A chiral polymer with alternating conjugated segments and (1R,2R)-1,2-diaminocyclohexane as a unit with C2 symmetry

Article abstract of DOI:10.1016/S0040-4039(01)00387-2

A chiral conjugated polymer was prepared via the palladium catalysed coupling reaction between 1,4-di-ynyl-2,5-dialkoxybenzene and a bis-5-bromothienyl derivative of (1R,2R)-1,2-diaminocyclohexane as a chiral unit with C₂ symmetry and a unit wi

Full text of DOI:10.1016/S0040-4039(01)00387-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 3073–3076
A chiral polymer with alternating conjugated segments and
(1R,2R)-1,2-diaminocyclohexane as a unit with C2 symmetry
Jean-Pierre Le`re-Porte, Joe¨l J. E. Moreau,* Franc¸oise Serein-Spirau and Salem Wakim
He´te´rochimie Mole´culaire et Macromole´culaire, CNRS UMR 5076, Ecole Nationale Supe´rieure de Chimie,
8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France
Received 6 February 2001
Abstract—A chiral conjugated polymer was prepared via the palladium catalysed coupling reaction between 1,4-di-ynyl-2,5-
dialkoxybenzene and a bis-5-bromothienyl derivative of (1R,2R)-1,2-diaminocyclohexane as a chiral unit with C2 symmetry and
a unit with complexing properties. The resulting chiral polymer of which it’s backbone consists of alternating conjugated segments
and chiral complexing units, showed intense luminescence properties and was used as a ligand in the rhodium-catalysed hydride
transfer reduction. © 2001 Elsevier Science Ltd. All rights reserved.
Owing to their remarkable electronic and photonic
properties, which result from the existence of an extended
conjugated p-system, conjugated organic polymers are
receiving much attention and very diverse applications of
these materials have emerged.1–4 Many efforts are being
made to control the bulk properties of these materials by
a design strategy based on molecular engineering.5–9 The
tuning of the optical properties of conjugated polymers
results from the control and variation of the structure of
the chain unit. For example, the use of organometallic
coupling reactions allowed a controlled synthesis of
various polymers and nano-architectures.6,7,10 Addition-
ally, the arrangement of the polymeric chain in the solid
is also of great importance since interchain interactions
and dimensionality also play a determining role in the
physical properties of the material11,12 which are critically
dependent on supramolecular features such as molecular
packing. High-charge mobility arose from well-defined
arrangement of the conjugated segments in supramolec-
ular assemblies.13 Conversely, solid-state quantum
efficiency is related to the degree of long-range order,
decreasing with increasing order.14 The coplanar orienta-
tion of conjugated polymer backbones is assumed to lead
to the formation of excimer complex which provides a
non emissive decay channel for the excited state. A helical
polymer, which conformation arises from the main chain
chirality15 may exhibit lower interactions which may help
to prevent the polymer chain from packing.16 Also, chiral
conjugated polymers17 are potentially useful in areas such
as photo- or electroluminescence, non-linear optics,
enantioselective sensing, molecular recognition or asym-
metric catalysis. Our current interest in the optical and
electro-optical properties of thienylene–phenylene
copolymers,12,18 for which emission with high quantum
yields was observed in solution,19 has led us to study the
synthesis of chiral structures as a means to decrease
interchain interaction in the solid state. We have been
interested in developing a quite general route from a
readily available chiral di-amine, having C2 symmetry
which could lead to helical conformation and which
would be capable of metal complexation for catalysis.
We prepared a chiral conjugated polymer using the easily
accessible
(1R,2R)-1,2-diaminocyclohexane20
as
described in Scheme 1. The preparation of the conjugated
segment was achieved from 2,5-dibromo-1,4-dioctoxy-
benzene 1. It was first reacted with trimethylsilylacetylene
(3 mol equiv.) in triethylamine in the presence of
(PPh3)2PdCl2/CuI which led to 1,4-bis[(trimethylsilyl)-
ethynyl)]-2,5-bis-(octoxy)benzene in 90% yield. Subse-
quent treatment with MeOH/THF in the presence of
KOH as base gave the desilylated di-yne 2 in 84% iso-
lated yield. The palladium catalysed coupling of 2
in the presence of Pd(PPh3)4, with 2-bromothio-
phene gave 5 which was isolated in 80% yield.†
† Selected characterisation data of 5: mp: 66.5°C; 1H NMR (l, ppm,
CDCl3): 7.27–7.32 (m, 4H, -Hth), 7.02 (dd J=4.9, 3.8 Hz 2H, Hth),
6.99 (s, 2H, Hph), 4.01 (t, J=6.3, 4H, O-CH2), 1.85 (m, 4H, CH2),
1.28–1.54 (m, 20H, CH2), 0.88 (t, J=6.1 Hz 6H, CH3); 13C NMR
(l, ppm, CDCl3): 14.1, 22.7, 25.9, 26.1, 29.3, 29.4, 31.8, 69.7, 88.1,
89.8, 113.8, 116.6, 123.5, 127.1, 127.4, 131.8, 153.5; MS (FAB+): 546
(M); absorption (CHCl3) umax nm (mmax/l mol−1 cm−1) 325 (21300);
377 (26800); emission (CHCl3) umax (nm) 430 (excitation umax=385
nm). Anal. calcd for C34H42O2S2: C, 74.68; H, 7.74%. Found: C,
74.69; H, 8.04%.
