From polynorbornene to the complementary polynorbornene by replication

Article abstract of DOI:10.1002/anie.200700472

In undergoing a DNA-like replication process, the single-stranded polynorbornene acts as a template for norbornene monomer adhesion through ester linkages (see scheme). Polymerization of the adhered monomers affords the corresponding unsymmetric double-st

Full text of DOI:10.1002/anie.200700472

Angewandte
Chemie
DOI: 10.1002/anie.200700472
Polynorbornene Replication
From Polynorbornene to the Complementary Polynorbornene by
Replication**
Nai-Ti Lin, Shu-Yi Lin, Shern-Long Lee, Chun-hsien Chen, Chao-Hsiung Hsu, Lian Pin Hwang,
Zhen-Yu Xie, Chung-Hsuan Chen, Shou-Ling Huang, and Tien-Yau Luh*
The double-helical structure, the act of replication linked to
the cell cycle, and the ability of transcription leading to RNA
[
1]
synthesis are unique features of DNA. Many approaches to
simulate the biological processes by replicating a host
molecule into its complementary molecule are known in the
[
2–6]
literature.
Particular attention has been focused on the use
of oligonucleotide hosts to furnish the nonenzymatic synthesis
[
3]
[4]
of complementary oligonucleotides or related analogues.
[
5]
[6]
Peptides and other synthetic systems have also been used
as templates for self-replication. We recently reported the
first helical double-stranded polymer 2 by ring-opening
[
7]
metathesis polymerization (ROMP) of a bisnorbornene
[
8]
derivative 1 [Eq. (1)]. The structure of 2 was unambiguously
proved by chemical hydrolysis, spectroscopic means, and STM
images. It is noteworthy that the linker between two
norbornene moieties in 2 is derived from 4-aminobenzyl
ester and ferrocene dicarboxylate. The aminobenzyl fragment
[
9]
in 2 is known to be particularly labile towards hydrolysis to
afford the corresponding single-stranded polymer 3 and
[
8]
ferrocene dicarboxylic acid 4 [Eq. (1)].
Recently, we have also established that single-stranded
polynorbornenes having endo pendant groups are rigid and
that all pendant groups may align coherently in the same
[
10]
direction. In particular, single-stranded polymers that have
[
+]
[
*] N.-T. Lin, Dr. S.-Y. Lin, S.-L. Lee, Prof. C.-h. Chen, C.-H. Hsu,
Prof. L. P. Hwang, S.-L. Huang, Prof. T.-Y. Luh
Department of Chemistry
[
#]
National Taiwan University
Taipei 106 (Taiwan)
Fax: (+886)2-2364-4971
E-mail: tyluh@ntu.edu.tw
an electron-withdrawing substituent on the aryl pendant
group (for example, 3), directly obtained from the corre-
sponding norbornene monomer by the Grubbs I catalyst, has
[
#]
C.-H. Hsu, Prof. L. P. Hwang
Institute of Atomic and Molecular Science
Academia Sinica
[
10b]
been shown to exhibit isotactic stereochemistry.
Accord-
Taipei 106 (Taiwan)
ingly, another norbornene monomer may be able to link to
these pendant groups. Because the neighboring norbornene
moieties may be in close proximity, ROMP may then take
place, leading to a double-stranded polymer. After hydrolysis,
a complementary polymer resulting from the replication of
the original polymer may be obtained. This strategy is
outlined in Figure 1. Herein, we report the first example of
the use of a polynorbornene derivative as a template to
exhibit replication ability for the formation of a complemen-
tary polymer.
[
$]
Z.-Y. Xie, Prof. C.-H. Chen
Genomic Research Center
Academia Sinica
Nangang, Taipei 115 (Taiwan)
+
[
] To whom correspondence concerning STM should be addressed:
chhchen@ntu.edu.tw.
#
[
] To whom correspondence concerning relaxation-time measure-
ments should be addressed: lphwang@ntu.edu.tw.
$
[
] To whom correspondence concerning MALDI-TOF investigations
should be addressed: winschen@gate.sinica.edu.tw.
