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90
Chemistry Letters Vol.38, No.2 (2009)
Synthesis of a Linked [1]–[1]Rotaxane
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ꢀ2
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ꢀ1
Susumu Tsuda, Jun Terao, Keisuke Tsurui, and Nobuaki Kambe
Department of Applied Chemistry, Graduate School of Engineering, Osaka University,
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2
-1 Yamadaoka, Suita, Osaka 565-0871
2
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering,
Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510
(Received October 27, 2008; CL-081029; E-mail: terao@scl.kyoto-u.ac.jp)
Highly organic soluble [1]–[1]rotaxane also known as
linked [3]rotaxane was synthesized by intramolecular self-inclu-
sion of a modified permethylated ꢀ-cyclodextrin (PM ꢀ-CD) to
form a pseudo[1]rotaxane followed by dimerization. The NMR
spectroscopy of thus formed rotaxane suggests that the diphen-
ylacetylene units are fully encapsulated by the PM ꢀ-CDs. The
Stern–Volmer analysis of fluorescence quenching using a violo-
gen analogue shows that the PM ꢀ-CDs inhibits electron transfer
efficiently suggesting a high coverage ratio of the ꢁ-conjugated
axle with the PM ꢀ-CDs.
Rotaxanes have attracted considerable attention because of
their unique physical properties and potential applications in
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molecular devices. It is known that the encapsulation of ꢁ-con-
jugated systems can lead to an enhancement in their chemical
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stability and fluorescence efficiency. Rotaxanes have usually
been synthesized by threading an axle molecule through a mac-
rocycle followed by capping with two bulky stoppers (Scheme 1,
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Method 1). We have revealed in a previous paper that an organ-
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ic soluble [1]rotaxane also known as linked [2]rotaxane can be
synthesized in a good yield by the intramolecular self-inclusion
Scheme 2. Synthesis of a linked [1]–[1]rotaxane 3.
of a lipophilic PM ꢀ-CD carrying a rigid ꢁ-conjugated axle moi-
ety followed by capping with a small aniline unit as a stopper
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(
[
Scheme 1, Method 2). Herein, we report the synthesis of a
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1]–[1]rotaxane without using any bulky stopper molecules via
intramolecular self-inclusion and dimerization of thus-formed
pseudo[1]rotaxanes (Scheme 1, Method 3).
Scheme 2 shows our strategy for the synthesis of [1]–
[
1]rotaxane. The reaction of 6-O-monotosyl PM ꢀ-CD with 2-
iodo-5-acetamidophenol results in a modified PM ꢀ-CD iodide
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in 98% yield. The Sonogashira coupling of 1 with (4-
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ethynylphenylethynyl)trimethylsilane followed by deprotection
of the trimethylsilyl group gave an ethynyldiphenylacetylene-
linked PM ꢀ-CD 2 in 71% yield.
The intramolecular self-inclusion phenomenon of 2 has
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Figure 1. The aromatic region of 400 MHz H NMR spectra of
2 in several solvents. 1) CDCl3 at rt; 2) CD OD at rt; 3)
D2O:CD3OD:pyridine = 10:5:1 at 50 C.
been confirmed by using solvent- and concentration-dependent
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3
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ꢁ
H NMR methods. As shown in Figure 1, the NMR spectrum
of 2 in CDCl3 reveals the exclusion of the diphenylacetylene
moiety from the cavity of the PM ꢀ-CD. A spectrum in CD3OD
showed an equilibrium mixture of two species, 2 and its supra-
molecular complex (pseudo[1]rotaxane) 2 . When a more hydro-
philic medium, D2O/CD3OD/pyridine-d5 (10/5/1) was used at
inclusion
Method 1
+
capping
0
Method 2
self-inclusion
ꢁ
0
5
0 C, this complex 2 formed quantitatively. The fact that there
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was no change in the H NMR spectra at different concentrations
in the hydrophilic medium (Eglinton coupling conditions) indi-
cated that the intramolecular self-inclusion complex (pseudo-
dimerization
Method 3
0
Scheme 1. Synthetic strategies for rotaxanes.
[1]rotaxane) 2 was selectively generated from 2.
Copyright Ó 2009 The Chemical Society of Japan