Cholesteric silatranes

Article abstract of DOI:10.1080/15421400590955118

Most liquid crystals with a C₃ symmetry have so far been discotic systems, e.g., triphenylenes. In the context of calamitic systems research has concentrated on triptycenes and to a lesser extend on silatranes. In this contribution we report our investigations of cholesteric silatrane systems, which have been obtained by connecting a tribenzylsilatrane core to cholesteric groups via alkyl and siloxane spacer groups.

Full text of DOI:10.1080/15421400590955118

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Cholesteric Silatranes
R. Alvarez a & G. H. Mehl a
a Department of Chemistry , University of Hull ,
Hull, UK
Published online: 31 Aug 2006.
To cite this article: R. Alvarez & G. H. Mehl (2005) Cholesteric Silatranes,
Molecular Crystals and Liquid Crystals, 439:1, 259/[2125]-267/[2133], DOI:
10.1080/15421400590955118
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Mol. Cryst. Liq. Cryst., Vol. 439, pp. 259=[2125]–267=[2133], 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590955118
Cholesteric Silatranes
R. Alvarez
G. H. Mehl
Department of Chemistry, University of Hull, Hull, UK
Most liquid crystals with a C3 symmetry have so far been discotic systems, e.g., tri-
phenylenes. In the context of calamitic systems research has concentrated on trip-
tycenes and to a lesser extend on silatranes. In this contribution we report our
investigations of cholesteric silatrane systems, which have been obtained by con-
necting a tribenzylsilatrane core to cholesteric groups via alkyl and siloxane spacer
groups.
Keywords: liquid crystal; silatrane; benzylsilatrane; cholesteric
INTRODUCTION
Shape anisotropy based on convex molecules is an important require-
ment in the design of novel thermotropic liquid crystals. Recent
computer simulation studies have shown that the formation of low
ordered liquid crystalline mesophases developed from non-convex
anisotropic mesogens with an overall ‘‘fan shape’’ is possible [1,2].
Research using triptycenes or tribenzosilatranes functionalised with
alkyl chains has demonstrated that such systems can exhibit meso-
morphic behaviour, however usually highly ordered phase structures
have been observed [3,4]. Extension of triptycene along one of the
molecular axes has indicated that the formation of high free volume
assemblies is possible [5]. This is very interesting from a technological
point of view [6]. Tribenzosilatranes, which have a threefold sym-
metry, a strong internal dipole and which have the potential to be
extended in a broadly similar manner as the reported triptycenes
can functionalised with suitable mesogens via alkyl siloxane spacers
and nematic phase behaviour has been observed [7]. In this contribution
Address correspondence to G. H. Mehl, Department of Chemistry, University of Hull,
Hull, HU6 7RX, UK. E-mail: g.h.mehl@hull.ac.uk
259=[2125]

260=[2126]
R. Alvarez and G. H. Mehl
SCHEME 1 Reagents and conditions: i) 11-bromo-1-undecene, KOH, KI,
water, ethanol; ii) cholesterol, DCC, DMAP, DCM; iii) tetramethyldisiloxane,
Karstedt’s catalyst, dry toluene; iv) vinyltribenzosiltrane 1, Karstedt’s cata-
lyst, dry toluene.

Cholesteric Silatranes
261=[2127]
FIGURE 1 Schematic structure of tribenzosilatrane 1.
we report the results of our investigation of mesomorphic systems based
on two tribenzosilatranes linked to chiral mesogens as it is shown in
Scheme 1. Cholesteric groups were selected as the chiral mesogens.
The tribenzosilatrane unit 1 shown in Figure 1 was selected as a model
for a non-convex propeller shaped system, which exhibit three aromatic
‘‘blades’’ constituted by benzene rings interconnected via a nitrogen and
a silicon-oxy function resulting in a large internal dipole moment. The
free electron pair of the nitrogen atom is directed towards the apical
silicon enabling the subsistence of a silicon with a fivefold functionality
[8,9].
SYNTHESIS
The synthetic approach towards the nematic silatranes is convergent
with the silatrane units and the rod shaped chiral groups being pre-
pared separately and coupled in the last step. The synthesis of the tri-
benzosilatrane unit
1 follows broadly the literature reported
methodology [10]. Compound 1 was obtained in a four step synthesis
discussed in detail elsewhere [7].
The synthesis of two silatrane functionalised cholesterol derivatives 11
and 12 is shown in Scheme 1. Firstly, commercially available 4-hydroxy-
benzoic acid 2 and compound 3 were reacted with 11-bromo-1-undecene
in the presence of potassium hydroxide and few crystals of potassium iod-
ide in refluxing ethanol: water mixture furnishing compounds 4 and 5
respectively, afterrecrystallisationoftheproducts fromglacialaceticacid.
Cholesterol derivatives 7 and 8 were synthesised by coupling either
compound 4 or compound 5 with commercially available cholesterol 6
via an esterification reaction using DCC and DMAP in dichloro-
methane as solvent to give materials 7 and 8 as white solids after puri-
fication by column chromatography. Compounds 7 and 8 were
transformed in hydrosilylation reactions using 1,1,3,3 tetramethyldisi-
loxane to the siloxane functionalised materials 9 and 10. These

