Synthesis of new liquid crystals with 2,3,4,9-tetrahydro-1H-fluorene moiety

Article abstract of DOI:10.1080/15421406.2011.570542

A new range of liquid crystals containing the 2,3,4,9-tetrahydro-1H- fluorene system have been prepared and its thermal properties were examined. The mesomorphic properties are compared with those of the fluorene and trans-cyclohexane derivatives. Introdu

Full text of DOI:10.1080/15421406.2011.570542

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Synthesis of New Liquid Crystals with
2,3,4,9-Tetrahydro-1H-Fluorene Moiety
R. Ch. Geivandov a , V. Mezhnev a & T. Geivandova a
a Moscow Fine Organic Synthesis & Liquid Crystal Laboratory
“INCORFIN” Ltd. Co , Moscow, Russia
Published online: 14 Jun 2011.
To cite this article: R. Ch. Geivandov , V. Mezhnev & T. Geivandova (2011) Synthesis of New Liquid
Crystals with 2,3,4,9-Tetrahydro-1H-Fluorene Moiety, Molecular Crystals and Liquid Crystals, 542:1,
141/[663]-148/[670], DOI: 10.1080/15421406.2011.570542
To link to this article: http://dx.doi.org/10.1080/15421406.2011.570542
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Mol. Cryst. Liq. Cryst., Vol. 542: pp. 141=[663]–148=[670], 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.570542
Synthesis of New Liquid Crystals with
2,3,4,9-Tetrahydro-1H-Fluorene Moiety
R. CH. GEIVANDOV, V. MEZHNEV, AND
T. GEIVANDOVA
Moscow Fine Organic Synthesis & Liquid Crystal Laboratory
‘‘INCORFIN’’ Ltd. Co, Moscow, Russia
A new range of liquid crystals containing the 2,3,4,9-tetrahydro-1H-fluorene system
have been prepared and its thermal properties were examined. The mesomorphic
properties are compared with those of the fluorene and trans-cyclohexane deriva-
tives. Introduction of 2,3,4,9-tetrahydro-1H-fluorene in typical liquid crystal struc-
ture leads to decrease of melting point and the clearing point values. The chemical
structures and mesogenic properties for the reported novel compounds are discussed.
Keywords 2,3,4,9-tetrahydro-1H-fluorene; cross-coupling reaction; liquid
crystals
1. Introduction
The introduction of alicyclic fragment in the mesogenic moiety of potential
liquid-crystalline structure leads to a change in the basic properties of mesomorphic
compounds (temperature phase transitions, optical and dielectric anisotropy,
viscosity, etc.) [1–4]. In continuation of our systematic studies [5–7] in the area of
synthesis of mesomorphic structures, containing mono-, bi- and trialicyclic units,
we synthesized a new class of liquid crystals – 2,7-disubstituted 2,3,4,9-tetrahydro-
1H-fluorene derivatives and investigated their temperature phase transitions.
2. Results and Discussion
2.1. Synthesis of Target Intermediates and Liquid Crystal
We thought it was interesting to synthesize an analogue of fluorene, where one of
benzene rings is partially hydrogenated. We have concentrated our efforts on the
development of convenient synthetic methods for key intermediate products,
which provide a new class of liquid crystals – 2,7-disubstituted 2,3,4,9-tetrahydro-
1H-fluorene derivatives. The syntheses were carried out according to Schemes 1
and 2.
Address correspondence to R. Ch. Geivandov, Moscow Fine Organic Synthesis & Liquid
Crystal Laboratory ‘‘INCORFIN’’ Ltd. Co, 27, Burakova str., 111123 Moscow, Russia. Tel.:
þ7-916-0518677; E-mail: r.ch.geivandov@gmail.com
141=[663]

