A convenient synthetic route to tetrahydropyran-based liquid crystals

Article abstract of DOI:10.1002/ejoc.200600160

The tetrahydropyran moiety has been identified as a highly advantageous addition to the toolbox for the design of nematic liquid crystals for LCD applications. A new synthetic procedure based on the Lewis acid catalysed ring opening of oxetanes by lithium

Full text of DOI:10.1002/ejoc.200600160

FULL PAPER
DOI: 10.1002/ejoc.200600160
A Convenient Synthetic Route to Tetrahydropyran-Based Liquid Crystals
Peer Kirsch*[a] and David Maillard[b]
Keywords: Density functional calculations / Liquid crystals / Mesogens / Oxetane / Tetrahydropyran
The tetrahydropyran moiety has been identified as a highly
advantageous addition to the toolbox for the design of ne-
matic liquid crystals for LCD applications. A new synthetic
procedure based on the Lewis acid catalysed ring opening
of oxetanes by lithium iminoenolates followed by reductive
dehydroxylation of the resulting hemiketal provides a conve-
nient preparative access to this class of materials.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
using liquid crystal mixtures with high dielectric aniso-
tropies (∆ε). Reflective and transflective LCDs make no or
Introduction
The ubiquitous use of mobile electronic devices, such as only rather limited use of energy-consuming backlights. For
cellular phones, personal digital assistants (PDAs) or video these even more power-efficient applications, very low bire-
games, has created a strong demand for very robust active fringence (∆n) of the material is required in addition.[2]
matrix liquid crystal displays (AM-LCDs) with long life- Combining extremely high reliability with strong polarity
times even under extreme environmental conditions.[1] As a and low birefringence is a very challenging task for the de-
consequence, the nematic liquid crystals used in such LCD veloper of liquid crystals.
devices are required to show extreme endurance under pho-
The liquid crystals currently in use for active matrix
tochemical, thermal and oxidative stress. The manufactur- LCDs are so-called superfluorinated materials (SFMs),
ers of LCD panels can achieve significant reductions in de- mostly based on phenylbicyclohexyl mesogenic core struc-
vice production cost and extensions of battery lifetimes by tures with fluorine-containing polar terminal groups (1;
Scheme 1. Examples of liquid crystals used for mobile devices, combining polarity (i.e., high ∆ε) with low birefringence (∆n) (1–5),
together with an example of a new material based on a tetrahydropyran subunit (6).
Scheme 1).[2] The polarities of materials based on this basic
[a] Merck Ltd. Japan, Technical Center Atsugi,
structure can be enhanced by insertion of polar bridges, such
as esters (2) or difluorooxymethylene groups (3),[3] into their
mesogenic core structures. An alternative means to achieve
higher dielectric anisotropy is the replacement of one cyclo-
hexane moiety by a polar 1,3-dioxane unit (4).[4] If even lower
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, 64293 Darmstadt, Germany
3326
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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A Convenient Synthetic Route to Tetrahydropyran-Based Liquid Crystals
FULL PAPER
birefringence is required, (perfluoroalkyl)bicyclohexyl sys-
tems (5) without polarizable olefinic substructures are used.
With increasing polarity of the liquid crystal mixture it
becomes more and more challenging to design materials that
will also maintain high voltage holding ratios and specific
resistivities after extended heat load or UV irradiation. In
particular, some esters and dioxane derivatives show insuf-
ficient reliability under harsh conditions, probably due to the
formation of oxidative or hydrolytic decomposition products.
Against this background, we have recently been focusing
our attention on liquid crystals containing tetrahydropyran
(THP) moieties as polar rings within their mesogenic core
structures (e.g., 6). Somewhat counterintuitively, the local
dipole moment of the THP moiety is quite effective for in-
creasing the dielectric anisotropy in spite of its oblique ori-
entation relative to the long molecular axis.
