Preparation of two new [2]rotaxanes based on the pillar[5]arene/alkane recognition motif

Article abstract of DOI:10.1016/j.tetlet.2012.05.028

Based on the pillar[5]arene/alkane recognition motif, two [2]rotaxanes were successfully prepared. Their formation was confirmed by NMR spectroscopy and ESI mass spectrometry. We also demonstrated that 3,5-dinitrophenyl group and 3,5-bis(trifluoromethyl)phenyl group are big enough to work as stoppers for DPPillar[5]arene, which lays a foundation for the preparation of more complex and functional supramolecular structures.

Full text of DOI:10.1016/j.tetlet.2012.05.028

Tetrahedron Letters 53 (2012) 3668–3671
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Preparation of two new [2]rotaxanes based on the pillar[5]arene/alkane
recognition motif
⇑
Shengyi Dong, Chengyou Han, Bo Zheng, Mingming Zhang, Feihe Huang
Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Based on the pillar[5]arene/alkane recognition motif, two [2]rotaxanes were successfully prepared. Their
formation was confirmed by NMR spectroscopy and ESI mass spectrometry. We also demonstrated that
3,5-dinitrophenyl group and 3,5-bis(trifluoromethyl)phenyl group are big enough to work as stoppers for
DPPillar[5]arene, which lays a foundation for the preparation of more complex and functional supramo-
lecular structures.
Received 25 March 2012
Revised 26 April 2012
Accepted 4 May 2012
Available online 11 May 2012
Ó 2012 Published by Elsevier Ltd.
Keywords:
Pillararene
Rotaxane
Self-assembly
Host–guest system
Supramolecular chemistry
The creation of artificial supramolecular assemblies with fasci-
nating structures and desired properties has been fundamental to
supramolecular chemistry and material science.1 Rotaxane, as the
basic representative of mechanically interlocked threaded struc-
tures, has been one of the most active frontiers in supramolecular
chemistry and received a great deal of attention.2 Due to their un-
ique topological structures and physical and mechanical proper-
ties, rotaxanes have been widely used in the preparation of
molecular devices, molecular machines, and supramolecular poly-
mers.3 Pillar[5]arenes, as a new type of macrocyclic hosts, are ex-
pected to show many advantages over traditional macrocyclic
hosts due to their symmetrical pillar-shaped architectures.4
Though, pillar[5]arenes have been used in the field of supramolec-
ular assemblies and host–guest chemistry based on their unique
structures, rotaxanes based on pillar[5]arenes have been rarely re-
ported. The only example was reported by the Stoddart’s group.4r
They exploited a new guest molecule, 1,8-diaminooctane, to pre-
pare a [2]rotaxane with DMPillar[5]arene by reacting the primary
amino groups of the guest with 3,5-di-tertbutylbenzaldehyde.
However, the yield of this [2]rotaxane was quite low (7%).
investigate the inclusion properties of pillararenes by us and other
research groups.4e–g,i–n,r,s For example, our group investigated the
different binding abilities between four constitutional isomers of
BMpillar[5]arene (1-butoxy-4-methoxypillar[5]arene) and a tri-
methyl ammonium hexafluorophospate with a long alkyl chain.4g
Cao et al. studied host–guest properties between pillar[5]arenes
and terminal dibromoalkanes both in solution and in the solid
In our previous work, we demonstrated that pillar[5]arenes
could form [2]pseudorotaxanes with long alkyl chains driven by
C-HÁ Á Á
p
interactions4d and this recognition motif was successfully
applied for the formation of supramolecular assemblies including
linear supramolecular polymers4o and a [c2]daisy chain.4q Later,
alkyl chains with different substitutes have also been used to
Figure 1. Partial 1H NMR spectra (400 MHz, CDCl3, 22 °C) of 5.00 mM DPP5 (a), a
mixture of 5.00 mM DPP5 and 1 (b) and 5.00 mM 1 (c).
⇑
Corresponding author.
E-mail address: fhuang@zju.edu.cn (F. Huang).
0040-4039/$ - see front matter Ó 2012 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.tetlet.2012.05.028