142=[664]
R. Ch. Geivandov et al.
Scheme 1. Synthesis of 7-Bromo-2-pentyl-2,3,4,9-tetrahydro-1H-fluorene (6).
The Scheme 1 in included the following basic subsequent steps:
. obtaining the b-chloracroleine 2 using Vilsmeier-Haack-Arnold method [8] by
action of POCl3-DMF complex on ketone 1,
. cross-coupling of b-chloracroleine 2 with 4-bromophenylboronic acid 3,
. reduction of aldehyde group in compound 4 using lithium aluminum hydride
to obtain alcohol 5,
. intramolecular dehydration-cyclization of alcohol 5 in the presence of acidic
catalysts to obtain compounds 6 with tetrahydrofluorene system.
Although aryl chloride is less reactive due to higher energy barrier for the oxi-
dative insertion of the palladium catalyst into the C-C bond, the Suzuki coupling
of vinyl chlorides takes place at a sufficient pace. Moreover, the presence of an
electron-accepting group leads to an increase of the reaction rate and allows the
use of such derivatives as b-chloracroleine.
The b-chloracroleine 2 easily went into cross-coupling reaction and the
compound 4 was produced in 82% yield.
We have conducted wide investigation while searching for the optimal con-
ditions of cyclization of alcohol 5 by trying various acidic catalysts and reaction con-
ditions variants. The best result for cyclization of alcohol 5 into the compound 6 has
been achieved under action of polyphosphoric acid (PPA). The yield of compound 6
was approximately 35%.
Scheme 2. Synthesis of liquid crystals (9–12) by cross-coupling reaction.

Synthesis of New Liquid Crystals
143=[665]
Table 1. Cross- coupling reaction of 7-bromo-2-pentyl-2,3,4,9-tetrahydro-1H-
fluorene (6) with substituted phenylboronic acids (8a-d)
Yield,
%
Ms:
m=z (Mþ)
Phenylboronic acids
Product
68
77
334
352
44
76
341
346
Compound 6 have been cyanated using cooper cyanide and yielded nitrile 7.
The cross-coupling reaction of compound 6 with 4-substituted phenylboronic
acids (8a-d) in the presence of 10% Pd=C lead to liquid crystals 9–12 (Scheme 2,
Table 1).
2.2. Liquid Crystalline Properties
The 2,3,4,9-tetrahydro-1H-fluorene is a partially saturated derivative of fluorene.
The fluorene moiety differs from a biphenyl unit since it is almost flat and just
slightly bent in the plane of the rings. Previous work on fluorene containing meso-
gens showed them to exhibit mainly smectic phases or very short nematic ranges [9].
Molecule of 2,3,4,9-tetrahydro-1H-fluorene (A) in contrast to fluorene (B) is not
flat at all and has a rigid-flexible configuration; both structures are shown on
Figure 1.
In this regard, when comparing properties of LC molecules, we also present data
on the temperatures for similar trans-cyclohexane derivatives, whose structures
combine a rigid-flexible configuration of the core molecule.
Figure 1. Molecular structures of 2,3,4,9-tetrahydro-1H-fluorene (A) and fluorene (B). (Figure
appears in color online.)

144=[666]
R. Ch. Geivandov et al.
As one can see from Table 2, compound 7 and fluorene derivative 7b with
terminal nitrile group exhibit monotropic nematic phase, while compound 7a has
a normal liquid crystalline temperature range.
We have synthesized three units compounds of 2,3,4,9-tetrahydro-1H-fluorene
(A). Compounds 9, 10, 11, 12 (Table 2) have, in contrast to described corresponding
trans-cyclohexane derivatives (9a, 11a, 12a), lower melting point and give way
mainly to thermal stability.
The compounds containing tetrahydro-1H-fluorene units 9–12 were found to
have nematic and smectic liquid crystalline phase with relatively low phase transition
temperature and wide liquid crystalline temperature range.
Table 2. Transition temperatures (ꢀC) for 7-substituted -2-pentyl-2,3,4,9-tetrahydro-
1H-fluorenes, fluorene and trans-cyclohexane derivatives
Compound
Molecular structureꢁ
Cr
S
N
I
Ref.
7
52
–
ꢂ
(34)
–
7A
7B
31
66
–
–
ꢂ
ꢂ
55
[10]
[9]
(24.5)
9
76
–
–
ꢂ
ꢂ
141
152
–
9a
100
[11]
10
64
–
ꢂ
104
–
11
89
96
110
–
ꢂ
ꢂ
ꢂ
178
222
216
–
11a
11b
[12]
[13]
128
–
12
110
80
130
–
ꢂ
ꢂ
183
165
–
12a
[14]
ꢁ
=
R C5H11.