Scheme 3. Synthesis of the different 3-substituted oxetane precur-
sors 9, 11 and 13: a) Br2, PPh3, pyridine, CH2Cl2, room temp., 18 h
(65%). b) Bu3SnOMe, 50 °C, 100 mbar, distillative removal of
MeOH, then 200 °C, 90 mbar, distillative collection of 9 (86%).
c) 1. nBuLi (1.0 equiv.), THF; 0 °C, 40 min, 2. TsCl, THF, room
temp., 1 h, 3. nBuLi (1.0 equiv.), THF, addition at 0 °C, then 60 °C,
4 h (11: 65%, 13: 75%).
Table 1. Preparation of the cyclohexylimines (15a–j) from the corre-
sponding methyl ketones (14a–j): a) cyclohexylamine, cat. TsOH,
toluene, reflux with azeotropic water removal.
This report is mainly centered on the synthesis of the
THP building block. A more general account of the chemis-
try, together with a detailed discussion of the unique
physicochemical properties of THP-based liquid crystals,
will be the subject of a forthcoming publication.
A major obstacle for the practical application of liquid-
crystalline tetrahydropyran derivatives is their rather incon-
venient synthetic accessibility. Most methods reported in
the literature are based on the derivatization of carbo-
hydrate precursors,[5] and these reactions usually give single
enantiomers of the products, whereas we were interested
primarily in racemic liquid crystals. Other major ap-
proaches include the use of lactone derivatives as intermedi-
ates,[6] ring-closing olefin metathesis[7] or application of the
Ferrier reaction.[8] All these methods suffer either from low
yields or from limited structural scope, too narrow for the
synthesis of typical highly fluorinated liquid crystals.
Ketone
14a
R
Imine
15a
Yield [%]
n-C3H7
CF3
C2F5
n-C3F7
Ph
80[a]
39[a–c]
44[a–c]
32[a–c]
82[a,b]
95
14b
14c
14d
14e
14f
14g
14h
14i
15b
15c
15d
15e
15f
15g
15h
15i
C6H4-4-CF3
C6H4-4-OCF3
C6H4-4-Br
83
87[d]
91
C6H4-4-n-C3H7
14j
C6H3-3,5-F2
15j
84
[a] Benzene was used instead of toluene as solvent, due to the low
boiling point of the ketone. [b] No pTsOH was used. [c] Reaction
was performed in a Soxhlet apparatus charged with MgSO4. The
imines were purified by distillation under reduced pressure (15b:
b.p. 60–62 °C, 14–15 mbar; 15c: b.p. 76–78 °C, 30–33 mbar; 15d:
b.p. 73–75 °C, 22–24 mbar). [d] 15h is a colourless solid that was
purified by recrystallization from toluene.
Results and Discussion
In order to gain access to liquid-crystalline tetrahydropy-
ran derivatives, we developed a new synthetic method based
Scheme 2. General pathway for the synthesis of tetrahydropyran-based liquid crystals. The mechanism of the Lewis acid catalysed oxetane
ring-opening is depicted in the box.
Eur. J. Org. Chem. 2006, 3326–3331
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P. Kirsch, D. Maillard
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on the Lewis acid catalysed ring-opening of 3-substituted
The oxetane precursors are readily available from the
oxetanes with lithium iminoenolates (Scheme 2).[9] This step corresponding 1,3-diols (Scheme 3). Compounds contain-
furnishes a hemiketal intermediate, which is reductively de- ing primary alkyl substituents (9) were synthesized by cycli-
hydroxylated to give the corresponding trans-2,5-disubsti- zation of the 3-bromo alcohols with tributyl(methoxy)-
tuted tetrahydropyran derivative.
tin.[10] Oxetanes with more bulky substituents (such as 11)
Table 2. Synthesis of the tetrahydropyran-based liquid crystals 17a–j, 19, 21 and 23: a) 1. TMP, nBuLi, THF, –78 °C, 20 min, 2. imine,
THF, –70 to –60 °C, 40 min. 3. oxetane, BF3·OEt2, THF, –60 °C, 1.5 h, then room temp., 10 h. b) Et3SiH, BF3·OEt2, CH2Cl2, –70 °C,
3 h, then –10 °C. The dihydropyrans 24 and 25 (box) were formed as byproducts during the reduction of 16c and 16d.
