Liquid crystals based on axially fluorinated 1,3-dioxanes: Synthesis, properties and computational study

Article abstract of DOI:10.1002/ejoc.200600529

A new class of liquid crystals with strongly negative dielectric anisotropy has been prepared by the exploitation of the somewhat "ambivalent" characteristics of the 1,3-dioxane structure with respect to its polarity and by the axial fluorination of this

Full text of DOI:10.1002/ejoc.200600529

FULL PAPER
DOI: 10.1002/ejoc.200600529
Liquid Crystals Based on Axially Fluorinated 1,3-Dioxanes: Synthesis,
Properties and Computational Study
Peer Kirsch*[a] Alexander Hahn,[b] Roland Fröhlich,[c] and Günter Haufe[c]
Keywords: Liquid crystals / Dielectric anisotropy, negative / DFT calculations / Mesogens / Fluorine / 4-ax-Fluoro-1,3-
dioxane
A new class of liquid crystals with strongly negative dielec-
tric anisotropy has been prepared by the exploitation of the
somewhat “ambivalent” characteristics of the 1,3-dioxane
structure with respect to its polarity and by the axial fluorina-
tion of this structure, which gives these compounds attractive
mesogenic and electrooptic properties. The increased chemi-
cal stability of these materials compared with their carbocy-
clic analogues has been analyzed and explained by molecu-
lar modelling.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
Introduction
Recently, liquid crystal displays (LCD) have become an
integral part of our daily lives.[1,2] The transition from cath-
ode ray tube (CRT) based displays to LCD technology was
initiated by notebook PCs and gained additional impetus
from the replacement of desktop PC monitors by flat panel
displays. Currently, the market for LCD TV sets continues
to grow rapidly. Technologically, LCD TV is dominated by
the so-called vertical alignment (VA) technology, which re-
quires nematic liquid crystals with negative dielectric an-
isotropy (∆ε).[3] The advantages of this technology are fast
switching times, high brightness and an excellent contrast
ratio.
Scheme 1. Examples of liquid crystals with negative dielectric an-
isotropy (∆ε).
Unfortunately, axially fluorinated cyclohexane deriva-
tives (such as 2) never found their way into industrial appli-
cation because of their propensity to unpredictably and au-
tocatalytically eliminate hydrofluoric acid.
With very few exceptions,[4] most liquid crystals that are
in practical use for VA LCDs are based on a 2,3-difluo-
robenzene subunit as their polar group (such as 1,
Scheme 1).[3d] This functionality conveys a dipole moment
perpendicular to the molecular long axis and thus gives a
negative value to ∆ε. Nevertheless, there is a need for other
dielectrically negative materials with low rotational vis-
cosity and with lower birefringence (∆n). One class of mate-
rials fulfilling these requirements are axially fluorinated cy-
clohexane derivatives.[5] Besides their low rotational vis-
cosity (γ1), these compounds also have low birefringence
because their substructures show low polarizability.
Because of the attractive mesogenic and electrooptic
property profile of this substance class, we sought to im-
prove the stability problem. In addition, it was our aim to
obtain structurally related materials with low birefringence
and an even more strongly negative dielectric anisotropy.
In liquid crystals with positive ∆ε, one method to increase
the dielectric anisotropy is to include trans-1,3-dioxane-2,5-
diyl building blocks into the mesogenic core structure
(Scheme 2).[2] This works because the dioxane carbon–oxy-
gen bonds add their dipole moments to that of the polar
terminal group into the direction of the long molecular axis.
Thus, the dielectric anisotropy of 4 (∆ε = 9.7) can be in-
creased by 7 units by the replacement of one cyclohexane
unit by 1,3-dioxane (5, ∆ε = 17.0). However, the 1,3-dioxane
unit has a somewhat ambivalent character as a polar ring
element (Scheme 2); the orientation of its local group dipole
moment is oblique relative to the long molecular axis of the
resulting liquid crystal. Therefore, it can be used to enhance
the perpendicular dipole moment of a lateral polar group
in dielectrically negative liquid crystals (3)[6] as well.
[a] Merck Ltd. Japan, Atsugi Technical Center,
4084 Nakatsu, Aikawa-machi, Aiko-gun, Kanagawa 243-0303,
Japan
Fax: +81-46-285-3838
E-mail: peer_kirsch@merck.co.jp
[b] Merck KGaA, Liquid Crystals Division,
Frankfurter Str. 250, 64271 Darmstadt, Germany
[c] Institute for Organic Chemistry, University of Münster,
48149 Münster, Germany
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P. Kirsch, A. Hahn, R. Fröhlich, G. Haufe
FULL PAPER
yields of the alkylation procedure are very low in the case
of 12, the purification of the product is easier than that
of the electrophilic fluorination route. Alkylfluoromalonic
esters 8 and 12 were carefully reduced to corresponding
diols 9 and 13,[8] which were subsequently converted into
bis(trimethylsilyl) ethers 10 and 14. With the use of trimeth-
ylsilyl triflate as a catalyst,[9,10] the ethers were condensed
under very mild conditions with trans-4-propylcyclohexyl
carbaldehyde and butyric aldehyde, respectively. Pure trans-
trans target compounds 3, 15, and 16 were isolated by their
repeated crystallization from n-heptane.
