1,3,4-Thiadiazole-2-carboxylate esters: New synthetic methodology for the preparation of an elusive family of self-organizing materials

Article abstract of DOI:10.1039/b704085g

A series of alkyl 5-(4-octyloxyphenyl)-1,3,4-thiadiazole-2-carboxylates has been prepared via a novel chemoselective ring-closure of appropriate tricarbonyl precursors. This paper represents the first report of a general and reproducible synthetic methodology for the preparation of 1,3,4-thiadiazole-2- carboxylate esters. The 1,3,4-thiadiazole esters synthesized were designed to evaluate the feasibility of this new core unit for potential ferroelectric applications. The core unit was found to promote strongly the formation of tilted smectic phases (the smectic C phase in particular). The 1,3,4-thiadiazole ester was found to be a stronger promoter of the smectic C phase than its phenyl-based counterpart. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b704085g

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
1,3,4-Thiadiazole-2-carboxylate esters: new synthetic methodology for the
preparation of an elusive family of self-organizing materials
Brian Sybo, Patrick Bradley, Alan Grubb, Seth Miller, Katie J. W. Proctor, Lucy Clowes, M. Ruth Lawrie,
Paul Sampson and Alexander J. Seed*
Received 19th March 2007, Accepted 29th May 2007
First published as an Advance Article on the web 11th June 2007
DOI: 10.1039/b704085g
A series of alkyl 5-(4-octyloxyphenyl)-1,3,4-thiadiazole-2-carboxylates has been prepared via a
novel chemoselective ring-closure of appropriate tricarbonyl precursors. This paper represents the
first report of a general and reproducible synthetic methodology for the preparation of
1,3,4-thiadiazole-2-carboxylate esters. The 1,3,4-thiadiazole esters synthesized were designed to
evaluate the feasibility of this new core unit for potential ferroelectric applications. The core unit
was found to promote strongly the formation of tilted smectic phases (the smectic C phase in
particular). The 1,3,4-thiadiazole ester was found to be a stronger promoter of the smectic C
phase than its phenyl-based counterpart.
For some time, we have been interested in the synthesis of
Introduction
new heterocyclic materials for ferroelectric and antiferro-
1,3,4-Thiadiazoles are relatively common in the liquid crystal
electric applications. As part of this program, we have focused
(LC) literature although the variety of structural modifications
on ester-linked materials, since the ester linkage is well known
that have been studied are actually very limited. The majority
to be an excellent promoter of tilted smectics such as the
smectic C (SmC) phase.17 1,3,4-Thiadiazoles have also been
of systems have the 1,3,4-thiadiazole core substituted at the 2
and 5-positions by aryl units or a combination of aryl and
noted to be exceptionally good promoters of the SmC
alkyl/cycloalkyl units.1–7 Typically, these materials (4) are
prepared via sulfurization of appropriately substituted
N,N9-diacyldiazanes 3 that are, in turn, prepared by the
reaction of hydrazides 1 with acid chlorides 2 (Fig. 1). Early
reports utilized phosphorus pentasulfide as the sulfurization
reagent although Lawesson’s reagent soon emerged as the
most reliable and reproducible reagent for this task.8,9
In 1995, Tschierske et al. reported a novel synthesis of a
2-alkylsulfanyl-5-aryl-1,3,4-thiadiazole for nonlinear optical
(NLO) applications.10 However, this appears to be the only
compound of this type in the LC literature.
phase.18–20 We have identified 1,3,4-thiadiazole-2-carboxylates
as worthy targets with potentially low viscosities, high
dielectric biaxialities (due to the large heterocyclic dipole of
3.0 D),21 and more linear structures than other common five-
membered heterocycles (thus leading to higher mesophase
thermal stabilities). Recently, we reported our preliminary
results in this area with the synthesis of the first 1,3,4-
thiadiazole-2-carboxylate ester; the synthesis reported was
low-yielding and reaction conditions were not optimized.22
Subsequently, we decided to target a number of 1,3,4-
thiadiazole-2-carboxylate esters (14–17) that would allow us
to assess the feasibility of this new core as a promoter of SmC
phase behavior and to fully optimize the synthetic methodol-
ogy that was used to prepare this new class of materials.
A significant body of work has been reported on the
synthesis of 1,3,4-thiadiazoles that are attached to aromatic
systems by amide, azo, and imine linking groups.11–16 These
compounds were synthesized by the dehydration of thiosemi-
carbazides (or cyclization of thiosemicarbazones) to give
2-amino-5-aryl-1,3,4-thiadiazoles that were then subjected to
standard amine transformations. To our knowledge, these
compounds constitute the only 1,3,4-thiadiazole-linked meso-
gens in the LC literature prior to our work.