Imine
R
Oxetane
X
Hemiketal: yield (%)
THP: yield (%), trans (%)
15a
15b
15c
15d
15e
15f
15g
15h
15i
n-C3H7
CF3
C2F5
n-C3F7
Ph
11
11
11
11
11
11
11
11
11
11
9
–
–
–
–
H
16a: 59
16b: 24
16c: 10
16d: 15
16e: 69
16f: 81
16g: 84
16h: 60
16i: 51
16j: 7
17a: 99, 91
17b: 55[a], 80
17c: 30[a,b], –
17d: 59[a,c], –
17e: 94, 93
17f: 91, 91
17g: 93, 94
17h: 82, 92
17i: 98, 60
17j: 88, 95
19: 91, 92
C6H4-4-CF3
C6H4-4-OCF3
C6H4-4-Br
C6H4-4-n-C3H7
C6H3-3,5-F2
C6H4-4-Br
n-C3H7
4-CF3
4-OCF3
4-Br
4-n-C3H7
3,5-F2
4-Br
–
15j
15h
15a
15h
18: 74
13
13
20: 48
22: 20
21: 82, 88
C6H4-4-Br
4-Br
23: 69, 71
[a] The reaction mixture was heated to 40 °C until completion of the reduction. [b] Crude reduction product, mixture of 17c (29%) and
24 (71%). [c] Crude reduction product, mixture of 17d (29%) and 25 (71%).
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A Convenient Synthetic Route to Tetrahydropyran-Based Liquid Crystals
FULL PAPER
were prepared directly from the diols by a one-pot acti- polar fluorinated liquid crystals, their solubilities in all
vation/cyclization procedure.[11] The same method was also kinds of nematic host mixtures are excellent. This is also
applied for the synthesis of the 3-(trans-5-alkyl-1,3-dioxan- apparent in the order of the melting points, with 4 (74 °C)
2-yl)oxetane 13 from the diol 12.[4]
Ͼ 1 (66 °C) Ͼ 6 (57 °C). In the neat materials, slight tend-
Necessary requirements for the iminoenolate building encies towards the formation of smectic phases are ob-
block are sufficient reactivity to accomplish the opening of served, but these are less pronounced than in 1,3-dioxane
the oxetane ring, convenient synthetic accessibility and ease derivatives with similar structures.[4] The virtual clearing
of the hydrolytic workup after the addition reaction. As temperatures are generally a little lower than those of their
also reported by J.-F. Le Borgne[12] for similar ring-opening cyclohexane analogues (e.g., 1: 74.3 °C Ǟ 6: 62.3 °C).
reactions of epoxides, lithium (cyclohexylimino)enolates
Table 3. Physical properties of some selected pyran-based liquid
were found to fit this property profile. They were readily
crystals in comparison to conventional materials (1–5) with similar
prepared by acid-catalysed condensation of the correspond-
ing methyl ketones with cyclohexylamine with azeotropic
removal of water.[13] For the more reactive and volatile
methyl perfluoroalkyl ketones the procedure had to be
slightly modified, the acid catalysis not being necessary.[14]
Except for 15b–d and 15h, the imines were isolated as yel-
low or brown liquids after simple concentration and fil-
tration of the reaction mixtures and were used directly for
the next step (Table 1).
areas of application.[17]
No.
Phase sequence
∆εvirt
∆nvirt
TNI,virt
γ1,virt
1
2
C 66 N 94.1 I
C 56 N 117.2 I
C 44 N 105.3 I
C 74 N (51.2) I
C 19 SH? (8) SB? 41 I
C 57 N (50.9) I
C 28 SB 62 I
9.7
11.1
10.5
17.0
6.8
12.7
–0.5
8.8
13.2
6.7
–
4.7
3.6
0.0750
0.0670
0.0668
0.0680
0.0594
0.0778
0.0482
0.0606
0.0853
0.0831
–
74.3
115.0
91.5
63.2
–44.4
62.3
36.1
–57.6
104.2
134.2
–
160
175
145
201
63
182
56
3
4
5
6
17a
17b
17f
C 59 I
C 73 SB 115.5 I
–
The deprotonation of the imines to afford the corre-
sponding lithium (cyclohexylimino)enolates was achieved
by use of lithium 2,2,6,6-tetramethylpiperidine (LiTMP)
(prepared in situ from n-butyllithium and 2,2,6,6-tetrameth-
ylpiperidine) as a powerful base with low nucleophilicity.