Synthesis of New Liquid Crystals
145=[667]
The wide liquid crystalline temperature range of compound 11 and fluorene
derivative 11b are almost comparable, but compound 11 have lower melting and
clearing points as compared to compound 11b.
3. Experimental
All commercially available substances were purchased from Aldrich, Merck, Acros,
Fluka or Lancaster and were used without further purification. Solvents were puri-
fied and dried if necessary according to standard procedures. All reactions were
monitored by thin-layer chromatography on silica gel (Merck, Kieselgel 60, F254).
UV-light with wavelength of k ¼ 254 nm was used for detection.
3.1. Melting and Clearing Points
The temperatures of phase transitions and liquid crystalline phases were observed
with Mettler FP 90 coupled with a polarizing Olympus BH-2 microscope. Measured
temperatures from the polarizing microscope have been corrected by calibration with
standard materials.
3.2. NMR Spectroscopy
Confirmations of the intermediate and final product structures were accomplished
1
by H, 13C, 19F, NMR spectroscopy (Bruker AM-300 and Bruker Avance II 300
spectrometers) at ambient temperature. Chemical shifts values (d) are relative to
Me4Si and are given in ppm (references solvents are CDCl3 and DMSO-d6).
3.3. Mass Spectrometry
Mass-spectra were obtained directly using Finningan MAT INCOS 50 spectrometer
(EI, 70 eV).
2-Chloro-5-pentylcyclohex-1-ene-1-carbaldehyde (2). To solution of DMF (10.3 ml)
in dry chloroform (25 ml) at 5–10 ꢀC was added phosphoryl chloride (10.1 ml) and
the mixture was stirred for 10 minutes. Then the 4-pentylcyclohexanone 1
(16.8 ml, 0.1 mol) in chloroform (25 ml) was added drop wise at 20 ꢀC and the
mixture was heated at 55–60 ꢀC for 3 hour. The reaction mixture was poured into
an aqueous sodium acetate solution (35 g sodium acetate in 120 ml water),
extracted with chloroform (3 ꢃ 30 ml), and the combined chloroform extracts were
washed successively with water and dried (Na2SO4). The solvent was removed
in vacuo and the residue was distilled to give a colourless liquid.
Yield 82%; bp 92–94 ꢀC at 0.24 mmHg; nD20 1.4925; 1H NMR(CDCl3) d 0.85 (3H,
t, CH3), 1.2–1.4 (8H, m, C5H11), 1.45–1.8 (4H, m, cyclohexane), 2.47 (3H, m, cyclo-
hexane), 11,5 (H, s, CHO); ms m=z 214 (Mþ).
2-(4-Bromophenyl)-5-pentylcyclohex-1-ene-1-carbaldehyde (4). A mixture of
chloroacroleine 2 (2,59 mmol), 4-bromophenyl boronic acid (3) (2,83 mmol),
tetrabutylammonium bromide (2,60 mmol), palladium acetate (0,053mmol, 2 mol %)
and potassium carbonate (6,45 mmol) was added to a 10 mL round-bottom flask.
Deionized water (5 ml) was added, and the reaction was stirred vigorously for