Synthetic studies
A survey of the organic literature reveals that 1,3,4-thiadia-
zole-2-carboxylate esters with a carbon-based group at the
5-position are extremely rare. The synthesis of such esters has
been reported only once (from an ortho ester)23 and the
methodology used was limited to a methyl chain being
attached to the ethereal oxygen. Unfortunately this procedure
is not amenable to large scale synthesis or to the preparation of
liquid crystalline precursors.
Department of Chemistry, Kent State University, Kent, OH, 44242-001,
USA. E-mail: aseed@kent.edu; Fax: +1 330 672-3816;
Tel: +1 330 672-9528
Fig. 1 Typical synthesis of 2,5-disubstituted-1,3,4-thiadiazoles.
3406 | J. Mater. Chem., 2007, 17, 3406–3411
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anhydrous THF.28 The use of fresh Lawesson’s reagent was
found to be very important with older reagent giving reduced
yields (in some cases a 25% reduction in yield was noted).
Careful monitoring of the reaction was also found to be impor-
tant as extended reaction times were found to lead to the pro-
duction of significant quantities of unidentified byproducts.
At this point we had intended to hydrolyze the resulting
ester 12 to the carboxylic acid derivative and subsequently to
esterify with appropriate alcohols.29 Unfortunately, the 1,3,4-
thiadiazole-2-carboxylic acid proved to be unstable in solution
and underwent spontaneous decarboxylation (the solid car-
boxylic acids also underwent decarboxylation over several
days). Attempted esterification of 5-(4-methoxyphenyl)-1,3,4-
thiadiazole-2-carboxylic acid with p-cresol using DCC/DMAP
gave an unsatisfactory yield (25%) of the ester with the
remainder of the material being lost to decarboxylation.
Decarboxylation has been observed previously in 5-amino-
1,3,4-thiadiazole-2-carboxylic acids.30–33 We therefore decided
to isolate the sodium salt of the carboxylic acid (13, a stable
molecule unless heated in solution upon which decarboxyla-
tion was observed). Direct conversion of 13 into the
corresponding acid chloride derivative followed by in situ
esterification with the desired alcohols was expected to afford
the desired ester adducts. However, this reaction proved to be
extremely sensitive to temperature and competitive decarbox-
ylation was again observed unless the reaction temperature
was carefully maintained at 26 to 28 uC. For example,
reaction temperatures of 210 to 217 uC gave .20%
Fig. 2 Tanaka’s chemoselective synthesis of 1,3-thiazole-2-carboxy-
late ester 6.
Tanaka et al. have reported the chemoselective reaction of
ethoxalylaminoacetophenone 5 with phosphorus pentasulfide
to give ethyl 5-phenyl-1,3-thiazole-2-carboxylate 6 (Fig. 2).24
We reasoned that the analogous diacyldiazane derivative
(11, Scheme 1) might be similarly cyclized to a 1,3,4-
thiadiazole-2-carboxylate ester using a suitable sulfurization
agent. Further precedent for the chemoselective sulfurization
of an amide carbonyl in the presence of an ester carbonyl was
reported in the same year by Scheibye et al. using Lawesson’s
reagent.25 With this knowledge in hand, we targeted the key
diazane 11 as a suitable 1,3,4-thiadiazole precursor (Scheme 1).
A standard Williamson etherification was used for the
preparation of ether 8 from phenol 7.26 Benzohydrazide 9 was
obtained by the reaction of a large excess of hydrazine hydrate
with ester 8.27 The key tricarbonyl intermediate 11 was
prepared by the reaction of ethyl oxalyl chloride (10) with
benzohydrazide 9. These materials are extremely polar,
difficult to purify by chromatography, and were used crude
in the subsequent cyclization step. Sulfurization and ring-
closure of 11 was carried out using Lawesson’s reagent in
Scheme 1 Synthesis of alkyl 1,3,4-thiadiazole-2-carboxylates 14–17.
