Synthesis of some novel aromatic alkynyl silanes: Mesomorphic characterization of ethynyl-substituted rod-shaped molecules

Article abstract of DOI:10.1080/15421406.2013.822299

The synthesis and characterization of 10 new rod-shaped substituted benzoates possessing the 4-(2-trimethylsilyl)ethynyl group in terminal phenyl position is reported employing the simple and efficient Sonogashira cross-coupling and DCC esterification. 4′

Full text of DOI:10.1080/15421406.2013.822299

This article was downloaded by: [University of Newcastle, Australia]
On: 31 December 2014, At: 14:59
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Molecular Crystals and Liquid Crystals
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/gmcl20
Synthesis of Some Novel Aromatic
Alkynyl Silanes: Mesomorphic
Characterization of Ethynyl-Substituted
Rod-Shaped Molecules
H. T. Srinivasa a b & S. Hariprasad a
a Department of Studies in Chemistry , Bangalore University ,
Bangalore , India
b Raman Research Institute , Sadashivanagara , Bangalore , India
Published online: 30 Dec 2013.
Click for updates
To cite this article: H. T. Srinivasa & S. Hariprasad (2014) Synthesis of Some Novel Aromatic Alkynyl
Silanes: Mesomorphic Characterization of Ethynyl-Substituted Rod-Shaped Molecules, Molecular
Crystals and Liquid Crystals, 588:1, 17-27, DOI: 10.1080/15421406.2013.822299
To link to this article: http://dx.doi.org/10.1080/15421406.2013.822299
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &

Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

Mol. Cryst. Liq. Cryst., Vol. 588: pp. 17–27, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.822299
Synthesis of Some Novel Aromatic Alkynyl Silanes:
Mesomorphic Characterization of
Ethynyl-Substituted Rod-Shaped Molecules
H. T. SRINIVASA1,2 AND S. HARIPRASAD1,∗
1Department of Studies in Chemistry, Bangalore University, Bangalore, India
2Raman Research Institute, Sadashivanagara, Bangalore, India
The synthesis and characterization of 10 new rod-shaped substituted benzoates pos-
sessing the 4-(2-trimethylsilyl)ethynyl group in terminal phenyl position is reported
employing the simple and efficient Sonogashira cross-coupling and DCC esterifica-
tion. 4ꢀ-Dodecylbiphenyl-4-carboxylic(2-trimethylsilylethynyl)-phenyl ester (3i), 4-(4ꢀ-
hexadecyloxybenzyloxy)carboxylic-4-trimethylsilanylethynyl-phenyl ester (3j), and1,4-
bis(4-(pentyloxy)phenyl)buta-1,3-diyne (7) compounds were evaluated for liquid crystal
property.
[Supplemental materials are available for this article. Go to the publisher’s online edition
of Molecular Crystals and Liquid Crystals to view the free supplemental file.]
Keywords Liquid crystals; Smectic mesophase; Sonogashira coupling; 2-trime-
thylsilylethynyl
Introduction
Liquid crystals form an important class in material chemistry. In particular, liquid
crystals possessing the trimethylsilyl- acetylenic group have gained importance in the
field of synthetic chemistry such as for masking anionic synthons to prepare metal-
coordinated complexes [1,2]. The ethynyl silanes have also been used to prepare many
interesting classes of polycyclic aromatic compounds [3]. Very few reports are found
in the literature on molecular structure containing trimethylsilyl- acetylene moiety
functionalized in material science. Metallomesogens of platinum metal-carbon sigma
bond having di(arylethynyl)bis(trimethylphosphine)platinum(II) complex [4] and β-D-
galactopyranoside end-groups containing chiral amphiphilic mesogenic materials with self-
assembled nanostructures compounds have been reported [5]. Liquid crystals consisting of
a rigid and linear molecular framework with extended π-conjugated electronic structure
of difluoroterphenyl core with short and bulky trimethylsilyl terminal groups provide sta-
bility to the mesophase [6]. The pyrimidinylphenyldiacetylenes [7], azotolanes [8,9], 1,4-
bis(phenylethynyl)benzene [10], 1,4-bis(2-naphthyl)-1,3-butadiynes [11], and high bire-
fringence property having diphenyl-diacetylene-based nematogens [12] are also reported.
Alkyl-substituted oligo(p-phenylene)ethynylenes [13] and 1,2-(4,4-dialkoxyaryl)acetylene
∗Address correspondence to S. Hariprasad, Department of Studies in Chemistry, Banga-
lore University, Central College Campus, Bangalore 560001, India. Tel.: +918022961351; Fax:
+918022961331. E-mail: hariprasad@bub.ernet.in
17