A nonnucleophilic base is essential because the subsequent
oxetane ring-opening is catalysed by the Lewis acid boron
trifluoride–diethyl ether, which would otherwise be deacti-
vated. These reactions were conducted between –78 and
–60 °C, furnishing equilibrium mixtures of two hemiketal
isomers and the corresponding δ-hydroxy ketone (see also
Scheme 2). The subsequent reductive dehydroxylation of
the hemiketal was achieved with a combination of triethyl-
silane and boron trifluoride–diethyl ether in dichlorometh-
ane.[15] For most of the substrates (R2 = aryl or alkyl; see
Scheme 2 and Table 2) the reaction proceeds at tempera-
tures between –70 and –10 °C. For the less reactive perfluo-
roalkyl hemiketals 16b–d the temperature had to be raised
to 40 °C in order to drive the conversion to completion.
Even under these more vigorous conditions, the THP deriv-
atives 17c and 17d were accompanied by large quantities of
their elimination products, the corresponding dihydro-
pyrans 24 and 25. The reason for this lack of reactivity is
the destabilization of the transient α-perfluoroalkyl carbo-
cations formed during the reduction by the negative induc-
tive effects (–I) of the trifluoromethyl, pentafluoroethyl and
perfluoro-n-propyl groups.[16] The trans/cis isomer ratios of
the dehydroxylation products are typically in the range of
8:1 to 9:1.
351
179
–
216
78
17g C 71 SB 86 N 136.6 I
17h
17j
21
C 98 N 176.8 I
C 64 N (53.7) I
C 64 SB (53) I
0.0863
0.0506
44.1
–9.6
In addition to their favourable electrooptical and meso-
genic properties, the tetrahydropyran-based materials show
excellent reliability profiles.[2]
Conclusions
A new, convenient synthetic procedure allows the synthe-
sis of a variety of liquid crystals containing tetrahydropyran
moieties in their mesogenic core structures. The materials
have a highly advantageous combination of high dielectric
anisotropy (∆ε) and low birefringence (∆n), together with
excellent mesogenic properties and high chemical stability.
The synthetic methodology is generally applicable not only
for the preparation of materials but also for tetrahydropy-
ran-based natural products, pharmaceuticals and agro-
chemicals.
Experimental Section
General: The following synthetic procedures have been selected as
examples for the preparation of the different classes of liquid crys-
tals. The hemiketal intermediates (16b, 16f and 20) each exist in
equilibrium between two cyclic hemiketal isomers and the open δ-
hydroxy ketone. The compounds have not been fully characterized,
but their purities and identities were verified by HPLC and GC-
MS. All reactions were carried out in dry solvents and under nitro-
gen.
The very low yield of 16j is caused by ortho-lithiation
between the two aromatic fluorine substituents, competing
with the formation of the lithium iminoenolate by the very
strong base LiTMP.
The liquid crystals containing tetrahydropyran subunits
show some highly advantageous characteristics (Table 3):[17]
as expected, they have high dielectric anisotropies (∆ε) in
combination with low birefringences (∆n) and low rota-
Trifluoromethyl Hemiketal 16b: A mixture of TMP (2,2,6,6-tet-
ramethylpiperidine, 4.75 mL, 28.0 mmol) and THF (30 mL) was
treated dropwise at –78 °C with nBuLi (15% solution in n-hexane)
(17.50 mL, 27.90 mmol). The resulting mixture was stirred at the
tional viscosities (γ1). In contrast with many other highly same temperature for 20 min, followed by dropwise addition of a
Eur. J. Org. Chem. 2006, 3326–3331
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P. Kirsch, D. Maillard
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solution of 1,1,1-trifluoroacetone N-cyclohexylimine (15b, 5.41 g,
28.0 mmol) in THF (30 mL). The resulting deep violet solution was
stirred between –60 °C and –70 °C for 15 min, and a solution of 3-
(trans-4-propylcyclohexyl)oxetane (11, 4.00 g, 21.9 mmol) in THF
(20 mL) was added, followed by BF3·OEt2 (6.28 mL, 50.0 mmol).