* Corresponding author.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00387-2

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J.-P. Le`re-Porte et al. / Tetrahedron Letters 42 (2001) 3073–3076
OC8H17
Br
Br
Br
CHO
S
C8H17O
3
1
i)
, MS 3A°
NH2
i) Me3SiCCH / Et3N
PdCl2(PPh3)2/CuI
ii) KOH/MeOH
H2N
ii) NaBH4, MeOH
OC8H17
HN
NH
Pd(PPh3)4/CuI
/i-Pr2NH
C8H17O
S
S
2, 84%
Br
Br
4, 85%
Pd(PPh3)4/CuI
/i-Pr2NH
Br
S
HN
NH
OC8H17
OC8H17
S
S
S
S
n
C8H17O
C8H17O
6, 90%
5, 80%
Scheme 1. Synthesis of monomeric units of 4 and 5 and of polymer 6.
The chiral unit 4 was then prepared from (1R,2R)-1,2-
the collected fractions of 6 were evaluated by analytical
GPC column calibrated with polystyrene standards:
Mw=11900, Mw/Mn=1.7, corresponding to an average
degree of polymerisation of ten. Solutions of 6 in THF
showed specific optical rotation [h]2D5=−65° (c=0.9,
THF), higher than that of the monomeric chiral unit 4
[h]2D5=−38.6° (c=1.33, CHCl3).
diaminocyclohexane which upon reaction with 5-bromo-
,
2-thiophenecarboxaldehyde 3 in CH2Cl2 using 3 A
molecular sieves gave 84% yield of an intermediate
(1R,2R)-N,N%-bis(5-bromothiophen-2-ylmethyl)-1,2-
diiminocyclohexane. The latter was reduced by sodium
borohydride in dry methanol at room temperature to give
(1R,2R)-N,N%-bis(5-bromothiophen-2-ylmethyl)-1,2-
diaminocyclohexane 4 in 85% yield.‡ The polycondensa-
tion reaction was then achieved using equimolar quanti-
ties of 4 (1.02 mmol, 474 mg) and 2 (390 mg) in the
presence of palladium tetrakistriphenylphosphine (4 mol
%) and cuprous iodide (4 mol %) in a toluene/di-isopro-
pyl amine mixture (25 mL/10 mL).21 The resulting
polymer 6 was soluble in THF, CHCl3 and CH2Cl2. THF
solutions of 6 were purified by preparative gel permeation
chromatography (GPC), over polystyrene bio-beads S-
The absorption–emission properties of the conjugated
structure were studied. We compared the properties of
the rigid conjugated molecule 5 with those of the poly-
meric material 6. The latter contains the same unsatu-
rated unit as an alternating conjugated substructure
separated by the chiral unit which limits conjugation
length by isolating the unsaturated segments. The UV–
vis spectrum of CHCl3 solutions of 5 showed absorption
for values of umax=325 nm (m=21300), 377 nm (m=
26800). The emission spectrum (CHCl3 solutions)
revealed umax=438 nm (uexcitation=385 nm). The absorp-
tion spectrum of polymer 6 in dilute CHCl3 solutions
showed p–p* transition maxima at umax=329 (m=18700),
385 (m=25000) nm. It is quite similar to that of the
isolated structure 5, indicating similar conjugation prop-
erties in the two cases in agreement with a confinement
of the conjugated length within the region between the
chiral units. The polymer emitted strong fluorescence in
the green region under UV irradiation (uexcitation=385
nm) in dilute CHCl3 solutions and showed two photo-
luminescent bands at umax=497 and 519 (shoulder) nm.