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decarboxylation byproduct while temperatures of 24 to 27 uC
gave 5–17% decarboxylation (determined from 1H NMR
analyses of the crude products). Reaction temperatures
of 26 to 28 uC gave only 0–4% decarboxylation and yields
of the purified final esterification products 14–17 were good
(51–61%).
of the compound under examination into a host material that
displays the phase in question (a SmC phase in this case). The
mixture obtained is examined by polarizing optical microscopy
and the phase transition temperature is measured as usual (the
value will be modulated by the dopant). By making several
mixtures with different concentrations of the compound under
examination in the host, one can obtain a number of
concentration-dependent transition temperatures and extra-
polate to 100% of the compound under investigation. The
phase transition temperature so obtained is known as a virtual
LC phase transition temperature.
Our initial attempts to purify these obtained esters using
silica chromatography proved to be slow and difficult due to
acid–base interactions involving the thiadiazole nitrogens;
however, pre-treatment of the silica with a triethylamine-
containing solvent mixture alleviated this problem and led to
efficient purification. It is interesting to note that attempted
chromatography using basic alumina led to complete loss of
the product.
The C8 (20) and C10 (21) phenyl analogs do not exhibit
SmC phases and virtual LC phase values were not recorded by
the authors. Fortunately, Gray’s original paper also gives
details of the degree of cooling of the SmA phases of 20 and
21, with recrystallization occurring at 80 uC and 69 uC
Mesophase properties
respectively, without the observation of
a SmC phase.
The transition temperatures for the alkyl 1,3,4-thiadiazole-2-
carboxylates (14–17) are given in Table 1, and the transition
temperatures for the analogous phenyl carboxylates 18–21
(synthesized by Gray and Goodby)34,35 are presented in
Table 2.
Compounds 20 and 21 recrystallize at 2.5 uC and 15.5 uC
below the observed SmC phases in compounds 16 and 17
respectively.
The introduction of disubstituted 5-membered heterocycles
as replacements for 1,4-disubstituted benzene core units in
mesogenic structures has, until now, always resulted in a
reduction of mesophase thermal stability.36–40 This is due to
the introduction of a bend to the molecular structure that
invariably results in a reduced packing ability. This observa-
tion is likely to be due to the interplay of polarity/conforma-
tional issues; ongoing studies will attempt to elucidate the
factors responsible for this phenomenon. It should be noted,
however, that prior to this work Joachimi et al.18 and Zab
et al.19 had independently reported that 1,3,4-thiadiazoles had
higher SmC phase transitions than the analogous linear 2,5-
disubstituted pyrimidine derivatives. Our observation is there-
fore not altogether unexpected.
The melting points of the thiadiazole derivatives are all
higher than the corresponding phenyl derivatives whereas the
mesophase thermal stabilities of the thiadiazoles are higher
than the phenyl analogs. Compounds 15, 16, and 17 exhibit
SmC phases at temperatures of 71.3 uC, 82.5 uC, and 84.5 uC
respectively. The phenyl analog of 15 (compound 19) does not
exhibit a smectic C phase but a virtual LC phase was measured
at 15.3 uC lower than the thiadiazole derivative.
When a compound does not exhibit a LC phase, virtual LC
phase values can be obtained by dissolving known quantities
Table 1 Transition temperatures of alkyl 1,3,4-thiadiazole-2-carbox-
ylates 14–17a
Thiadiazoles 14–17 also possess SmA phases and com-
pounds 15, 16, and 17 have phase transitions 13.4 uC, 12.5 uC,
and 9.8 uC higher than those of phenyl analogs 19, 20, and 21,
respectively. The SmA phases of thiadiazole 14 and phenyl
analog 18 are comparable (85.5 uC and 86 uC, respectively).
Esters 14–17 have been exposed to bright sunlight for over
six months and have not shown any changes in color or
mesophase transitions, thus indicating very good photochemi-
cal and thermal stability.
n
Compound number Cryst
SmC
99.4 —
79.3 (?
87.0 (?
91.8 (?
SmA
Iso Liq
85.5) ?
4 14
?
?
?
?
(?
?
?
6 15
8 16
10 17
71.3)
82.5)
84.5
95.4
92.5
?
?
?
88.8) ?
a
() indicates a monotropic transition.
Experimental
Confirmation of the structures of intermediates and products
was obtained by 1H (400 MHz, Bruker Avance 400 MHz
spectrometer using Topspin version 1.3 software; tetramethyl-
silane was used as internal standard) and 13C (100 MHz) NMR
spectroscopy in CDCl3, unless another solvent is stated.
Elemental analyses were performed by Atlantic Microlabs
Inc. (Norcross, GA).
