Helix-sense-selective polymerization of phenylacetylene having two hydroxy groups using a chiral catalytic system

Article abstract of DOI:10.1021/ja021233o

We have found a simple and novel synthetic method for obtaining a chiral polymer from an achiral monomer by using a chiral catalytic system. The chirality of the polymer was caused only by a one-handed helical backbone, and the polymer had no other chiral

Full text of DOI:10.1021/ja021233o

Published on Web 05/02/2003
Helix-Sense-Selective Polymerization of Phenylacetylene Having Two Hydroxy
Groups Using a Chiral Catalytic System
Toshiki Aoki,* Takashi Kaneko, Naoki Maruyama, Atsushi Sumi, Masahiko Takahashi,
Takashi Sato, and Masahiro Teraguchi
Department of Chemistry and Chemical Engineering, Faculty of Engineering, Niigata UniVersity, Ikarashi 2-8050,
Niigata 950-2181, Japan
Received October 1, 2002; E-mail: toshaoki@eng.niigata-u.ac.jp
Table 1. Helix-Sense-Selective Polymerization of 1 by a Chiral
Syntheses of many kinds of polymers having a chiral main chain
have been reported.1 However, there are only a few examples of
chiral polymers whose chiral properties arise only from their chiral
conformational structures, such as one-handed helicity, without the
coexistence of any other chiral moieties such as chiral side groups,
chiral end groups, or asymmetric carbons in their main chain.2
Nonsubstituted polyacetylenes have aroused interest because of
their noteworthy physical properties such as conductivity and optical
nonlinear susceptibility. Recently, polyacetylenes that have a chiral
conformation have received much attention because the chiral
structures might enhance these unique properties and create new
properties. Such chiral substituted polyacetylenes have been
synthesized by polymerization of monomers having bulky chiral
substituents.3 However, after the chiral side groups were removed
by hydrolysis, the resulting polymer did not maintain its chiral
conformation in solution.4 Akagi et al. synthesized a chiral helical
nonsubstituted insoluble polyacetylene by polymerization of acety-
lene in a chiral nematic reaction field.5 However, there are no reports
of obtaining chiral helical substituted soluble polyacetylenes having
no other chiral moieties by polymerizing an achiral substituted
acetylene with a chiral catalyst.
We report herein that we have found a simple and novel synthetic
method for obtaining a chiral polymer from an achiral substituted
acetylene monomer by using a chiral catalytic system. The polymer
contains only a one-handed helical backbone and no other chiral
structures in the side or end groups. In addition, the helical
conformation was stable in solution. This is the first example of
helix-sense-selective polymerization of substituted acetylenes whose
chiral helicity is stable in solution without the aid of other chiral
substituents or other small molecules.6 We will discuss the reason
for the stability and propose a molecular model.
We polymerized an achiral phenylacetylene (1) using a chiral
catalytic system consisting of a rhodium dimeric complex, [Rh-
(nbd)Cl]2 (nbd ) 2,5-norbornadiene), as a catalyst and a chiral
amine, (R)-1-phenylethylamine ((R)-5), as a cocatalyst (Table 1,
no. 1). The polymer showed Cotton effects at wavelengths around
430 and 310 nm where there are no UV absorptions of 1 and 5
(Figures 1a and 2a). The absorption band at 430 nm is assigned to
the conjugated main chain, and the peaks at 310 nm may arise from
a chiral position between adjacent pendant groups. The intensity
of the band at 430 nm was similar, and the peaks at 310 nm were
a little stronger as compared with other chiral helical polyphenyl-
acetylenes having chiral side groups.7 The [R]D value of poly1 was
-262° (c 0.30, CHCL3). Therefore, we realized the first helix-sense-
selective polymerization of a substituted acetylene using a chiral
catalyst. When we used (R)-N,N-dimethyl-1-phenylethylamine ((R)-
6) instead of (R)-5, a chiral polymer, whose [θ] was lower than
that of (R)-5, was also obtained (Table 1, no. 3). The helical sense
was controlled by changing the chirality of the cocatalyst (Table
1, nos. 3 and 4). On the other hand, no helix-sense-selective
Catalyst at Room Temperaturea
poly(monomer/comonomer)
(composition)
[θ]310/103 c
no.
