Linear oligomers composed of a photochromically contractible and extendable Janus [2]rotaxane

Article abstract of DOI:10.1039/b604033k

The synthesis and characterization of N,N′-p-xylylene-linked oligo-Janus [2]rotaxanes based on a permethylated α-cyclodextrin and their contractible and extendable nature coupled with photochromism are described. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b604033k

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Linear oligomers composed of a photochromically contractible and
extendable Janus [2]rotaxane
Susumu Tsuda, Yoshio Aso and Takahiro Kaneda*
Received (in Cambridge, UK) 17th March 2006, Accepted 1st June 2006
First published as an Advance Article on the web 14th June 2006
DOI: 10.1039/b604033k
redox-driven3 molecular muscle. All real and potential4 molecular
The synthesis and characterization of N,N9-p-xylylene-linked
muscles consist of only one contractible and extendable unit.
Therefore, it is a challenge to access compounds containing more
than two such units. Here, we report on the first synthesis and
characterization of the title compounds up to the pentamer
and their contractible and extendable nature coupled with
photochromism.5
oligo-Janus [2]rotaxanes based on a permethylated a-cyclo-
dextrin and their contractible and extendable nature coupled
with photochromism are described.
Among various types of interlocked compounds, molecules that
can be contracted and extended by external stimuli are quite rare.1
Two examples of such molecules are known: a metal ion-2 and a
We carried out the condensation of a hermaphroditic cyclo-
dextrin (CD), 6-(deoxy-p-(p-(3-carboxypropoxy)phenylazo)phe-
noxy)-a-CD permethylate (2),6 and a water-soluble stopper,
3,5-dimethyl-4-(29-(20-hydroxyethoxy)ethoxy)aniline (1)6 in 1 : 1
MeOH–H2O at 0 uC–rt in the presence of 1-ethyl-3-(3-dimethyl-
aminopropyl)carbodiimide (EDC) (Scheme 1). The ‘‘solubility’’ of
The Institute of Scientific and Industrial Research, Osaka University,
8-1 Mihogaoka, Ibaraki, Osaka, 567-0047, Japan.
E-mail: kaneda@sanken.osaka-u.ac.jp; Fax: +81 6-6879-8479;
Tel: +81 6-6879-8476
Scheme 1 Synthesis of linear oligo-Janus [2]rotaxanes 4a–d and their photoisomerization.
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2 in the medium tended to be higher at 0 uC than at rt, probably
due to the self-assembly to a Janus [2]peudorotaxne. A 5 g-scale
experiment gave the key intermediate 37 (3.2 g, 56%) as a yellow
solid after column chromatography on SiO2. The Janus rotaxane 3
was treated with NaH in dry DMF at 0 uC and then at rt, and with
p-bis(bromomethyl)benzene. After recycling preparative GPC, a
linear dimer 4a,7 trimer 4b,7 tetramer 4c,7 and pentamer 4d7 were
obtained as yellow solids in 33, 14, 4, and 2% yields, respectively,
accompanied with recovery of 3 (53%). Passing the crude products
through the columns three times brought about their base line
separation. The MALDI-TOF-MS spectra of 3 and 4a–d showed
[M + Na]+ ion peaks at m/z 3424, 6929, 10432, 13937, and 17438
corresponding to each oligomer. Unexpectedly, we found the
products to be N,N9-p-xylylene-linked oligomers rather than the
expected O,O9-linked isomers as discussed below.
established using d-labeled derivatives.8 The proton Hc of 3 is
bound in the center of the CD cavity as previously described.8,9 All
1
the present oligomers 4 belong to point group C2 and their H
NMR spectra in the aromatic region are consistent with their
symmetry. The two hermaphroditic CDs of a terminal Janus unit
are no longer equivalent, whereas the protons of the inner Janus
units are expected to be exposed to
a similar magnetic
environment. Indeed, the protons Ha and Ha9, and Hb and
Hb9 appeared as overlapped signals, whereas the protons Hc
and Hc9, and Hd and Hd9, appeared as two separated
doublets with different intensities, except dimer 4a. In considera-
tion of the relative intensities, we assigned the signals in the lower
and higher fields to the protons Hc and Hd, and Hc9 and Hd9,
respectively.
In oligomers 4, only the new singlets that appeared at ca. 7.1
and 6.6 ppm could be reasonably assigned to the inner, stoppered
aryl protons He9 and p-xylylene ring protons Hf9, respectively.