Table 2 Transition temperatures of phenyl carboxylates 18–21
synthesized by Gray and Goodbya
n
Compound number Cryst
SmC
SmA
Iso Liq
The progress of reactions was monitored using either TLC
[aluminium backed silica gel plates (Sigma-Aldrich, 200 mm
4
6
8
18
19
20
?
?
?
?
56 (?
72 [?
56)
56]
?
?
(?
?
86
82
80)
79
?
?
?
?
˚
layer thickness, 2–25 mm particle size and 60 A pore size)] or
80
75
—
—
GC [Shimadzu gas chromatograph GC-14A with a RestekTM
RTX-5 capillary column (30 m) and Shimadzu class VP
software].
10 21
a
() indicates a monotropic transition, [] indicates a virtual transition.
3408 | J. Mater. Chem., 2007, 17, 3406–3411
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Transition temperatures of the final products were measured
using a Mettler FP82HT hot-stage and FP90 control unit in
conjunction with a Leica Laborlux 12PolS polarizing micro-
scope. All transitions are quoted from microscopic measure-
ments and are given upon cooling at a rate of 5 uC per minute.
For all compounds melting points were recorded using the
same apparatus at a heating rate of 5 uC per minute. The
control unit was calibrated with three Merck standards
(benzophenone, benzoic acid and caffeine). Differential scan-
ning calorimetry (DSC) measurements were performed using a
TA Instruments Differential Scanning Calorimeter 2920 at
heating and cooling rates of 5 uC per minute (unless otherwise
stated) with indium as internal standard.
and an excess of ethanol (53 mL) was added to the filtrate with
stirring. The solvent was removed in vacuo and the crude
product was dried in vacuo (P2O5) overnight to afford an off-
white solid (97–98% pure by NMR analysis) that was used in
the next step without purification. Yield 10.42 g (100%). 1H
NMR (CDCl3) d 0.91 (t, 3H, J = 7.0 Hz), 1.29–1.52 (m, 10H),
1.44 (t, 3H, J = 7.0 Hz), 1.83 (quint, 2H, J = 7.4 Hz), 4.03 (t,
2H, J = 6.6 Hz), 4.44 (q, 2H, J = 7.2 Hz), 6.97 (d, 2H, J =
8.8 Hz), 7.82 (d, 2H, J = 8.8 Hz), 8.86 (1H, br d, J = 6.2 Hz),
9.87 (1H, br d, J = 6.2 Hz). 13C NMR (CDCl3) d 164.18,
163.02, 158.65, 152.97, 129.51, 122.59, 114.67, 68.46, 63.86,
31.95, 29.47, 29.37, 29.22, 26.12, 22.81, 14.26, 14.12.
Triethylamine was dried by distillation over calcium hydride
under dry argon, and was stored over potassium hydroxide.
Anhydrous tetrahydrofuran was obtained by distillation from
benzophenone ketyl. Butan-1-ol, octan-1-ol, and decan-1-ol
were dried by distillation from anhydrous potassium carbonate
under dry nitrogen and were stored over anhydrous potassium
carbonate. Petroleum ether was freshly distilled and stored in
amber bottles. Toluene was dried over sodium metal.
Ethyl 5-(4-octyloxyphenyl)-1,3,4-thiadiazole-2-carboxylate (12)
Lawesson’s reagent (7.01 g, 17.3 mmol) was added to a
solution of 11 (10.00 g, 27.44 mmol) in anhydrous THF
(200 mL) under dry nitrogen. The reaction mixture was stirred
for 29 h at room temperature (NMR and TLC analysis
revealed a near completion of the reaction) before the crude
product was purified by column chromatography [silica
gel/ethyl acetate : petroleum ether, 1 : 3] to afford a white
All chromatographic separations were performed using flash
column chromatography on silica gel [Fisher Davisil1 silica
1
solid. Yield 7.07 g (71%), mp 105.3 uC. H NMR (CDCl3) d
0.91 (t, 3H, J = 7.1 Hz), 1.26–1.43 (m, 10H), 1.49 (t, 3H, J =
7.1 Hz), 1.84 (quint, 2H, J = 7.2 Hz), 4.05 (t, 2H, J = 6.6 Hz),
4.55 (q, 2H, J = 7.2 Hz), 7.01 (d, 2H, J = 8.8 Hz), 7.98 (d, 2H,
J = 8.8 Hz). 13C NMR (CDCl3) d 172.49, 162.44, 158.95,
158.68, 130.05, 121.77, 115.27, 68.44, 63.20, 31.91, 29.44,
29.33, 29.20, 26.08, 22.76, 14.30, 14.21. Anal. Calcd for
C19H26N2O3S: C, 62.96; H, 7.23; N 7.73; S 8.85. Found: C,
62.50; H, 7.27; N, 7.82; S, 8.90%.
