Green chemistry approach to the synthesis of liquid crystalline materials

Article abstract of DOI:10.1080/15421400701826795

This article presents a simple, rapid, economic and environmentally acceptable microwave promoted "green chemistry" approach to the synthesis of some conventional and non-conventional liquid crystalline materials. Alkylation of 4'-hydroxy-4-biphenylcarbonitrile with various alkyl halides under microwave irradiation furnished alkoxycynobiphenyl-based calamitic liquid crystals in very high yield within one minute. Similarly, terminally-functionalized alkoxycyanobiphenyls and alkoxycyanobiphenyl dimers can also be prepared. Microwave irradiation of hexaalkoxytriphenylene in the presence of ionic liquids gives monohydroxy-pentaalkoxytriphenylene in about 30% yield. Novel imidazolium-based ionic liquid crystalline dimers can be easily prepared via quaternization reaction using microwaves. Classical reaction conditions failed to produce these dimers.

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Green Chemistry Approach
to the Synthesis of Liquid
Crystalline Materials
a
a
Sandeep Kumar , Hari Krishna Bisoyi & Santanu
a
Kumar Pal
a
Raman Research Institute , Bangalore, India
Published online: 22 Sep 2010.
To cite this article: Sandeep Kumar , Hari Krishna Bisoyi & Santanu Kumar Pal (2008)
Green Chemistry Approach to the Synthesis of Liquid Crystalline Materials, Molecular
Crystals and Liquid Crystals, 480:1, 287-294, DOI: 10.1080/15421400701826795
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Mol. Cryst. Liq. Cryst., Vol. 480, pp. 287–294, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701826795
Green Chemistry Approach to the Synthesis
of Liquid Crystalline Materials
Sandeep Kumar, Hari Krishna Bisoyi,
Santanu Kumar Pal
Raman Research Institute, Bangalore, India
This article presents a simple, rapid, economic and environmentally acceptable
microwave promoted ‘‘green chemistry’’ approach to the synthesis of some
conventional and non-conventional liquid crystalline materials. Alkylation of
0
4
-hydroxy-4-biphenylcarbonitrile with various alkyl halides under microwave
irradiation furnished alkoxycynobiphenyl-based calamitic liquid crystals in very
high yield within one minute. Similarly, terminally-functionalized alkoxycyano-
biphenyls and alkoxycyanobiphenyl dimers can also be prepared. Microwave
irradiation of hexaalkoxytriphenylene in the presence of ionic liquids gives
monohydroxy-pentaalkoxytriphenylene in about 30% yield. Novel imidazolium-
based ionic liquid crystalline dimers can be easily prepared via quaternization
reaction using microwaves. Classical reaction conditions failed to produce these
dimers.
INTRODUCTION
Developing green methodologies is one of the main themes of modern
synthetic chemistry. In this context, use of microwaves and ionic
liquids are powerful tools. The high-speed chemical synthesis using
microwaves has attracted a considerable attention in the past decade.
After the discovery of microwave dielectric heating to facilitate organic
reactions in 1986 [1,2], almost all types of organic reactions have been
performed using the efficiency of microwave flash heating [3]. This is
not only due to the fact that all organic reactions proceed significantly
faster and more selective than under thermal conditions, but
also because of the operational simplicity, high yields of products
and cleaner reactions with easier work-up. Enormous accelerations
in reaction time can be achieved using microwave irradiation. In
Address correspondence to Sandeep Kumar, Raman Research Institute, C. V. Raman
Avenue, Sadashivanagar, Bangalore 560 080, India. E-mail: skumar@rri.res.in
287