A very large Stokes shift (112 nm) is observed for the
polymer 6. The photoluminescence emission of 6 also
remains very strong in the solid state (umax=556 and 580
nm), consistent with the absence of p-stacking interac-
tions in the solid material. The observed intense photo-
luminescence is probably related to the non-planar
structure of the polymeric material resulting from the
1
X1 (200–400 mesh) with THF as the eluent. The H
NMR spectrum recorded at 400 MHz and infrared
spectroscopy data of 6 are consistent with the polymer
structure shown in Scheme 1.§ The molecular weights of
‡
Selected characterisation data of 4: [h]2D5=−38.6° (c=1.33, CHCl3);
1H NMR (l, ppm, CDCl3): 6.86 (d, J=3.67, 2H, Hth), 6.65 (d,
J=3.7 Hz, 2H, Hth), 4.02 (d, J=15.3 Hz, 2H, -NH-CH2), 3.80 (d,
J=15.28 Hz, 2H, -NH-CH2), 2.06–2.26 (m, 4H, cyclohexyl), 1.95 (s,
2H, -NH-), 1.68–1.74 (m, 2H, cyclohexyl), 0.97–1.25 (m, 4H cyclo-
hexyl); 13C NMR (l, ppm, CDCl3): 24.8, 31.4, 45.8 (CH2-NH-),
60.3 (CH-NH), 110.6 124.5, 129.3, 141.1; wmax (KBr)/cm−1 : 3299
(N-H); MS (FAB+): 465 (M+1). Anal. calcd for C16H20Br2N2S2 C,
41.39; H, 4.34; N, 6.03%. Found: C, 41.57; H, 4.39; N, 6.16%.
§
Selected characterisation data of 6: 1H NMR (l, ppm, CDCl3): 7.12
(s, 2H, Hth), 6.94 (s, 2H , HPh), 6.83 (s, 2H , Hth), 4.12–3.75 (broad
singlet, 8H, CH2), 2.31–2.08 (broad singlet, 4H, cyclohexyl), 1.84–
1.78 (broad singlet, 8H, NH and CH2), 1.49–1.25 (broad singlets, 28
H, CH2), 0.85 (broad singlet, 6H, CH3); wmax (KBr)/cm−1 3298
(N-H), 2199 (CꢀC). Raman shift (solid state) cm−1 2201 (CꢀC).
Anal. calcd for C42H56N2O2S2 C, 73.64; H, 8.24; N, 4.09 %. Found:
C, 73.0; H, 8.15; N, 3.61 %.

J.-P. Le`re-Porte et al. / Tetrahedron Letters 42 (2001) 3073–3076
3075
S
H
N
/ 0.5 [RhCl(cod)]2
N
H
S
(1)
Ph
Ph
2 mol %
C
O
C
OH
H3C
iPrOH/THF (1/1), [Rh]: 3mol.%,
KOH: 1 mol.%, 20°C, 6 days
H3C
H
catalyst ligand: monomer 4 (R), ee 32%
catalyst ligand: polymer 6 (R), ee 58%
presence of alternating chiral units in the main chain. It
leads to lower interchain interactions and limits the
p-stacking14 of the photo-luminescent centre.
References
1. Skotheim, T. Handbook of Conducting Polymers; Marcel
Dekker: New York, 1986.
Also, trans-1,2-diamino-cyclohexane derivatives are of
great interest as ligands for catalysis.22 Rhodium com-
plexes of (1R,2R)-1,2-diaminocyclohexane ligands have
been used in asymmetric hydrogen-transfer reduction of
prochiral ketones.23,24 Rhodium complexes of 4 and 6
were formed upon reaction with 0.5 mol of
[Rh(cod)Cl]2 in THF,¶ and these were used as catalysts
in the asymmetric reduction of acetophenoneꢀꢀ (Eq. (1)).
Under identical conditions, the use of monomer 4 as a
ligand gave (R)-1-phenylethanol (93% yield; ee=33%)
and the use of polymer 6 as a ligand led to lower
conversion but higher enantioselectivity (46% yield;
ee=58%). The enhanced selectivity may arise from a
helical conformation of the polymeric structure arising
from the presence of alternating chiral C2 units and a
rigid conjugated segment in the main chain. It provides
an additional contribution to the chirality of the cata-
lytic species and results in higher ee compared to the
value obtained using the isolated monomeric chiral
unit.
2. Nalwa, H. S. Handbook of Organic Conductive
Molecules and Polymers; Wiley & Sons: New York,
1996.
3. Skotheim, T.; Reynolds, J.; Elsenbaumer, R. Handbook
of Conducting Polymers; Marcel Dekker: New York,
1998.
4. Miyata, S.; Nalwa, H. S. In Organic Electroluminescent
Material and Devices; Gordon and Breach: Amsterdam,
1997.
5. Roncali, J. Chem. Rev. 1992, 92, 711–738 and refer-
ences cited therein.
6. Tour, J. M. Chem. Rev. 1996, 96, 537–553 and refer-
ences therein.
7. McCullough, R. D. Adv. Mater. 1998, 10, 93–116 and
references cited therein.
8. Schopf, G.; Kossmehl, G. A. Adv. Polym. Sci. 1997,
129, 1–166.
9. Roncali, J. Chem. Rev. 1997, 97, 173–205.
10. Le`re-Porte, J.-P.; Moreau, J. J. E.; Torreilles, C. Eur. J.
Org. Chem. 2001, 1249–1258.