˚
gel (60 A, 55–75 mm particle size, grade 1740)] unless otherwise
stated.
Sonication was carried out using a Fisher Scientific sonic
bath (model number FS20H).
Syntheses of compounds 841 and 942 have been previously
reported.
Methyl 4-octyloxybenzoate (8)
A waxy white solid was obtained. Yield 38.72 g (74%). 1H
NMR (CDCl3) d 0.91 (t, 3H, J = 7.1 Hz), 1.25–1.53 (m, 10H),
1.82 (quint, 2H, J = 7.1 Hz), 3.91 (s, 3H), 4.03 (t, 2H, J =
6.6 Hz), 6.92 (d, 2H, J = 8.8 Hz), 8.00 (d, 2H, J = 8.8 Hz). 13C
NMR (CDCl3) d 166.70, 163.02, 131.58, 122.36, 114.02, 68.15,
51.66, 31.93, 29.48, 29.36, 29.24, 26.10, 22.76, 14.14.
Sodium 5-(4-octyloxyphenyl)-1,3,4-thiadiazole-2-carboxylate
(13)
A mixture of 12 (7.00 g, 19.3 mmol), ethanol (140 mL), water
(47 mL) and sodium hydroxide (1.62 g, 40.5 mmol) was stirred
for 24 h at room temperature (TLC analysis revealed
completion of the reaction). The product was filtered off,
washed with diethyl ether, and dried in vacuo (P2O5). The
product was used in the next step without purification. Yield
6.72 g (98%). 1H NMR (DMSO-d6) d 0.87 (t, 3H, J = 6.7 Hz),
1.21–1.38 (m, 8H), 1.42 (quint, 2H, J = 6.7 Hz), 1.73 (quint,
2H, J = 6.7 Hz), 4.04 (t, 2H, J = 6.5 Hz), 7.06 (d, 2H, J =
8.9 Hz), 7.87 (d, 2H, J = 8.9 Hz). 13C NMR (CDCl3) d 171.95,
169.93, 161.25, 159.74, 129.52 (2C), 123.24, 115.59 (2C), 68.22,
31.69, 29.21, 29.18, 29.02, 25.93, 22.61, 14.43.
4-Octyloxybenzohydrazide (9)
A white solid was obtained. Yield 31.72 g (82%), mp 90–91 uC
(lit.42 82–84 uC). 1H NMR (CDCl3) d 0.91 (t, 3H, J = 6.8 Hz),
1.26–1.52 (m, 10H), 1.82 (quint, 2H, J = 6.8 Hz), 2.06 (br s,
2H), 4.01 (t, 2H, J = 6.4 Hz), 6.94 (d, 2H, J = 8.8 Hz), 7.72 (d,
2H, J = 8.8 Hz), 7.78 (br s, 1H). 13C NMR (CDCl3) d 168.34,
162.03, 128.62, 124.54, 114.32, 68.16, 31.74, 29.26, 29.15,
29.06, 25.93, 22.59, 14.03.
Butyl 5-(4-octyloxyphenyl)-1,3,4-thiadiazole-2-carboxylate (14)
1-(Ethyl oxalyl)-2-(4-octyloxybenzoyl)diazane (11)
A suspension of 13 (0.50 g, 1.4 mmol) in anhydrous toluene
(5 mL) under dry nitrogen was subjected to sonication for
30 min. The mixture was cooled to 26 uC before thionyl
chloride (0.17 mL 2.3 mmol, d. 1.63 g mL21) was added drop-
wise at 26 uC. The reaction mixture was allowed to stir for
1K h at 26 uC before anhydrous toluene (7.5 mL) was added
at 24 to 26 uC and the reaction mixture was allowed to stir for
1 h at 24 to 27 uC. Anhydrous pyridine (0.2 mL, 2 mmol, d.
0.9700 g mL21) was added dropwise at 24 to 25 uC.
