Synthesis of a mechanically linked oligo[2]rotaxane

Article abstract of DOI:10.1016/S0040-4039(03)00242-9

A bifunctional [2]rotaxane, bearing two free functional groups each in the ring and axial parts, was synthesized, followed by its polycondensation with methylene diphenyl diisocyanate leading to a mechanically linked oligo[2]rotaxane.

Full text of DOI:10.1016/S0040-4039(03)00242-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 2307–2310
Synthesis of a mechanically linked oligo[2]rotaxane
Abdelhak Belaissaoui, Satoru Shimada, Akihiro Ohishi and Nobuyuki Tamaoki*
Institute for Materials and Chemical Process, National Institute of Advanced Industrial Science and Technology, Central 5,
1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
Received 18 November 2002; revised 19 January 2003; accepted 27 January 2003
Abstract—A bifunctional [2]rotaxane, bearing two free functional groups each in the ring and axial parts, was synthesized,
followed by its polycondensation with methylene diphenyl diisocyanate leading to a mechanically linked oligo[2]rotaxane. © 2003
Elsevier Science Ltd. All rights reserved.
Synthesis of polymers, in which monomers are con-
nected through mechanical bonds,1 is attracting grow-
ing interest, not only for its basic physical properties,
i.e. thermal, rheological2 and stimuli response,3 but also
for its possible applications as potential low-tempera-
ture elastomers. Although polymers containing cate-
nane structures in the main chain have been
For the polyrotaxane synthesis, reports in the
literature6,11 have shown carboxylic acid or hydroxy-
methyl groups directly attached to the tetracationic
synthesized,4 mechanically linked polyrotaxanes5
b
(Scheme 1) have not been successfully attempted,
except for the synthesis of functional rotaxanes and
their dimerization6,7 or polypseudorotaxanes.8 The
functional rotaxane synthesized by Tsivgoulis and col-
laborators has
a potential to be converted to
poly[2]rotaxane by deprotection–condensation cycles.6
Our strategy towards the polymer, in which monomers
are linked by rotaxane interaction, is to synthesize a
[2]rotaxane, bearing two free functional groups in the
ring and the axial parts, respectively, followed by its
polycondensation or polyaddition to obtain
a
poly[2]rotaxane (Scheme 1). In the case of using two
similar functional groups, a poly[2]rotaxane could be
formed with three mechanically linked head-to-head,
tail-to-tail and head-to-tail monomers (Scheme 2).
Scheme 1. Schematic view of the polymerization reaction
from a [2]rotaxane a bearing two functional groups A and B
leading to a mechanically linked poly[2]rotaxane b.
In order to design the suitable structure for the bi-func-
tional rotaxane, we have synthesized several mono- or
bi-functional rotaxanes, with a lower synthetic yield
compared to non-functional rotaxanes. This is
explained by steric and/or electronic effects6,9 of the
functional groups incorporated in the rotaxane ring,
during the self-assembly10 reaction.
Scheme 2. Schematic view of head-to-head I, tail-to-tail II
and head-to-tail III apparent monomers.
* Corresponding author. Tel.: +81-29-861-4671; fax: +81-29-861-4673;
e-mail: n.tamaoki@aist.go.jp
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00242-9