In contrast to the previously described axially fluorinated
cyclohexane derivatives (e.g., 2),[5] all newly synthesized
fluorodioxane-based liquid crystals, as well as their precur-
sors, are chemically stable even at elevated temperatures
without any precautions such as the addition of acid scav-
engers. In no case was the elimination of hydrofluoric acid
observed.
Unfortunately, from the application point of view, the
new liquid crystals 3, 15, and 16 have high melting points.
Their solubility in organic solvents and in the nematic
screening mixtures[11] is very poor (Table 1), which impeded
experimental determination of their electrooptic properties.
Compounds 15 and 16 show only smectic B phases – in
the case of 15 only a monotropic one – at relatively high
Scheme 2. Schematic representation of the underlying concept for
the design of ax-5-fluoro-1,3-dioxane derived liquid crystals.
Therefore, liquid crystals based on an axially fluorinated
1,3-dioxane substructure were chosen as candidates for ma-
terials with low birefringence and enhanced negative dielec-
tric anisotropy. The influence of the orientation of the
fluorodioxane moiety within the mesogenic core structure
on mesogenic and electrooptic properties was another topic
of our interest.
Results and Discussion
The synthesis of our target compounds (Scheme 3) was temperatures. Only our primary target, compound 3, was
centered around malonic ester precursors 6 and 11, which sufficiently soluble in the nematic screening host to be able
were deprotonated and fluorinated with the electrophilic to obtain the full set of its “virtual”[11] properties, such as
fluorination reagent F-TEDA [Selectfluor; 1-(chloro- its clearing temperature (TNI,virt), dielectric anisotropy (∆ε-
methyl)-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetra- virt), and birefringence (∆nvirt). The birefringence is – as ex-
fluoroborate)] to furnish fluoromalonates 8 and 12.[7] Alter- pected – in the same range as that for carbocyclic analogue
natively, the two alkylfluoromalonates can be obtained by 2. The dielectric anisotropy of 3 (∆ε = –4.8) shows a pro-
the alkylation of diethyl fluoromalonate (7).[8] Although the nounced increase compared with that of 2 (∆ε = –2.2),
Scheme 3. Synthesis of axially fluorinated 1,3-dioxane derivatives 3, 15, and 16. a) 1. NaH, THF; 2. Selectfluor (8: 66%, 12: 77%). b) 1.
NaH, THF, 2 h, 0–45 °C; 2. 1-Bromopropane; 18 h, room temp. (68%). c) LiAlH4, THF, toluene, 2 h, –30–30 °C (9: 51%, 13: 50%). d)
Me3SiCl, NEt3, DMF, 2 h, room temp. (10: 69%, 14: 90%). e) trans-4-Propylcyclohexyl carbaldehyde, cat. Me3SiOTf, CH2Cl2, –78 °C
(3: 24%, 16: 38%). f) 1. NaH, cis-4-propyl-bromocyclohexane, 15 h, 0 °C to reflux (6%). g) Butyric aldehyde, cat. Me3SiOTf, CH2Cl2,
–78 °C (15: 48%).
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Liquid Crystals Based on Axially Fluorinated 1,3-Dioxanes
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which was intended by the design of the materials. Another electrostatic component increases the lattice energy. A very
remarkable feature, particularly of material 15, is the ex- similar effect has been observed for the crystal structure
tremely large discrepancy between its “real” (monotropic) of liquid crystal 17,[5a] which has two ax-fluorocyclohexane
crystalline-smectic B phase transition temperature (97 °C) units. In liquid crystal 17, a ladder-like stacked arrangement
and the “virtual” crystalline-nematic phase transition is stabilized by aligned C–F bonds (with a C···F distance of
(6.3 °C). Also for compound 3, which shows no real meso- 3.575 Å), which stabilizes the packing by their dipole-dipole
phase, the virtual clearing point is unusually low (TNI,virt
=
interactions. In the case of solid 17 the anti-parallel ar-
14.8 °C) compared for example with carbocyclic analogue rangement of the two C–F bonds is stabilized by the crystal
2 (TNI,virt = 87.3 °C).
packing; in solution, the liquid crystalline host is composed
of two cyclohexane subunits that are twisted around the
central axis by ca. 75°, which breaks the symmetry of the
molecule and provides the compound with a negative di-
electric anisotropy (∆nvirt = –2.5).
Table 1. Comparison of the physical properties of liquid crystals
2–5 and 15–17. The virtual electrooptical parameters (∆nvirt) and
clearing points (TNI,virt) were extrapolated from Merck mixture
ZLI-4792. The dielectric anisotropies ∆εvirt of 1, 2, 3, and 17 were
extrapolated from ZLI-2857 and those of 4 and 5 from ZLI-
4792.[11] (C = crystalline, SB = smectic B, I = isotropic. The phase
transition temperatures are given in °C, the number in parentheses
denotes a monotropic phase transition).