Anhydrous triethylamine (6.1 mL, 44 mmol, d. 0.726 g mL21
)
was added to a solution of 9 (8.0 g, 29 mmol) in anhydrous
THF (200 mL) under dry nitrogen. The reaction mixture was
stirred for 10 min at room temperature before ethyl oxalyl
chloride (10) (3.9 mL, 35 mmol, d. 1.22 g mL21) was added
dropwise to the mixture at room temperature over approxi-
mately 15 min. The reaction mixture was stirred at room
temperature for a further 20 h (TLC analysis revealed the
completion of the reaction). The reaction mixture was filtered
Anhydrous butan-1-ol (0.34 mL, 3.7 mmol, d. 0.8100 g mL21
)
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in anhydrous toluene (2 mL) was added dropwise at 24
to25 uC and the reaction mixture was allowed to slowly warm
to room temperature before being stirred overnight. The
mixture was sequentially washed with saturated sodium
bicarbonate (2 6 100 mL) and deionized water (2 6 75 mL)
before being dried (MgSO4). The mixture was concentrated in
vacuo followed by purification using column chromatography
(the silica column was pre-treated with a mixture of 5%
triethylamine, 10% ethyl acetate, and 85% petroleum ether)
[silica gel/petroleum ether : ethyl acetate, 1 : 9]. The product
was crystallized from petroleum ether and dried in vacuo
(P2O5) to afford a white solid. Yield 0.28 g (51%). Transitions
(uC) Cryst 99.4 (SmA 85.5) Iso Liq [recrystallized (Rec.) 80.7].
1H NMR (CDCl3) d 0.92 (t, 3H, J = 7.0 Hz), 1.02 (t, 3H, J =
7.4 Hz), 1.29–1.57 (m, 14H), 1.84 (quint, 2H, J = 7.2 Hz), 4.05
(t, 2H, J = 6.6 Hz), 4.49 (t, 2H, J = 6.8 Hz), 7.01 (d, 2H, J =
8.3 Hz), 7.98 (d, 2H, J = 8.8 Hz). 13C NMR (CDCl3) d 172.46,
162.37, 159.04, 158.62, 130.03 (2C), 121.77, 115.22 (2C), 68.39,
66.95, 31.81, 30.53, 29.33, 29.23, 29.10, 25.99, 22.66, 19.10,
14.11, 13.69. Anal. Calcd for C21H30N2O3S: C, 64.58; H, 7.74;
N, 7.17; S, 8.21. Found: C, 64.84; H, 7.80; N, 7.22; S, 8.21%.
30 min. The mixture was cooled to 24 uC before thionyl
chloride (0.35 mL, 4.8 mmol, d. 1.6310 g mL21) was added
dropwise at 24 uC and the reaction mixture was stirred for
1.05 h at 23 to 24 uC. Anhydrous toluene (15 mL) was added
dropwise at 26 to 27 uC. The mixture was stirred for 1 h at
26 to 28 uC. Anhydrous pyridine (0.46 mL, 5.6 mmol, d.
0.9700 g mL21) was added dropwise at 26 uC. Anhydrous
octan-1-ol (1.20 mL, 7.62 mmol, d. 0.8270 g mL21) in
anhydrous toluene (3 mL) was added dropwise at 26 to
29 uC before the reaction mixture was slowly allowed to warm
to room temperature overnight. The pyridinium salt was
filtered off and the filtrate was washed successively with
saturated sodium bicarbonate (2 6 100 mL) and deionized
water (2 6 75 mL) before being dried (MgSO4). The solution
was concentrated in vacuo followed by purification using
column chromatography (the column was pre-treated with a
mixture of 5% triethylamine, 10% ethyl acetate, and 85% pet-
roleum ether) [silica gel/petroleum ether : ethyl acetate, 1 : 9].
The product was crystallized from methanol using a minimum
volume of added water. Yield 0.6688 g (54%). Transitions (uC)
Cryst 87.0 (SmC 82.5) SmA 92.5 Iso Liq (Rec. 81.6). 1H NMR
(CDCl3) d 0.91 (t, 3H, J = 7.0 Hz), 0.92 (t, 3H, J = 7.0 Hz),
1.28–1.54 (m, 20H), 1.79–1.89 (m, 4H), 4.04 (t, 2H, J = 6.6 Hz),
4.46 (t, 2H, J = 6.8 Hz), 7.00 (d, 2H, J = 8.8 Hz), 7.97 (d, 2H,
J = 9.2 Hz). 13C NMR (CDCl3) d 172.46, 162.37, 159.04,
158.64, 130.03 (2C), 121.78, 115.22 (2C), 68.39, 67.26, 31.81,
31.77, 29.33, 29.23, 29.17, 29.15, 29.10, 28.52, 25.99, 25.83,
22.66, 22.64, 14.11, 14.10. Anal. Calcd for C25H38N2O3S: C,
67.23; H, 8.58; N, 6.27; S, 7.18. Found: C, 67.21; H, 8.62; N,
6.35; S, 7.31%.