18
H. T. Srinivasa and S. Hariprasad
with solid polymorphism [14, 15] and mesomorphism showing triptycene derivatives [16]
have been achieved using different synthetic routes and in the presence of different concen-
trations of palladium catalysts. The reported synthetic strategies used for the preparation
of trimethylsilylethynyl containing biological molecules and rod, disc, and banana-shaped
compounds suffer from certain synthetic procedures [17–19]. Recently, Hanasaki et al. and
Liao et al. reported polar groups containing symmetric and achiral swallow tail-derived
siloxane dimers [20,21].
Our research laboratory focuses on the synthesis and reactions of novel organosilicon
reagents [22–27]. In continuation of our studies and in the light of the importance of the
aforesaid work in literature, it was in our interest to attempt the preparation and study of
liquid crystal properties of some low molar mass silyl- substituted molecules [28].
In this article, we report the simple and efficient synthesis of some 4-(2-
trimethylsilyl)ethynyl phenyl ester derivatives using Sonogashira cross-coupling reaction
and DCC esterification to obtain alkynyl aromatics. Their mesomorphic studies are reported.
Experimental
Materials and Characterization
All chemicals were purchased from commercial sources (Aldrich, Merck, and Fluka,
Bangalore, Karnataka, India). Triethylamine and N,N-dimethylformamide were distilled
and stored on molecular sieves before use. IR spectra were recorded on Shimadzu FTIR-
8400 spectrophotometer. 1H-NMR and 13C-NMR spectra were recorded in CDCl3 with a
Bruker AMX 400 MHz and a Bruker AMX 100 MHz spectrometer, respectively, using
tetramethylsilane as an internal standard. Mass spectral analysis was carried on a Shimadzu
GC-MS QP5050A instrument equipped with a 30 m × 0.32 mm BP-5 capillary column.
Elemental analysis was carried out by a Carlo-Erba 1106 analyzer. Stability was checked
by a Perkin-Elmer TGA 4000 analyzer. Melting points, thermal behavior, and enthalpies
were examined using a combination of differential scanning calorimetry (DSC) heating
from room temperature to isotropic temperatures at the scan rate of 5◦C/min (Perkin-Elmer
Model Pyris 1D) and polarizing optical microscopy (POM) using Olympus BX50 micro-
scope equipped with a heating hot stage Mettler FP82HT and a central processor Mettler
FP80. Melting points remain uncorrected. X-ray diffraction (XRD) studies were carried out
on unoriented samples using Cu–Kα (λ = 1.54 Å) radiation from a Rigaku Ultrax 18 rotat-
ing anode generator (5.4 kW) monochromated with a graphite crystal. The samples were
held in sealed Lindemann capillary tubes (0.7 mm diameter) and the diffraction patterns
were collected on a two-dimensional Marresearch image plate.
Synthesis and Analytical Data. All benzoic acid derivatives were prepared according to the
reported literature procedures [6, 29, 30]. 1-(4ꢀ;-Hydroxyphenyl)-2-trimethylsilylacetylene
(2) [20] was used to prepare the compounds 3a–j. 1-(4ꢀ;-n-Pentyloxyphenyl)-2-
trimethylsilylacetylene (5) was used for the preparation of compounds 7 and 8, respectively,
according to Scheme 1.
All chemical structures are shown in Table 1.
Reagents: a) Ethynyltrimethylsilane, PdCl2, TPP, NEt3, Cu(OAc)2, 80◦C, 24 h. b) Ben-
zoic acids, DCC, DMAP, dry CH2Cl2, 45 min. c) n-C5H11Br, K2CO3, 85◦C, methylethylke-
tone. d) 2% aqueous K2CO3, MEOH, rt. e) Cu(OAc)2, methanol/pyridine, reflux, 24 h. f)
5% Pd-C, H2, 1,4-dioxane, 60◦C, 12 h.