The increased stability of ax-5-fluoro-1,3-dioxanes com-
pared with their carbocyclic analogues can be rationalized
and explained by DFT calculations[13] based on their struc-
ture and the supposed decomposition mechanism. Remark-
ably, the isodesmic equation [B3LYP/6-311+G(2d,p)//
B3LYP/6-31G(d)+ZPE level of theory] in Scheme 4 shows
that a switch of the fluorine atom from fluorocyclohexane
18 to fluorodioxane 21 actually increases the energy of the
system by 3.5 kcalmol–1. At the same time, the bond length
decreases from 1.421 Å in 18 to 1.404 Å in 21, which indi-
cates a strengthening of the C–F bond. This apparent para-
doxical result can be explained by the repulsive interaction
between the axial lone electron pairs of the two oxygen
atoms and the lone pairs of electrons on the axial fluorine.
The resulting increase in the transannular strain explains
Compound
Phase sequence
TNI,virt
∆εvirt
∆nvirt
1
2
3
C 67 N 145.3 I
C 75 SB 94 I
137.1
87.3
14.8
74.3
63.2
6.3
–2.8
–2.2
–4.7
9.7
17.0
–
0.095
0.054
0.055
0.075
0.068
–
C 112 I
4
5
15
16
17
C 66 N 94.1 I
C 74 N (51.2) I
C 97 SB (97) I
C 109 SB Ͼ200, dec.
C 100 SB 153 I[a]
–
–
–2.5
–
105.7
0.035
[a] Decomposition.
The crystal structure of 3[12] provides some insight into the higher energy of the system in spite of the stronger C–
the reason for the high melting point and poor solubility F bond.
of the compound. The crystal packing is dominated by an
The decomposition reaction of axially fluorinated cyclo-
eclipsed stacking of the single molecules with a nearly per- hexane derivatives is autocatalytically promoted by hydro-
fect alignment of the axial C–F bonds (with a nonbonded fluoric acid, and it is assumed to proceed via tertiary cat-
C···F distance of 3.413 Å) and of the local dipole moments ionic intermediate 22 (Scheme 5), which is stabilized by in-
from the dioxane substructure (Figure 1). Thus, a strong ductive effects and by hyperconjugation. The abstraction of
Figure 1. The packing of axially fluorinated dioxane (3) (top and bottom left) and cyclohexane-based molecules (17) (middle and bottom
right) is dominated by a strong correlation of the local C–F dipole moments.
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P. Kirsch, A. Hahn, R. Fröhlich, G. Haufe
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Scheme 4. DFT calculations [B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d)+ZPE level of theory][13] show that the axial C–F bond is shorter
(i.e. more stable) in the dioxane system 21 compared with that of the cyclohexane analogue 18. The overall energy increase of 3.5 kcalmol–1
in the isodesmic equation is caused by the additional transannular strain that is induced by the repulsion of the axial electron pairs of
the oxygen atoms and the fluorine substituent.
Scheme 5. Proposed mechanism for the autocatalytic elimination of HF from tertiary axially fluorinated cyclohexanes and 1,3-dioxanes
(top). The thermodynamic stabilization of the axial carbon–fluorine bond in 21, or in other words, the destabilization of the carbocationic
intermediate 23 by the presence of ring oxygen atoms, is estimated to be 10.2 kcalmol–1 [bottom: isodesmic equation at the B3LYP/6-
311+G(2d,p)//B3LYP/6-31G(d) + ZPE level of theory].[13]
8: A suspension of NaH (80% in paraffin, washed before use with
dry THF, 1.2 g, 40 mmol) in THF (70 mL) was cooled to 0 °C. A
solution of diethyl n-propylmalonate (6, 6.7 g, 33 mmol) in THF
(30 mL) was added dropwise while the cold bath was maintained.
The reaction was stirred for 2 h at 0 °C. The temperature was then
raised to room temperature over the course of 1 h and raised again
to 45 °C over a one hour period. The mixture was then stirred for
18 h at room temperature. After the reaction was cooled to 0 °C, a
suspension of Selectfluor (17.0 g, 44 mmol) in acetonitrile (100 mL)
was added dropwise. The mixture was stirred for 2 h at 0 °C and
the fluoride ion as well as the subsequent deprotonation is
facilitated by the HF catalyst. The isodesmic equation de-
picted in Scheme 5 indicates that carbocationic dioxane in-
termediate 23, afforded from the elimination reaction, is de-
stabilized by 10.2 kcalmol–1 compared with 22. This is pre-
dominantly due to the combined negative inductive effects
(–I) of the two ring oxygen atoms. In other words, the C–F
bond in 21 is stabilized (compared with that of 18) by this
amount of energy with regard to its heterolytic dissoci-
ation.[14] A secondary contribution is the release of transan- for 2 h at room temperature.CH2Cl2 (300 mL) was then added fol-
lowed by HCl (1 , 100 mL). The organic layer was separated and
washed with saturated aqueous NaHCO3 solution and then with
water. The solution was dried with Na2SO4, the solvent removed
in vacuo, and the crude product purified by Kugelrohr distillation
(100 °C, 0.3 mbar) to yield product 8 (4.8 g, 66%) as a colorless oil
nular strain by “shifting” the fluorine substituent from 21
to 18.