S. Dong et al. / Tetrahedron Letters 53 (2012) 3668–3671
3669
NO2
NO2
O2N
O2N
HO
O2N
COOH
O
O
O
O
O
R
O
R
EDC/DMAP
DPPillar[5]arene
CHCl3/2 hours
O
NO2
R
O
R
+
R
O
OR
O
R
O
R
O2N
COCl
Et3N/ice-bath
R
O
O
R
O
O
NO2
O
O
NO2
O
O
O2N
NO2
NO2
O2N
O2N
R = n-propyl
1
3
2
Scheme 1. Synthetic route of [2]rotaxane 3.
Figure 2. Partial 1H NMR spectra (400 MHz, CDCl3, 22 °C) of dumbbell-shaped
component 2 (a), [2]rotaxane 3 (b) and DPP5 (c).
Scheme 2. Synthetic route of [2]rotaxane 4.
state.4s To further explore the possible application of the pil-
lar[5]arene/alkane recognition motif and to develop new synthetic
methods to efficiently prepare [2]rotaxanes, we are interested in
the fabrication of pseudorotaxane- and rotaxane-type threaded
structures based on the pillar[5]arene/alkane recognition motif.
Herein, we report the preparation of two new [2]rotaxanes by
the complexation between a DPPillar[5]arene and a 1,10-decane-
diol with 3,5-dinitrophenyl or 3,5-bis(trifluoromethyl)phenyl as
stoppers.
DPPillar[5]arene (1,4-dipropoxypillar[5]arene, DPP5) was suc-
cessfully synthesized by the condensation of 1,4-dipropoxyben-
zene and paraformaldehyde with [BF3ÁO(C2H5)2] as a catalyst
according to the literature.4c First of all, we have to confirm which
kind of block group is big enough to work as a stopper for DPPil-
lar[5]arene. On the basis of the analysis of previously reported
crystal structures of pillar[5]arenes,4d,e we chose easily available
3,5-dinitrophenyl as stoppers and synthesized the semi-blocked
rod-like component 1 and the dumbbell-shaped component 2
(Scheme S1 in Supplementary data) via the esterification reaction
of 3,5-dinitrobenzoic acid with decane-1,10-diol.
The complexation between DPPillar[5]arene and the semi-
blocked rod-like component 1 in chloroform was first investigated.
The 1H NMR spectrum of an equimolar solution of compound 1 and
DPPillar[5]arene in chloroform-d shows a fast-exchange complex-
ation on the 1H NMR time scale at 22 °C (Fig. 1). After complexa-
tion, phenyl protons H1 and H2 of compound 1 shifted downfield
slightly due to the deshielding effect of the electron-rich cavity
of DPPillar[5]arene. Methylene protons Hb of DPPillar[5]arene
shifted upfield from 3.78 to 3.74 ppm while H12 in compound 1
shifted upfield from 3.62 to 3.52 ppm. Larger chemical shift
changes were observed for the alkyl chain protons of compound
1. Methylene protons H11 and H5–10 moved upfield from 1.55 and
1.25–1.40 ppm, to 1.38 ppm and 1.10–1.25 ppm, respectively.
These chemical shift changes demonstrated that DPPillar[5]arene
and the semi-blocked rod-like component 1 can form a pseudoro-
taxane with the alkyl chain of compound 1 locating in the electron-
rich cavity of DPPillar[5]arene in chloroform. However, no
chemical shift changes were found for compound 2 (dumbbell-