Synthesis of 2-(Trimethylsilylethynyl)Benzoates
19
Scheme 1. General synthetic method used for the preparation of substituted 4(trimethylsi-
lyl)ethynylbenzoates (3a–j), 1,4-bis[4-(n-pentyloxy)phenyl]buta-1,3-diyne (7), and 1,4-bis[4-
(pentyloxy)phenyl]butane (8).
Preparation of 1-(4ꢀ-Hydroxyphenyl)-2-trimethylsilylacetylene (2) [20]. To a reflux-
ing suspension of iodophenol (2.5 g, 1.0 mmol), copper acetate monohydrate (1.35 g,
1.07 mmol), and palladium chloride (1.8 g, 1.07 mmol) in triethylamine (10 mL) in an inert
atmosphere was added dropwise trimethylsilylacetylene (1.36 g, 1.20 mmol) over 30 min.
The reaction mixture was further refluxed at 80◦C for 24 h under nitrogen atmosphere
and monitored by TLC for completion of reaction. The mixture was cooled and filtered
to remove the solids. The volatile solvents were evaporated under reduced pressure, and
the resulting crude light brown colored viscous oil was extracted into ether, washed with
saturated potassium bicarbonate solution, dried over anhydrous sodium sulfate, and con-
centrated. The silylated product was purified by column chromatography on silica gel with
3% ethyl acetate in hexane as eluent. Yield = 1.58 g (79%).
General Procedure for the Preparation of 4-(2-trimethylsilylethynyl) phenylesters (3a–j)
A mixture of 4-(2-(trimethylsilyl)ethynyl)phenol (1 mmol), benzoic acid derivatives
(1 mmol), N,N-dicyclohexylcarbodiimide (DCC) (1.2 mmol), and catalytic quantity of N,N-
dimethylaminopyrimidine (DMAP) in anhydrous dichloromethane (10 mL) was stirred at
room temperature for 45 min. After completion of reaction, the N,N-dicyclohexylurea
formed was filtered off and the filtrate diluted with dichloromethane. This solution was
washed successively with water (2 × 30 mL), 5% aqueous sodium hydroxide solution (2 ×
25 mL), water (2 × 25 mL), and dried over anhydrous sodium sulfate. The crude product
was chromatographed on silica gel (100–200 mesh size) using 2% ethyl acetate in petroleum
ether as an eluent. Removal of solvent afforded white solid, which was recrystallized from
hexane.
4ꢀ-(2-Trimethylsilylethynyl)phenyl-4-cyanobenzoate (3a). IR: 2926, 2821, 2233,
1
2160, 1747, 1597, 1502, 1454, 1404, 1257, 1195, 1066, 1016, 761, 686 cm−1; H NMR
(ppm, CDCl3): 8.2 (d, 2H, J = 2.1 Hz), 7.8 (d, 2H, J = 2.0 Hz), 7.5 (d, 2H, J = 1.9 Hz),

20
H. T. Srinivasa and S. Hariprasad
Table 1. Chemical structures and melting points (in ◦C) of compounds 3a–j
Compound
Chemical Structure
Melting Points
137–139
3a
3b
3c
86–88
126–128
3d
3e
165–167
Viscous liquid
3f
50–52
3g
3h
3i
84–85
70–72
90–92
3j
102–104
7.1 (d, 2H, J = 1.8 Hz), 0.26 (s, 9H); 13C NMR (ppm, CDCl3): 164.6, 159.7, 150.9, 133.1,
130.5, 129.6, 122.6, 121.6, 120.3, 114.5, 104.2, 94.3, 55.5, 0.02; GC-MS: 319.8 (m/e,
relative intensity), 318.8 (4.02), 304.9 (0.96), 303.8 (3.17), 276.2 (0.59), 174 (3.29), 146
(6.65), 130 (100), 115 (2.56), 105 (4.46), 102 (48.54), 75 (12.81), 51 (9.83), 42 (6.46);
C19H17NO2Si requires C, 71.44; H, 5.36; N, 4.38; found: C, 71.19; H, 5. 19; N, 4.50.