We have previously shown that the ferrocene moiety
provides a unique linker for double-stranded polymers
because it may serve as a filling to prop up the two polymeric
backbones. Moreover, the ferrocene unit may increase the
solubility of the polymer because of the flexibility of the
[
**] This work was supported by the National Science Council and
National Taiwan University. We thank Professor Hsiao-Ching Yang
for helpful discussion.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Communications
“
anhydrous hydroxide” (5 equiv KOtBu, 2 equiv H O,
2
[
12]
[13]
THF)
gave 6 in 70% yield. Mitsunobu reaction
of 7
with 6 (DIAD, Ph P) gave 8 in 40% yield. ROMP of 8 with
3
[
7]
7
mol% Grubbs I catalyst afforded polymer 9 as a pale
yellow solid in 92% yield. Gel permeation chromatography
(
(
GPC) analysis suggested that 9 had an M value of 12000
n
PDI = 1.3), corresponding to an average of 20 repeat units,
[
11]
which were consistent with the data from the end-group
1
analysis based on H NMR spectrum (an average of 19 repeat
units). Attempts to use MALDI-TOF MS for the absolute
molecular-weight determination were unsuccessful, presum-
ably owing to the instability of the aminobenzyl ester moieties
in 9.
To test the stability of ferrocene-containing polynorbor-
nene under MALDI-TOF MS conditions, polymer 14, with an
ethylene bridge between the ferrocene unit and the 4-
Figure 1. Strategy for polynorbornene replication.
[
11]
skeleton that results from twisting of the two cyclopenta-
aminophenyl group, was synthesized.
Because of the
[
8]
dienyl ligands. Accordingly, we have incorporated the
ferrocene moiety into the single-stranded polymer
absence of the aminobenzyl ester group in 14, the MALDI-
TOF mass spectrum was obtained satisfactorily. It is note-
worthy that the average molecular weights for 14 obtained by
[
11]
(
Scheme 1). Treatment of 5 with TBDMSCl in the presence
of imidazole and subsequent hydrolysis of the ester with
gel permeation chromatography (M = 12000, PDI = 1.1),
n
Scheme 1. Reaction conditions: a) TBDMSCl, imidazole, RT, 4 h, quantitative; b) KOtBu, H O, RT, 20 h, 70%; c) DIAD, PPh , 7, THF, RT, 40%;
2
3
d) 7 mol% [(Cy P) Cl Ru=CHPh], CH Cl , RT, 2 h, 92%; e) TBAF, THF, 08C, 5 h, quantitative; f) DMAP, Et N, 11, RT, 10 h, 80%; g) 5 mol%
3
2
2
2
2
3
[
(Cy P) Cl Ru=CHPh], CH Cl , RT, 50 min, quantitative.
3 2 2 2 2
4
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Chemie
MALDI-TOF (M = 12000, PDI = 1.3), and end-group anal-
n
1
ysis with H NMR spectroscopy (number of repeat units: 21)
were comparable. These results suggested that GPC or end-
group analysis may provide reliable results for the analysis of
ferrocene-containing polynorbornenes.
The silyl protective groups in 9 were removed (TBAF,
8C, 5 h, quantitative) to give the alcohol 10, which was
Figure 2. High-impedance STM images of 13. a) Typical images
0
acquired from different runs (image size for each frame: 52”52 nm,
Ebias =1.0 V, itunneling =12 pA, height mode). b) High-resolution STM
image (15.7”15.7 nm, Ebias =0.7 V, itunneling =20 pA, current mode). The
simulated structure is magnified and placed at the lower left part of
esterified with acid chloride 11 (DMAP/Et N, RT), leading to
3
the formation of polymer 12. This latter esterification
1
procedure was repeated and the H NMR spectrum showed
(b). In these images, 13 adopts a ladderlike conformation which
that at least 94% of 12 was incorporated into this polymer.
resides at the flat terrace rather than at a step edge or domain
GPC analysis suggested that 12 had an M value of 12000
n
boundaries so that artefacts arising from the highly oriented pyrolytic
(
PDI = 1.2), corresponding to an average of 17 repeat units.