The solution was stirred at –60 °C for 2.5 h and was then allowed
to warm to room temperature. After 10 h of stirring at room tem-
perature, the reaction was quenched by addition of a saturated
aqueous NH4Cl solution (80 mL). The aqueous phase was sepa-
rated and extracted with tert-butyl methyl ether (2×50 mL). The
combined organic phases were washed with brine (40 mL), dried
with Na2SO4 and filtered, and the solvents were evaporated to dry-
ness. The residue was purified by chromatography (silica gel; n-
heptane/ethyl acetate, 7:1) to afford a yellow oil (2.66 g). In order
to remove residual traces of the cyclohexylimine, the crude product
was dissolved in a mixture of tert-butyl methyl ether (40 mL) and
hydrochloric acid (10%, 20 mL). The biphasic mixture was vigor-
ously stirred for 15 min, the layers were separated, the organic
phase was dried with Na2SO4 and filtered, and the solvents were
evaporated. The hemiketal 16b was obtained as colourless crystals
(1.58 g, 24%), which were used for the subsequent synthetic step
without further characterization.
mixture was stirred between –60 and –70 °C for 40 min before ad-
dition of a solution of 3-(trans-4-propylcyclohexyl)oxetane (11,
3.0 g, 16.5 mmol) in THF (20 mL) and subsequent slow addition
of BF3·OEt2 (5.10 mL, 40.60 mmol). The solution was stirred at
–60 °C for an additional 90 min, and was then allowed to warm
to room temperature. After the mixture had been stirred at room
temperature for 10 h, saturated aqueous NH4Cl (70 mL) was
added, and the aqueous phase was separated and extracted twice
with tert-butyl methyl ether (30 mL). The combined organic phases
were washed with brine (30 mL), dried with sodium sulfate, filtered
and concentrated in vacuo. The residue was chromatographed (sil-
ica gel; n-heptane/ethyl acetate, 8:1) to afford 16f (4.92 g, 81%) as
a colourless solid, which was used for the subsequent synthetic step
without further characterization.
The hemiketals 16a, 16e, 16g–j, 18, 20 and 22 were prepared ac-
cording to an analogous procedure.
Liquid Crystal 17f: A solution of the adduct 16f (3.0 g, 8.10 mmol),
dissolved in dichloromethane (100 mL), was treated dropwise at
–70 °C with triethylsilane (2.10 mL, 13.0 mmol) and then with
BF3·OEt2 (0.85 mL, 13.0 mmol). The mixture was stirred at the
same temperature for 3 h and was then allowed to warm to –10 °C.
The reaction was quenched and the mixture neutralized by careful
addition of aqueous sodium hydroxide (10%). The organic phase
was separated, and the aqueous phase was extracted with dichloro-
methane (2×40 mL). The combined organic layers were washed
with brine (30 mL), dried with sodium sulfate and concentrated in
vacuo to afford colourless crystals of 17f (2.64 g, 91%), containing
91% (HPLC) of the desired trans isomer. Repeated recrystallization
from n-heptane afforded trans,trans-17f with high purity (Ͼ99% by
HPLC). For mesophase sequence and electrooptical characteristics
The pentafluoroethyl and perfluoro-n-propyl hemiketals 16c and
16d were prepared according to an analogous procedure.