Synthesis of 2-(Trimethylsilylethynyl)Benzoates
21
4ꢀ-(2-Trimethylsilylethynyl)phenyl-4-nitrobenzoate (3b). IR: 2922, 2854, 1745, 1527,
1
1498, 1456, 1348, 1261, 1201, 1078, 1014, 850 cm−1; H NMR (ppm, CDCl3): 8.3 (s,
4H), 7.5 (d, 2H, J = 8.8 Hz), 7.2 (d, 2H, J = 8.8 Hz), 0.26 (s, 9H); 13C NMR (ppm,
CDCl3): 162.9, 151, 150.4, 134.7, 133.3, 131.3, 123.8, 121.5, 121.4, 103.9, 94.9, 0.02;
GC-MS: 339 (m/e, relative intensity), 207 (1.43), 193 (0.26), 150 (19.08), 146 (1.5), 134
(2.75), 120 (2.10), 104 (11.77), 76 (11.92), 73 (11.46), 63 (1.58), 55 (7.05), 44 (64.42),
40 (100); C18H17NO4Si requires C, 63.70; H, 5.05; N, 4.13; found: C, 63.86; H, 4.99; N,
4.17.
4ꢀ-(2-Trimethylsilylethynyl)phenyl-3-methoxybenzoate (3c). IR: 2960, 2837, 2158,
1
1730, 1600, 1504, 1465, 1276, 1199, 1039, 908, 746 cm−1; H NMR (ppm, CDCl3): 7.7
(m, 1H), 7.6 (m, 1H), 7.5 (d, 2H, J = 2.0 Hz), 7.4 (t, 1H, J = 8.0 Hz), 7.1 (m, 3H), 3.9
(s, 3H), 0.26 (s, 9H); 13C NMR (ppm, CDCl3): 163.2, 150.4, 133.3, 133.1, 132.4, 130.6,
121.4, 117.7, 117.1, 103, 94.8, 0.02; GC-MS: 324 (m/e, relative intensity), 309 (0.55), 174
(2.67), 146 (4.43), 136 (8.25), 135 (100), 107 (20.88), 92 (14.73), 77 (25.86), 64 (7.60), 63
(6.37), 53 (2.77), 50 (2.59), 43 (3.19); C19H20O3Si requires C, 70.34; H, 6.21; found: C,
70.38; H, 6.10.
4ꢀ-(2-Trimethylsilylethynyl)phenyl-4-benzyloxybenzoate (3d). IR: 2922, 2852, 2162,
1
1724, 1606, 1462, 1280, 1074, 987, 744 cm−1; H NMR (ppm, CDCl3): 8.1 (d, 2H, J =
9.0 Hz), 7.5 (d, 2H, J = 8.8 Hz), 7.4 (m, 5H), 7.1 (d, 2H, J = 8.8 Hz), 7.0 (d, 2H, J =
9.0 Hz), 5.1 (s, 2H), 0.26 (s, 9H); C25H24O3Si requires C, 74.97; H, 6.04; found: C, 75.03;
H, 6.18.
4ꢀ-(2-Trimethylsilylethynyl)phenyl-3,4,5-tris(n-hexyloxy)benzoate (3e). IR: 2929,
2856, 2119, 1735, 1585, 1502, 1452, 1338, 1247, 1188, 1045, 842 cm−1; 1H NMR (ppm,
CDCl3):7.5 (d, 2H, J = 8.8 Hz), 7.4 (d, 2H, J = 2.6 Hz), 7.1 (d, 2H, J = 8.7 Hz), 4.0 (m,
6H), 1.8–1.2 (m, 24H), 0.9 (m, 9H), 0.26 (s, 9H); C36H54O5Si requires C, 72.68; H, 9.15;
found: C, 72.51; H, 9.21.
4ꢀ-(2-Trimethylsilylethynyl)phenyl-3,4-bis(n-decyloxy)benzoate (3f). IR: 2920, 2852,
2152, 1735, 1597, 1462, 1377, 1273, 1138, 1018, 842, 754 cm−1; 1H NMR (ppm, CDCl3):
7.8 (d, 1H, J = 2.0 Hz), 7.6 (d, 1H, J = 2.0 Hz), 7.5 (d, 2H, J = 8.8 Hz), 7.1 (d, 2H, J =
1.8 Hz), 6.9 (d, 1H, J = 8.8 Hz), 4.2 (t, 4H, J = 6.5 Hz), 1.8 (m, 4H), 1.6–1.2 (m, 28H),
0.9 (tt, 6H, J = 3.1 Hz and 3.0 Hz), 0.25 (s, 9H); C38H58O4Si requires C, 75.20; H, 9.63;
found: C, 75.11; H, 9.59.
4ꢀ-(2-Trimethylsilylethynyl)phenyl-4-n-heptyloxybenzoate (3g). [5] IR: 2906, 2854,
2160, 1741, 1608, 1510, 1454, 1377, 1251, 1201, 1165, 1066, 842 cm−1; 1H NMR (ppm,
CDCl3): 8.1 (d, 2H, J = 2.0 Hz), 7.5 (d, 2H, J = 1.9 Hz), 7.1 (d, 2H, J = 1.8 Hz),
6.9 (d, 2H, J = 1.7 Hz), 4.0 (t, 2H, J = 6.5 Hz), 1.8–1.2 (m, 10H), 0.9 (t, 3H, J
= 6.7 Hz), 0.25 (s, 9H); C25H32O3Si requires C, 73.49; H, 7.89; found: C, 73.58; H,
8.01.
4ꢀ-(2-Trimethylsilylethynyl)phenyl-4-n-tetradecyloxy)benzoate (3h). [5] IR: 2924,
2854, 2158, 1732, 1606, 1508, 1458, 1251, 1165, 1068, 842 cm−1; 1H NMR (ppm, CDCl3):
8.1 (d, 2H, J = 2.0 Hz), 7.5 (d, 2H, J = 1.9 Hz), 7.1 (d, 2H, J = 1.8 Hz), 6.9 (d, 2H, J =
1.8 Hz), 4.0 (t, 2H, J = 6.5 Hz), 1.8–1.2 (m, 24H), 0.9 (t, 3H, J = 6.6 Hz), 0.25 (s, 9H);
C32H46O3Si requires C, 75.84; H, 9.15; found: C, 76.01; H, 9.08.
4ꢀ-(n-Dodecyl-biphenyl-4-carboxylic-(2-trimethylsilylethynyl)phenyl ester (3i). IR:
2922, 2852, 2162, 1730, 1606, 1502, 1454, 1377, 1247, 1203, 1080, 840 cm−1; 1H NMR
(ppm, CDCl3): 8.2 (d, 2H, J = 1.9 Hz), 7.7 (d, 2H, J = 2.0 Hz), 7.5 (m, 4H), 7.3 (d, 2H,
J = 8.1 Hz), 7.1 (d, 2H, J = 2.0 Hz), 2.6 (t, 2H, J = 7.5 Hz), 1.6–1.2 (m, 20H), 0.9 (t,
3H, J = 6.8 Hz), 0.26 (s, 9H); C36H50O2Si requires C, 79.65; H, 9.28; found: C, 79.80; H,
9.31.

