Model studies towards liquid crystalline dendrimers with mesogenic repeat units throughout the structure

Article abstract of DOI:10.1016/j.tetlet.2006.05.107

Model studies towards liquid crystal dendrimers in which the mesogenic units are based on alkoxyalkylbiphenyls and repeat through the structure are described; the parent monomers show smectic mesophases demonstrating the suitability of the motifs.

Full text of DOI:10.1016/j.tetlet.2006.05.107

Tetrahedron Letters 47 (2006) 5569–5572
Model studies towards liquid crystalline dendrimers with
mesogenic repeat units throughout the structure
Andrew N. Cammidge* and Amy S. H. King
Wolfson Materials and Catalysis Centre, School of Chemical Sciences and Pharmacy, University of East Anglia,
Norwich NR4 7TJ, UK
Received 30 March 2006; revised 8 May 2006; accepted 18 May 2006
Abstract—Model studies towards liquid crystal dendrimers in which the mesogenic units are based on alkoxyalkylbiphenyls and
repeat through the structure are described; the parent monomers show smectic mesophases demonstrating the suitability of the
motifs.
Ó 2006 Elsevier Ltd. All rights reserved.
The interplay between microscopic and macroscopic
behaviour lies at the heart of molecular-based materials
research and a superb example is provided by the field of
liquid crystals. The dendrimeric molecular architecture
has proved extremely fascinating, not least because the
properties of dendrimers tend to be significantly differ-
ent from traditional polymers based on related repeat
units.1 The dendrimeric architecture, in which branching
repeat units radiate from a central core, contrasts
directly with the parallel molecular (‘unit’) alignment in
liquid crystal phases. Despite this apparent dichotomy,
a number of liquid crystal dendrimers are known.2 In
most cases, these materials are prepared by substituting
mesogenic units onto the termini of classical dendrimers
(termed side-chain dendrimers3 by analogy to side-chain
polymers). Liquid crystal dendrimers in which the
mesogenic units repeat through the structure (termed
main-chain dendrimers3) are rare3,4 and model studies
towards such materials are described herein.
design was based on knowledge of the ‘parent’ biphenyl
mesogens where it is known that alkoxyalkylbiphenyls
are liquid crystalline6 but bisalkoxy derivatives are not.7
The alkoxyalkylbiphenyl unit was therefore incorpo-
rated in the molecular design and the first targets
(repeating units) were formulated as 2 and 3. The syn-
thesis of model monomers is shown in Scheme 1. Both
syntheses follow similar routes and, in order to provide
rapid access to key intermediates, rely on statistical
RO
R
OR
OR
Archetypal mesogens-
alkoxyalkylbiphenyls
RO
1
O
Biphenyl units, which can be considered as the archetypal
liquid crystal cores,5 were chosen as the basis for investi-
gation. Perhaps the simplest biphenyl unit which could be
used in dendrimer construction is 3,40,5-trisubstituted
biphenyl itself. This building block was briefly investi-
gated but, as expected, model compounds of type 1
proved to be non-mesogenic. More elaborate bis-
biphenyls were therefore targeted (Fig. 1). The molecular
O
2
O
O
3
O
O
biphenyl
alkoxy
alkoxy
biphenyl
alkyl
Keywords: Dendrimers; Liquid crystals; Polycatenar.
*
Corresponding author. Tel./fax: +44 01603 592011; e-mail:
a.cammidge@uea.ac.uk
Figure 1. Dendrimer repeat units containing the alkoxyalkylbiphenyl
motif.
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.05.107