Summary and Conclusion
1
(88% purity by GC, contains ca. 10% of unreacted 6). H NMR
ax-5-Fluoro-1,3-dioxane has been identified as a highly
effective structural building block that imparts a strongly
negative dielectric anisotropy in liquid crystals that are suit-
able for LCD applications. Compared with the analogous
(250 MHz, CDCl3, 303 K): δ = 4.29 (q, J = 7.3 Hz, 4 H), 2.23–2.06
(m, 2 H), 1.51–1.37 (m, 2 H), 1.22 (t, J = 6.8 Hz, 6 H), 0.97 (t, J
= 7.5 Hz, 3 H) ppm. MS (EI = 70 eV): m/z (%) = 221 (0.5) [M +
1]+, 178 (27), 160 (10), 150 (11), 148 (24), 122 (13), 119 (40), 102
materials that are based on axially fluorinated cyclohexane, (23), 99 (19), 91 (77), 73 (22), 55 (16), 43 (12), 29 (100).
the dioxane substructure does not only increase the dipole
moment perpendicular to the long molecular axis, but it
also significantly improves the chemical stability of the
compound. The observed stabilization of the tertiary alkyl
9: A suspension of LiAlH4 (4.0 g, 105 mmol) in a mixture of tolu-
ene (25 mL) and THF (90 mL) was cooled to –30 °C. At this tem-
perature, a solution of 8 (purity 90.5% by GC, 15.5 g, 64 mmol) in
THF (20 mL) was added dropwise. The mixture was warmed to
fluoride – compared with its carbocyclic analogue – can be 30 °C (under careful temperature control; the reaction was occa-
sionally cooled with a water bath). This temperature was main-
tained for 20 min and then stirred for 90 min at room temperature
under temperature control. The reaction mixture was ice-cooled
and quenched by the careful addition of a solution of Na2CO3
(29 g) in water (12 mL). The precipitate was filtered, the filtrate
dried with Na2SO4, and the solvents evaporated to dryness. The
crude product was purified by chromatography (silica gel, n-hex-
ane/ethyl acetate 3:2), to yield two fractions. The first fraction
(1.5 g) was discarded. The second fraction afforded 9 (5.5 g, 51%)
as a colorless oil (80% purity by GC, contains ca. 10% of the non-
attributed to the inductive destabilization of the corre-
sponding carbocation by the ring oxygen atoms.
Experimental Section
General: All reactions – in particular those with NaH and LiAlH4 –
were carried out in carefully dried solvents and under a nitrogen
atmosphere. The reagents trans-4-propylcyclohexyl carbaldehyde
and diethyl trans-4-propylcyclohexylmalonate were obtained from
Merck KGaA. The mesophase sequence and electrooptical param-
eters for compound 3, 15, and 16 are gathered in Table 1.
1
fluorinated diol analogue). H NMR (250 MHz, CDCl3, 303 K): δ
= 3.83–3.66 (m, 4 H), 2.90 (br. s, 2 H), 1.71–1.56 (m, 2 H), 1.47–
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Liquid Crystals Based on Axially Fluorinated 1,3-Dioxanes
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1.35 (m, 2 H), 0.95 (t, J = 7.1 Hz, 3 H) ppm. 19F NMR (280 MHz,
CDCl3, 303 K): δ = –172.52 (mc, 1F) ppm. MS (EI = 70 eV): m/z
ous phases were extracted once with CH2Cl2. The combined or-
ganic phases were dried with Na2SO4, the solvent was removed in
(%) = 137 (2) [M + 1]+, 131 (5), 103 (7), 98 (8), 88 (100), 75 (16), vacuo, and the crude product was purified by chromatography (sil-
67 (13), 60 (75), 41 (32), 29 (30).