22
H. T. Srinivasa and S. Hariprasad
4-(4ꢀ-n-Hexadecyloxybenzoyloxy)carboxylic-4-trimethylsilylethynyl-phenyl
ester
(3j). IR: 2922, 2852, 2158, 1738, 1608, 1512, 1462, 1377, 1282, 1166, 1018, 846 cm−1
;
1H NMR (ppm, CDCl3): 8.2 (dd, 2H, J = 2.0 Hz & 8.8 Hz), 8.1 (d, 2H, J = 8.8 Hz), 7.7
(d, 1H, J = 8.8 Hz), 7.5 (d, 1H, J = 8.8 Hz), 7.3 (d, 2H, J = 8.5 Hz), 7.1 (d, 1H, J =
8.7 Hz), 7.0 (m, 3H), 4.0 (t, 2H, J = 6.5 Hz), 1.8–1.2 (m, 28H), 0.9 (t, 3H, J = 6.6 Hz),
0.26 (s, 9H); C41H54O5Si requires C, 75.19; H, 8.31; found: C, 75.23; H, 8.52.
Preparation of 1-(4ꢀ-n-Pentyloxyphenyl)-2-trimethylsilylacetylene (5). [4,10] A mix-
ture of 1-iodo-4-(pentyloxy)benzene (2 g, 0.34 mmol), copper acetate monohydrate (1.2 g,
0.85 mmol), and palladium chloride (1.6 g, 0.85 mmol) in triethylamine (5 mL) was stirred
in nitrogen atmosphere at room temperature. Trimethylsilylacetylene (0.76 g, 0.40 mmol)
was added dropwise over 30 min and after completion of addition the reaction mixture
refluxed at 80◦C overnight. The reaction was monitored by TLC. After completion of
reaction, the mixture was cooled and filtered. The volatile solvents were removed under
reduced pressure. The resulting crude light brown colored viscous oil was extracted into
ether, washed with saturated potassium bicarbonate solution, dried over anhydrous sodium
sulfate, and concentrated. Finally, the silylated product was purified by column chromatog-
raphy on silica gel with 8:2 ratio of hexane–ethylacetate as an eluent. Yield = 1.26 g
(73%).
Preparation of 1-Ethynyl-4ꢀ-(n-pentyloxy)benzene (6) [12]. To the 1-(4ꢀ-n-pentylo-
xyphenyl)-2-trimethylsilylacetylene (1 g, 0.38 mmol) was added potassium carbonate
(1.0 g, 0.76 mmol) in cold methanol at ambient temperature. The resultant reaction mixture
was stirred for 4 h under inert atmosphere. The reaction was monitored by TLC. After com-
plete conversion of starting material into product, the reaction mixture was filtered. The
volatile solvents were removed under reduced pressure. The resulting crude light brown
colored viscous residue was extracted into ether, washed with saturated potassium bicar-
bonate solution, dried over anhydrous sodium sulfate, and concentrated. The crude product
was purified by column chromatography on silica gel using 2:8 ratio of ethyl acetate and
hexane as mobile phase. Viscous liquid, Yield = 0.530 g, 76%.
1,4-Bis[4ꢀ-(n-pentyloxy)phenyl]buta-1,3-diyne (7). [12,13] To a solution of dry pyri-
dine (5 mL) and methanol (5 mL) was added 1-ethynyl-4-(pentyloxy)benzene (0.250 g,
1 mmol) and copper acetate (0.601 g, 2.5 mmol) under argon atmosphere. The resultant
reaction mixture was refluxed at 80◦C for 24 h. The reaction mixture was cooled, filtered,
and solvents were removed under reduced pressure. The crude brown colored product
was purified by column chromatography on silica gel using 10% ethyl acetate in petroleum
ether. The product was crystallized in chloroform as colorless crystal; IR: 2916, 2856, 2119,
1735, 1585, 1502, 1431, 1338, 1247, 1188, 1045, 842 cm−1; 1H NMR (ppm, CDCl3): 7.43
(d, 4H, J = 8.7 Hz), 6.83 (d, 4H, J = 8.7 Hz), 3.95 (t, 4H, J = 6.5 Hz), 1.8–1.40 (m, 12H),
0.93 (t, 6H, J = 4.5 Hz); 13C NMR (ppm, CDCl3): 159.8, 134, 114.6, 113.7, 81.3, 72.9,
68.21, 28.3, 28.1, 22.42, 13.9; C26H30O2 requires C, 83.38; H, 8.07; found: C, 82.29; H,
7.92.
1,4-Bis[4ꢀ(n-pentyloxy)phenyl]butane (8). A mixture of compound 7 (0.2 g, 1 mmol)
dissolved in ethyl acetate (20 mL) and 5% Pd–C catalyst (0.2 g, 3 mmol) was stirred in an
atmosphere of hydrogen for 12 h. The resulting mixture was filtered and the solvent removed
under reduced pressure. The solid material obtained was recrystalized from ethanol. Yield:
1
0.172 g; Mp: 75–77◦C; IR: 2922, 2852, 1654, 1456, 1375, 1244, 1174, 1018 cm−1; H
NMR (ppm, CDCl3): 7.06 (d, 4H, J = 8.8 Hz), 6.81 (d, 4H, J = 8.8 Hz), 3.94 (t, 4H, J =
6.5 Hz), 2.8 (s, 4H), 1.79–1.25 (m, 16H), 0.94 (t, 6H, J = 4.6 Hz); C26H39O2 requires C,
81.62; H, 10.01; found: C, 81.53, H, 10.22.