262=[2128]
R. Alvarez and G. H. Mehl
compounds were reacted in the final step of the synthesis with triben-
zosilatrane 1 under hydrosilylation reaction conditions to yield the
final products 11 and 12 as white solids after purification by column
chromatography. The chemical purity of the compounds was verified
by spectroscopic methods. For the final products it should be noted
that some b-addition to the vinyl groups of the benzyl silatrane unit
took place when it was connected to the cholesteryl derivatives. This
occurred in a yield of less than 5% of the total product and it could
be detected in the 29Si spectra of the final systems (signal at
ꢀ20.63 ppm). However the by-products could not be removed by chro-
matographic methods or by recrystallisation techniques.
RESULTS
The synthesis of cholesterol derivatives 7 and 8 and their combination
with tribenzosilatrane 1 has been successfully achieved yielding final
products 11 and 12 respectively. The results of the calorimetric analy-
sis (DSC) of the transitions, confirmed by optical polarising microscopy
are shown in Table 1.
Cholesteryl derivatives 7 and 8 exhibit enantiotropic liquid crystal
behaviour with isotropisation temperatures at 203ꢁC (Nꢂ–Iso) for 7
and at 291ꢁC (SmA–Iso) for 8. The molecular structure of 8 contains
an additional benzene ring compared to 7 resulting in a increase of
about 90ꢁC in the isotropisation temperature and the suppression of
the Nꢂ phase exhibited by its analogue, material 7.
The enthalpy associated with the transitions to the isotropic state
are in the same range, 1.65 J=g for 7 and 1.19 J=g for 8. This is
unusual, as the SmA phase is higher ordered than the Nꢂ phase. How-
ever, comparing the reduced molar transition entropies
(DSmol=R) ¼ (DHmol=RT) with values of 0.29 for 7 and 0.19 for 8 it is
observed that this effect is partly due to the large difference in the
TABLE 1 Transition Temperatures as Determined by DSCa
Comp
Transition temperatures (T=ꢁC) (DH ¼ J=g; DCp ¼ J=gꢁC)
7
8
11
12
Cr 99 SmA 172 (0.66) Nꢂ 203 (1.65) Iso
Cr 95 SmA 291(1.19) Iso
Tg 25 (0.21 ) SmCꢂ (0.45) 60 Nꢂ 72 (2.63) Iso
Cr 98 Nꢂ 164 (1.98) Iso
aDH ¼ enthalpy of the transition, DCp ¼ heat capacity of glass transition, Cr ¼
crystalline, SmCꢂ ¼ chiral smectic C, SmA ¼ smectic A, Nꢂ ¼ chiral nematic, Iso ¼
isotropic liquid, Tg ¼ glass transition temperature; Comp ¼ compound.

