,
809, 760, 715, 541, 413. Anal. for C39H51F8NO5 calc. C:
61.17, H: 6.71, N: 1.83, F: 19.85%; found C: 61.28, H: 6.67,
N: 1.81, F: 19.52%.
1
4t, 2H, J ˆ 14 Hz, OCH2CF2), 6.40 4tt, J1 ˆ 52 Hz,
J2 ˆ 6 Hz, 1H, HCF2), 7.29 4d, 1H, J ˆ 9 Hz, ArH), 7.97
4d, 1H, J ˆ 9 Hz, ArH), 8.10 4s, 1H, ArH). 19F NMR
4CD3COCD3) d 4ppm): 43.3 4m, 2F, CF2), 48.6 4m, 2F,
CF2), 54.0 4m, 2F, CF2), 61.5 4d, J1 ˆ 56 Hz, 2F, HCF2). MS
Cholesteryl
p-2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-hexadec-
1
¯uorononoxy-m-nitrobenzoate 44c): Yield 87%. H NMR
4CDCl3) d 4ppm): 0.60±0.70 4s, 6H), 0.85±2.10 4m, 35H),
2.40±2.50 4m, 2H), 4.55±4.65 4m, 1H), 4.75 4t, 2H, J ˆ
14 Hz, OCH2CF2), 5.38 4m, 1H), 6.30 4tt, J1 ˆ 52 Hz,
‡
‡
4m/z): 397 4M , 40.77), 167 4m-NO2C6H3COOH , 90.76),
‡
150 4m-NO2C6H3CO , 100.00).

208
K. Wang et al. / Journal of Fluorine Chemistry 109 -2001) 205±208
[3] J.D. Laffitte, M. Mauzac, R.J. Twieg, H.T. Nguyen, G. Sigaud, Liq.
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J2 ˆ 6 Hz, 1H, HCF2), 7.35 4d, 1H, J ˆ 9 Hz, ArH), 8.40 4d,
1H, J ˆ 9 Hz, ArH), 8.60 4s, 1H, ArH). 19F NMR 4CDCl3) d
4ppm): 41.5 4m, 2F, CF2), 44.3 4m, 8F, 4CF2)4), 45.7 4m, 2F,
CF2), 52.0 4m, 2F, CF2), 59.5 4d, J1 ˆ 56 Hz, 2F, HCF2). MS
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‡
‡
4m/z): 965 4M , 2.77), 368 4OCh , 100.00). IR 4KBr) cmÀ1
:
2950, 1721, 1616, 1535, 1351, 1292, 1214, 1151, 1029, 836,
810, 761, 709, 655, 634, 535, 413. Anal. for C43H51F16NO5,
calc. C: 53.47, H: 5.32, N: 1.46, F: 31.47%; found C: 53.61,
H: 5.63, N: 1.46, F: 31.23%.
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