A Novel Type of Liquid Crystals Based on Axially Fluorinated Cyclohexane Units

Full text of DOI:10.1002/(SICI)1521-3773(19980302)37:4<484::AID-ANIE484>3.0.CO;2-6

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A Novel Type of Liquid Crystals Based on
Axially Fluorinated Cyclohexane Units**
Peer Kirsch* and Kazuaki Tarumi
The recent boom in the development of multimedia liquid
crystal displays (LCDs) has created a strong demand for new
liquid crystalline materials with high clearing temperatures,
high dielectric anisotropy (De), and low rotational viscosity
(g1).[1] Other desired properties with increasing importance
for active matrix displays (AMDs) are high specific resistance
(SR) and good voltage holding ratio (VHR) of LC mixtures.[2]
Liquid crystals with highly fluorinated aromatic substructures
meet most of these criteria and are now commonly used as
major components of LC mixtures for AMDs with fast
response times.[1]
Because of the strong dipole moment of the C ± F bond,
arenes with terminal or lateral fluorine substituents are well
suited for achieving a strongly positive or negative dielectric
anisotropy De.[3] Unfortunately, the price to pay for such
a high De is a decrease in the clearing temperature by
about 25 ± 30 K per fluorine substituent.[1] Especially fluori-
nated two-ring liquid crystals, whose major advantage is
their low rotational viscosity, suffer from very low clearing
points. Therefore, we are investigating structural modifica-
tions that improve the clearing points without losing the
advantageous properties of fluorinated two- and three-ring
systems.
Currently, most active matrix displays use liquid crystal
mixtures with a positive dielectric anisotropy. In contrast, the
recently introduced VA-TFT LCD (vertical alignment thin
film transistor LCD)[4]Ðwhich offers a superior picture
quality with a wide viewing angle (1608), high contrast, and
video-compatible switching times (<20 ms)Ðis based on a
liquid crystal mixture with negative dielectric anisotropy and
very low birefringence (Dn ꢀ 0.08). In the first generation of
technically applicable materials (1, De ˆ 8.2; Scheme 1)[5]
the negative De was achieved by means of the strong
perpendicular dipole moment of a cyano group attached to
an axial position of a dialkylbicyclohexane as mesogenic core
structure. Unfortunately, nitriles in general cannot be used in
AMDs because of their tendency to reduce the mixtureꢁs
voltage holding ratio (VHR), which is a highly critical
parameter for any kind of AMD technology.[1, 2] Nowadays,
most AMD mixtures with negative De are based on second-
generation materials such as the laterally difluorinated
phenylbicyclohexanes 2 (De ˆ 2.7) or 3[6] (De ˆ 5.9).
Scheme 1. Examples for first- (1) and second-generation (2, 3) materials
with negative dielectric anisotropy.
A general disadvantage of all compounds with the 2,3-
difluorobenzene subunit is their relatively high rotational
viscosity (2: g1 ˆ 184; 3: g1 ˆ 413 mPas), which leads to longer
switching times. In addition, recent experience indicates that
materials containing no heteroatoms other than fluorine
display the best VHR values. Therefore, it would be desirable
to replace compounds such as 3 (which contains an ether
group) by a pure fluorohydrocarbon.
To reduce the driving voltage and therefore power con-
sumption of VA-TFT displays, we looked for new liquid
crystals with strongly negative dielectric anisotropy, low
birefringence, high clearing points, and low rotational viscos-
ity. These new materials should contain no aromatic sub-
structures (which increase Dn) and no non-fluorine hetero-
atoms (which sometimes decrease the VHR).
A rational approach to this problem would be the attach-
ment of one or more perpendicular dipoles to a nonaromatic
mesogenic core structure. Especially suitable for this purpose
are the tertiary axial positions of cyclohexane substructures of
dialkyl bicyclohexanes, bis(cyclohexyl)ethanes, or their ho-
mologues (see 1). If the axial hydrogen atom is replaced by
fluorine, dipole moments perpendicular to the long molecular
axis are created (4 ± 6, Scheme 2). If more than one axial
fluorine atom is introduced, the resulting C ± F dipoles can be
oriented parallel (6), which leads to mutual enhancement, or
antiparallel (5), so that they cancel each other.
Scheme 2. Dielectrically negative target compounds with purely aliphatic
mesogenic core structures containing axially fluorinated cyclohexyl sub-
units (R1, R2 are combinations of C2H5, C3H7, and C5H11).