cocat.
yield (%) MW/106 b (deg‚cm/dmol)
1
2
poly1
poly1
poly1
poly1
poly1/2 (92/8)
poly1/2 (70/30)
poly1/2 (53/47)
poly2
poly3
poly4
(R)-5
none
(R)-6
(S)-6
(R)-5
(R)-5
(R)-5
(R)-5
(R)-5
(R)-5
(R)-5
36.5
83.4
63.7
88.5
13.1
14.2
10.8
33.2
33.7
95.8
85.0
5.17
2.04
2.28
2.46
8.69
7.31
8.69
2.97
6.02
e
31.1
0
3.90
-3.07
11.1
3d
4d
5
6
7
8
1.36
0
0
0
e
9
10d
11
polyTMSPA
0.17
0
a In toluene for 3 h, [monomer] ) 0.1 mol/L, [[Rh(nbd)Cl]2] ) 0.2 mmol/
L, [cocat.] ) 0.05 mol/L. b By GPC (polystyrene, THF). c In chloroform.
d In toluene for 1 h, [monomer] ) 0.1 mol/L, [[Rh(nbd)Cl]2] ) 1 mmol/L,
[cocat.] ) 0.02 mol/L. e Insoluble.
polymerizations occurred in the case of monomer 2 and p-
trimethylsilylphenylacetyelene (TMSPA) that have no hydroxy
groups (Table 1, nos. 8 and 11).
Monomer 4 gave only an insoluble polymer. Intermolecular
hydrogen bonds may make poly4 insoluble. The length of the
dodecyl groups in poly1 may be enough to avoid such intermo-
lecular hydrogen bonds, whereas hexyl groups may be too short.
The polyphenylacetylene without chiral substituents derived from
the chiral homopolymer showed no CD as described above.4 On
the other hand, the chiral structure of poly1 in this study was stable
in chloroform at room temperature for 5 months. Moreover, even
when the solution was heated to 50 °C, almost no change was de-
tected in CD (see Supporting Information). Because 2 and TMSPA
did not give a chiral polymer, the hydroxy groups in 1 were thought
to play a very important role in this stable chiral structure. This
stability may be caused by intramolecular hydrogen bonds between
hydroxy groups in different monomer units. To confirm this, we
measured the CD and IR in various two-component solvents which
have different polarities. As the content of polar DMSO increased,
the CD signal became smaller (Figure 1b-d) and then disappeared
(Figure 1e).8 Simultaneously, the OH absorption at 3300 cm-1
shifted to higher wavelength, indicating that the hydrogen bonds
became weaker (see Supporting Information). Therefore, the
intramolecular hydrogen bonds were found to be effective in making
the chiral structure stable.9 When excess amounts of (R)-5 were
added to an achiral poly1 (Table 1, no. 2) solution, no induced CD
was observed in contrast to the case in poly(monocarboxyphenyl-
acetylene).10 Therefore, poly1 has a stable conformation.
In addition, we copolymerized 1 with 2 (Table 1, nos. 5-7).
The CD and UV spectra of the resulting copolymers are shown in
9
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10.1021/ja021233o CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
probable one. The most probable model has a main chain that is
highly strained cis-transoid. This highly strained structure is in
agreement with the observation that poly1 absorbed at shorter UV
wavelengths in a hydrophobic solvent than in a polar solvent (Figure
1a,e) and the fact that poly1 absorbed at shorter wavelengths than
poly2 (Figure 2a,e). In addition, a WAXD measurement supported
the observed difference in the UV spectra because the radius of
the columnar11 of poly1 was shorter than that of poly2 (see
Supporting Information).
In conclusion, we have found two new findings: one is the helix-
sense-selective polymerization of an achiral acetylene by a chiral
catalyst, and the other is that the chiral helical conformation of the
obtained polymer is stabilized by intramolecular hydrogen bonds
in solution without the use of extra reagents. On the other hand,
the chiral helicities of the polyisocyanide of Nolte,1a,2c prepared
by optical resolution, and the poly(trityl methacrylate) of Okamoto,2a,b
synthesized by asymmetric polymerization, were stabilized by bulky
substituents. Additionally, Yashima’s polymer,6 having an induced
CD, needed the coexistence of an excess of achiral amino alcohols
to maintain the chiral helicity, and Masuda’s polyacetylene,9 whose
helicity was stabilized by intramolecular hydrogen bonds, was
obtained from a chiral monomer and contained many chiral
substituents. Our new asymmetric polymerization reported here is
expected to be applicable to other monomers which can make
hydrogen bonds. Investigations of the degree of the one-handed-
ness12 and the polymerization mechanism13 are now in progress.