Synthesis of 2-(Trimethylsilylethynyl)Benzoates
23
Table 2. Transition temperatures and enthalpies (ꢀH kJ/mol−1) of compounds 3i and 3j
and 7; heating/cooling scan rates at 5◦C min.
Compound
Heating Scan (◦C)
Cooling Scan (◦C)
3ia
3ja
7
Cr 90 SmA 160 Iso
Cr 102 SmA 176 Iso
Cr 120.3(89.2) N 157.3(4.35) Iso
Iso 157 SmA 70 Cr
Iso 173 SmA 80 Cr
Iso 156.5(4.56) N 116.9(85.65) Cr
aTemperatures determined by POM; SmA = Smectic A phase; N = Nematic phase.
Results and Discussion
Synthesis
1-(4ꢀ-Hydroxyphenyl)-2-trimethylsilylacetylene (2) was used to prepare rod-shaped
molecules 3a–j by simple and efficient DCC coupling with various substituted benzoic
acids according to the general procedure given in the experimental section. Our procedure
simplifies laborious synthesis of compound 2 to get a range of compounds with variation
of the terminal unit. All compounds that we have synthesized constituted of polar termi-
nal substituents like cyano-, nitro-, groups with two aryl ring compounds as indicated in
3a–j.
In further studies, we prepared the dialkoxydiphenyldiacetylene compound 7. We have
converted 4-iodophenol 1 into 4-(pentyloxy)iodobenzene 4 by Williamson etherification
using appropriate quantity of 1-bromopentane. Subsequently, 1-(4ꢀ-n-pentyloxyphenyl)-2-
trimethylsilylacetylene was prepared by palladium-catalyzed Sonogashira coupling, fol-
lowed by basic hydrolysis using potassium carbonate and methanol as a proton source to
get 1-ethynyl-4-(pentyloxy)benzene 6 (Scheme 1). Compound 6 was used for Sonogashira
homo-coupling reaction to obtain compound 7, which was subjected to reduction with 5%
palladium on charcoal and hydrogen gas at atmospheric pressure and room temperature to
get 8 with flexible n-butyl spacer. The chemical structures were confirmed by comparison
with the corresponding literature [12,13].
The compounds 3a–j were evaluated for the liquid crystalline properties using DSC
and POM techniques. Compounds 3a and 3b did not exhibit liquid crystalline property. The
compounds with methoxy- 3c, benzyloxy- 3d, tri-n-hexyloxy- 3e, and di-n-decyloxy- 3f
moieties also hardly showed any liquid crystalline property. This prompted us to increase
the substituent by one more aryl ring to verify the structure–property relationships of the
liquid crystals.
In this regard, we synthesized the 2-trimethylsilylacetylene group containing known
monotropic SmA phase compound 3g and nonmesogenic compound 3h [11]. These com-
pounds 3g and 3h and their respective acetylenes show thermodynamically unstable
mesophases, probably due to steric hindrance of the much larger trimethylsilyl group
present in the terminal position of the molecule.
To overcome this, and to achieve thermodynamically stable mesophase, we prepared
the compounds 3i and 3j by increasing the length of the molecule with one phenyl ring and
a single long alkyl chain. Compound 3i, a biphenyl derivative without connecting group;
and 3j with ester connecting group possess long alkyl chains which favor the mesomorphic
property.