[*] Dr. P. Kirsch, Dr. K. Tarumi
Merck KGaA, Business Unit Liquid Crystals
D-64271 Darmstadt (Germany)
Fax: (‡49)6151-72-2593
E-mail: peer.kirsch@merck.de
In the search for new materials with improved mesogenic
and viscoelastic properties in the 1980s, many attempts were
made to synthesize liquid crystals containing axially fluori-
nated cyclohexane building blocks. The few examples known
up to now were prepared by reaction of the respective tertiary
cyclohexanol with diethylaminosulfur trifluoride (DAST),[5, 7]
or by treating an alkylcyclohexane derivative with diluted
[**] We thank A. Ruhl and R. Sander for experimental assistance, Dr. M.
Bremer for helpful discussions about computational chemistry, and
Dr. J. Krause, J. Haas, and H. Heldmann for the physical evaluation of
the new substances. Part of this work was performed under the
management of the Association of Super-Advanced Electronics
Industries (ASET) in the R & D program of the Japanese Ministry
of International Trade and Industry (MITI) supported by the New
Energy and Industrial Development Organization (NEDO).
484
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elemental fluorine.[8] Both procedures are potentially hazard-
ous and unsuitable for large-scale syntheses. Furthermore,
most of the axially fluorinated cyclohexane derivatives
decomposed during purification or on extended storage. Here
we report on an alternative approach, which also allows more
complex compounds containing two or more axial fluorine
atoms to be synthesized.
The syntheses of the axially monofluorinated bicyclohex-
anes 4 started from the corresponding cyclohexylcyclohex-
anones 7 (Scheme 3). A Wittig reaction furnished the
Scheme 5. Synthesis of 6a and 6b: a) H2, 5% Pd/C, toluene (89%).
‡
b) H7C3PPh3 Br , KOtBu, THF; 108C !room temperature, 2 h (70%).
c) LiAlH4, THF; reflux, 3 h (88%). d) CBr4, PPh3, CH3CN; 58C !room
temperature, 18 h (69%). e) PPh3, 1,3-dimethylhexahydro-2-pyrimidinone
(DMPU); 758C, 18 h (63%). f) KOtBu, THF; 108C !room temperature,
2 h (22a: 56%, 22b: 29%). g) 19.5 equiv of 70% HF/pyridine, CH2Cl2;
158C !108C, 10 min (6a: 17%, 6b: 18%).
Scheme 3. Synthesis
of
the
fluorobicyclohexanes
4a ± d:
‡
a) R'CH2PPh3 Br , KOtBu, THF; 108C !room temperature, 2 h (80 ±
90%). b) 4 equiv of 70% HF/pyridine, CH2Cl2; room temperature, 18 h
(15 ± 50%; R2 ˆ R'CH2).
alkylidenebicyclohexanes 8, which were diluted with dichloro-
methane and treated with 70% HF/pyridine (Olahꢁs re-
agent).[9] The crude products were extracted with n-heptane,
and the extracts filtered through silica gel. Crystallization
from n-heptane furnished the pure axially fluorinated com-
pounds 4a ± d in 15 ± 50% yield, mainly depending on the
solubility of the product.[10] The more complex compounds
5a/b and 6a/b were synthesized analogously (Schemes 4
and 5).
In addition, we were interested in the general influence of
axial fluorination on the physical properties of liquid crystals
with different mesogenic core structures, including those with
positive dielectric anisotropy. Therefore, we synthesized some
representative compounds for comparison with their non-
fluorinated analogues (23 ± 29, Scheme 6).
Scheme 6. Dielectrically positive target compounds and their nonfluori-
nated analogues.
Scheme 4. Synthesis of the symmetric (5a) and unsymmetric (5b)
‡
‡
difluorobicyclohexanes: a) H11C5PPh3 Br or H7C3PPh3 Br , KOtBu,
THF; 108C !room temperature, 2 h (10: 85%, 12: 74%). b) 122 equiv
of 70% HF/pyridine, CH2Cl2; room temperature, 18 h (5a: 32%, 5b:
26%). c) HCOOH, toluene; room temperature, 18 h (97%).
d) 1. C5H11MgBr, THF; 2. cat. TsOH, ethylene glycol, toluene; azeotropic
removal of water (32%).