[
15]
graphite substrate would be unlikely. c) Histogram of length (d)
1
However, the end-group analysis based on the H NM R
spectrum showed that 12 may have an average of 19 repeat
units. Again, it was not possible to determine the molecular
weight of 12 by MALDI-TOF MS owing to instability of the
polymer under laser-bombardment conditions.
distribution plotted from 548 polymeric molecules (bin size: 1 nm).
A simulated structure is superimposed on the image of the
double strand, and a magnified view of the structure is given
at the lower left corner of the image (Figure 2b). One
phenylene ring (on the left side of ferrocene in Figure 2b)
might be tilted from the surface normal and positioned closer
to the substrate surface than the other phenylene ring, which
might hang above the substrate in between the norbornene
and ferrocene units. Because of better tunneling conditions,
the phenylene ring on the left side of the ferrocene moieties
would exhibit a more distinct contrast than the right one.
Polymer 13 represents the first unsymmetrical double-
stranded polymer in which the two constituent strands are
complementary to each other and linked by the ferrocene
Under high-dilution conditions, metathesis of 12 with
[
7,14]
Grubbs I catalyst (5 mol%)
gave efficiently the corre-
sponding double-stranded polymer 13. Owing to solubility
problems, we were unable to determine the M value by GPC.
n
The end-group analysis was also difficult because of signifi-
cant overlap of characteristic peaks. The characteristic peak
for the olefinic carbon atoms of the norbornene moiety in 12
(d = 135.6 ppm) was no longer observable for 13. The signals
at d = 171.0 and 166.5 ppm are attributed to the two carbonyl
carbon atoms, whereas the signals at d = 150.9 and 148.5 ppm
are characteristic for the ipso carbon atoms of the amino-
substituted aryl rings. The simplicity of the spectrum indicated
that 13 might adopt a regular rigid structure.
[
8]
units. Unlike 2, only ladderlike structures were observed by
STMfor 13, and neither helical nor supercoiled structures
were observed in the STMimages. The relationship between
the structure of the double-stranded polymer and the solid-
state morphology remains unclear at this stage.
Scanning tunneling microscopy (STM) was employed to
gain detailed structural information for 13. Figure 2a shows
four 52 ” 52-nm frames acquired from different trials of
experiments. It is noteworthy that no dendritic branching was
observed for 13. From more than 500 polymers 13, the
nominal width and length of 13 were 2.8 Æ 0.2 and 10.5 Æ
Polymer 13 was hydrolyzed under “anhydrous hydroxide”
[
12]
conditions (excess KOtBu, 4 equiv H O in THF, 708C) to
2
afford 15 and ferrocene derivative 17 [Eq. (2)]. Attempts to
[
16]
2
.7 nm, respectively, where the standard deviation was
isolate polymer 16 were unsuccessful. Polymer 15, obtained
derived from the Gaussian fitting shown in Figure 2c.
Figure 2b exhibits the resolved STMimage of one double
strand of 13. Although the contrast of the image is obscured
from being exported from the NanoScopeIIIa program, there
are clearly resolved fine rows nearly perpendicular to the
polymer long axis with a spacing of about 0.56 nm. These
results suggested that 13 may have an average of 19 repeat
units and are consistent with the average number of repeat
units of the precursors 9 and 12 described above.
from the neutralization of the aqueous solution, was treated
in situ with CsF and MeI to give the corresponding methyl
[
17]
ester 18 (GPC: M = 5000, PDI = 1.3; MALDI-TOF: M =
n
n
4500, PDI = 1.2) in 50% overall yield of isolated product from
15. Polymer 18 showed spectroscopic properties identical to
those of the polymer synthesized directly from the ROMP of
[
10]
the corresponding monomer and had 18 repeat units, which
were comparable to those of 12 and 13. Since the number of
the repeat units of 18 is comparable with those of the
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Communications
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Keywords: ferrocenes · polynorbornenes · replication ·
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