Cholesteric Silatranes
263=[2129]
transition temperatures in these two systems in their different phases.
In the final reaction step the benzyl silatrane molecule 1 was
appended to mesogens 9 and 10 furnishing 11 and 12 respectively,
which exhibit enantiotropic mesomorphism. Material 11 exhibits
SmCꢂ (DH ¼ 0.45 J=g) and Nꢂ phase behaviour up to 72ꢁC, where the
compound clears to the isotropic with an associated enthalpy
DH ¼ 2.63 J=g. Material 12 exhibits only one mesophase. It clears at
164ꢁC from the cholesteric phase to the isotropic and the enthalpy
associated to this transition was 1.98 J=g. Notable is the presence of
a glass transition observed for 11 (DCp ¼ 0.21 J=gꢁC) and the lack of
crystalline phases. Nevertheless, material 12 has a similar structure
as the analogue 11 and exhibits a low temperature crystalline state
with melting point at 98ꢁC.
CONCLUSION
The functionalisation of a tribenzosilatrane unit at the apex silicon
with a vinyl group allows for the attachment of cholesteryl groups
via siloxane and alkyl spacer units. The connection of tribenzosila-
trane moieties to cholesteryl derivatives leads to the formation of
chiral phases. SmCꢂ and Nꢂ mesophase behaviour have been detected
for the tribenzosilatrane functionalised cholesteric mesogens. To the
best of our knowledge these materials are the first silatranes which
form chiral mesophases. Compared to the non-substituted analogues,
the silatrane functionalised systems exhibit transition temperatures
which are considerably lower, reduced by about 150ꢁC than what
has been observed in the non-substituted derivatives. Noticeable is
that phase sequence and phase structures can be altered when com-
pared to non-substituted cholesteryl derivatives.
EXPERIMENTAL
All materials and solvents were used as purchased unless mentioned
otherwise. Column chromatography was performed on silica gel (Fluoro-
chem, 35–70m, 60 A). The purity of the synthesis products was checked by
Thin Layer Chromatography (TLC) using silica gel 60 F254 (Merck)
plates and subsequent detection was performed by UV fluorescence
(254 nm). 1H, 13C and 29Si NMR spectra were recorded on a JEOL
1
Lambda 400 spectrometer (400, 100 and 55 MHz, respectively, for H,
13C and 29Si) using tetramethysilane as internal standard. Chemicals
shifts are given in ppm. Elemental analyses were carried out with a
Fisons EA 1108 CHN analyzer. Transition temperatures were measured
with a Mettler FP52 heating stage and FP5 control unit in conjunction

264=[2130]
R. Alvarez and G. H. Mehl
with an Olympus BH2 polarizing microscope and were confirmed with
DSC measurements (Perkin-Elmer DSC 7, indium as standard).
Compound 4
Commercially available 4-hydroxybenzoic acid 2 (2.00 g, 14.5 mmol)
was dissolved in hot ethanol (75 ml) and water (5 ml) together with
potassium hydroxide (2.03 g, 0.036 mol) and few crystals of potassium
iodide in a 250 ml three necked round bottom flask equipped with
reflux condenser and magnetic stirrer. Afterwards 11-bromo-1-unde-
cene (5.06 g, 21.7 mmol) was added and the mixture was refluxed for
24 hours. After this time a solution of potassium hydroxide (10%) in
70% ethanol was added to hydrolyse any ester formed and refluxing
was continued for an additional 3 hours. Then, the solution was
allowed to cool down to room temperature and finally acidified with
concentrated hydrochloric acid until acid pH, giving a white solid.
Recrystallisation of the solid from glacial acetic acid gave the desired
1
product as a white powder (2.15 g, 51.13%); H NMR (DMSO) d: 1.30
(12H, m), 1.78 (2H, qui), 2.02 (2H, q), 4.01 (2H, t), 4.92 (2H, m), 5.77
(1H, m), 6.92 (2H, d), 8.0 4(2H, d); 13C NMR (DMSO) d: 25.95, 28.89,
29.05, 29.09, 29.31, 29.39, 29.47, 33.79, 68.25, 114.17, 121.16,
132.31, 139.22, 163.52, 171.30; Anal: C18H26O3(290.40); Calc C:
74.45, H: 9.02%; Found C:74.47, H: 9.19%; MS (m=z): 290 (M)þ ,
279, 214, 197, 167, 149, 138, 121, 97, 78, 69, 55 (100%).
Compound 5
Same procedure as described for compound 4. Compound 3 (1.00 g,
4.67 mmol), 11-bromo-1-undecene (2.18 g, 9.35 mmol). Recrystallisa-
tion of the solid from glacial acetic acid gave the desired material as
a white powder (1.26 g, 74%); RF ¼ 0.39 [CH2Cl2:ether (60:1)]; 1H
NMR (DMSO) d: 1.21–1.41 (12H, m), 1.70 (2H, qui), 1.99 (2H, q),
3.99 (2H, t), 4.95 (2H, m), 5.76 (1H, m), 7.01 (2H, d), 7.65 (2H, d),
7.74 (2H, d), 7.95 (2H, d); 13C NMR (DMSO) d: 26.01, 29.05, 29.24,
29.54, 29.64, 34.09, 68.13, 116.03, 125.80, 128.35, 128.62, 129.75,
130.09, 144.49, 158.12, 167.35; Anal: C24H30O3(366.50); Calc C:
78.65, H: 8.25%; Found C: 77.63, H: 8.53%; MS (m=z): 366 (M)þ ,
214 (100%), 197, 185, 169, 152, 141, 115, 97, 83, 69.
Compound 7
A mixture of compound 4 (0.43 g, 1.48 mmol), cholesterol (0.69 g,
1.78 mmol), 4-dimethylaminopyridine (100 mg) and p-toluensulponic
acid (50 mg) was dissolved in 40 ml of dichloromethane. The stirred