Earlier publications on axially fluorinated cyclohexane
derivatives[7, 8] mention the instability of these substances.
Therefore, we first explored the limitations on the use of, for
example, axially fluorinated phenylcyclohexane derivatives in
the synthesis of more complex liquid crystalline materials.
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The starting materials for all the newly synthesized com-
pounds are the corresponding cyclohexanones, which were
prepared by literature methods.[11] The hydrofluorination
procedure functions very well for all alkylidenecyclohexylar-
enes with electron-withdrawing substituents and provides the
axially fluorinated phenylcyclohexanes in good yields. An
example is the smooth conversion of 32 into 23a (Scheme 7).
Scheme 7. Synthesis of the axially fluorinated phenylcyclohexane deriva-
‡
tive 23a and the unexpected reaction product 34: a) H11C5PPh3 Br ,
KOtBu, THF; 108C !room temperature, 2 h (32: 57%, 33: 83%).
b) CH2Cl2, ca. 100 equiv of 70% HF/pyridine; room temperature, 18 h
(23a: 19%, 34: quantitative according to GC, yield of isolated product
58%)
Scheme 8. Synthesis of the axially fluorinated phenylcyclohexane building
blocks 43 and 44 and subsequent coupling reactions: a) 1. 35: BuLi, KOtBu,
THF; 1008C, 30 min; 36: BuLi, THF; 758C, 30 min. 2. I2, THF;
metalation temperature ! 258C (37: 61%, 38: 65%). b) 98% HCOOH,
In contrast, for compound 33, which contains an electron-
donating methoxy group, the same hydrofluorination condi-
tions result in quantitative formation of the benzobicyclooc-
tane derivative 34. The unexpected formation of this product
can be explained by the intramolecular electrophilic attack of
an intermediate carbocationic species on the activated
methoxyphenyl group.[12]
‡
toluene; room temperature, 18 h (39: 79%, 40: 70%). c) H7C3PPh3 Br ,
KOtBu, THF; 108C !room temperature, 2 h (41: 84%, 42: 70%).
d) CH2Cl2, 4 equiv of 70% HF/pyridine; 258C !room temperature, 1 h
(43: 45%, 44: 44%). e) N-Methylpyrrolidone, 0.1 equiv of 2,6-bis-tert-
butylpyridine, 1.3 equiv of CuCN; 1208C, 2 h (24a: 66%, 25a: 25%). f) 4-
Ethylphenylacetylene, 0.026 equiv CuI, 0.004 equiv [PdCl2(PPh3)2], NEt3;
room temperature, 3 h (29%).
As shown in Scheme 8, the building blocks 43 and 44 can be
converted under drastic conditions into either the nitriles 24a
and 25a (2 h at 1208C) or, in the case of 44, into the
cyclohexyltolane 26a (NEt3 solvent, transition metal catalyst).
To inhibit autocatalytic dehydrofluorination, the cyanation
reaction was conducted in the presence of a small amount of
2,6-bis-tert-butylpyridine as a proton scavenger. The yields of
both reactions were in the same range as for the analogous
synthesis of the nonfluorinated liquid crystals 24b, 25b, and
26b.
Under the same conditions as for the phenylcyclohexane
derivatives, hydrofluorination with HF/pyridine in dichloro-
methane can also be used to prepare the more complex three-
ring systems 27a, 28a, and 29a (Scheme 6). The starting
materials are the corresponding phenylcyclohexylcyclohexa-
nones and the biphenylcyclohexanone.
fications required for an optimal display performance. Many
of the two-ring compounds display no mesophases as pure
substances (see also Table 1) but are nevertheless indispen-
sable components of the LC mixtures currently in use for
displays. For this reason, the physical behavior of a new liquid
Table 1. Physical properties of the axially fluorinated cyclohexane deriv-
atives compared to their nonfluorinated analogues. To determine the
extrapolated clearing points TNI,extr, dielectric anisotropies De, birefringen-
ces Dn, and rotational viscosities g1, see ref. [13]. The increase in the
extrapolated clearing points DTNI is calculated with respect to the
corresponding nonfluorinated cyclohexane derivatives. The temperatures
are given in 8C, and the g1 values in mPas. C ˆ crystalline, N ˆ nematic, I ˆ
isotropic. Numbers in parentheses denote monotropic phase transitions.