ica gel, n-heptane/methyl tert-butyl ether 4:1) to yield product 12
(19 g, 77%) as a colorless oil (86% purity by GC). 1H NMR
(250 MHz, CDCl3, 303 K): δ = 4.30 (q, J = 7.1 Hz, 4 H), 2.44–2.22
(m, 1 H), 1.85–1.76 (m, 2 H), 1.65–1.55 (m, 2 H), 1.40–1.10 (m, 13
H), 1.05–0.84 (m, 5 H) ppm. 13C NMR (75 MHz, CDCl3, 303 K):
δ = 166.1 (d, JC,F = 25.8 Hz), 97.3 (d, JC,F = 203.0 Hz), 62.4, 42.7
10: A solution of 9 (80% by GC, 5.0 g, 29 mmol) in DMF (50 mL)
and triethylamine (22.2 mL, 160 mmol) was treated dropwise with
Me3SiCl (10.1 mL, 80 mmol) while a maximum temperature of
30 °C was maintained. The reaction was stirred for 2 h at room
temperature and n-pentane (100 mL) was added. The organic phase
was separated, washed twice with ice-cold saturated aqueous
NaHCO3, once with ice-cold HCl (3%), again with aqueous
NaHCO3, and finally with brine. The collected aqueous layers were
extracted with n-pentane. All organic phases were combined, dried
with Na2SO4, and the solvents evaporated to dryness. The crude
product (8.8 g) was subjected to Kugelrohr distillation. Three frac-
tions were obtained. Fraction 1 (1.0 g, b.p. 63–65 °C/0.4 mbar, 71%
purity by GC), fraction 2 (6.0 g, b.p. 65–68 °C/0.4 mbar, 84% pu-
rity) and fraction 3 (0.7 g, 68–69 °C/0.4 mbar, 88% purity). Frac-
(d, JC,F = 20.6 Hz), 39.4, 36.7, 32.4, 25.9, 19.9, 14.3, 14.1 ppm. 19
F
NMR (235 MHz, CDCl3, 300 K): δ = –178.39 (d, J = 27.5 Hz, 1F)
ppm. MS (EI = 70 eV): m/z (%) = 303 (1) [M]+, 178 (100), 150 (21),
122 (19), 83 (5), 81 (8), 67 (7), 55 (11).
13: A suspension of LiAlH4 (5.7 g, 150 mmol) in a mixture of tolu-
ene (18 mL) and THF (140 mL) was cooled to –30 °C. At this tem-
perature, a solution of 12 (purity 85% by GC, 29.0 g, 82 mmol) in
THF (20 mL) was added dropwise. The reaction was stirred for
30 min, and the mixture was warmed to room temperature (under
careful temperature control and a water bath if necessary to cool
tions 2 and 3 were combined to yield 10 (6.7 g, 69%) as a colorless
1
oil. H NMR (250 MHz, CDCl3, 303 K): δ = 3.65–3.43 (m, 4 H), the reaction). The reaction was then stirred for an additional 3 h.
1.59–1.43 (m, 2 H), 1.38–1.23 (m, 2 H), 0.85 (t, J = 7.7 Hz, 3 H),
0.03 (s, 18 H) ppm. MS (EI = 70 eV): m/z (%) = 280 (0.5) [M]+,
265 (20), 260 (29), 217 (75), 191 (10), 173 (12), 170 (12), 159 (23),
147 (38), 107 (21), 103 (71), 77 (32), 75 (25), 73 (100), 45 (13).
The reaction mixture was ice-cooled and quenched by the careful
addition of a solution of Na2CO3 (41 g) in water (18 mL). After
30 min, the precipitate was filtered off and washed with methyl tert-
butyl ether. The filtrate was dried with Na2SO4 and the solvents
evaporated to dryness. The crude product (24 g) was crystallized
from acetonitrile (ca. 75 mL) at –20 °C to afford 13 (10.0 g, 50%)
as a colorless solid (86% purity by GC). 1H NMR (250 MHz,
CDCl3, 303 K): δ = 3.89 (d, J = 6.3 Hz, 2 H), 3.81 (dd, J = 8.5 Hz,
J = 3.4 Hz, 2 H), 2.50 (br. t, J = 6.3 Hz, 2 H), 1.86–1.70 (m, 5 H),
1.38–1.26 (m, 2 H), 1.20–1.05 (m, 5 H), 0.99–0.84 (m, 5 H) ppm.
13C NMR (75 MHz, CDCl3, 303 K): δ = 99.1 (d, JC,F = 172.6 Hz),
63.6 (d, JC,F = 25.8 Hz), 41.0 (d, JC,F = 20.0 Hz), 39.3, 37.0, 32.8,
26.3 (d, JC,F = 5.3 Hz), 19.8, 14.2 ppm. 19F NMR (235 MHz,
CDCl3, 300 K): δ = –176.20 (mc, 1F) ppm. MS (EI = 70 eV): m/z
(%) = 218 (1) [M]+, 198 (10), 180 (14), 170 (100), 149 (21), 141 (13),
128 (22), 123 (67), 121 (34), 109 (25), 107 (27), 95 (37), 93 (28), 81
(94), 69 (90), 67 (70), 57 (23), 55 (84), 43 (32), 41 (84).