24
H. T. Srinivasa and S. Hariprasad
Liquid Crystal Property
Melting and isotropic transition points, which are liquid crystalline behaviors, were deduced
by the combination of DSC and polarized optical microscopic observations. Liquid crystal
phase transition temperatures are shown in Table 2.
The compounds 3i and 3j show a fan-shaped texture of Smectic A phase enantiotropi-
cally (Fig. 1(a)). Compound 3i has lower mesophase stability and width of the mesophase
than that of the molecule 3j. The ester linkage of 3j gives additional stability in relation to
the physical properties such as mesophase range and melting and isotropic temperatures.
Figure 1. Optical photomicrographs: a) compound 3j Smectic A phase at 125◦C and b) compound
7 at 140◦C Nematic phase obtained on cooling from isotropic states.

Synthesis of 2-(Trimethylsilylethynyl)Benzoates
25
Figure 2. A DSC thermogram obtained for compound 7 showing phase transition; heating and
cooling cycles scan rate at 5◦C/min.
Figure 3. X-ray diffractogram of compound 3i in the mesophase at 110◦C.

26
H. T. Srinivasa and S. Hariprasad
Figure 4. Representative TGA curves of compounds 3a and 3j.
Compound 7 shows a long-range Nematic mesophase (Fig. 1(b)) at about 40◦C, which
◦
melts from crystalline to liquid crystalline state at 120.3 C (89.25) (Fig. 2), and then it
goes into isotropic state at 157.3◦C (4.35). Upon cooling from isotropic liquid, a peculiar
Schleiren texture of Nematic phase appears at 156.5◦C (−4.56). The material crystallizes
at 116.9◦C (−85.65).
The reduction and formation of 8 too did not show liquid crystalline property. This
is due to the introduction of flexibility at the center of the low molar mass molecules
leading to disruption of molecular packing and therefore, loss of the liquid crystalline
property.
X-ray analysis was carried out for the compound 3i for further confirmation of the
mesophase. The intensity versus 2θ plot was derived from the diffraction pattern as shown
in Fig. 3. In the small angle region, a sharp peak corresponding to spacing of 35.09 Å
and in the wide angle region a spacing of 4.63 Å were indicative of liquid-like or-
der, local molecular structure, and typical scattering pattern, thus confirming the SmA
mesophase.
The thermal stability of compounds 3a and 3j was studied by ThermoGravimetric
Analysis, with the representative TGA curves shown in Fig. 4. According to TGA analysis,
compounds 3a and 3j have good thermal stability up to 200–250◦C and temperatures at
which 100% weight loss occurred were between 210◦C and 500◦C.
This study illustrated the effect of highly polar and nonpolar alkoxy substituents in
the molecular structure on the liquid crystal property of rod-shaped molecules derived
from aryl 2-(trimethylsilyl)ethynyl. We observed that the polar substituent at the terminal
position does not favor mesomorphism in the low molecular weight compounds possessing
an ethynyltrimethylsilyl group.
Conclusions
Ten novel 4-trimethylsilylethynyl group substituted benzoates and aryloxydiacetylene rod-
shaped molecules have been synthesized with simple and efficient coupling procedures. All
chemical structures have been confirmed by standard spectroscopic techniques. The bulky