Cholesteric Silatranes
265=[2131]
mixture was cooled to 0ꢁC and addition of N,N0-dicyclohexylcarbodii-
mide (0.93 g, 4.5 mmol) to the reaction mixture was carried out. After
five minutes stirring at 0ꢁC the temperature was allowed to rise to
room temperature and stirring was continued for 20 hours. Solid urea
(white solid) was filtered off and the filtrate was evaporated to dry-
ness. Purification of the solid residue was carried out by column chro-
matography using a gradient mixture of dichloromethane-hexane as
eluent giving the desired product as a white solid (0.720 g, 74%);
RF ¼ 0.29 [CH2Cl2:hexane (1:1)]; 1H NMR (CDCl3) d: 0.69 (3H, s),
0.85–2.08 (54H, m), 2.44 (2H, d), 3.98 (2H, t), 4.79 (1H, m), 4.91 (2H,
m), 5.40 (1H, d), 5.78 (1H, m), 6.88 (2H, d), 7.97 (2H, d); 13C NMR
(CDCl3) d: 14.70, 21.55, 22.22, 23.88, 25.40, 25.66, 26.66, 27.13,
28.80, 30.77, 30.85, 31.08, 31.74, 31.94, 32.17, 32.24, 32.32, 34.71,
34.77, 36.63, 38.63, 39.02, 39.49, 39.89, 41.12, 42.35, 45.15, 52.87,
58.96, 59.53, 71.00, 77.00, 116.78, 116.98, 125.49, 125.82, 134.33,
142.03, 142.62, 165.64, 168.65; Anal: C45H70O3(659.035); Calc C:
82.01, H: 10.71%; Found C: 82.10, H: 11.00%; MS (m=z): 659 (Mþ ),
368 (100%), 353, 318, 247, 213, 195, 147, 121, 95, 55.
Compound 8
Same procedure as described for compound 7. Compound 5 (0.20 g,
0.54 mmol), cholesterol (0.42 g, 1.09 mmol), N,N0-diciclohexylcarbodii-
mide (0.34 g, 1.64 mmol). The desired product was obtained as a white
solid after purification by column chromatography (0.310 g, 77%);
1
RF ¼ 0.64 (CH2Cl2); H NMR (CDCl3) d: 0.66 (3H, s), 0.83–1.65 (45H,
m), 1.67–2.07 (9H, m), 2.43 (2H, d), 3.97 (2H, t), 4.81 (1H, m), 4.91
(2H, m), 5.41 (1H, d), 5.76 (1H, m), 6.95 (2H, d), 7.52 (4H, m), 8.05
(2H, d); 13C NMR (CDCl3) d: 11.82, 18.69, 19.34, 21.02, 22.54, 22.80,
23.83, 24.25, 26.00, 27.88, 27.98, 28.21, 28.89, 29.08, 29.22, 29.35,
29.40, 29.49, 31.83, 31.89, 33.77, 35.78, 36.15, 36.60, 37.00, 38.22,
39.48, 39.70, 42.26, 50.00, 56.10, 56.64, 68.04, 74.44, 114.11, 114.83,
122.70, 126.25, 128.23, 128.79, 130.00, 132.15, 139.13, 139.63,
145.01, 159.33, 165.87; Anal: C51H74O3(735.14); Calc C: 83.32, H:
10.15%; Found C: 83.58, H: 10.37%; MS (m=z): 735 (M)þ , 663, 484,
454, 424, 368, 366 (100%), 247, 214, 197, 162, 145, 121, 55.
Compound 9
To a stirred solution of compound 7 (0.4 g, 0.607 mmol) and 1,1,3,3-
tetramethyldisiloxane (1.63 g, 0.012 mol) in 15 ml of dry toluene was
added Karstedt’s catalyst [platinum(0)-1,3-divinyl-1,1,3,3-tetra-
methyl-disiloxane complex, 25 ml, 5% xilene]. The mixture was aerated