2
88
S. Kumar et al.
majority of reactions, only thermal effect has been found, however,
non-thermal effects do play a role in some reactions.
On the other hand ionic liquids, a class of organic salts with usually
low melting points, have attracted considerable attention. Due to
their unique characteristics such as non-volatility, thermal stability,
non-flammability, extremely high ionic conductivity, very low vapour
pressure, reusability and diverse solvating ability they can be used
as environmentally benign solvent media to replace conventional
volatile organic solvents in many chemical processes. Ionic liquids
serve the dual purpose of solvent as well as reagent and allow easy iso-
lation of the product. Surprisingly these green methodologies have not
been applied for the synthesis of liquid crystalline materials.
Alkoxycyanobiphenyls are indispensable ingredients of the liquid
crystal layer used in liquid crystal display devices. Because of their
commercial importance, a number of cyanobiphenyl derivatives have
been extensively studied for various physical properties [4]. Their
0
dimers, i.e., a,x-bis (4 -cyanobiphenyl-4-yloxy) alkanes, have served
the purpose as model compounds to understand the more complex
polymeric systems [4]. In the family of discotic liquid crystals,
triphenylene derivatives are the most widely synthesized and well
studied materials [5]. However, the potential utility of their dimers,
oligomers and polymers has been back held because of the synthetic
problems in obtaining mono-functionalized triphenylenes. Imidazo-
lium-based ionic liquid crystals have attracted considerable attention
recently because of their anisotropic ionic conductivity [6].
Here we present an efficient, simple, rapid, economic and environ-
mentally acceptable microwave assisted and=or ionic liquid mediated
green chemistry approach to the synthesis of (i) alkoxycyanobiphenyls
0
(
ii) a,x-bis(4 -cyanobiphinyl-4-yloxy)alkanes (alkoxycyanobiphenyl dimers)
(
iii) x-bromoalkoxycyanobiphenyls (iv) monohydroxy-pentaalkoxy-
triphenylenes and (v) novel imidazolium-based alkoxycyanobiphenyl
dimers.
MICROWAVE-ASSISTED SYNTHESIS
OF ALKOXYCYANOBIPHENYLS
The traditional synthesis of alkoxycyanobiphenyls involves alkylation
0
of 4 -hydroxy-4-biphenylcarbonitriles with an appropriate alkyl halide
[
4
4]. The reaction is usually carried out in moderate yield by heating
-hydroxy-4-biphenylcarbonitriles with an appropriate alkyl halide
0
in the presence of a base and in polar solvents for 24 hours. We
have observed that this reaction (Scheme 1) could be finished in
about one minute in very high yield by using microwave heating [7].

Green Chemistry Approach to the Synthesis of LCs
289
SCHEME 1 Synthesis of alkoxycyanobiphenyls using microwaves.
0
Thus, when a mixture of 4 -hydroxy-4-biphenylcarbonitrile (1 equivalent),
octyl bromide and sodium carbonate (2 equivalent of each) in
1
-methylpyrrolidinone was irradiated with 360 W microwave power
for 1 minute (30 s X 2), about 80%of pure 4-octyloxycyanobiphenyl
was isolated. The use of potassium carbonate under similar reaction
conditions afforded about 90% of the product. The best yield of
0
0
4
-(octyloxy)[1,1 -biphenyl]-4-carbonitrile (98%) was obtained using
cesium carbonate as a base. Other solvents such as, DMSO, DMF,
PEG furnished slightly lower yield. By using 600 W microwave
power, the same reaction can be accomplished in 30 seconds in 94%
yield.
MICROWAVE-ASSISTED SYNTHESIS OF
ALKOXYCYANOBIPHENYL DIMERS
Physical properties of liquid crystalline dimers are significantly differ-
ent to that of conventional low molar mass liquid crystals. Dimers
represent ideal model compounds for polymers or networks, due to
their ease of purification and characterization and the possibility of
freezing, in their mesophase, to a glassy state. Dimers containing
two calamitic units show interesting mesomorphic behaviour depend-
ing on the length of the spacer and structure of the linking group. A
variety of liquid crystalline dimers are known in the literature [8,9].
The most extensively studied series of liquid crystalline dimers is
0
the a,x-bis(4 -cyanobiphenyl-4-yloxy)alkanes [9]. The symmetrical
dimers have been generally prepared by a single-step alkylation of
0
4
-hydroxy-4-biphenylcarbonitrile with 0.5 equivalent of the appropriate
a,x-dibromoalkanes. Using the reaction conditions mentioned for the
preparation of alkoxycyanobiphenyls, their dimers can be easily pre-
pared in about 65% yield within one minute (Scheme 2). Thus, when
0
a mixture of 1,6-dibromohexane (1 equivalent) and 4 -hydroxy-4-
biphenylcarbonitrile (2 equivalent) in the presence of cesium carbon-
ate in 1-methylpyrrolidinone was irradiated with 360 W microwave
0
power for 1 minute (30 s X 2), about 65% of pure a,x-bis(4 -cyanobiphe-
nyl-4-hexyloxy)alkane can be isolated. The use of 600 W microwave
power furnished the reaction in 30 s only.