Liquid Crystal 17b: A solution of the hemiketal 16b (1.35 g,
4.57 mmol) in dichloromethane (50 mL) was treated dropwise at
–70 °C with triethylsilane (1.21 mL, 7.50 mmol), followed by
BF3·OEt2 (0.49 mL, 7.50 mmol). The reaction mixture was allowed
to warm to room temperature over 1 h and was then heated to
40 °C and stirred until the complete disappearance of the starting
material 16b was confirmed by TLC after ca. 6 h. After cooling in
an ice bath, the mixture was hydrolysed and neutralized by careful
addition of a solution of aqueous sodium hydroxide (10%). The
aqueous phase was extracted twice with dichloromethane (30 mL),
and the combined organic phases were washed with brine (30 mL),
dried with Na2SO4 and concentrated in vacuo to afford a colourless
oil (1.22 g). Purification by column chromatography (silica gel; n-
heptane/ethyl acetate, 8:1) afforded 17b (0.70 g, 55%), containing
80% (HPLC) trans isomer. Repeated recrystallization from n-hep-
tane afforded pure (96.5% by HPLC) trans-trans-17b. For melting
point and electrooptical characteristics see Table 3. 1H NMR
(500 MHz, CDCl3, 303 K): δ = 4.13 (ddd, J = 11.2 Hz, J = 4.0 Hz,
J = 2.0 Hz, 1 H), 3.68–3.54 (m, 1 H), 3.19 (t, J = 11.2 Hz, 1 H),
2.05–1.95 (m, 1 H), 1.88–1.37 (m, 7 H), 1.35–1.07 (m, 6 H), 1.05–
0.91 (m, 3 H), 0.87 (t, J = 7.3 Hz, 3 H), 0.84–0.76 (m, 2 H) ppm.
13C NMR (75 MHz, CDCl3, 303 K): δ = 124.0 (q, 1JCF = 278.8 Hz,
CF3), 75.2 (q, 2JCF = 31.1 Hz, CHCF3), 72.0, 40.3, 39.6, 39.5, 37.2,
33.0, 30.0, 29.7, 25.7, 24.3, 19.8, 14.2 ppm. 19F NMR (235 MHz,
1
see Table 3. H NMR (500 MHz, CDCl3, 303 K): δ = 7.58 (d, J =
8.4 Hz, 2 H, ar-H), 7.44 (d, J = 8.4 Hz, 2 H, ar-H), 4.29 (d, J =
11.0 Hz, 1 H), 4.19 (dq, J = 11.0 Hz, J = 3.5 Hz, 1 H), 3.33 (t, J
= 11.0 Hz, 1 H), 2.03–1.86 (m, 2 H), 1.81–1.69 (m, 3 H), 1.60–1.23
(m, 6 H), 1.19–1.11 (m, 3 H), 1.07–0.98 (m, 3 H), 0.88 (t, J =
7.3 Hz, 3 H, CH3), 0.89–0.81 (m, 2 H) ppm. 13C NMR (75 MHz,
2
CDCl3, 303 K): δ = 147.1, 129.3 (q, JCF = 32.3 Hz, ar-C-CF3),
3
1
125.8, 125.0 (d, JCF = 3.5 Hz, ar-C), 124.1 (q, JCF = 272.3 Hz,
CF3), 79.1, 72.3, 40.8, 40.0, 39.6, 37.3, 34.2, 33.2, 30.2, 29.8, 27.7,
19.8, 14.2 ppm. 19F NMR (235 MHz, CDCl3, 300 K): δ = –60.68
(s, 3 F, CF3). MS (EI, 70 eV): m/z (%) = 354 (55) [M]+, 335 (15),
323 (15), 210 (17), 198 (12), 185 (12), 172 (37), 164 (26), 159 (20),
123 (40), 109 (52), 95 (44), 83 (100), 79 (17), 69 (79), 55 (77).
The liquid crystals 17a, 17e, 17g–j, 19, 21 and 23 were prepared
according to an analogous procedure.
Hemiketal 20: The dioxanyl hemiketal 20 was prepared analogously
to 16f. The crude product was purified by chromatography (silica
gel; n-heptane/ethyl acetate, 3:1) to furnish 20 as a yellowish solid
in 48% yield. The product was used without further characteriza-
tion as starting material for 21.
3
CDCl3, 300 K): δ = –79.6 (d, JHF = 6.3 Hz, 3 F, CF3). MS (EI,
70 eV): m/z (%) = 278 (18) [M]+, 235 (24), 139 (16), 124 (58), 109
(19), 95 (25), 83 (78), 69 (100), 57 (15), 55 (61).
