Design and efficient synthesis of a pillar[5]arene-based [1]rotaxane

Article abstract of DOI:10.1039/c3cc48014c

A pillar[5]arene-based [1]rotaxane was simply prepared in a high yield of 73%. Its stucture was confirmed by ¹H NMR, 2D NMR spectra, mass spectra and theoretical calculation. This journal is

Full text of DOI:10.1039/c3cc48014c

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Design and efficient synthesis of a
pillar[5]arene-based [1]rotaxane†
Cite this: Chem. Commun., 2014,
50, 1021
Binyuan Xia and Min Xue*
Received 18th October 2013,
Accepted 13th November 2013
DOI: 10.1039/c3cc48014c
www.rsc.org/chemcomm
A pillar[5]arene-based [1]rotaxane was simply prepared in a high host–guest chemistry and fabricated numerous pseudorotaxane
yield of 73%. Its stucture was confirmed by 1H NMR, 2D NMR structures. Stoddart et al.11 prepared a [2]rotaxane using 1,4-dimethoxyl-
spectra, mass spectra and theoretical calculation.
pillar[5]arene (DMpillar[5]arene) as the wheel and the product obtained
from reaction of 1,8-aminooctane and 3,5-di-tert-butylbenzaldehyde
Mechanically interlocked molecules (MIMs) exist extensively in as the axle in a yield of 7%. Then, Ogoshi et al.12 and Huang et al.13
nature which have been artificially synthesized and widely applied both constructed [2]rotaxanes or [3]rotaxanes by click chemistry.
in the fields of nanotechnology and biology.1,2 Rotaxanes, one of the
Previously, Huang et al.14 found that a pillar[5]arene is able to
most important classes of mechanically interlocked molecules, have recognize alkyl chains by C–HÁÁÁp interactions. It is an interesting
attracted considerable attention due to their beautiful architectures, and unique driving force for recognition between macrocyclic hosts
topological importance and great applications in materials science, and guests.11 However, the C–HÁÁÁp interaction is so weak that the
nanotechnology, and biological science.3 In order to efficiently yields of rotaxanes synthesized based on C–HÁÁÁp interactions are
obtain various rotaxanes, scientists have developed several synthetic usually low.11,13 It is known that perfect pre-organization and high
strategies, such as ‘‘threading-followed-by-stoppering’’,4a ‘‘threading- association strength are two crucial points in the fabrication of
followed-by-shrinking’’,4b ‘‘threading-followed-by-swelling’’,4c and mechanically interlocked molecules. Based on this consideration,
template-directed methods.4d [1]Rotaxanes, whose wheels and axles we designed and synthesized an alkyl chain guest with a stopper
are linked together by covalent bonds, play an important role in the on one side and a primary amine group on the other side.
rotaxane family. Because of their unique architectures, [1]rotaxanes Moreover, monocarboxylic acid-functionalised pillar[5]arene is avail-
are rather attractive but very difficult to synthesize efficiently using able through a co-oligomerization reaction. In the next step, we put
traditional methods. Until recently there have only been a few the guest and the monocarboxylic acid-functionalised pillar[5]arene
reports on the synthesis and properties of [1]rotaxanes. For example, together in solution. C–HÁÁÁp interactions made it possible for the
Hiratani et al.5 synthesized a fluorescent [1]rotaxane that can be used alkyl chain part to thread through the cavity of the pillar[5]arene.
as a sensor for the detection of lithium ions. Qu et al.6 constructed a Meanwhile, the primary amine group and the carboxylic acid group
ferrocene-based [1]rotaxane with two recognition sites which could are expected to form an ion pair complex15 and self-assemble16 in
perform switchable molecular motions under external stimuli. solution. In this way, not only was the perfect pre-organization of
Lucarini et al.7 fabricated a nitroxide-based [1]rotaxane to stabilize pseudorotaxane achieved, but also a much higher host–guest asso-
free-radicals. However, efficient syntheses of [1]rotaxanes using other ciation strength was realized (Scheme 1). Because of the ion pair
macrocycles and methods remain challenging.
interaction, the primary amine group on 2 and the carboxylic acid
Pillararenes are a new class of macrocyclic hosts.8,9 They not only group on 3 have a good opportunity to combine via a covalent bond
have symmetric and rigid structures, but also exhibit electron-rich and form an amide-linked pillar[5]arene-based [1]rotaxane.
cavities since they are made up of five or more aromatic ring units.
Firstly, the guest 2 and monocarboxylic acid-functionalised
So they performed efficient host–guest recognitions for electron- pillar[5]arene 3 were easily prepared (see ESI†). Then, they were
deficient molecules or even some neutral compounds.10–14 In recent mixed in solution. ESI mass spectrometry of the mixture
years, Li et al.10 have paid great attention to pillararene-based revealed an ion peak at m/z 1104.2 (Fig. S13, ESI†), which gave