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A. Belaissaoui et al. / Tetrahedron Letters 44 (2003) 2307–2310
component, are inactive towards esterification reac-
tions. Furthermore, the functional groups reactivity in
the rotaxane can be significantly decreased, owing to
the steric and/or electronic effects of the rotaxane
structure.
Scheme 4. Synthesis of 3,5-bis(bromomethyl)(hydroxy-
methyl)benzene 11. Reagents and conditions: (i) LAH/THF
(100%); (ii) CBr4/PPh3/THF (35%).
In our molecular design of
a
bifunctionalized
[2]rotaxane, we chose not to introduce a protecting
group in the rotaxane structure, as a result of the
limited stability of the bis(4,4%-bipyridinium)cyclophane
derivative under basic conditions. Under acidic condi-
tions, other groups such as specific silyloxy groups can
display instability. In addition, the rotaxane structures
are known to be unstable to heat in solution.
spectrum recorded in acetonitril-d3 (Fig. 1), is in accor-
dance with the assigned structure. The protons of the
1,4-dioxybenzene ring show an upfield shift from 6.84
ppm for the axial component 8 to 3.4–4.0 ppm (over-
lapped by the resonances from the OCH2 protons) for
[2]rotaxane 13·4PF6, as observed for the related rotax-
anes.12,15 The electrospray mass spectrometry shows a
peak at 1822.8 assigned to (Mr−PF6)+ ion.†
The key step for synthesis of the axial component, 8,
corresponds to the introduction of functionality to the
thread by incorporating two hydroxymethyl groups in
the main chain of the polyether derivative, followed by
monosilylation of 7 to introduce the second stopping
group (Scheme 3).
The copolymerization of rotaxane 13·4PF6 with methyl-
ene diphenyl diisocyanate bridging unit was carried out
in acetonitrile and at room temperature. The reaction
1
was followed by H NMR, Mass Spectrometry and Gel
3,5-Bis(bromomethyl)(hydroxymethyl)benzene11,13
11
Permeation Chromatography (GPC) analysis at 500
nm, using DMF/10 mM LiBr, as eluent. In the reaction
mixture, only compounds based on rotaxane structure
absorb at 500 nm wavelength, due to the charge trans-
fer interaction between the complementary p-electron
rich and deficient aromatic units. 1H NMR of the
product is shown with the monomer of the rotaxane in
Figure 1. The resonances around l 4.4, associated with
methylene protons of the two hydroxymethyl groups of
the starting difunctional [2]rotaxane 13·4PF6, are no
longer present in the 1H NMR spectrum of the product.
(Scheme 4) was obtained by bis-bromination reaction
of 1,3,5-tris(hydroxymethyl)benzene14 10.
The donor–acceptor template-directed synthesis of the
[2]rotaxane 13·4PF6 was attempted by reacting 8, 11
and 12·2PF6, in DMF, at ambient temperature and
subjected to ultra high-pressure 10 kbar for 110 h
(Scheme 5). The bifunctionalized [2]rotaxane 13·4PF6
was obtained in relatively low yield 5.5%. The 1H NMR
Scheme 3. Synthesis of the axial part 8. Reagents and condi-
tions: (i)10 t-BuOK, t-BuOH, reflux (92 h), 100%; (ii) (i-
Pr)3SiCl, imidazole, DMF, 46%; (iii) Et3N (1.5 equiv.),
CH2Cl2, TsCl (1.5 equiv.), rt (overnight), 98%; (iv) NaH 60%
in oil (1.17 equiv.)/DMF, bis(hydroxymethyl)-p-cresol (1.17
equiv.), room temperature 17 h then 8 h at 60°C, 83%; (v)
imidazole (2.10 equiv.)/DMF, (i-Pr)3SiCl (1.05 equiv.), 42%.
Scheme 5. From the monomeric [2]rotaxane 13·4PF6 to
poly[2]rotaxane 14·n(4PF6).
† Mr=Molecular ion of the rotaxane monomer, Mi=Molecular ion
of the methylenediphenyldiisocyanate.