Liquid Crystal 21: The liquid crystal 21 was prepared from the
dioxanyl hemiketal 20 analogously to the synthesis of 17e. The
crude product was purified by chromatography (silica gel; n-hep-
tane/ethyl acetate, 5:1) to furnish 20 as a colourless solid (82%
yield) containing 88% (HPLC) of the desired trans isomer. Two
consecutive recrystallizations from ethanol afforded trans,trans-21
with high purity (100% purity by HPLC). For mesophase sequence
The liquid crystals 17c and 17d were prepared from the correspond-
ing methyl perfluoroalkyl N-cyclohexylketimines according to an
analogous procedure. Nevertheless, 17c and 17d could not be iso-
lated cleanly because they were accompanied by the corresponding
dihydropyranes 24 and 25, respectively.
Hemiketal 16f: A solution of TMP (3.90 mL, 22.90 mmol) in tetra-
hydrofuran (30 mL) was treated dropwise at –78 °C with nBuLi
(15% solution in hexane, 14.37 mL, 22.90 mmol). The mixture was
stirred at the same temperature for 20 min, and a solution of 4-
(trifluoromethyl)acetophenone N-cyclohexylimine (15f, 6.19 g,
23.00 mmol) in THF (30 mL) was then added slowly. The resulting
1
and electrooptical characteristics see Table 3. H NMR (500 MHz,
CDCl3, 303 K): δ = 4.18 (d, J = 5.3 Hz, 1 H), 4.08–3.99 (m, 3 H),
3.28–3.12 (m, 4 H), 2.03–1.76 (m, 3 H), 1.66–1.61 (m, 1 H), 1.52–
1.15 (m, 8 H), 1.02–0.95 (m, 2 H), 0.90 (t, J = 6.9 Hz, 3 H), 0.89
(t, J = 7.3 Hz, 3 H) ppm. 13C NMR (75 MHz, CDCl3, 303 K): δ =
3330
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A Convenient Synthetic Route to Tetrahydropyran-Based Liquid Crystals
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130; f) X. Teng, D. R. Cefalo, R. R. Schrock, A. H. Hoveyda,
J. Am. Chem. Soc. 2002, 124, 10779–10784; g) L. C. Usher, M.
Estrella-Jimenez, I. Ghiviriga, D. L. Wright, Angew. Chem. Int.
Ed. 2002, 41, 4560–4562.
a) R. J. Ferrier, W. G. Overend, M. A. E. Ryan, J. Chem. Soc.
1962, 3667–3670; b) G. Grynkiewicz, A. Zamojski, Z. Natur-
forsch. 1980, 35, 1024–1027.
102.9, 77.5, 72.2, 68.9, 40.6, 38.5, 34.1, 31.0, 30.4, 24.8, 19.5, 18.8,
14.2 ppm. MS (EI, 70 eV): m/z (%) = 256 (3) [M]+, 213 (18), 129
(100), 83 (40), 55 (44).
[8]
Acknowledgments
[9]
Oxetane ring-opening by lithium ester enolates: M. Yamaguchi,
K. Shibata, I. Hirao, Tetrahedron Lett. 1984, 25, 1159–1162.
a) B. Delmond, J.-C. Pommier, J. Valade, Tetrahedron Lett.
1969, 10, 25, 2089; b) B. Delmond, J.-C. Pommier, J. Valade, J.
Organomet. Chem. 1973, 47, 337–350.
We thank J. Haas, H. Heldmann and K. Altenburg for the physical
characterization of the new compounds. D. M. gratefully acknowl-
edges a Marie Curie Fellowship of the European Commission
(G5TR-CT-2002-00093).
[10]
[11]
P. Picard, D. Leclercq, J. P. Bats, J. Moulines, Synthesis 1981,
550.
[1] a) D. Pauluth, K. Tarumi, J. Mater. Chem. 2004, 14, 1219–
[12]
J.-F. Le Borgne, J. Organomet. Chem. 1976, 122, 139–143.