Entry
X
Mesophases
TNI,extr
DTNI
De
Dn
g1
23a
23b
24a
24b
25a
25b
26a
26b
27a
27b
28a
28b
29a
29b
F
H
F
H
F
H
F
H
F
H
F
H
F
H
C 32 I
C 14 I
C 50 I
C 39 N ( 11) I
C 64 I
28.0
44.2
15.6
33.6
50.0
63.3
218.3
211.0
126.4
116.0
89.4
74.8
43.4
25.0
‡ 16.2
5.9 0.079
5.6 0.089
±
38
±
All newly synthesized axially fluorinated cyclohexane
derivatives are stable on extended storage at ambient temper-
ature. In addition, their stability at elevated temperatures is
clearly demonstrated by their successful conversion to cyano
and alkinyl derivatives.
The evaluation of all the newly synthesized materials
concentrated on mesogenic properties and electrooptic data,
extrapolated from a standard LC mixture.[13] In LC displays
pure liquid crystals are never used, but mixtures of typically 10
to 15 liquid crystalline components. Blending is the only
method to achieve a broad nematic phase range and to adjust
electrooptic and viscoelastic properties exactly to the speci-
±
‡ 18.0 27.1 0.124
±
26.0 0.123
98
‡ 13.3 34.7 0.114 231
C 57 I
±
‡7.3
±
32.6 0.110
0.1 0.236
1.3 0.234
4.9 0.080 293
6.4 0.079 160
9.8 0.082 301
9.7 0.075 171
99
±
±
C 107 N, decomp
C 81 N 197 I
C 62 N, decomp
C 46 N 124 I
C 73 N 115 I
C 66 N 94 I
C 73 N (48.8) I
C 64 I
‡0.4
±
‡ 14.6
±
‡ 18.4 16.0 0.126 292
±
15.2 0.135 173
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crystal in a complex mixture is of utmost importance for
applied liquid crystal research.
clearing point (TNI,extr) by more than 50 K. Also the melting
points and the SB ± I transition temperatures of the pure
compounds 4 are increased by about 10 ± 20 K. The general
tendency to form smectic B (SB) mesophases is not influenced
by fluorination. As expected, the birefringence of 4 is even
slightly lower than that of the nonfluorinated compounds 23.
The dielectric anisotropy De is between 2.1 and 2.4.
Remarkably, the rotational viscosities of the axially mono-
fluorinated bicyclohexanes are in the range of 50 ± 75 mPas, in
contrast to 184 mPas for the currently used material 2
(Table 3). With exception of the lower birefringence and the
much lower g1 values, the axially fluorinated bicyclohexanes
have quite similiar properties to aromatic compounds such
as 2.
For all dielectrically positive materials, axial fluorination
has a significant effect on the mesogenic behavior: The
melting points and the real and extrapolated[13] clearing points
are increased by 10 ± 20 K. Compounds 26a and 27a decom-
pose with loss of HFabove 1208C. The nematic phase range of
28a is broader by 14 K than than that of its nonfluorinated
analogue 28b. For 29a there is a monotropic nematic phase,
while 29b exhibits no mesophase.
For 23 ± 29 with a positive De, axial fluorination has only a
marginal effect on the electrooptic characteristics. For the
weakly polar compounds 26a/b and 27a/b the dielectric
anisotropy becomes smaller; for the other, more polar
substances it increases slightly. Fluorination has no significant
effect on the birefringence.
The rotational viscosity (g1) of a liquid crystal mixture is
directly proportional to its switching time. To obtain the fast-
switching LC mixtures required for multimedia displays, it is
desirable to use single components with a g1 value as low as
possible. For this reason, the extrapolated rotational viscos-
ity[13] of a liquid crystalline compound is one of the most
important parameters for evaluating new materials. Since
there is also an empirical correlation of g1 with the clearing
point and polarity of LC materials, it must always be seen in
relation to these parameters. For all the newly synthesized
compounds, axial fluorination approximately doubles the
rotational viscosity. This increase probably outweighs the
advantage of the higher clearing points for the three-ring
compounds. However, for highly polar two-ring systems such
as 25a, the increased clearing point is a dramatic improve-
ment and makes axially fluorinated phenylcyclohexanes a
very promising class of substances for future application in
AMDs.