Hexyl 5-(4-octyloxyphenyl)-1,3,4-thiadiazole-2-carboxylate (15)
A suspension of 13 (1.00 g, 2.81 mmol) in anhydrous toluene
(10 mL) was subjected to sonication under dry nitrogen for
30 min. The mixture was cooled to 26 uC before thionyl
chloride (0.34 mL, 4.7 mmol, d. 1.6310 g mL21) was added
dropwise at 23 to 24 uC and the mixture was stirred for 1.05 h
at 26 to 27 uC. Anhydrous toluene (14 mL) was added
dropwise at 28 uC and the mixture was stirred for 1 h at 26 to
27 uC. Anhydrous pyridine (0.46 mL, 5.6 mmol, d.
0.9700 g mL21) was added dropwise at 24 to 26 uC.
Decyl 5-(4-octyloxyphenyl)-1,3,4-thiadiazole-2-carboxylate (17)
Anhydrous hexan-1-ol (0.93 mL, 7.5 mmol, d. 0.8200 g mL21
)
A suspension of 13 (1.00 g, 2.81 mmol) in anhydrous toluene
(10 mL) under dry nitrogen was subjected to sonication for
30 min. The reaction mixture was cooled to 26 uC before
thionyl chloride (0.35 mL, 4.8 mmol, d. 1.6310 g mL21) was
added dropwise at 26 uC and the reaction mixture stirred for
1 h 20 min at 24 to 26 uC. Anhydrous toluene (15 mL) was
added dropwise at 24 to 26 uC and the reaction mixture was
stirred for 1 h at 25 to 26 uC. Anhydrous pyridine (0.46 mL,
5.6 mmol, d. 0.9700 g mL21) was added dropwise at 25 to
26 uC. Decan-1-ol (1.45 mL, 7.59 mmol, d. 0.8290 g mL21) in
anhydrous toluene (4 mL) was added dropwise at 23 to 26 uC
before the reaction mixture was allowed slowly to warm to
room temperature overnight. The pyridinium salt was filtered
off and the filtrate was washed successively with saturated
sodium bicarbonate (2 6 100 mL) and deionized water (2 6
75 mL) before being dried (MgSO4). The solution was
concentrated in vacuo followed by purification using column
chromatography (the column was pre-treated with a mixture
of 5% triethylamine, 10% ethyl acetate, and 85% petroleum
ether) [silica gel/petroleum ether : ethyl acetate, 1 : 9]. The
product was crystallized from methanol using a minimum
volume of added water. Yield 0.820 g (61%). Transitions (uC)
Cryst 91.8 (SmC 84.5 SmA 88.8) Iso Liq (Rec. 73.5). 1H NMR
(CDCl3) d 0.90 (t, 3H, J = 6.8 Hz), 0.92 (t, 3H, J = 6.8 Hz),
1.25–1.54 (m, 24H), 1.79–1.89 (m, 4H), 4.05 (t, 2H, J = 6.6 Hz),
4.47 (t, 2H, J = 6.8 Hz), 7.01 (d, 2H, J = 8.8 Hz), 7.98 (d, 2H,
in anhydrous toluene (3 mL) was added dropwise at 26 to
27 uC before the reaction mixture was slowly allowed to warm
to room temperature overnight. The pyridinium salt was
filtered off and the reaction mixture was washed successively
with saturated sodium bicarbonate (2 6 100 mL) and deio-
nized water (2 6 75 mL), and dried (MgSO4). The solution
was concentrated in vacuo followed by purification using
column chromatography (the silica column was pre-treated
with 5% triethylamine, 10% ethyl acetate, and 85% petroleum
ether) [silica gel/petroleum ether : ethyl acetate, 1 : 9]. The
product was crystallized from methanol using a minimum
volume of added water. Yield 0.6534 g (57%). Transitions (uC)
Cryst 79.3 (SmC 71.3) SmA 95.4 Iso Liq (Rec. 69.0). 1H NMR
(CDCl3) d 0.92 (t, 3H, J = 7.2 Hz), 0.93 (t, 3H, J = 7.2 Hz),
1.29–1.54 (m, 16H), 1.80–1.89 (m, 4H), 4.05 (t, 2H, J = 6.6 Hz),
4.48 (t, 2H, J = 6.8 Hz), 7.01 (d, 2H, J = 8.8 Hz), 7.98 (d, 2H,
J = 8.8 Hz). 13C NMR (CDCl3) d 172.45, 162.37, 159.03,
158.62, 130.02 (2C), 121.77, 115.21 (2C), 68.38, 67.24, 31.80,
31.37, 29.30, 29.22, 29.09, 28.48, 26.00, 25.49, 22.66, 22.51,
14.00, 14.00. Anal. Calcd for C23H34N2O3S: C, 65.99; H, 8.19;
N, 6.69; S, 7.66. Found: C, 65.75; H, 8.07; N, 6.84; S, 7.81%.