266=[2132]
R. Alvarez and G. H. Mehl
for a couple of minutes using a glass pipette in order to activate the
catalyst. After stirring for 2 h the reaction at room temperature the
solvent was removed under reduced pressure and the residue was pur-
ified by column chromatography using dichloromethane-hexane gradi-
ent mixture to obtain the desired compound 9 as a white solid which
was reacted in the next step without further purification or analysis
(0.430 g, 89%); RF ¼ 0.33 [CH2Cl2:hexane(1:1)].
Compound 10
Compound 10 was synthesised following the same procedure described
for compound 9. Compound 8 (0.17 g, 0.23 mmol) and 1, 1, 3, 3-
tetramethyldisiloxane (0.93 g, 6.93 mmol). The desired product was
obtained as a white solid after purification by column chromatography
using hexane-dicholoromethane gradient mixture as eluent and it was
employed in the following synthetic step without further analysis or
purification (0.160 g, 80%); RF ¼ 0.67 (CH2Cl2).
Compound 11
The above mentioned material was synthesised following a similar pro-
cedure as described for the synthesis of compound 9. Compound 9
(0.160 g, 0.202 mmol), compound 1 (0.070 g, 0.202 mmol). Reaction time
20 h. Purification was carried out by column chromatography using a
hexane-dichloromethane gradient mixture as eluent to yield the desired
product as a white solid (0.055 g, 24%); RF ¼ 0.12 [CH2Cl2):hexane (1:2)];
1H NMR (CDCl3) d: 0.00 (6H, s), 0,04 (6H, s), 0.44 (2H, t). 0.58 (3H, s),
0.74–1.95 (60H, m), 2.34 (2H, d), 3.85 (2H, t), 4.69 (1H, m), 5.30 (1H,
d), 6.77 (2H, d), 6.80 (3H, m), 6.88 (3H, m), 7.00 (3H, m), 7.62 (3H, dd),
7.87 (2H, d); 13C NMR (CDCl3) d: ꢀ0.14, 0.46, 5.23, 10.37, 11.84, 18.45,
18.70, 19.37, 21.03, 22.55, 22.82, 23.35, 23.81, 24.28, 25.93, 27.92,
28.00, 28.22, 29.09, 29.34, 29.41, 29.54, 29.59, 31.86, 31.91, 33.47,
35.78, 36.17, 36.63, 37.03, 38.27, 39.50, 39.72, 42.29, 50.01, 56.10,
56.67, 68.16, 74.14, 113.93, 117.76, 121.83, 122.64, 122.93, 126.57,
128.82, 131.48, 136.59, 139.76, 153.77, 162.80, 165.81; 29Si NMR DEPT
(CDCl3) d: ꢀ57.67, ꢀ20.64, 7.92, 9.08; Anal: C69H99NO7Si3(1138.78);
Calc C: 72.77, H: 8.76, N: 1.23%; Found C: 72.65, H: 8.87, N: 1.21%;
MS (m=z) MALDI-TOF: 1161.28 (M þ Na)þ .
Compound 12
The target material was synthesised following a procedure described
for the synthesis of compound 11. Compound 10 (0.160 g, 0.184 mmol),
compound 1 (0.190 g, 0.552 mmol). Reaction time 20 h. Purification of

Cholesteric Silatranes
267=[2133]
the final residue was carried out by column chromatography using a
gradient mixture of hexane-dichloromethane as eluent to give the
desired product as a white solid (0.146 g, 65%); RF ¼ 0.82 (CH2Cl2);
1H NMR (CDCl3) d: 0.01 (6H, s), 0.05 (6H, s), 0.47 (2H, t), 0.60 (3H,
s), 0.76–1.95 (60H, m), 2.38 (2H, d), 3.87 (2H, t), 4.74 (1H, m), 5.33
(1H, d), 6.81–6.92 (8H, m), 7.00 (3H, m), 7.45 (2H, d), 7.51 (2H, d),
7.64 (3H, dd), 7.97 (2H, d); 13C NMR (CDCl3) d: ꢀ0.15, 0.46, 5.23,
10.36, 11.84, 14.12, 18.45, 18.70, 19.37, 21.02, 22.55, 22.64, 22.82,
23.35, 23.81, 24.27, 26.00, 27.89, 28.00, 28.23, 29.38, 29.43, 29.57,
29.60, 31.58, 31.85, 31.92, 33.48, 35.78, 36.16, 36.63, 37.02, 38.23,
39.50, 39.71, 42.29, 50.01, 56.10, 56.66, 68.09, 74.48, 114.87, 117.75,
121.83, 122.74, 126.32, 126.56, 128.27, 128.82, 130.03, 132.18,
136.58, 139.67, 145.07, 153.77, 159.37, 165.95; 29Si NMR DEPT
(CDCl3) d: ꢀ57.67, ꢀ20.63, 7.95, 9.08; Anal: C75H103NO7Si3 (1214.88);
Calc C: 74.15, H: 8.55, N: 1.15%; Found C: 74.37, H: 8.84, N: 1.17%;
MS (m=z) MALDI-TOF: 1237.88 (M þ Na)þ .
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