Table 3. Physical properties of some conventional materials 1, 2, and 3 with
negative dielectric anisotropy. Clearing points TNI,extr, birefringences Dn,
and rotational viscosities g1 are extrapolated from Merck liquid crystal
mixture ZLI-4792, and dielectric anisotropies De from ZLI-2857.[13] The
temperatures are given in 8C, and the g1 values in mPas. C ˆ crystalline,
SB ˆ smectic B, N ˆ nematic, I ˆ isotropic. Values in parentheses denote
monotropic phase transitions.
Entry
Mesophases
TNI,extr
De
Dn
g1
1
2
3
C 34 N (19) I
C 58 N 106.2 I
C 79 SB (78) N 184.5 I 175.4
14.1
107.8
8.2
2.7
5.9
0.027
0.092
0.096
> 400
184
413
The axially difluorinated bicyclohexanes 5 were synthesized
in order to test the scope of our hydrofluorination method. We
wanted to determine whether a second axial fluorine sub-
stituent leads to an increase in clearing point comparable to
that resulting from the first one. This could result in
extrapolated clearing points exceeding 1508C. As shown in
Table 1, the second fluorine substituent causes a slight
decrease in the clearing point compared to the first substitu-
ent. The difluorinated compounds have low solubility in
organic solvents. Since the C F bonds were assumed to be
aligned in opposite directions, we expected no significant
For the dielectrically negative materials, the results are
somewhat different. Compared to the nonfluorinated di-
alkylbicyclohexanes 45 (Table 2),[14] the first axial fluorine
substituent causes a dramatic increase in the extrapolated
Table 2. Physical properties of the axially fluorinated bicyclohexanes 4 and 5 compared to their nonfluorinated analogues 45. Clearing points TNI,extr
,
dielectric anisotropies De, birefringences Dn, and rotational viscosities g1 are extrapolated from ZLI-4792, and the De values for the dielecrically negative
materials 4a ± d, 5a and 5b from ZLI-2857.[13] The increase in the extrapolated clearing points DTNI is calculated with respect to the corresponding
nonfluorinated dialkylbicyclohexanes 45. The temperatures are given in 8C, and the g1 values in mPas. C ˆ crystalline, SB ˆ smectic B, I ˆ isotropic.
Entry
R1
R2
X
Y
Mesophases
TNI,extr
DTNI
De
2.0
Dn
g1
4a
4b
4c
4d
5b
5a
45a
45b
45c
45d
C2H5
C2H5
C3H7
C5H11
C5H11
C5H11
C2H5
C2H5
C5H11
C5H11
C3H7
C5H11
C2H5
C3H7
C3H7
C5H11
C3H7
C5H11
C3H7
C5H11
H
H
H
H
F
F
F
F
F
F
F
H
H
H
H
C 9 SB 80 I
? SB 95 I
C 31 SB 80 I
C 52 SB 109 I
C 142 I[a]
C 100 SB 153 I[a]
50.1
67.7
28.5
100.7
89.2
105.7
16.7
10.8
56.2
77.0
‡ 33.4
‡ 56.9
‡ 11.8
‡ 44.5
‡ 33.0
‡ 28.7
±
±
±
±
0.036
0.034
0.048
0.048
0.040
0.035
0.050
0.052
0.049
±
52
76
51
±
2.1
2.4
2.1
2.5
2.5
1.1
1.1
0.5
±
F
188
23
25
31
±
H
H
H
H
C
C
3 SB 68.2 I
16 SB 77 I
C 22 SB 98 I
C 41 SB 114 I
±
[a] Decomposition.
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Table 4. Physical properties of liquid crystals based on ethylene-bridged axial
fluorocyclohexane derivatives compared to their nonfluorinated analogues. Clear-
ing points TNI,extr, dielectric anisotropies De, birefringences Dn, and rotational
viscosities g1 are extrapolated from ZLI-4792, and the De values for the dielecrically
negative materials 6a and 6b from ZLI-2857.[13] The temperatures are given in 8C,
and the rotational viscosities g1 in mPas. C ˆ crystalline, SB ˆ smectic B, I ˆ
isotropic.
dielectric anisotropy. To our surprise, 5a and 5b had De
values that surpass those of their monofluorinated analogues.