5570
A. N. Cammidge, A. S. H. King / Tetrahedron Letters 47 (2006) 5569–5572
Pd(0), Tol, EtOH,
H2O, Na2CO3,
reflux 25 h (30%)
MeO
B(OH)2
MgBr
O
O
ClCO(CH2)4COCl
NH2NH2
7
Br
Br
(CH2)4
Br
Br
(CH2)6
Br
or
MeO
AlCl3, 40 oC, 3 h
(66%)
KOH, EtOH, trigol
(88%)
Ni(0), Et2O,
reflux 22 h
(33%)
4
5
6
8
(88%)
MeO
MeO
Ni(0), 23 h (83%)
Br
OMe
(CH2)6
OMe
MeO
(CH2)6
OMe
9
17
OMe
Br
Br
OMe
MeO
OMe
NaOMe
HBr,
HOAc (69%)
Mg
CuI, DMF
reflux 23 h
(46%)
10
B(OH)2
16
14
15
Br
Br
MgBr
Pd(0), 48 h (50%)
MeO
HO
HO
(CH2)6
OH
MeO
(CH2)6
OMe
11
HBr, HOAc
reflux 19 h (73%)
18
HO
HO
RBr
K2CO3, EtOH
(40-61%)
(CH2)6
OH
12
RO
RO
RBr
K2CO3, EtOH, reflux 20 h (42-68%)
(CH2)6
OR
RO
RO
19
(CH2)6
OR
13
Scheme 1. Synthesis of model tricatenar monomers.
Table 1. Thermal properties of model monomers 19 on cooling
cross-coupling reactions for desymmetrisation. Friedel–
Crafts reaction between bromobenzene (used as solvent)
and adipoyl chloride8,9 gave diketone 5, which was
reduced8 using the Huang–Minlon modification of the
Wolff–Kishner reaction. Suzuki coupling between di-
bromide 6 and 4-methoxyphenylboronic acid 7 gave an
acceptable yield of singly coupled product 9 (unreacted
starting material was recycled). Similar Suzuki coupling
between 9 and 10 gave 11, whereas isomer 17 was most
conveniently prepared using a nickel-catalysed Kumada
coupling employing the Grignard reagent formed from
15. Trimethoxy intermediates 11 and 17 were demethyl-
ated using HBr/HOAc to give triols 12 and 18. Realkyl-
ation gave the two series of model monomers 13 and 19.
R
Transition temperatures (isotropic
liquid–smectic mesophase) °C
n-Hexyl
n-Heptyl
n-Octyl
n-Decyl
n-Dodecyl
n-Octanoyl
51
40
39
39
35
47
All materials show a monotropic mesophase which persists to room
temperature. A more detailed table is presented in Supplementary
data.
Table 2. Thermal properties of model monomers 13 on cooling
Liquid crystalline compounds, which present a structure
which is a hybrid between rod-like and disc-like mole-
cules, are termed ‘polycatenar’ liquid crystals10 and 13
and 19 are therefore elaborated versions of tricatenar
compounds. Their liquid crystalline behaviour was
investigated using polarising optical microscopy
(POM) and differential scanning calorimetry (DSC),
and the results are summarised in Tables 1 and 2.
R
Transition temperatures (isotropic liquid
(I)–smectic mesophase (S) – crystal (Cr)) °C
n-Hexyl
n-Heptyl
n-Octyl
n-Decyl
n-Dodecyl
I 109 S 84 Cr
I 108 S 84 Cr
I 108 S 83 Cr
I 104 S 70 Cr
I 108 S 87 Cr
All materials show an enantiotropic mesophase. A more detailed table
is presented in the Supplementary data.
Both series of compounds show liquid crystal behaviour.
Compounds in series 13 develop monotropic smectic
phases on cooling, whereas those of series 19 show enan-
tiotropic smectic phases. The phases were assigned as
smectic A based on the textures observed under POM.
It is worth noting that alkoxyalkylbiphenyls typically
show smectic liquid crystal behaviour also.