146=[668]
R. Ch. Geivandov et al.
3 hours at 45 ꢀC. The reaction mixture became dark and non-homogenous. Then the
mixture was diluted with water (15 ml), and the product was extracted with EtOAc
(3 ꢃ 30 ml). The organic phase was stirred over charcoal for 30 min and dried
(MgSO4). After filtration and concentration in vacuo, the residue was distilled to
give a colourless liquid.
Yield is 68%; bp183–190 ꢀC at 0.120 mm Hg; nD20 1.5660; mp 28–30 ꢀC; 1H
NMR(CDCl3) d 0.9(3H, s, CH3), 1.2–1.4 (8H, m, C5H11), 1.45–1.95(4H, m, cyclo-
hexane), 2.5–2.7(3H, m, cyclohexane), 7,1(2H, d, aromatic). 7,5(2H, d, aromatic),
9,48(1H, s, CHO); ms m=z 334 (Mþ).
[2-(4-Bromophenyl)-5-pentylcyclohex-1-en-1-yl]methanol (5). Ether solution (8 ml)
of aldehyde 4 (2.1 g, 6.25 mmol) was added to a stirred suspension of lithium
aluminum hydride (0.300 g, (7.9 mmol) in dry ether (6 ml) at 0 ꢀC. The temperature
was kept at 0 ꢀC for 1 hour. Then reaction mixture was poured into NH4Cl
solution, the product was extracted with ether (3 ꢃ 30 mL), combined extracts
were washed with water and dried (Na2SO4). The solvent was removed in vacuo
and the residue was purified by column chromatography (silica gel=n-hexane).
1
Yield 90%; m.p. 55–56 ꢀC; H NMR (CDCl3) d 0.9(3H, t, CH3), 1.2–1.45 (8H, m,
C5H11), 1.75–1.92 (4H, m, cyclohexane), 2.22–2.45 (3H, m, cyclohexane), 3.92
(2H, s, CH2OH), 7,0(2H, d, aromatic). 7,42(2H, d, aromatic); 13C NMR (DMSO)
=
=
d 141.70(Carom-C C) 135.64(C C-Carom), 132.86(Carom), 131.92(Carom), 129.93
(Carom), 120.54(C-Br), 63.94(CH2OH), 36.57(Ctret), 32.80, 34.0, 33.7, 32.3, 29.55,
26.69, 22.75(CH2), 14.14(CH3); ms m=z 337 (Mþ).
7-Bromo-2-pentyl-2,3,4,9-tetrahydro-1H-Fluorene (6). A mixture of (1.5 g,
4.7 mmol) compound 5 and polyphosphoric acid (PPA) (50 g) was heated for 20
minutes at 135–137 ꢀC. The reaction mixture was diluted with water and the
product was extracted with ether (3 ꢃ 50 ml). The combined organic extracts were
washed with water and dried (Na2SO4). The solvent was removed in vacuo and the
residue was purified by column chromatography (silica gel=n-hexane).
1
Yield 35%; m.p. 79–80 ꢀC (acetone); H NMR (CDCl3) d 0.92(3H, t, CH3),
1.25–1.48 (8H, m, C5H11), 1.65–1.8(1H, s, cyclohexane), 1.9–2.1(2H, m, cyclohex-
ane), 2.3–2.55 (3H, m, cyclohexane), 3.2 (2H, s, CH2), 7,0(1H, d, aromatic).
7,38(1H, d, aromatic), 7,5(1H, d, aromatic); 13C-NMR (DMSO) d 145.06(Carom
-
=
=
C C) 141.91(C C-Carom), 135.49(Carom), 129.04 (Carom), 126.61 (Carom),
118.73(C-Br), 117.49(Carom), 40.42(CH2), 36.41(Ctret), 34.44, 32.54, 32.23, 29.03,
26.87, 22.80, 22.00(CH2), 14.21(CH3); ms m=z 318 (Mþ).
2-Pentyl -2,3,4,9-tetrahydro-1H-Fluorene-7-carbonitrile (7). A mixture of 7-Bromo-
2-pentyl-2,3,4,9-tetrahydro-1H-fluorene 6 (0.175 g, 0.55 mmol), copper (I) cyanide
(0.58 g, 0.32 mmol) and N-methyl-2-pyrrolidone (5.5 ml) was refluxed in a
three-neck flask for 5 hours. Toluene (5 ml) was added, the mixture was cooled,
and a solution consisting of ferric chloride (0.65 g), conc. hydrochloric acid (0.16 ml)
and water (7.5 ml) was added and it was stirred for 30 minutes at 60–70 ꢀC. The
resulting two layers were separated and the resulting organic layer was washed
with dilute hydrochloric acid, then with dilute aqueous sodium hydroxide, and
further with water. After a small amount of solid matter was eliminated by
filtration, toluene was distilled off in vacuo. The residue was purified by column
chromatography (silica gel=n-hexane).