1227; b) D. Pauluth, K. Tarumi, J. Soc. Inf. Disp. 2005, 13,
693–702.
[13] D. G. Norton, V. E. Havy, F. C. Davis, L. J. Mitchell, S. A. Bal-
lard, J. Org. Chem. 1954, 19, 1054.
[14] P. Margaretha, Helv. Chim. Acta 1982, 65, 290.
[2] P. Kirsch, M. Bremer, Angew. Chem. 2000, 112, 4384–4405; An-
gew. Chem. Int. Ed. 2000, 39, 4216–4235; and references cited
therein.
[3] P. Kirsch, M. Bremer, A. Taugerbeck, T. Wallmichrath, Angew.
Chem. 2001, 113, 1528–1532; Angew. Chem. Int. Ed. 2001, 40,
1480–1484.
[4] P. Kirsch, E. Poetsch, Adv. Mater. 1998, 10, 602–606.
[5] a) V. Vill, H.-W. Tunger, K. Diekmann, Tetrahedron: Asym-
metry 1994, 5, 2443–2446; b) V. Vill, H.-W. Tunger, J. Chem.
Soc. Chem. Commun. 1995, 10, 1047–1048; c) V. Vill, H.-W.
[15] a) Review: D. N. Kursanov, Z. N. Parnes, N. M. Loim, Synthe-
sis 1974, 633–651; b) F. A. Carey, H. S. Tremper, J. Org. Chem.
1971, 36, 758–761; c) C. T. West, S. J. Donnelly, D. A. Kooistra,
M. P. Doyle, J. Org. Chem. 1973, 38, 2675–2681; d) J. L. Fry,
M. Orfanopoulos, M. G. Adlington, W. R. Dittmann Jr, S. B.
Silverman, J. Org. Chem. 1978, 43, 374–375; e) G. A. Kraus,
K. A. Frazier, B. D. Roth, M. J. Taschner, K. Neuenschwander,
J. Org. Chem. 1981, 46, 2417–2419.
Tunger, Liebigs Ann. Chem. 1995, 6, 1055–1060; d) V. Vill, H.- [16] P. Kirsch, Modern Fluoroorganic Chemistry: Synthesis Reactiv-
W. Tunger, M. von Minden, J. Mater. Chem. 1996, 6, 739–746;
e) B. Bertini, C. Moineau, D. Sinou, G. Gesekus, V. Vill, Eur.
J. Org. Chem. 2001, 2, 375–382; f) B. Bertini, M. Perrin, D.
Sinou, A. Thozet, V. Vill, J. Carbohydr. Chem. 2003, 22, 685–
704.
ity, Applications, Wiley-VCH, Weinheim, Germany, 2004.
[17] The “virtual” parameters TNI,virt, ∆εvirt, ∆nvirt and γ1,virt were
determined by linear extrapolation from a 10% w/w solution
in the commercially available Merck mixture ZLI-4792 (TNI
=
92.8 °C, ∆ε = 5.3, ∆n = 0.0964). The extrapolated values are
corrected empirically for differences in the order parameter in-
duced by the analyte. For the pure substances, the phase transi-
tion temperatures were measured by differential scanning calo-
rimetry (DSC), whilst the phase type was assigned by optical
polarization microscopy.
[6] J. Parsch, J. W. Engels, Helv. Chim. Acta 2000, 83, 1791–1808.
[7] a) S. Chang, R. H. Grubbs, J. Org. Chem. 1998, 63, 864–866;
b) J. D. Rainier, S. P. Allwein, J. Org. Chem. 1998, 63, 5310–
5311; c) J. M. Percy, S. Pintat, Chem. Commun. 2000, 607–608;
d) M. de Rosa, A. Solladie-Cavallo, A. Scettri, Tetrahedron
Lett. 2000, 41, 1593–1596; e) M. A. Walters, F. La, P.
Deshmukh, D. O. Omecinsky, J. Comb. Chem. 2002, 4, 125–
Received: February 24, 2006
Published Online: May 29, 2006
Eur. J. Org. Chem. 2006, 3326–3331
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3331