A. N. Cammidge, A. S. H. King / Tetrahedron Letters 47 (2006) 5569–5572
5571
BuLi, THF
MeO
I
B(OH)
-78 oC then
2 , Pd(0)
Br
(CH2)6
Br
7
I
(CH2)6
I2, 3 h (80%)
I
(CH2)6
OMe
6
(30%)
20
21
1. HBr,HOAc
(80-85%)
BnO
BnO
Pd(0) (58%)
OBn
X
(CH2)6
OBn
O(CH2)6Br
(CH2)6
O(CH2)6X
R'
2. Br(CH2)6Br
K2CO3, EtOH,
(60-61%)
BnO
22 X = Br
23 X = I
26 X = Br
27 X = I
R
OBn
R
25
BnO
R'
B(OH)2
Pd(0)
OBn
(31-74%)
24
30, K2CO3, EtOH
reflux 48 h
B(OH)2
(CH2)6
BnO
BnO
(43%)
30, K2CO3, EtOH
reflux 48 h
O(CH2)6X
(CH2)6
28 X = Br
29 X = I
(CH2)6
(39%)
OH
HO
Br
Br
1.
O
O
(CH2)6
MeO
B(OH)2
(CH2)6
O
O
Pd(0) (48%)
2. HBr, HOAc (70%)
Br
30
OH
31 R = H, R' = OBn
32 R = OBn, R' = H
O
BnO
(CH2)6
O
R
(CH2)6
R'
Scheme 2. Synthesis of first generation protected dendrimers.
The syntheses of these model compounds (‘monomers’)
therefore demonstrates their suitability as mesogenic
repeat units for the construction of dendrimeric archi-
tectures. However, the syntheses and intermediates are
not themselves directly suitable for use in the synthesis
of dendrimers. For dendrimer synthesis, a more elabo-
rate repeat unit is required which is selectively pro-
tected/functionalised permitting the dendrimer to be
constructed generation-on-generation. The syntheses of
these units and the syntheses of the first-generation, pro-
tected dendrimers are shown in Scheme 2. The syntheses
again start from symmetric dibromide 6. In some syn-
theses, the bromides of 6 were converted to iodides (giv-
ing 20) via lithium–halogen exchange to improve the
subsequent cross-coupling reactions. Statistical Suzuki
reaction between 20 and 7 proceeded smoothly and both
bromide 9 and iodide 21 were demethylated using HBr/
HOAc. The resulting phenols were realkylated using
excess 1,6-dibromohexane to give 22 and 23, respectively.
Separate model studies indicated that benzyl protecting
groups would be suitable in the repeating units, so bis-
benzyloxyphenyl boronic acids 24 and 25 were prepared
(see Supplementary data) and coupled with 22/23. As
expected, cross coupling with the aryl iodide derivatives
proceeded more readily and in higher yields. The final
isolated products from these reactions were mixtures
of alkyl bromides and iodides, the latter resulting from
simple substitution with the iodide generated during
cross coupling. Formation of such mixtures is not detri-
mental to the overall synthesis as both can be used in the
dendrimer growth steps. Indeed, 26/27 and 28/29 have
been attached to a ‘core’ trisphenol 3011 to complete
the model study and produce the first generation pro-
tected dendrimers 31 and 3212 which, as expected, are
not themselves liquid crystalline.
In conclusion, we have described the synthesis of two
new series of elaborated tricatenar liquid crystals based
on bisbiphenyl units. The materials were targeted as
models for mesogenic dendrimer repeat units and have
therefore been shown to be suitable in this respect.
The syntheses have been adapted to produce differen-
tially functionalised/protected derivatives which can be
used as building blocks for dendrimer construction,
and the synthesis of the first generation protected den-
drimers is described.
Acknowledgements
The authors are grateful for the support received from
the EPSRC Mass Spectrometry Service (Swansea).