Synthesis of 2-(Trimethylsilylethynyl)Benzoates
27
and tetrahedron geometry of the trimethylsilyl group does not favor the mesomorphic
property without elongation of the molecular length. On the other hand, only terminal
monoalkylated extended phenyl ring compounds show mesomorphic property.
Acknowledgments
We are thankful to Mrs. K. N. Vasudha, RRI, Bengaluru, for thermal and X-ray analysis.
Grateful thanks to the NMR Research Center, Indian Institute of Science, for NMR spec-
tral analysis. Financial support by the University Grants Commission and Department of
Science and Technology, New Delhi, India, is gratefully acknowledged.
References
[1] Yamanishi, H., Tomita, I., Ohta, K., & Endo. T. (2001). Mol. Cryst. Liq. Cryst., 369, 47.
[2] Celanire, S., Marlin, F., Baldwin, J. E., & Adlington, R. M. (2005). Tetrahedron, 61, 3025.
[3] Schafer, C., Herrmann, F., & Mattay. J. (2008). Beilstein. J. Org. Chem., 4, 1.
[4] Kaharu, T., Matsubara, H., & Takahashi. S. (1992). J. Mater. Chem., 2, 43.
[5] Ho, M. S., & Hsu, C. S. (2010). Liq. Cryst., 37, 293.
[6] Radini, I. A., & Hird, M. (2009). Liq. Cryst., 36, 1417.
[7] Hudson, C. M., Shenoy, R. A., Neubert, M. E., Brown, G. H., & Petschek, R. G. (1999). Liq.
Cryst., 26, 241.
[8] Mohamed Ahmed, M. S., & Mori. A. (2004). Tetrahedron, 60, 9977.
[9] Skelton, G. W., Dong, D., Tuffin, R. P., & Kelly, S. M. (2003). J. Mater. Chem., 13, 450.
[10] Lydon, D. P., Albesa-Jove, D., Shearman, G. C., Seddon, J. M., Howard, J. A. K., Mardera, T.
B., & Low, P. J. (2008). Liq. Cryst., 35, 119.
[11] Hsu, H. F., Lai, Y. H., Lin, S Y., Lin, W. C., & Chenl, J. F. (2003). Liq. Cryst., 30, 325.
[12] Arakawa, Y., Nakajima, S., Ishige, R., Uchimura, M., Kang, S., Konishi, G., & Watanabe, J.
(2012). J. Mater. Chem., 22, 8394.
[13] Yatabe, T., & Kawanishi, Y. (2011). Liq. Cryst., 39, 1.
[14] Pugh, C., & Percec. V. (1999). Mol. Cryst. Liq. Cryst., 178, 193.
[15] Lee, C. H., Jeong, N. H., Nam, K. D., & Kitamura. T. (1999). J. Ind & Eng. Chem., 5, 40.
[16] Long, T. M., & Swager, T. M. (2002). J. Mater. Chem., 12, 3407.
[17] Hennrich, G., Ortiz, P. D., Cavero, E., Hanes, R. E., & Serrano, J. L. (2008). Eur. J. Org. Chem.,
4575.
[18] Demus, D., Goodby, G., Gray, G. W., Spiess, H. W., & V. Vill. (1998). Hand Book of Liquid
Crystals, vols. 1–3, Wiley-VCH: Weinhem.
[19] Yashima, E., Huang, S., Matsushima, T., & Okamoto. Y. (1995). Macromolecules, 28, 4184.
[20] Hanasaki, T., Kamei, Y., Mandai, A., Uno, K., & Kaneko, K. (2011). Liq. Cryst., 38, 841.
[21] Liao, C. T., Wu, Z. L., Wu, N. C., Liu, Z. Y., Jiang, M. H., Zou, S. F., & Lee, J. Y. (2010). Mol.
Cryst. Liq. Cryst., 533, 3.
[22] Srinivasa, H. T., & Hariprasad, S. (2012). Org. Commun., 5(3), 128.
[23] Srinivasa, H. T., & Hariprasad, S. (2012). Org. Chem. Ind. J., 8, 130.
[24] Divya Jyothi, & Hariprasad, S. (2012). ARKIVOC, 6, 194.
[25] Hariprasad, S., & Nagendrappa, G. (1993). Tetrahedron, 49, 3387.
[26] Hariprasad, S., & Nagendrappa, G. (1997). Indian J. Chem., Sect. B: Org. Chem. Incl. Med.
Chem., 36, 1016.
[27] Venkatesha, M. A., & Hariprasad, S. (2012). J. Bulg. Chem. Soc., 44, 155.
[28] Naciri, J., Carboni, C., & Georgel, A. K. (2003). Liq. Cryst., 30, 219.
[29] Radhika, S., Srinivasa, H. T., & Sadashiva, B. K. (2011). Liq. Cryst., 38, 785.
[30] Cowling, S. J., Hall, A. W., & Goodby, J. W. (2011). J. Mater. Chem., 21, 9031.