3: A mixture of 10 (85% purity, 6.5 g, 20 mmol), CH2Cl2 (80 mL)
and Me3SiOTf (0.6 mL, 3 mmol) was cooled to –70 °C.A solution
of trans-4-propylcyclohexyl carbaldehyde (86% purity, 3.9 g,
25 mmol) in CH2Cl2 (20 mL) was then added dropwise at the same
temperature. After stirring for 2 h at –70 °C, the reaction was
quenched by the addition of pyridine (2.4 mL), and the mixture
was warmed to room temperature. Saturated aqueous NaHCO3
(100 mL) was added. The reaction was stirred for 15 min, the or-
ganic layer separated, and the aqueous layer extracted with
CH2Cl2. The organic phases were combined, washed with water,
dried with Na2SO4, and the solvents evaporated to dryness. The
crude product (8.7 g) was purified by chromatography (silica gel,
n-hexane/methyl tert-butyl ether 4:1) and subsequently crystallized
from n-hexane at –20 °C to furnish product 3 (1.3 g, 24%) as color- 14: A solution of 13 (89% by GC, 10.0 g, 41 mmol) in DMF
1
less crystals. M.p. 112 °C (n-hexane). H NMR (250 MHz, CDCl3,
303 K): δ = 4.18 (d, J = 5.3 Hz, 1 H), 4.13 (t, J = 13.2 Hz, 2 H),
3.60 (dd, J = 35.3 Hz, J = 13.2 Hz, 2 H), 1.92–1.72 (m, 4 H), 1.63–
0.97 (m, 14 H), 0.93 (t, J = 6.6 Hz, 3 H), 0.86 (t, J = 6.8 Hz, 3 H)
ppm. 13C NMR (75 MHz, CDCl3, 303 K): δ = 105.4, 89.6 (d, JC,F
= 178.7 Hz), 72.4 (d, JC,F = 21.9 Hz), 42.6, 40.1, 37.7, 36.9 (d, JC,F
= 21.4 Hz), 32.8, 27.7, 20.4, 15.7 (d, JC,F = 3.5 Hz), 14.8 ppm. 19F
(65 mL) and triethylamine (30.5 mL, 220 mmol) was treated drop-
wise with Me3SiCl (13.9 mL, 110 mmol) while a temperature of
30 °C was maintained. The reaction was stirred for 2 h at room
temperature and n-pentane (150 mL) was added. The organic phase
was separated, washed twice with ice-cold saturated aqueous
NaHCO3, once with ice-cold HCl (3%), again with aqueous
NaHCO3, and finally with brine. The collected aqueous layers were
NMR (235 MHz, CDCl3, 300 K): δ = –169.60 (mc, 1F) ppm. MS extracted with n-pentane. All organic phases were combined, dried
(EI = 70 eV): m/z (%) = 272 (0.6) [M]+, 271 (2.5), 147 (100), 81
(15), 55 (10), 41 (10). C16H29FO2: calcd. C 70.5, H 10.7; found C
70.7, H 10.6.
with Na2SO4, and the solvents evaporated to dryness. Crude prod-
uct 14 (16.0 g, 90%, 83% purity by GC) was used for the next
reaction step without further purification. 1H NMR (250 MHz,
CDCl3, 303 K): δ = 3.83–3.63 (m 4 H), 1.85–1.70 (m, 6 H), 1.40–
1.00 (m, 8 H), 0.88 (t, J = 7.3 Hz, 3 H), 0.12 (s, 18 H) ppm. 13C
NMR (75 MHz, CDCl3, 303 K): δ = 99.6 (d, JC,F = 175.8 Hz), 62.6
(d, JC,F = 27.6 Hz), 40.6 (d, JC,F = 20.8 Hz), 40.2, 37.8, 33.7, 27.0,
26.9, 20.4, 14.9 ppm. 19F NMR (280 MHz, CDCl3, 300 K): δ =
–173.90 (mc, 1F) ppm. MS (EI = 70 eV): m/z (%) = 362 (1) [M]+,
347 (10), 342 (25), 252 (17), 241 (11), 217 (100), 191 (9), 163 (19),
147 (20), 129 (12), 121 (12), 103 (36), 73 (60), 55 (14), 41 (12).
12: A suspension of NaH (60% in paraffin, washed before use with
dry THF, 6.4 g, 160 mmol) in THF (200 mL) was cooled to 2 °C.
A solution of diethyl trans-4-propylcyclohexylmalonate (11, purity
96.6%, 31.3 g, 106 mmol) in THF (25 mL) was added dropwise
while the reaction was cooled. The reaction was stirred for 2 h at
2 °C. The temperature was then raised to room temperature over
the course of 1 h and raised again to 40 °C over a three hour
period. The mixture was cooled to 5 °C, and a suspension of Se-
lectfluor (50.0 g, 130 mmol) in acetonitrile (200 mL) was added in
small portions. The mixture was stirred for 3 d at room tempera-
ture.CH2Cl2 (500 mL) was then added followed by water (300 mL).
The organic layer was separated and washed with HCl (1 ⁾, satu-
rated aqueous NaHCO3 and then with water. The combined aque-
15: A mixture of 14 (8.0 g, 22 mmol), CH2Cl2 (50 mL), and Me3Si-
OTf (0.5 g, 2 mmol) was cooled to –70 °C. A solution of butyric
aldehyde (2.3 mL, 25 mmol) in CH2Cl2 (10 mL) was then added
dropwise at the same temperature. The reaction was stirred for 2 h
at –70 °C. The reaction was quenched by the addition of pyridine
Eur. J. Org. Chem. 2006, 4819–4824
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4823

P. Kirsch, A. Hahn, R. Fröhlich, G. Haufe
FULL PAPER
proximated by the long molecular axis. The correlation be-
tween ∆ε, the dipole moment µ, and the angle β between the
molecular dipole and the director is the following: ∆ε Ϸ ∆α –
F (µ2/2kT) (1–3cos2β) S; ∆α is the anisotropy of the polarizabil-
ity, S the order parameter: a) W. Maier, G. Meier, Z. Natur-
forschg. 1961, 16a, 262–267; b) D. Demus, G. Pelzl, Z. Chem.