11. Lorwer, R. S.; McCullough, R. D. Chem. Mater. 2000,
12, 3214–3221.
12. Le`re-Porte, J.-P.; Moreau, J. J. E.; Serein-Spirau, F.;
Torreilles, C.; Righi, A.; Sauvajol, J.-L.; Brunet, M. J.
Mater. Chem. 2000, 10, 927–932.
13. Schoonbeek, F. S.; Van Esch, J. H.; Wegewijs, B.; Rep,
D. B. A.; De Haas, M. P.; Klapwijk, T. M.; Kellog, R.
M.; Feringa, B. L. Angew. Chem., Int. Ed. 1999, 38,
1393–1397.
14. Weder, C.; Wrighton, M. S. Macromolecules 1996, 29,
5157–5165.
In conclusion, compared to the monomeric units, chiral
conjugated polymers containing a chiral diamine unit
and a conjugated unit exhibit upgraded properties. The
chiral unit can serve as a catalyst ligand, leading to
enhanced selectivity, also strong photoluminescence
was observed. We are currently studying the physical
properties of polymer 6 and its use to prepare electro-
luminescent diodes, owing to the observed large Stokes
shift and intense solid state photoluminescence.25 Also,
the reported synthetic route can be extended to a
variety of diamines as chiral units and to a variety of
conjugated segments using palladium catalysed cou-
pling reactions.
15. Wulff, G. Angew. Chem., Int. Ed. Engl. 1989, 28, 21–
37.
16. Zheng, L.; Urian, R. C.; Liu, Y.; Jen, A. K.-Y.; Pu, L.
Chem. Mater. 2000, 12, 13–15.
¶
Evidence for the complexation of the ligand 4 was obtained by
UV–vis spectroscopy. THF solutions of [Rh(cod)Cl]2 exhibited an
absorption at 350 nm which was red-shifted to 375 nm in the
presence of 4. The high absorption of 6 in this range, prevents
measurements for the polymer.
17. (a) Pu, L. Acta Polym. 1997, 48, 116–141; (b) Pu, L.
Chem. Rev. 1998, 98, 2405–2494.
18. (a) Le`re-Porte, J.-P.; Moreau, J. J. E.; Torreilles, C.
Synth. Metals 1999, 101, 588–589; (b) Bouachrine, M.;
Le`re-Porte, J.-P.; Moreau, J. J. E.; Serein-Spirau, F.;
Torreilles, C. J. Mater. Chem. 2000, 10, 263–268.
19. Le`re-Porte, J.-P.; Moreau, J. J. E.; Torreilles, C. Synth.
Metals 1999, 101, 104.
ꢀꢀ
The experiments were performed at room temperature using a 1/1
THF/i-PrOH solvent mixture, molar ratios Rh/substrate: 0.03; Rh/
KOH: 0.1. The reaction was followed by capillary GC and the
enantioselectivity was determined by HPLC using a Chiracel-OD
column.

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J.-P. Le`re-Porte et al. / Tetrahedron Letters 42 (2001) 3073–3076
20. Larrow, J. F.; Jacobsen, E. N. J. Org. Chem. 1994, 59,
1939–1942.
21. Sanechika, K.; Yamamoto, T.; Yamamoto, Y. Bull.
Chem. Soc. Jpn. 1984, 57, 752–762.
718; (d) Hashiguchi, H.; Fujii, A.; Takehara, J.; Ikariya,
T.; Noyori, R. J. Am. Chem. Soc. 1995, 117, 7562–7563;
(e) Pu¨ntener, P.; Schwink, L.; Knochel, P. Tetrahedron
Lett. 1996, 37, 8165–8168.
22. Bennani, Y. L.; Hanessian, S. Chem. Rev. 1977, 97,
3161–3195.
24. (a) Adima, A.; Moreau, J. J. E.; Wong Chi Man, M. J.
Mater. Chem. 1997, 7, 2331–2333; (b) Adima, A.;
Moreau, J. J. E.; Wong Chi Man, M. Chirality 2000, 12,
411–420.
25. (a) McCullough, R. D.; Ewbank, P. C.; in Ref. 3, pp.
225–258; (b) Friend, R. H.; Greenham, N. C.; in Ref. 3,
pp. 823–880.
23. (a) Zassinovich, G.; Mestroni, G.; Gladiali, S. Chem. Rev.
1992, 92, 1051–1069 and references cited therein; (b)
Gamez, P.; Fache, F.; Mangeney, P.; Lemaire, M. Tetra-
hedron Lett. 1993, 34, 6897–6898; (c) Gamez, P.; Fache,
F.; Lemaire, M. Tetrahedron: Asymmetry 1995, 6, 705–
.
.