Thermoresponsive shuttling of rotaxane containing trichloroacetate ion

Article abstract of DOI:10.1021/ol301771w

A thermoresponsive rotaxane shuttling system was developed with a trichloroacetate counteranion of an ammonium/crown ether-type rotaxane. Chemoselective thermal decomposition of the ammonium trichloroacetate moiety on the rotaxane yielded the corresponding nonionic rotaxane accompanied by a positional change of the crown ether on the axle. The rotaxane skeleton facilitated effective dissociation of the acid, markedly lowering the thermal decomposition temperature.

Full text of DOI:10.1021/ol301771w

ORGANIC
LETTERS
2012
Vol. 14, No. 16
4122–4125
Thermoresponsive Shuttling of Rotaxane
Containing Trichloroacetate Ion
Yoko Abe, Hisashi Okamura, Kazuko Nakazono, Yasuhito Koyama, Satoshi Uchida,
and Toshikazu Takata*
Department of Organic and Polymeric Materials, Tokyo Institute of Technology,
2-12-1 (H-126), Ookayama, Meguro, Tokyo 152-8552, Japan
ttakata@polymer.titech.ac.jp
Received June 27, 2012
ABSTRACT
A thermoresponsive rotaxane shuttling system was developed with a trichloroacetate counteranion of an ammonium/crown ether-type rotaxane.
Chemoselective thermal decomposition of the ammonium trichloroacetate moiety on the rotaxane yielded the corresponding nonionic rotaxane
accompanied by a positional change of the crown ether on the axle. The rotaxane skeleton facilitated effective dissociation of the acid, markedly
lowering the thermal decomposition temperature.
Ammonium/crown ether-type rotaxane is a fascinating
member of the rotaxane class, which consists of crown
ether as the wheel component and dumbbell-shaped sec-
ammonium salt as the axle component.1 The ammonium
moiety localizes at the center of the crown ether cavity.
Deprotonation of the ammonium axle is quite difficult
because of both thermodynamic stabilization (delocalized
ammonium cation charge through hydrogen bonds with
the crown ether) and kinetic stabilization (because steric
hindrance prevents the approach of a base).2 However,
achieving deprotonation brings about a unique and attrac-
tive molecular switch, changing the location of the crown
ether from the ammonium to another position. Several
methods of producing nonionic rotaxanes, such as direct
neutralization with the help of a metastable position,3
N-acylation reaction of the ammonium moiety,4 tertiariza-
tion of the nitrogen atom,5 and counteranion exchange,6
have been developed and applied to versatile stimuli-
responsive systems.7 We have recently developed several
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J.; Fyfe, M. C. T.; Stoddart, J. F. J. Org. Chem. 2000, 65, 1937–1946.
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Chem. 2006, 71, 5093–5104. (b) Kihara, N.; Koike, Y.; Takata, T. Chem.
Lett. 2007, 36, 208–209. (c) Makita, Y.; Kihara, N.; Takata, T. J. Org.
Chem. 2008, 73, 9245–9250.
(1) For selected reviews, see: (a) Fyfe, M. C. T.; Stoddart, J. F. Adv.
Supramol. Chem. 1999, 5, 1–53. (b) Rowan, S. J.; Cantrill, S. J.; Cousins,
G. R. L.; Sanders, J. K. M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2002,
41, 898–952. (c) Sauvage, J.-P.; Dietrich-Buchecker, C. Molecular
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1–75. (e) Huang, F.; Gibson, H. W. Prog. Polym. Sci. 2005, 30, 982–
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49, 2397–2401. (b) Suzuki, S.; Nakazono, K.; Takata, T. Org. Lett. 2010,
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1018. (f) Takata, T. Polym. J. 2007, 38, 1–20. (g) Berna, J.; Bottari,
G.; Leigh, D. A.; Perez, E. M. Pure Appl. Chem. 2007, 79, 39–54.
(h) Champin, B.; MobianP.; Sauvage, J.-P. Chem. Soc. Rev. 2007, 36,
358–366. (i) Griffiths, K. E.; Stoddart, J. F. Pure Appl. Chem. 2008, 80, 485–
506. (j) Haussmann, P. C.; Stoddart, J. F. Chem. Rec. 2009, 9, 136–154. (k)
Thibeault,D.;Morin, J. F.Molecules 2010, 15, 3709–3730. (l) Tokunaga, Y.
Yuki Gosei Kagaku Kyokaishi (Japanese) 2011, 69, 62–72.
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r
10.1021/ol301771w
Published on Web 08/03/2012
2012 American Chemical Society