A. Belaissaoui et al. / Tetrahedron Letters 44 (2003) 2307–2310
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Figure 1. 1H NMR spectra of the rotaxane monomer 13·4PF6 (top) and the reaction product of 13·4PF6 and methylene diphenyl
diisocyanate (bottom) recorded in CD3CN at 25°C (270 MHz).
On the contrary, resonances around l 5.1 are observed
in the case of the product for the methylene groups
showing that, indeed, condensation has occurred.11 In
addition new resonances for aromatic protons for the
diphenylmethyl group in the linker are present around
l 7.3. The electrospray mass spectrometry of the crude
product shows peaks at 1822, 1947 and 2072 assigned
to (Mr−PF6)+, (2Mr+Mi−2PF6)2+, (Mr+Mi−PF6)+
ions,† respectively. The peaks corresponding to the
higher molecular weight could not be observed due to
the electrospray mass spectrometer limits. The
monomer GPC analysis, using DMF/10 mM LiBr as
eluent, displayed at high concentration abnormal peak
shape, achieving higher apparent molecular weight,
probably due to the aggregation. In the present case,
the degree of aggregation decreased considerably, by
reducing the monomer concentration during the injec-
tion process in the spectrometer. The GPC analysis of
the polyrotaxane, using polystyrene standards, in
DMF/10 mM LiBr gave 4514 apparent molecular
weight, as opposed to 1967.83 for the rotaxane
monomer 13·4PF6. The significantly low values can be
considered as a result of increased compact structure of
14·n(4PF6) due to the charge transfer interaction
between monomer units compared to the polystyrene
coil of similar molecular weight. The lower limit of the
degree of the polymerization was estimated as 2 based
on the apparent Mw in GPC and the electrospray mass
spectrometry, but further investigations are needed to
obtain the information on the real molecular size of the
obtained polymer.
Innovation Institute)-Polymer Objects Program. We
would like to express our sincerest thanks to Dr. M.
Asai, Dr. T. Shimizu and Dr. H. Matsuda.
References
1. (a) Amabilino, D. B.; Stoddart, J. F. Chem. Rev. 1995,
95, 2725–2828; (b) Sauvage, J.-P.; Dietrich-Buchecker, C.
Molecular Catenanes, Rotaxanes and Knots; Wiley-VCH,
1999.
2. Okumura, Y.; Ito, K. Adv. Mater. 2001, 13, 485–487.
3. Fujita, H.; Ooya, T.; Kurisawa, M.; Mori, H.; Terano,
M.; Yui, N. Macromol. Rapid Commun. 1996, 17, 509–
515.
4. (a) Hamers, C.; Kocian, O.; Raymo, F. M.; Stoddart, J.
F. Adv. Mater. 1998, 10, 1366–1369; (b) Weidmann, J.-L.;
Kern, J.-M.; Sauvage, J.-P.; Muscat, D.; Mullins, S.;
Kohler, W.; Rosenauer, C.; Rader, H. J.; Martin, K.;
Geerts, Y. Chem. Eur. J. 1999, 5, 1841–1851; (c) Shi-
mada, S.; Ishikawa, K.; Tamaoki, N. Acta Chem. Scand.
1998, 52, 374–376; (d) Weidmann, J.-L.; Kern, J.-M.;
Sauvage, J.-P.; Geerts, Y.; Muscat, D.; Muellen, K.
Chem. Commun. 1996, 10, 1243–1244.
5. (a) Herrmann, W.; Schneider, M.; Wenz, G. Angew.
Chem., Int. Ed. Engl. 1997, 36, 2511–2514; (b) Gong, C.;
Gibson, H. W. Angew. Chem., Int. Ed. Engl. 1998, 37,
310–314; (c) Park, K. M.; Heo, J.; Roh, S. G.; Jeon, Y.
M.; Whang, D.; Kim, K. Mol. Cryst. Liq. Cryst. 1999,
327, 65–70.
6. Werts, M. P. L.; Boogaard, M. V. D.; Hadziioannou, G.;
Tsivgoulis, G. M. Chem. Commun. 1999, 7, 623–624.
7. Dunnwald, T.; Jager, R.; Vogtle, F. Chem. Eur. J. 1997,
3, 2043–2051.
Acknowledgements
This work was supported by JCII (Japan Chemical

2310
A. Belaissaoui et al. / Tetrahedron Letters 44 (2003) 2307–2310
8. (a) Ashton, P. R.; Parsons, I. W.; Raymo, F. M.; Stod-
12. Anelli, P. L.; Ashton, P. R.; Ballardini, R.; Balzani, V.;
Delgado, M.; Gandolfi, M. T.; Goodnow, T. T.;
Kaifer, A. E.; Philp, D.; Pietraszkiewicz, M.; Prodi, L.;
Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.;
Stoddart, J. F.; Vicent, C.; Williams, D. J. J. Am.
Chem. Soc. 1992, 114, 193–218.
13. Ashton, P. R.; Shibata, K.; Shipway, A. N.; Stoddart,
J. F. Angew. Chem., Int. Ed. Engl. 1997, 36, 2781–2783.
14. Yamagiwa, Y.; Koreishi, Y.; Kiyozumi, S.; Kobayashi,
M.; Kamikawa, T.; Tsukino, M.; Goi, H.; Yamamoto,
M.; Munakata, M. Bull. Chem. Soc. Jpn. 1996, 69,
3317–3324.
dart, J. F.; White, A. J. P.; Williams, D. J.; Wolf, R.
Angew. Chem., Int. Ed. Engl. 1998, 37, 1913–1916; (b)
Yamaguchi, I.; Takenada, Y.; Osakada, K.;
Yamamoto, T. Macromolecules 1999, 32, 2051–2054.
9. Tamaoki, N.; Shimada, S. Acta Chem. Scand. 1997, 51,
1138–1140.
10. (a) Philp, D.; Stoddart, J. F. Angew. Chem., Int.
Ed. Engl. 1996, 35, 1154–1196; (b) Sijbesma, R. P.;
Meijer, E. W. Curr. Opin. Colloid Interface Sci. 1999, 4,
24–32.
11. Menzer, S.; White, A. J. P.; Williams, D. J.; Belohrad-
sky, M.; Hamers, C.; Raymo, F. M.; Shipway, A. N.;
Stoddart, J. F. Macromolecules 1998, 31, 295–307.
15. Anelli, P. L.; Spencer, N.; Stoddart, J. F. J. Am. Chem.
Soc. 1991, 113, 5131–5133.