From the following evidence, we identified those oligomers with
N,N9-p-xylylene-linked tertiary amides, where the protons Hf9 are
located in the shielding areas of two inner stoppered rings: (1) an
upfield shift by 0.45 ppm for Hf9 compared to the corresponding
chemical shift of N,N9-p-xylylene bis-p-methoxyacetanilide10 as a
reference compound; (2) an upfield shift by 0.2 ppm for He9
compared to He; and (3) no detected signals due to the inner amide
protons even in CDCl3 at rt and in d6-DMSO at rt and 100 uC.
The steric requirement for the stacking may be attributed to the
bulkiness of Janus units as a substituent. A few examples for
N-benzylation under similar conditions are available.11
Monomer 3 showed four independent doublets due to the
azobenzene protons Ha (6.77), Hb (7.66), Hc (8.43), and Hd (7.23),
and two singlets due to He (7.14) and NH (6.95 ppm) (Fig. 1(b)).
We could complete this assignment by comparing to that of a
previous Janus rotaxane whose assignment has already been
The irradiation of 3 in CDCl3 gave a mixture of photoisomers,
EE-, EZ-, and ZZ-3, and the molar ratio was found to be 75 :
20 : 5.12 We determined the ratio from the relative intensities of the
doublets at 8.43, 8.38, and 6.48 ppm whose signals can be assigned
to the respective EE-, EZ-, and ZZ-3 according to their symmetry
(Fig. 1(a)). Heating the mixture at 90 uC for a short time or setting
it aside at rt for several days completely reproduced the original
spectrum. Due to their bulkiness, the Z-azobenzene groups of
ZZ-3 should be placed outside the CD cavity, and the trimethylene
spacers attached to them should occupy the cavity instead.
According to molecular simulations,13 the distances between the
two amide-nitrogen atoms of EE- and ZZ-3 are estimated as ca. 30
˚
and 23 A, respectively. Therefore, the ZZ-isomer is shorter than
˚
the EE-isomer by 7 A; 3 is an interlocked compound exhibiting a
contractible/extendable nature coupled with the photochromism.
We also observed photochromism with oligomers, e.g., trimer
1
4b. There are so many possible photoisomers that the H NMR
spectrum is complicated (Fig. 1(e)). On the other hand, the
absorption spectral features of 4b are quite similar to those of 3
even after the irradiation12 (Fig. 2), suggesting that three Janus
units in 4b undergo photoisomerization independently. On the
assumption that each Janus unit undergoes photoisomerization
with the same probability, the existence probability of the
oligomers including at least one ZZ-Janus unit, i.e., species having
a contractible/extendable nature was as follows: 9.8, 14, 19, and
23% for the di-, tri-, tetra-, and pentamer, respectively. At an
˚
extreme, the molecules shorten by 14, 21, 28, and 35 A with Z4-,
Z6-, Z8-, and Z10-4, respectively, compared to the corresponding
E2n-4. However, the theoretical contents of all Z-isomers are
very small or trace: 0.25, 0.013, 6.3 6 1024, and 3.1 6 1025% for
Z4-, Z6-, Z8-, and Z10-4, respectively.
Fig. 1 1H NMR spectra (600 MHz, CDCl3) of: (a) 3 after irradiation; (b)
3; (c) 4a; (d) 4b; (e) 4b after irradiation; (f) 4c; and (g) 4d. Symbol ‘‘P’’
means signals due to photoisomeric mixtures.
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S. Magonov, S. D. Solares, W. A. Doddard, C.-M. Ho and
J. F. Stoddart, J. Am. Chem. Soc., 2005, 127, 9745.
4 (a) S. J. Rowan, S. J. Cantrill, J. F. Stoddart, A. J. P. White and
D. J. Williams, Org. Lett., 2000, 2, 759; (b) T. Fujimoto, Y. Sakata and
T. Kaneda, Chem. Commun., 2000, 2143; (c) H. Onagi, C. J.
Easton and S. F. Lincoln, Org. Lett., 2001, 3, 1041; (d) S.-H. Chiu,
S. J. Rowan, S. J. Cantrill, J. F. Stoddart, A. J. P. White and
D. J. Williams, Chem. Commun., 2002, 2948.
5 Photochromism of single [2]rotaxanes or light-driven molecular shuttles:
H. Murakami, A. Kawabuchi, K. Katoo, M. Kunitake and
N. Nakashima, J. Am. Chem. Soc., 1997, 119, 7605; I. Willner,
V. Pardo-Yissar, E. Katz and K. T. Ranji, J. Electroanal. Chem., 2001,
497, 172.