5572
A. N. Cammidge, A. S. H. King / Tetrahedron Letters 47 (2006) 5569–5572
7. Toyne, K. Thermotropic Liquid Crystals; J. Wiley: Chich-
Supplementary data
ester, 1987.
8. Baer, T.; Jaunin, R. Helv. Chim. Acta 1957, 233, 2245–
2248.
9. Cammidge, A. N.; Kelsey, R. D.; King, A. S. H.
Tetrahedron Lett. 1999, 40, 147–148.
10. Gharbia, M.; Gharbi, A.; Nguyen, H. T.; Malthete,
J. Curr. Opin. Colloid Interface. Sci. 2002, 7, 312–
325.
Full experimental details, characterisation data and
spectra are provided for all compounds along with more
detailed tables and description of mesophase behaviour.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2006.05.107.
11. (a) Smet, M.; Corens, D.; Dehaen, W. ARKIVOC 2003,
51–67; (b) Loeb, S. J.; Tramontozzi, D. A. Org. Biomol.
Chem. 2005, 13, 1393–1401.
References and notes
12. Selected data: Compound 31 mp 57–62 °C; (Found: C,
84.18; H, 7.14. C174H174O12. EtOH requires C, 84.37; H,
7.28); dH (270 MHz; CDCl3) 1.40–1.86 (48H, m), 2.64
(12H, m), 4.03 (12H, m), 5.09 (12H, s), 6.61 (3H, t, J 2.3),
6.83 (6H, d, J 2.3), 6.95–7.52 (90H, m); dC (67.9 MHz;
CDCl3) 25.91, 29.16, 29.27, 31.36, 35.56, 67.88, 70.17,
100.72, 106.46, 114.71, 114.81, 123.77, 126.54, 126.99,
127.59, 127.92, 127.99, 128.31, 128.60, 128.75, 133.56,
133.69, 136.91, 138.22, 138.36, 141.24, 141.85, 142.33,
143.41, 158.43, 158.81, 160.20; m/z (FAB) 2457.6 (MH+).
Compound 32 mp 125–130 °C; (Found: C, 84.50; H, 7.07.
1. Frechet, J. M. J.; Tomalia, D. A. Dendrimers and Dendritic
Polymers; Wiley: Chichester, 2001.
2. (a) Saez, I. M.; Goodby, J. W. J. Mater. Chem. 2005, 15,
26–40; (b) Guillon, D.; Deschenaux, R. Curr. Opin. Solid
State Mater. Sci. 2002, 6, 515–525; (c) Ponomarenko, S.;
Boiko, N. L.; Shibaev, V. P. Polym. Sci. Ser. C 2001, 43,
1–45; (d) Marcos, M.; Omenat, A.; Serrano, J. L. Comptes
Rendus Chim. 2003, 6, 947–957.
3. Gehringer, L.; Guillon, D.; Donnio, B. Macromolecules
2003, 36, 5593–5601.
4. (a) Gehringer, L.; Bourgogne, C.; Guillon, D.; Donnio, B.
J. Am. Chem. Soc. 2004, 126, 3856–3867; (b) Gehringer,
L.; Bourgogne, C.; Guillon, D.; Donnio, B. J. Mater.
Chem. 2005, 15, 1696–1703; (c) Campidelli, S.; Eng, C.;
Saez, I. M.; Goodby, J. W.; Deschenaux, R. Chem.
Commun 2003, 1520–1521.
5. Collings, P. J.; Hird, M. Introduction to Liquid Crystals;
Taylor and Francis: London, 1997.
6. (a) Demus, D.; Richter, L.; Rurup, C. E.; Sackman, H.;
Schubert, H. J. Phys. Paris 1975, 36, 349–353; (b) Osman,
M. A.; Huynh-Ba, T. Z. Z. Naturforsch. B 1983, 38, 1221–
1226.
C174H174O12. EtOH requires C, 84.37; H, 7.28%); dH
(270 MHz; CDCl3) 1.20–1.50 (12H, m), 1.55–1.75 (24H,
m), 1.85–1.95 (12H, m), 2.61 (12H, t, J 7.6,), 4.03 (12H,
m), 5.18 (6H, s), 5.20 (6H, s), 6.93–7.02 (15H, m), 7.07–
7.13 (6H, m), 7.18–7.23 (12H, m), 7.30–7.52 (48H, m), 7.60
(6H, d, J 8.6), 7.65 (3H, s); dC (67.9 MHz; CDCl3) 25.39,
29.18, 29.27, 31.41, 35.54, 67.95, 71.40, 71.52, 114.26,
114.73, 114.82, 115.33, 120.10, 123.79, 126.55, 126.65,
127.31, 127.42, 127.81, 127.94, 128.33, 128.49, 128.77,
133.58, 133.71, 134.93, 137.37, 138.23, 141.26, 141.56,
141.87, 148.43, 149.11, 158.45, 158.83; m/z (FAB) 2457.6
(MH+).