1981, 21, 1–9.
Reviews: a) S. G. Lal, G. P. Pez, R. G. Syvret, Chem. Rev. 1996,
96, 1737; b) S. D. Taylor, C. C. Kotoris, G. Hum, Tetrahedron
1999, 12431–12477; c) P. Kirsch, Modern Fluoroorganic Chem-
istry: Synthesis Reactivity, Applications, Wiley-VCH,
Weinheim, Germany, 2004, p. 73 ff, and references therein.
For a similar reaction sequence, see J. Hannah, R. L. Tolman,
J. D. Karkas, R. Liou, H. C. Perry, A. K. Field, J. Heterocycl.
Chem. 1989, 26, 1261–1271.
(2.3 mL), and the mixture was warmed to room temperature. Satu-
rated aqueous NaHCO3 (50 mL) was added. The reaction was
stirred for 15 min, the organic layer separated, and the aqueous
layer extracted with CH2Cl2. The organic phases were combined,
washed with water, dried with Na2SO4, and the solvents evaporated
to dryness. The crude product (10 g) was purified by chromatog-
raphy (silica gel, n-heptane/methyl tert-butyl ether 4:1) and sub-
sequently crystallized from n-heptane/methyl tert-butyl ether (4:1)
at –20 °C to furnish product 15 (2.9 g, 48%) as colorless crystals
(99.5% purity by HPLC). M.p. 97 °C (n-heptane/methyl tert-butyl
ether 4:1). 1H NMR (250 MHz, CDCl3, 303 K): δ = 4.48 (t, J =
5.5 Hz, 1 H), 4.04 (t, J = 12.8 Hz, 2 H), 3.62 (dd, J = 36.2 Hz, J
= 13.0 Hz, 2 H), 1.75–1.68 (m, 2 H), 1.65–1.52 (m, 4 H), 1.45–1.05
(m, 12 H), 0.84 (t, J = 7.1 Hz, 3 H), 0.74 (t, J = 7.1 Hz, 3 H) ppm.
[7]
[8]
[9]
R. Noyori, S. Murata, M. Suzuki, Tetrahedron 1981, 37, 3899–
3910.
P. Kirsch, E. Poetsch, Adv. Mater. 1998, 10, 602–606.
13C NMR (75 MHz, CDCl3, 303 K): δ = 102.5, 91.3 (d, JC,F
=
182.9 Hz), 71.8 (d, JC,F = 22.8 Hz), 42.5 (d, JC,F = 20.5 Hz), 40.4,
37.8, 37.6, 33.6, 26.5 (d, JC,F = 3.6 Hz), 20.8, 18.3, 15.3, 14.9 ppm.
19F NMR (235 MHz, CDCl3, 300 K): δ = –175.60 (mc, 1F) ppm.
MS (EI = 70 eV): m/z (%) = 271 (10) [M – H]+, 229 (100), 170 (7),
163 (15), 123 (8), 121 (9), 107 (11), 81 (15), 69 (16), 67 (14), 55
(18), 43 (14), 41 (18).
[10]
[11] The “virtual” parameters TNI,virt, ∆εvirt, and ∆nvirt were deter-
mined by linear extrapolation from a 5% w/w solution in the
commercially available Merck mixture ZLI-4792 (TNI
=
92.8 °C, ∆ε = 5.3, ∆n = 0.0964). The ∆εvirt values for the dielec-
trically negative compounds were obtained from ZLI-2857 (TNI
= 82.3 °C, ∆ε = –1.42, ∆n = 0.0776). The extrapolated values
are empirically corrected for the differences in the order param-
eter which are induced by the analyte. For the pure substances,
the phase transition temperatures were determined by differen-
tial scanning calorimetry (DSC), the mesophase types were as-
signed by optical polarization microscopy.
16: By the same method with trans-4-propylcyclohexyl carbal-
dehyde as the aldehyde component, 16 (38%) was obtained as col-
orless crystals (99.6% purity by HPLC). M.p. 109 °C (n-heptane/
1
methyl tert-butyl ether 4:1). H NMR (250 MHz, CDCl3, 303 K):
δ = 4.06–3.97 (m, 3 H), 3.58 (dd, J = 35.3 Hz, J = 13.2 Hz, 2 H),
1.83–1.55 (m, 8 H), 1.44–0.88 (m, 20 H), 0.80 (mc, 6 H) ppm. 13C
[12] a) Crystal structure data for 3 (C16H29FO2), by crystallization
from n-hexane: monoclinic, P21/n, a = 4.841(1) Å, b =
9.731(1) Å, c = 35.171(1) Å, α = γ = 90°, β = 92.47(1)°, V =
1655.3(4) Å3, Z = 4, ρcalcd. = 1.093 g·cm–1, R(F) = 6.5% for
2709 observed independent reflections (2.17°ՅϑՅ25.00°).