Synthesis of New Liquid Crystals
147=[669]
Yield 65%; transitions (ꢀC) Cr 52 N (34) I; 1H NMR (CDCl3) d 0.92(3H, t, CH3),
1.25–1.45 (8H, m, C5H11), 1.65–1.8(1H, s, cyclohexane), 1.9–2.15(2H, m, cyclohex-
ane), 2.3–2.62(3H, m, cyclohexane), 3.3(2H, s, CH2), 7,0(1H, d, aromatic).
7,22(1H, aromatic), 7,58(1H, aromatic); ms m=z 265 (Mþ).
General procedure for cross- coupling reaction of 7-bromo-2-pentyl-2,3,4,9-tetrahydro-
1H-fluorene (5) with substituted phenylboronic acids (8a-d). 7-Bromo-2-pentyl-
2,3,4,9-tetrahydro-1H-fluorene 6 (4.38 mmol), substituted phenylboronic acid 8a-d
(4.38 mmol), 10% Pd=C (0.01 g), triphenylphosphine (0.18 mmol) and sodium
carbonate (8.76 mmol) were heated in a mixture of toluene (1 ml), ethanol (0.5 ml)
and water (0.3 ml) for 24 hours at 80 ꢀC. The reaction mixture was diluted with
water, the product was extracted with EtOAc and the combined extracts were
washed with water, filtered, dried (Na2SO4). The solvents were removed in vacuo
and the residue was purified by column chromatography (silica gel=n-hexane).
7-(4-Fluorophenyl)-2-pentyl-2,3,4,9-tetrahydro-1H-fluorene
1
(9). Yield
68%;
transitions (ꢀC) Cr 76 N 141 I; H NMR (CDCl3) d 0.91 (3H, t), 1.20–2.7 (15H,
m), 3.3 (2H, m), 7.05–7.6 (7H, m, arom). 19F-NMR (DMSO): d 138.5.
7-(3,4-Difluorophenyl)-2-pentyl-2,3,4,9-tetrahydro-1H-fluorene (10). Yield 77%;
1
transitions (ꢀC) Cr 64 N 104 I; H NMR (CDCl3) d 0.91 (3H, t), 1.20–1.5 (8H,
m), 1.75 (1H, s), 1.9–2.2 (2H, m), 2.3–2.65 (3H, m), 3.3 (2H, s, CH2), 7.1–7.55
(6H, m, arom). 19F NMR (CDCl3): d 138.5 (d, 1 F), 142.45(d, 1 F).
4-(2-Pentyl-2,3,4,9-tetrahydro-1H-fluoren-7-yl)benzonitrile
(11). Yield
44%;
transitions (ꢀC) Cr 89 S 110 N 178 I;1H NMR (CDCl3) d 0.91 (3H, t), 1.25–2.6
(15H, m), 3.3 (2H, m), 7.25–7.7 (7H, m, arom.).
7-(4-Methoxyphenyl)-2-pentyl-2,3,4,9-tetrahydro-1H-fluorene (12). Yield 76%;
1
transitions (ꢀC) Cr 110 S 130 N 183 I; H NMR (CDCl3) d 0.91 (3H, t), 1.25–2.6
(15H, m), 3.29 (2H, m), 3.85 (3H, s, OCH3), 7.0–7.6 (7H, m, arom.).
4. Conclusions and Perspectives
For the first time a new class of nematic liquid crystal compounds containing
2,3,4,9-tetrahydro-1H-fluorene system has been synthesized and characterized.
We can conclude that the replacement of rigid fragments in the typical LC
molecules with rigid-flexible fragments changes the whole set of their properties.
Molecular factors can be divided into three groups – geometry, dispersion, and
polarity. They all have an impact on the mesomorphic behavior of molecules.
The relative importance of each factor depends on the characteristics of
chemical, electronic and spatial structure of the molecule. In this regard, sequential
Figure 2. Structures of 2,3,4,4a,9-tetrahydro-2H-fluorene (C) and trans-2,3,4,4a,9,9a-
hexahydro-1H-fluorene (D) mesogenic moiety.

148=[670]
R. Ch. Geivandov et al.
analysis of each of these factors on the example of the LC classes allows us to esti-
mate the contribution of factor in the total effect of changes in respect of thermal
stability and mesophase range.
In future, we plan to synthesize new LC structures, containing 2,3,4,4a,9-
tetrahydro-2H-fluorene (C) and trans-2,3,4,4a,9,9a-hexahydro-1H-fluorene (D)
systems (Figure 2).
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