6 The physical properties of the compounds will be reported elsewhere.
7 3: mp: >300 uC. Anal. found: C, 55.85; H, 7.21; N, 2.36. Calcd for
C162H250N6O70?4H2O: C, 56.01; H, 7.49; N, 2.42. 4a: mp: 200–203 uC.
Anal. found: C, 56.40; H, 7.26; N, 2.31. Calcd for
C332H506N12O140?8H2O: C, 56.56; H, 7.46; N, 2.38. 4b: mp: 198–
201 uC. Anal. found: C, 56.74; H, 7.36; N, 2.57. Calcd for
C502H762N18O210?12H2O: C, 56.74; H, 7.46; N, 2.37. 4c: mp: 203–
205 Cu. Anal. found: C, 57.06; H, 7.45; N, 2.24. Calcd for
C672H1018N24O280?16H2O: C, 56.83; H, 7.45; N, 2.37. 4d: mp: 200–
203 uC. 1H NMR (270 MHz, CDCl3, 50 uC): d 8.43 (d, J = 8.8 Hz, 4H),
8.38 (d, J = 8.8 Hz, 16H), 7.66–7.62 (m, 20H), 7.23 (d, J = 8.8 Hz, 4H),
7.16–7.15 (m, 20H), 7.09 (s, 16H), 6.96 (s, 2H), 6.77–6.75 (m, 20H), 6.60
(s, 16H), 5.06–4.85 (m, 60H, CD-H1), 4.81–4.75 (q, 16H), 4.36–2.87 (m,
970H, CD-H, OMe, OCH2, OCH2CH2O), 2.45–2.01 (m, 110H,
COCH2, Me, CH2, OH) ppm.
Fig. 2 Absorption spectra ([c]azobenzene = 2.9 6 1025 mol l21, CHCl3, rt)
of 3 (black) and 4b (red) before (dotted line) and after (solid line)
irradiation.
This work was partially supported by a Grant-in-Aid for
Scientific Research (C), No.14654113, from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
8 S. Tsuda, G. Fukuhara, T. Fujimoto, Y. Aso and T. Kaneda, in
preparation.
9 T. Fujimoto, Y. Uejima, H. Imaki, N. Kawarabayashi, J. H. Jung,
Y. Sakata and T. Kaneda, Chem. Lett., 2000, 564; T. Fujimoto,
Y. Sakata and T. Kaneda, Chem. Lett., 2000, 764; T. Fujimoto,
A. Nakamura, Y. Inoue, Y. Sakata and T. Kaneda, Tetrahedron Lett.,
2001, 42, 7987.
10 The compound was prepared from p-methoxyacetanilide and
p-bis(bromomethyl)benzene under the same conditions as were used
for synthesis of compounds 4. The chemical shift of p-xylylene ring
protons, 7.07 ppm, was found to be normal.
Notes and references
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Molecular Catenanes, Rotaxanes and Knots, ed J.-P. Sauvage and
C. Dietrich-Buchecker, Wiley-VCH, Weinheim, 1999; F. Arico`,
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11 B. Staskun, J. Org. Chem., 1979, 44, 875; A. S. Kyei, K. Tchabanenko,
J. E. Baldwin and R. M. Adlington, Tetrahedron Lett., 2004, 45, 8931.
12 The irradiation was carried out with a 350 W high-pressure mercury
arc lamp equipped with a CuSO4 aqueous solution filter for isolating
the light of 366 nm, and required 20 min to reach a photostationary
state.
2 M. C. Jime´nez, C. Dietrich-Buchecker and J.-P. Sauvage, Angew.
Chem., Int. Ed., 2000, 39, 3284; M. C. Jimenez-Molero, C. Dietrich-
Buchecker and J.-P. Sauvage, Chem.–Eur. J., 2002, 8, 1456;
M. C. Jimenez-Molero, C. Dietrich-Buchecker and J.-P. Sauvage,
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3 Y. Liu, A. H. Flood, P. A. Bonvallet, S. A. Vignon, B. H. Northrop,
H.-R. Tseng, J. O. Jeppesen, T. J. Huang, B. Brough, M. Baller,
13 InsightII/Discover, Applied Biosystems, San Diego, CS, USA.
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