evidence for the formation of the 1 : 1 complex between 3 and 2.
Afterwards, they reacted in the presence of compounds EDC
Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China.
and DMAP at À20 1C for about 24 h to afford [1]rotaxane 4 in a
E-mail: xuemin@zju.edu.cn; Fax: +86-571-8795-3189; Tel: +86-571-8795-3189
high yield of 73%. No other products were obtained in this
coupling reaction.
† Electronic supplementary information (ESI) available: Synthetic procedures and
characterization. See DOI: 10.1039/c3cc48014c
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result. The association constant of the complexation between 2 and
3 in chloroform was 1.52 (Æ0.075) Â 104 MÀ1 (Fig. S15, ESI†), which
is substantially higher than that of the complexation between
pillar[5]arene and alkyl chain guests.13
Further 1H NMR studies were carried out to ensure the formation of
a mechanically interlocked molecular structure. The 1H NMR spectrum
of 4 is also shown in Fig. 1 (spectrum d). Compared with the spectrum
of 3*2, there existed only one set of signals for the complexed protons
in spectrum d, without the uncomplexed-proton signals, indicating the
formation of an interlocked structure. We also found that the
methylene peaks of H1–4 (see the 2D COSY spectrum of 4 in Fig. S14,
ESI†) shifted below 0 ppm and similar phenomena were observed from
the 1H NMR spectrum of 4 in high polar solvent DMSO-d6 (Fig. S16,
ESI†), which demonstrated the formation of [1]rotaxane.
The conformation of [1]rotaxane was also confirmed by a 2D NOESY
NMR study (Fig. 2). Correlation signals can be observed between
protons H14 and H15 on the pillar[5]arene unit and methylene protons
H
1–9 on the alkyl chain, respectively. However, there is no correlation
Scheme 1 Chemical structures of the compounds used in this article and
schematic illustrations of the synthetic route for [1]rotaxane 4.
between H14 and H10 since the distance between them is too far.
We also found that phenyl protons H13 on the pillar[5]arene unit
1H NMR spectra of 3, a mixture of 2 and 3, and 2 in chloroform-d
at 25 1C are shown in Fig. 1 (spectra a–c). We found that there exist
two sets of signals in the mixture spectrum (spectrum b) for
uncomplexed host 3, uncomplexed guest 2, and the complex 3*2,
indicating slow-exchange complexation between 2 and 3 on the
1H NMR time scale. The appearance of a new NMR resonance
implied that guest 2 was threaded through the cavity of host 3. After
complexation, the methylene peaks of 2 were shifted upfield signifi-
cantly (Dd = À2.21, À3.04 and À3.20 ppm for H1–6, H7,8 and H9,
respectively) due to the shielding effect of the electron-rich cavity of
host 3, while the peaks corresponding to protons H11 on the benzene
ring of 2 shifted slightly downfield. At the same time, the signals
related to H12 and H13 on 3 shifted downfield obviously due to a
deshielding effect. These phenomena confirmed that compound 3
was able to form an interpenetration structure with compound 2,
which is necessary for the construction of rotaxanes. The stoichio-
metry of the host–guest complex was determined to be 1 : 1 from
integration of all peaks, in accordance with the ESI experimental
Fig. 2 Partial 2D NOESY spectrum of a chloroform-d solution of [1]rotax-
ane 4 at 25 1C.
Fig. 1 Full 1H NMR spectra (400 MHz, chloroform-d, 25 1C) of: (a) 3;
(b) the 1 : 1 mixture of 2 and 3; (c) 2; (d) [1]rotaxane 4.
Fig. 3 The energy-minimized structure of [1]rotaxane 4: (a) side view,
(b) top view, (c) bottom view. All hydrogens were omitted for clarity.
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correlated with H1–7, while no NOE signals were found between
H13 and other alkyl chain protons. These studies further
verified the formation of the threaded structure.
The energy-minimized structure (Fig. 3) shows that the alkyl
chain threads through the cavity of the pillar[5]arene and
extends out in a liner manner, forming a mechanically inter-
locked molecular structure.
In conclusion, we have combined C–HÁÁÁp and ion-pair interac-
tions to fabricate a pillar[5]arene-based [1]rotaxane in a high yield. It is
the first pillararene-based [1]rotaxane after the previously reported
pseudo[1]rotaxane by Stoddart and coworkers.17 Here, C–HÁÁÁp inter-
actions ensured the interpenetration of the host and guest, while ion-
pair recognition provided a fixed-position effect and much higher
host–guest interaction. In this way, the pseudorotaxane was highly
pre-organized and the yield of [1]rotaxane was reasonably high. We
are considering to modify the [1]rotaxane with functional groups and
try to apply it in the construction of molecular machines.
This work was supported by the National Natural Science
Foundation of China (31002701), and the China Postdoctoral
Science Foundation (2013M541767).
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