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S. Kumar et al.
SCHEME 2 Synthesis of alkoxycyanobiphenyl dimers using microwaves.
MICROWAVE-ASSISTED SYNTHESIS OF
x-BROMOALKOXYCYANOBIPHENYLS
While the symmetrical dimers can be easily prepared by a single-step
0
alkylation of 4 -hydroxy-4-biphenylcarbonitrile with 0.5 equivalent of
an appropriate a,x-dibromoalkanes, a two-step process is required to
prepare unsymmetrical dimers. In a two-step procedure, the
hydroxyl-functionalized core is first reacted with an excess of appro-
priate a,x-dibromoalkane to obtain the x-brominated product
(
Scheme 3) that can be further reacted with the same or different
hydroxyl-functionalized core to get the symmetrical or unsymmetrical
0
dimer. When 4 -hydroxy-4-biphenylcarbonitrile was irradiated with
excess of 1,6-dibromohexane as mentioned above, x-bromohexyloxy-
cyanobiphenyl was formed exclusively in 80% yield. The x-bromoalk-
oxycyanobiphenyl is a key intermediate in the synthesis of various
other liquid crystalline materials, such as, trimers [10], tetramers
[
11] and terminally functionalized alkoxycyanobiphenyls [12].
MICROWAVE-ASSISTED SYNTHESIS OF MONOHYDROXY-
PENTAALKOXYTRIPHENYLENES
Monohydroxy-pentaalkoxytriphenylenes are valuable precursors for
the preparation of a variety of discotic liquid crystals. A number of
methods have been developed to prepare monohydroxy-functionalized
SCHEME 3 Synthesis of x-bromoalkoxycyanobiphenyls using microwaves.

Green Chemistry Approach to the Synthesis of LCs
291
SCHEME 4 Synthesis of monohydroxy-pentaalkoxytriphenylenes using
microwaves.
triphenylene derivatives [13]. Recently ionic liquids and particularly
imidazolium-based ionic liquids have attracted considerable attention
as environmentally benign solvents for various chemical reactions
due to their interesting properties, such as thermal stability, non-
flammability, very low vapour pressure and reusability [14]. The
ꢀ
ꢀ
anion, Br or I , of these ionic liquids may be used to cleave the aryl-
ether bond(s) of hexaalkoxytriphenylene and, thus, hydroxyl-functio-
nalized triphenylenes can be prepared (Scheme 4). With this idea,
we mixed hexapentyloxytriphenylene with 1-n-butyl-3-methyl imida-
zolium bromide (3 equivalents) and the mixture was subjected to
microwave irradiation for five minutes (1 minute X 5). The reaction
product on column chromatography yielded about 30% of mono-
hydroxy-pentapentyloxytriphenylene. It is noteworthy that no other
side product forms in the reaction and about 60% of the starting
material was recovered which can be recirculated.
SYNTHESIS OF NOVEL IMIDAZOLIUM-BASED IONIC
LIQUID CRYSTALLINE DIMERS
It is well known that ionic molecules form amphotropic liquid crystals
[
15]. They have great potential as ordered reaction media that can
impart selectivity in reactions by ordering reactants [16]. Alkali metal
soaps were the first salts identified as displaying liquid crystalline
properties, followed by alkylammonium, pyridinium, vinamidinium,
phosphonium salts [17]. A number of calamitic liquid crystalline
imidazolium salts have recently been prepared and the formation of
lamellar phases in these materials has been recognized [18]. Self-
assembly of a non-liquid crystalline imidazolium ionic liquid and a
hydroxyl-terminated liquid crystal leads to the formation of phase-
segregated layered structures on the nano scale. These materials have
been found to exhibit two-dimensional ionic conductivities with high
anisotropy [19]. Recently, Yoshio et al. reported one-dimensional ion

2
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S. Kumar et al.
SCHEME 5 Microwave promoted synthesis of ionic liquid crystalline dimers.
transport in self-organized columnar ionic liquids [6]. We have
recently reported a number of pyridinium and imidazolium-based
discotic ionic liquid crystals [20]. However, ionic liquid crystalline
dimers based on imidazolium moiety containing two mesogenic groups
have not yet been explored. To realize these dimers, we prepared
imidazole-substituted alkoxycyanobiphenyl by reacting imidazole with
x-bromoalkoxycyanobiphenyl (Scheme 5). The liquid crystalline imi-
dazolium dimer was obtained by microwave-assisted quaternization
of the above product with another molecule of x-brominated cyanobi-
phenyl. It is interesting to note that the quaternization under classical
reaction conditions does not yield the desired product. The ionic dim-
mer (n ¼ 7) shows a monotropic SmC phase. The crystalline compound
ꢁ
melts at 133 C to the isotropic phase on heating. The smectic C phase
ꢁ
appears at 102 C on cooling the isotropic liquid and remains stable
down to room temperature.
CONCLUSION
An efficient, simple, rapid, economic and environmentally acceptable
‘‘green chemistry’’ approach to the synthesis of various liquid crystal-
line materials has been described. The present facile and clean proto-
col can be employed for the synthesis of many other liquid crystalline
materials.
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