An explanation for this unexpected behavior is provided by
AM1 calculations (Scheme 9):[15] apart from the approxi-
mately centrosymmetric conformation A with f ˆ 163.18
(Decalcd
ˆ
0.4) there is a gauche conformation B with f ˆ
74.88 (Decalcd
ˆ
2.7). If one assumes a Boltzmann distribu-
Entry R1
R2
X
Mesophases
TNI,extr DTNI
De
Dn
g1
6a
6b
46a
46b
C3H7 C3H7
C3H7 C5H11
C3H7 C3H7
C5H11 C5H11
F
F
H
H
C 78 SB 104 I 76.5
C 68 SB 120 I 81.5
‡ 21.5
4.6 0.051 198
4.2 0.051 251
±
±
±
C 36 SB 73 I
55
±
±
±
C 44 SB 108 I 80
0.1 0.045 134
building blocks were successfully subjected to drastic reaction
conditions and provided yields in the same range as for the
nonfluorinated analogues. Comparison of the physical proper-
ties of the axially fluorinated liquid crystals with their
nonfluorinated analogues shows strongly increased clearing
points TNI accompanied by an (undesired) approximate
doubling of the rotational viscosity g1. At least for low-
viscosity, highly fluorinated two-ring liquid crystals, axial
fluorination offers an interesting and useful method for
improving their generally too low clearing points. We have
obtained a new generation of superior liquid crystals with
negative dielectric anisotropy. Due to their combination of
low birefrincence, low rotational viscosity, and high clearing
point, these materials are very well suited for application in
recently emerging superior active matrix display technologies.
Scheme 9. Influence of the torsion angle f (dihedral angle between the
neighboring central axial hydrogen atoms) on the conformational energy
&
^
DHf and the dielectric anisotropy De of 5a. ± ± ˆ DHf, ± ± ˆ De.
tion between conformer A with 42% and the energetically
1
slightly preferred (190 calmol ) conformer B with 58%
Experimental Section
occupation, this results in a calculated average Deres of 1.7
for 5a. Comparative calculations on 45d indicate that the
axial fluorination of 5a has no significant influence on this
conformer equilibrium, and that the usual bicyclohexanes
will show a similiar rotamer distribution. The g1 value of
188 mPas for compound 5a is quite high in comparison to the
monofluorinated analogues, but it is still in the same range as
for 2, which has a similiar De and extrapolated clearing point
4d: In a Teflon flask, a solution of 8d (100 g, 0.36 mol) in CH2Cl2 (200 mL)
was treated with 70% HF/pyridine (36.3 mL, 1.45 mol, 4 equiv) and stirred
at room temperature for 18 h. The mixture was poured onto ice (300 g) and
extracted three times with CH2Cl2 (100 mL). The combined organic
extracts were washed to neutrality with a saturated solution of aqueous
NaHCO3 and dried over MgSO4. After addition of pyridine (1%), the
solution was evaporated to dryness to give 98 g of crude hydrofluorination
product. The crude product was filtered over silica gel (n-heptane/pyridine
99/1) and crystallized from the same solvent at 208C (38.4 g, 36%, purity
according to HPLC: 99.5%; for mesophases see Table 2). 1H NMR
(500 MHz, CDCl3, 303 K): d ˆ 0.85 ± 1.56 (m, 34H), 1.70 ± 1.75 (m, 2H),
TNI
.
The compounds containing two ethylene-bridged axially
fluorinated cyclohexyl subunits in their mesogenic core
structure, 6a and 6b, have significantly higher extrapolated
clearing points than their nonfluorinated analogues (e.g.,
DTNI ˆ ‡21.5 K for 6a; Table 4). The general tendency to
form SB mesophases is not influenced by fluorination. The
dielectric anisotropy of the bis(fluorocyclohexyl)ethanes 6 lies
in the range of 4.2 to 4.6, in good agreement with the
predictions of AM1 calculations. These values are about
halfway between those of 2 and 3, but in contrast to 3 these
purely fluoroaliphatic materials have no adverse effect on the
VHR. The rotational viscosity of 6b is in the same range as
that of the less polar 2.
1.86 ± 1.92 (mc, 1H); 19F NMR (280 MHz, CDCl3, 303 K): d ˆ 160.4 (mc);
‡
MS (EI): m/z ˆ 276 [M
HF].