Supporting Information Available: Experimental procedures for
synthesis, CD, IR, WAXD charts, and models (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 1. CD and UV spectra of poly1 in various solvents at 20 °C. (a) In
CCl4, (b) in CCl4/DMSO (50/1), (c) in CCl4/DMSO (30/1), (d) in CCl4/
DMSO (20/1), (e) in CCl4/DMSO (10/1).
References
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(3) (a) Aoki, T.; Kokai, M.; Shinohara, K.; Oikawa, E. Chem. Lett. 1993,
2009. (b) Aoki, T.; Kobayashi, Y.; Kaneko, T.; Oikawa, E.; Yamamura,
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(4) Unpublished results from this laboratory concerning poly(L-menthyloxy-
carbonylphenylacetylene).
Figure 2. CD and UV spectra of copoly(1/2) in chloroform at 20 °C. (a)
poly1 (no. 1 in Table 1), (b) copoly(1/2) (92/8) (no. 5 in Table 1), (c) copoly-
(1/2) (70/30) (no. 6 in Table 1), (d) copoly(1/2) (53/47) (no. 7 in Table 1),
(e) copoly2 (no. 8 in Table 1).
(5) Akagi, K.; Piao, G.; Kaneko, S.; Sakamaki, K.; Shirakawa, H.; Kyotani,
M. Science 1998, 282, 1683.
Figure 3. Stereoviews of chiral helical poly1 (C12H25O groups are omitted).
(6) Yashima, E.; Maeda, K.; Okamoto, Y. Nature 1999, 399, 449.
(7) Values of [θ]430 and [θ]310 of other polyphenylacetylenes reported were,
for example, 2 × 103 and 1.8 × 104, 1 × 103 and 0.8 × 104, and 1 × 103
and 1.9 × 104 (deg‚cm2/dmol) from ref 3a, 3c, and 3d, respectively.
(8) When methanol or urea was used as a polar component instead of DMSO,
the intensity of the CD peaks decreased (see Supporting Information).
(9) A similar observation for a polymer from a chiral monomer was reported
by: Nomura, R.; Tabei, J.; Masuda, T. J. Am. Chem. Soc. 2001, 123, 8430.
(10) Yashima, E.; Matsushima, T.; Okamoto, Y. J. Am. Chem. Soc. 1995, 117,
11596.
(11) For columnar structures of monosubstituted polyacetylenes, see: Tabata,
M.; Sone, T.; Sadahiro, Y. Macromol. Chem. Phys. 1999, 200, 265.
(12) The degree of the one-handedness of the polymer is unclear at present.
(13) A chiral monomeric species, for example, Rh(nbd)Cl-(R)-5, is thought to
form in this system on the basis of the observation that [Rh(nbd)Cl]2
dissociates to form a monomeric species Rh(nbd)Cl-triethylamine on
addition of triethylamine, reported by: Tabata, M.; Yang, W.; Yokota,
K. J. Polym. Sci., Polym. Chem. Ed. 1994, 32, 1113.
(Both covalent and hydrogen bonds are shown in solid lines.)
Figure 2. Even when small amounts of 2 were incorporated into
the copolymer, the intensity of the CD signal decreased significantly
(Figure 2b). When the amount of 2 exceeded ca. 50%, the CD signal
disappeared (Figure 2d). Also, polymers of 3 with only one hydroxyl
group showed no CD.
Judging from these experimental results, we propose that the
helical conformation is maintained by hydrogen bonds between two
hydroxyl groups in adjacent phenyl groups. We postulate the
possible conformation in Figure 3. To make the molecular model,
the length of hydrogen bonds between hydroxyl groups in adjacent
monomer units was fixed at 2.7 Å. After making several models
using an MM2 program for minimization, we selected the most
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