Communications
DOI: 10.1002/anie.201102834
Supramolecular Catalysis
Artificial Molecular Clamp: A Novel Device for Synthetic
Polymerases**
Yoshinori Takashima, Motofumi Osaki, Yoshihiro Ishimaru, Hiroyasu Yamaguchi, and
Akira Harada*
[
29–31]
[32,33]
Renewable materials have attracted much attention from the
viewpoints of environmental protection and efficient utiliza-
tion of natural resources. Certain polyesters, polyamides, and
polylactides, which are synthesized by either biological
methods or chemical processes using a metal catalyst, have
been extensively investigated as biodegradable and renew-
able polymers. However, biological methods are inefficient,
and chemical processes involve harmful metals and organic
solvents. Thus, more efficient and environmentally benign
processes are necessary. In our studies, we hypothesized that
innovative syntheses are best developed using chemical
processes that take advantages of biological systems.
Herein, we successfully obtained synthetic polymerases
including an artificial molecular clamp to yield high-molec-
ular-weight polymers without solvents or co-catalysts. This
system is reminiscent of highly efficient DNA polymerases
including a sliding clamp where the ring-shaped protein
assembly of DNA polymerases plays an important role in the
tions,
and 1,3-dipolar cycloadditions.
The bases of
supramolecular catalysts are selective molecular recognition
and substrate activation. One limitation of these catalysts is
product inhibition because of their complex design, but
introduction of an artificial molecular clamp into supramolec-
ular catalysts can resolve the problems.
Herein, we show that cyclodextrins play an important role
as an artificial molecular clamp in polymerization reactions.
We selected b-CD as a supramolecular polymerization
catalyst because it does not require a highly reactive catalytic
center (metal complexes, cationic or anionic groups). CDs can
include and activate lactones, yielding an oligomer tethered to
[
34,35]
a single CD at the end of the polymer chain.
However,
the produced oligo(lactone)s bearing a b-CD unit did not
initiate the polymerization reaction. We hypothesized that an
artificial molecular CD clamp attached to the active site of the
b-CD plays an important role in the polymerization by
holding the polymer chain and consequently securing the
active site.
[
1–7]
replication of polynucleotides.
Although the clamp does
not have an active site, polymerization does not proceed well
without the clamp. Similarly, cyclodextrins (CDs) are ring-
shaped host molecules, which include various guests to form
First, we studied the polymerization activity of the a,b-
TPA-dimer linked with terephthalamide between the a- and
b-CDs for d-valerolactone (d-VL; Scheme 1). Polymeri-
zations of d-VL initiated by CD dimers were carried out by
stirring and heating a bulk mixture of the CD dimers and d-
VL ([d-VL]/[CD unit] = 50) at 1008C.
[
8–11]
supramolecular complexes such as rotaxanes.
An early example of supramolecular catalysis
hydrolysis of activated phenyl esters using CDs. These
[
12–14]
is the
[
15–20]
catalysts have also been utilized as enzyme models.
Moreover, modern supramolecular catalysts using host–
guest interactions have achieved highly efficient and selective
[
19,20]
reactions, including hydrolysis reactions,
CÀH bond
[23–25]
[26–28]
activation,
epoxidation of olefins,
Diels–Alder reac-
[*] Dr. Y. Takashima, Dr. M. Osaki, Dr. H. Yamaguchi, Prof. Dr. A. Harada
Department of Macromolecular Science
Graduate School of Science, Osaka University
Toyonaka, Osaka, 560-0043 (Japan)
Scheme 1. Polymerization of d-VL initiated by the a,b-TPA-dimer linked
with terephthalamide between the a-CD (gray-blue) and b-CD (yellow).
E-mail: harada@chem.sci.osaka-u.ac.jp
Prof. Dr. Y. Ishimaru
Department of Functional Materials Science
School of Engineering, Saitama University (Japan)
Table 1 summarizes the polymerization of d-VL by CDs.
Although intact a-CD did not initiate polymerization of d-VL
Prof. Dr. A. Harada
Core Research for Evolutional Science and Technology (CREST)
Japan Science and Technology Agency (JST)
Sanban-cho Building, 4F, 5 Sanban-cho, Chiyoda-ku
Tokyo 102-0075 (Japan)
(
Entry 1), intact b-CD and a mixture of a- and b-CD resulted
in low polymerization activities for d-VL under the same
conditions (Entries 2 and 3). In contrast, the a,b-TPA-dimer
displayed a significantly higher polymerization activity to give
[
**] This research was supported by the CREST project, Japan Science
and Technology Agency. The authors thank Seiji Adachi (Osaka
University) for his helpful advice on the measurement of 2D NOESY
spectra.
poly(d-VL) with M = 11000 (Entry 5). Because of the
n
absence of the active b-CD site, the a,a-TPA-dimer showed
a much lower polymerization activity (Entry 4). Thus, the
molecular a-CD clamp connected to the active b-CD site
through a covalent bond is important for the polymerization.
7524
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 7524 –7528