Received: August 28, 1997
Revised version: November 3, 1997 [Z10863/10864IE]
German version: Angew. Chem. 1998, 110, 501 ± 506
Keywords: cyclohexane ´ fluorine ´ liquid crystals ´ meso-
phases
[1] a) D. Pauluth, T. Geelhaar, Nachr. Chem. Tech. Lab. 1997, 45, 9 ± 15. b)
W. H. de Jeu, Physical Properties of Liquid Crystalline Materials,
Gordon & Breach, New York, 1980.
[2] A. Sasaki, T. Uchida, S. Miyagami, Japan Display ꢀ86 1986, 62.
[3] a) E. Bartmann, U. Finkenzeller, E. Poetsch, V. Reiffenrath, K.
Tarumi, 22. Freiburger Arbeitstagung Flüssigkristalle, March 31 ± April
2, 1993, contribution no. 8; b) E. Bartmann, J. Krause, K. Tarumi, 23.
Freiburger Arbeitstagung Flüssigkristalle, March 23 ± 25, 1994, contri-
bution no. 27.
Making use of Olahꢁs hydrofluorination method, we were
able to synthesize multigram quantities of a variety of new
liquid crystals with one or more axially fluorinated cyclo-
hexane substructures. In spite of some reports on the inherent
instability of this substance class, axially fluorinated synthetic
488
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Angew. Chem. Int. Ed. 1998, 37, No. 4

COMMUNICATIONS
with inverse electron demand;[2, 3] nevertheless they were
mostly ignored as building blocks in synthesis.[4] The latter can
be attributed to a lack of functionality at the nitrogen atom,
because previously known syntheses gave access to ªsimpleº
N,N-alkyl or aryl-substituted 1-alkynylamines only.[2a]
Herein we report on the synthesis of N-functionalized and
electronically tunable 1-alkynylamines/1-alkynylamides of
type I and II (Scheme 1), as well as their application in regio-
[4] K. Ohmuro, S. Kataoka, T. Sasaki, Y. Koike, SID ꢀ97 Digest 1997, 845.
[5] R. Eidenschink, L. Pohl, M. Römer, B. Scheuble (Merck KGaA),
DE-A 3231707 A1, 1982 [Chem. Abstr. 1984, 101, 63 770].
[6] a) A. E. F. Wächtler, V. Reiffenrath, J. Krause, G. Weber, U.
Finkenzeller (Merck KGaA), EP-B 0364538, 1989 [Chem. Abstr.
1995, 112, 66 886]; b) V. Reiffenrath, J. Krause, H. J. Plach, G. Weber,
Liq. Crystals 1989, 5, 159; c) K. Tarumi, M. Bremer, B. Schuler, IEICE
Trans. Electron. 1996, E79-C, 1035 ± 1039.
[7] a) N. Mongelli, F. Animati, R. DꢁAlessio, L. Zuliani, C. Gandolfi,
Synthesis 1988, 310 ± 313. b) R. Winters, Dissertation, Universität-GH-
Paderborn, Germany, 1991.
[8] S. Rozen, C. Gal, J. Org. Chem. 1987, 52, 2769 ± 2779.
[9] G. A. Olah, J. T. Welch, Y. D. Vankar, M. Nojima, I. Kerekes, J. A.
Olah, J. Org. Chem. 1979, 44, 3872 ± 3881.
[10] Since all axially fluorinated cyclohexane derivatives tend to decom-
pose under GLC conditions, the purity of the newly synthesized
compounds was determined by HPLC. The regio- and stereoselectiv-
ity was verified by 1H and 19F NMR. The mass spectra usually
‡
indicated elimination of HF and display only the [M
20] fragment
instead of the molecular ion. Additional evidence for the desired
trans ± trans arrangement of the axial fluorine atoms are the high
clearing points of all the newly synthesized compounds.
Scheme 1. 1-Alkynylamine and 1-alkynylamide building blocks of type I
and II. EWG ˆ electron-withdrawing group.
[11] a) D. Lednicer, D. E. Emmert, R. Lahti, A. D. Rudzik, J. Med. Chem.
1972, 15, 1239 ± 1243; b) T. Taiho, K. San-nohe, S. Kajiya, T. Suzuki, K.
Otsuka, T. Ito, J. Kamiya, M. Maruyama, ibid. 1989, 32, 351 ± 357; c) M.
Petrzilka, A. Germann, Mol. Cryst. Liq. Cryst. 1985, 131, 327 ± 342;
d) E. M. Sigman, G. L. Closs, J. Phys. Chem. 1991, 95, 5012 ± 5017.