The data were collected at 198 K. b) Crystal structure data for
17 (C22H40F2), by crystallization from n-heptane: monoclinic,
P21/n, a = 19.016(4) Å, b = 5.0060(10) Å, c = 22.038(4) Å, α =
NMR (75 MHz, CDCl3, 303 K): δ = 104.0, 89.6 (d, JC,F
=
180.7 Hz), 70.1 (d, JC,F = 22.7 Hz), 41.4, 40.7 (d, JC,F = 20.4 Hz),
38.9, 38.7, 36.5, 36.0, 31.9, 31.7, 26.5, 24.8, 24.7, 19.2, 19.1, 13.5
ppm. 19F NMR (235 MHz, CDCl3, 300 K): δ = –175.65 (mc, 1F)
ppm. MS (EI = 70 eV): m/z (%) = 353 (3) [M – H]+, 229 (100), 163
(10), 107 (5), 81 (9), 69 (14), 55 (12), 41 (9).
γ = 90°, β = 100.79(3)°, V = 2060.8(7) Å3, Z = 4, ρcalcd.
=
1.104 g·cm–1, R(F) = 5.7% for 4209 observed independent re-
flections (2.82°ՅϑՅ74.39°). The data were collected at 223 K.
CCDC-607632 (3) and CCDC-607631 (17) contain the supple-
mentary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[13] Gaussian 98, Revision A.6: M. J. Frisch, G. W. Trucks, H. B.
Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G.
Zakrzewski, J. A. Montgomery Jr, R. E. Stratmann, J. C. Bu-
rant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin,
M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R.
Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J.
Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma,
D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman,
J. Cioslowski, J. V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox,
T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C.
Gonzalez, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, J. L. Andres, C. Gonzalez, M. Head-Gor-
don, E. S. Replogle, and J. A. Pople, Gaussian, Inc., Pittsburgh,
PA, USA, 1998. The minimum geometries were optimized at
the B3LYP/6-31G(d) level of theory, and were verified to have
only positive eigenfrequencies. The energies were calculated at
the B3LYP/6-311+G(2d,p) level of theory, with the B3LYP/6-
31G(d) geometries and zero-point energies.
Acknowledgments
We thank Dr. J. Krause, V. Reiffenrath, J. Haas, H. Heldmann, K.
Altenburg, and Dr. M. Klasen-Memmer for the physical evaluation
of the newly synthesized substances, and Dr. M. Bremer for the
helpful discussions of the DFT calculations. Part of this work was
supported by the German Bundesministerium für Bildung und
Forschung (01 B 621/1).
[1] a) D. Pauluth, K. Tarumi, J. Mater. Chem. 2004, 14, 1219–
1227; b) D. Pauluth, K. Tarumi, J. SID 2005, 13, 693–702.
[2] a) P. Kirsch, M. Bremer, Angew. Chem. 2000, 112, 4384–4405;
Angew. Chem. Int. Ed. 2000, 39, 4216–4235.
[3] a) A. Takeda, S. Kataoka, T. Sasaki, H. Chida, H. Tsuda, K.
Ohmuro, Y. Koike, T. Sasabayashi, K. Okamoto, SID ’98 Di-
gest 1998, 1077; b) Y. Ishii, S. Mizushima, M. Hijikigawa, SID
’01 Digest 2001, 1090; c) K. H. Kim, K. Lee, S. B. Park, J. K.
Song, S. Kim, J. D. Souk, Asia Display 1998, 383; d) P. Kirsch,
V. Reiffenrath, M. Bremer, Synlett 1999, 389–396.
[4] M. Bremer, L. Lietzau, New J. Chem. 2005, 29, 72–74.
[5] a) P. Kirsch, K. Tarumi, Angew. Chem. 1998, 110, 501–506;
Angew. Chem. Int. Ed. Engl. 1998, 37, 484–489; b) P. Kirsch,
K. Tarumi, Liquid Cryst. 1999, 26, 449–452; c) R. W.
Hoffmann, T. Brandl, P. Kirsch, K. Harms, Synlett 2001, SI, [14] A similar effect caused by the destabilization of a cationic in-
960–963.
termediate is described in: A. Burchardt, T. Takahashi, Y.
Takeuchi, G. Haufe, J. Org. Chem. 2001, 66, 2078–2084.
Received: June 20, 2006
[6] The dielectric anisotropy is defined as ∆ε = εʈ –εЌ, the birefrin-
gence as ∆n = nʈ –nЌ, where ʈ stands for parallel and Ќ stands
for perpendicular to the nematic phase director, which is ap-
Published Online: August 24, 2006
4824
www.eurjoc.org
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2006, 4819–4824