3670
S. Dong et al. / Tetrahedron Letters 53 (2012) 3668–3671
Scheme 3. Cartoon representation of the formation of pseudorotaxanes and [2]rotaxanes.
shaped component) and DPPillar[5]arene in the 1H NMR experi-
ment when an equimolar solution of them in chloroform-d was
made (Fig. S12). This indicated that 3,5-dinitrophenyl groups are
big enough to work as stoppers for DPPillar[5]arene, which lays a
foundation for the following [2]rotaxane preparation.
We then prepared [2]rotaxane 3 using 3,5-dinitrophenyl groups
as the stoppers (Scheme 1).5,6 A mixture of DPPillar[5]arene and
the semi-blocked rod-like component 1 in chloroform was stirred
at room temperature for two hours. Then the solution was treated
with 3,5-dinitrobenzoic acid in the presence of DMAP and EDC to
afford [2]rotaxane 3 as a red solid in a yield of 7%. If 3,5-dinitroben-
zoyl chloride and triethylamine were used, [2]rotaxane 3 was
obtained in 35% yield. Partial proton NMR spectra of the dumb-
bell-shaped component 2, [2]rotaxane and DPPillar[5]arene in
chloroform-d are shown in Figure 2. The aromatic protons Ha and
The ESI mass spectrum of [2]rotaxane 3 has a peak at m/z
1616.8 (100%) corresponding to [3 + Na]+ (Fig. S8 inSupplementary
data), which further confirmed the formation of the mechanically
interlocked structure.4m
3,5-Bis(trifluoromethyl)phenyl group was also applied as stop-
pers to prepare a [2]rotaxane.7 [2]rotaxane 4 was successfully syn-
thesized with a yield of 20% in a similar way (Scheme 2).
Furthermore, differences between [2]rotaxanes 3 and 4 were
investigated by 1H NMR spectra. In 1H NMR spectrum of [2]rotax-
ane 4, protons of DPPillar[5]arene split into two sets of peaks due
to the non-symmetric structure of its dumbbell-shaped component
(Fig. S18 in Supplementary data). For example, the signal of pro-
tons H in DPPillar[5]arene was observed to split into two peaks
a
at 6.95 ppm and 6.85 ppm, respectively.4
In summary, we successfully synthesized two new [2]rotaxanes
based on the pillar[5]arene/alkane recognition motif by efficient
and convenient esterification reactions (Scheme 3). Considering
that the formation of the related pseudorotaxane-type threaded
structure intermediates during the preparation of these [2]rotax-
Hb in component 2 and H in DPPillar[5]arene shifted downfield.
a
Furthermore, methylene protons Hc in the dumbbell-shaped units
shifted upfield from 4.45 to 3.90 ppm due to the shielding effect.
While methylene protons H in DPPillar[5]arene shifted upfield
c
for about 0.05 ppm, methylene protons Hd shifted downfield for
about 0.1 ppm. Methylene protons Hd and He–g in the alkyl chain
of the dumbbell-shaped molecule were located in the cavity of
the DPPillar[5]arene. Therefore, due to the shielding effect, these
protons moved upfield greatly after the formation of [2]rotaxane
3. Protons Hd moved upfield from 2.75 to 0.90 ppm, and He–f and
Hg shifted upfield from 1.50 and 1.40 ppm to À0.25 and
À0.35 ppm, respectively.4,5 We also found similar chemical shift
changes from the 1H NMR spectrum of [2]rotaxane 3 in DMSO-d6
(Fig. S6 and S17), a highly polar solvent. In DMSO-d6, He–f and Hg
shifted upfield from 1.41 and 1.32 ppm to À0.29, À0.45 and
À0.62 ppm, respectively, indicating that the alkyl chain still lo-
cated in the cavity of DPPillar[5]arene. From the 2D NOESY NMR
spectrum of [2]rotaxane 3 in chloroform-d (Fig. S15 in Supplemen-
tary data), protons Hb on the dumbbell-shaped units were corre-
anes were mainly driven by weak C-HÁ Á Á
p interactions, their yields
(35% and 20%) are reasonable. Furthermore, we showed that the
3,5-dinitrophenyl group and the 3,5-bis(trifluoromethyl)phenyl
group are big enough to work as stoppers for DPPillar[5]arene.
Considering the easy availability of pillar[5]arenes, alkane deriva-
tive rod components, and these stoppers, the convenient synthetic
route, and the elegant mechanically interlocked structure, this kind
of [2]rotaxanes will be widely used in the construction of novel fas-
cinating supramolecular structures including molecular machines,
molecular devices, and supramolecular polymers.
Acknowledgments
This work was supported by the National Natural Science Foun-
dation of China (20834004, 21125417 and 91027006), the Funda-
mental Research Funds for the Central Universities (2010QNA3
008), the National Basic Research Program (2009CB930104), and
the Zhejiang Provincial Natural Science Foundation of China
(R4100009).
lated with protons Hb, Hd,
H
e
on DPPillar[5]arene, and the
aromatic protons H on DPPillar[5]arene were correlated with H
a
e–f, confirming the formation of a stable mechanically interlocked
structure between DPPillar[5]arene and the dumbbell-shaped
component 2.
The 13C NMR spectra of the dumbbell-shaped component 2,
[2]rotaxane 3 and DPPillar[5]arene also showed consistent changes
with the 1H NMR spectra (Fig. S13 and S14 inSupplementary data).
For example, Cf moved downfield from 67.1 to 67.9 ppm, and Ch
shifted upfield from 25.7 to 24.7 ppm, respectively.
Supplementary data
Supplementary data (Synthetic procedures, characterizations,
and other materials) associated with this article can be found, in

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