[12] A similiar type of reaction is described in L. Billet, G. Descotes, Bull.
Soc. Chim. Fr. 1971, 2626 ± 2682.
[13] TNI,extr, De, Dn, and g1 were determined by linear extrapolation from a
10% w/w solution in a standard host. For the dielectrically positive
compounds, the commercially available Merck mixture ZLI-4792
(TNI ˆ 92.88C, De ˆ 5.27, Dn ˆ 0.0964) was used. For the dielectrically
negative materials, De was extrapolated from ZLI-2857 (TNI ˆ 82.38C,
De ˆ 1.42, Dn ˆ 0.0776). For the pure substances the mesophases
were identified by optical microscopy, and the phase transition
temperatures by differential scanning calorimetry (DSC).
and stereoselective inter- and intramolecular [2‡2‡1] cyclo-
additions. The protective groups (PGs) of these compounds
perform two functions: On the one hand they might act as
temporary masking groups of the primary or secondary amine
moiety, on the other hand by varying their electron-with-
drawing capacitiesÐtransformation of the amine into a
carbamide, toluenesulfonamide, or trifluorsulfonamideÐa
tuning of electron density and reactivity of the neighboring
triple bond should be possible.
The key step for the synthesis of this new class of
compounds is the ethynylation of the amides 1a ± j with the
readily available trimethylsilylethynyliodonium triflate 2.[5]
Additions of nitrogen nucleophiles to alkynyliodonium salts,
in which 2,3-dihydropyrroles are formed via alkylidene
carbene intermediates and intramolecular 1,5-C ± H inser-
tions, were recently reported by Feldman et al.[6] With respect
to the cases studied herein, and in accordance with a very high
aptitude of silyl groups for 1,2-migrations towards carbenoid
centers such as in 3,[7] preferential formation of 1-alkynyl-
amines and 1-alkynylamides (Scheme 2) is expected. Indeed
the alkynes 4a ± j were obtained as single products after
deprotonation of 1a ± j with n-butyllithium in toluene fol-
lowed by addition of 2 at 20oC (Table 1).[8] Dihydropyrroles 5
were not observed. This method is compatible with other
functional groups (alkenyl-, alkynyl-, and alkoxycarbonyl
residues). However, in some cases a-branched amides (1h ± j)
gave lower yields, reflecting an increase of steric hindrance in
the nucleophilic addition of 1 to 2. Desilylation with
tetrabutylammonium fluoride (TBAF) in wet THF yielded
the 1-alkynylamides 6a ± j in 78 ± 98% (Table 1). The acety-
lene derivatives 6 are often crystalline, air stable, and widely
insensitive to hydrolysis. Unlike their N,N-dialkyl-substituted
analogues, the 1-alkynylamides 4 and 6 withstand aqueous
work-up procedures as well as chromatographic purification
on silica gel. This stability and the 13C NMR spectroscopic
data,[9] which are atypical in comparison to those for regular
ynamines, are results of electron-withdrawing features of the
protective group (PG) affecting the alkyne moiety.
[14] M. A. Osman, Mol. Cryst. Liq. Cryst. 1985, 131, 353 ± 360.
[15] M. Bremer, K. Tarumi, Adv. Mater. 1993, 5, 842 ± 848.
N-Functionalized 1-Alkynylamides:
New Building Blocks for Transition Metal
Mediated Inter- and Intramolecular
[2‡2‡1] Cycloadditions**
Bernhard Witulski* and Thomas Stengel
Functionalized alkynes are versatile building blocks for
transition metal mediated cycloadditions and cyclizations.[1]
An exception in this respect are 1-alkynylamines (yna-
mines).[2] These electron-rich acetylene derivatives have
certainly gained some significance with respect to trans-
formations with electrophiles and to Diels ± Alder reactions
[*] Dr. B. Witulski, Dipl.-Chem. T. Stengel
Fachbereich Chemie der Universität
Erwin Schrödinger Strasse, D-67663 Kaiserslautern (Germany)
Fax: (‡49)631-205-3921
E-mail: Bernhard@chemie.uni-kl.de
[**] This work was supported by the Fonds der Chemischen Industrie.
B. W. thanks the Fonds der Chemischen Industrie for a Liebig
fellowship and Prof. Dr. Manfred Regitz for his generous support.
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