Octyl 5-(4-octyloxyphenyl)-1,3,4-thiadiazole-2-carboxylate (16)
A suspension of 13 (1.00 g, 2.81 mmol) in anhydrous toluene
(10 mL) under dry nitrogen was subjected to sonication for
3410 | J. Mater. Chem., 2007, 17, 3406–3411
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J = 8.8 Hz). 13C NMR (CDCl3) d 172.46, 162.37, 159.03,
158.63, 130.03 (2C), 121.78, 115.22(2C), 68.38, 67.25, 31.89,
31.81, 29.52, 29.49, 29.33, 29.30, 29.23, 29.21, 29.10, 28.52,
25.99, 25.83, 22.68, 22.66, 14.12, 14.11. Anal. Calcd for
C27H42N2O3S: C, 68.31; H, 8.92; N, 5.90; S, 6.76. Found: C,
68.03; H, 8.81; N, 5.91; S, 6.73%.
13 M. Parra, J. Belmar, H. Zunza, C. Zun˜iga, S. Villouta and
R. Martinez, Bol. Soc. Chil. Quim., 1993, 38, 325–330.
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Z. Naturforsch., B: Chem. Sci., 1997, 52, 1533–1538.
15 Y. Xu, B. Li, H. Liu, Z. Guo, Z. Tai and Z. Xu, Liq. Cryst., 2002,
29, 199–202.
16 Y. Xu, Z.-G. Zhu and Z. Xu, Chin. J. Chem., 2001, 19, 870–876.
17 J. W. Goodby, in Ferroelectric liquid crystals. Principles, properties
and applications, ed. G. W. Taylor, Gordon and Breach,
Philadelphia, 1991, vol. 7, pp. 99–248.
Conclusions
¨
18 D. Joachimi, C. Tschierske, A. Ohlmann and W. Rettig, J. Mater.
Chem., 1994, 4, 1021–1027.
19 K. Zab, H. Kruth and C. Tschierske, Chem. Commun., 1996,
977–978.
20 C. Tschierske, D. Joachimi, H. Zaschke, H. Kresse, B. Linstro¨m,
G. Pelzl, D. Demus and G. Y. Bak, Mol. Cryst. Liq. Cryst., 1990,
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21 G. I. Kornis, in Comprehensive heterocyclic chemistry. Five
membered rings with more than two heteroatoms and fused
carbocyclic derivatives, ed. R. C. Storr, Elsevier Science, Oxford,
1996, vol. 4, pp. 379–408.
We have described the novel synthesis of a new family of
thiadiazole-2-carboxylate ester-based liquid crystals 14–17
using a chemoselective ring-closing methodology. Further
studies designed to investigate the scope of this chemistry are
in progress. These novel materials possess remarkably high
mesophase thermal stabilities that, surprisingly, exceed those
of the analogous phenyl derivatives. The materials have
proven to be both thermally and photochemically stable.
22 P. Bradley, P. Sampson and A. J. Seed, Liq. Cryst. Today, 2005, 14,
15–18.
23 H. Hagen, R.-D. Kohler and H. Fleig, Liebigs Ann. Chem., 1980,
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Acknowledgements
Financial support for this work through a Research Challenge
Grant from the Ohio Board of Regents, and from Kent State
University, is gratefully acknowledged. Support for AG and
SM through KSU’s REU-NSF program is acknowledged. In
addition we would like to thank Dr Mahinda Gangoda for
assistance in obtaining some of the NMR spectra and Dr
Robert J. Twieg for use of his DSC instrument.
24 C. Tanaka, K. Nasu, N. Yamamoto and M. Shibata, Chem.
Pharm. Bull., 1982, 30, 4195–4198.
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G. Rihs, Tetrahedron, 1982, 38, 3753–3760.
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28 V. M. Sonpatki, M. R. Herbert, L. M. Sandvoss and A. J. Seed,
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