rotaxane-incorporated polymer systems capable of am-
plifying subtle and precise rotaxane movements to mac-
roscopic polymer property changes, e.g., molecular
switch-linked reversible helical switches,8 stimuli-degradable
network polymers,9 and dynamic graft polymer systems.10
However, these polymer systems suffer serious limitations
on repeated use because accumulated residues of the salts
and catalysts used can degrade the polymer material. To
improve such polymer systems, we became intrigued by the
potential usefulness of the trichloroacetate ion (CCl3COOꢀ)
as a rotaxane counteranion. It is well-known that CCl3COOH
(bp 198 °C) undergoes decomposition at 167 °C to CO2
gas and volatile CHCl3.11 Therefore, we envisaged that
heating ammonium trichloroacetate-type rotaxane could
yield the free amine-type rotaxane without any residue.
The addition of CCl3COOH to the free amine-type rotaxane
residue could reproduce the original ammonium trichlor-
oacetate-type rotaxane, providing a novel switching system.
Herein, we describe the development of the accumula-
tion-free thermoresponsive rotaxane shuttling system dri-
ven by the chemoselective thermal decomposition of
ammonium trichloroacetate. In this study, it turned out
that the rotaxane skeleton facilitated effective dissociation
of the acid, markedly lowering the thermal decomposition
temperature of trichloroacetic acid.
Scheme 1. Synthesis of Trichloroacetate Ion Containing Ro-
taxane 4
Considering its potential for application in versatile
polymer systems, we investigated the solid state thermal
decomposition behavior of rotaxane 4 (shown in Scheme 2).
The reaction progress was confirmed by thermogravi-
metric analysis (TGA). The TGA profile of 4 exhibited
two weight-loss steps (Figure 1); the first degradation
occurred between 59 and 84 °C, and the second one
occurred at approximately 240 °C. When heating was
stopped at 200 °C it was apparent that chemoselective
thermal decomposition of the ammonium trichloroace-
tate moiety on rotaxane 4 had occurred, yielding the
nonionic rotaxane 6 as a single product with quantitative
Scheme 1 shows the synthesis of trichloroacetate ion
containing rotaxane via two pathways, the end-cap meth-
od (route A) and the counteranion exchange method
(route B). Treatment of sec-ammonium salt 1 as the axle
moiety in the presence of dibenzo-24-crown-8-ether
(DB24C8, 2) in CH2Cl2 gave pseudorotaxane 3. The end-
capping reaction of 3 with 3,5-dimethylbenzoic anhydride
gave a moderate yield (56%) of trichloroacetate ion con-
taining rotaxane 4 (Scheme 1, route A).12 Rotaxane 4
could also be synthesized via the counteranion exchange
method (route B)6b recently developed by our group. The
yield. The structure of 6 was confirmed by 1H NMR, 13
C
NMR, IR, and MS spectra.12 The measured weight loss
(15.7%), from 4 to 6, during the first degradation step
(59ꢀ84 °C) was almost the same as the calculated weight
loss expected from the loss of trichloroacetic acid from 4
(16.3%).
easily prepared PF6-type rotaxane 5 PF6ꢀ was exposed to
3
a solution of Bu4NþFꢀ (TBAF) in THF to give a dynamic
equilibrium mixture of (5 PF6ꢀ þ Bu4NþFꢀ) and (5 Fꢀ þ
3
3
Bu4NþPF6ꢀ). Successive deprotonation of 5 Fꢀ with sat.
3
aq NaHCO3 proceeded smoothly to give a nonionic
rotaxane because of the low thermodynamic stability of
5 Fꢀ on the basis of the HSAB principle.13 Subsequent
treatment of the neutral rotaxane with CCl3COOH and
repeated precipitation of the crude material into Et2O
afforded rotaxane 4 at 86% overall yield.
3
(8) (a) Nakazono, K.; Fukasawa, K.; Sato, T.; Koyama, Y.; Takata,
T. Polym. J. 2010, 42, 208–215. (b) Ishiwari, F.; Fukasawa, K.; Sato, T.;
Nakazono, K.; Koyama, Y.; Takata, T. Chem.;Eur. J. 2011, 17, 12067–
12075. (c) Ishiwari, F.; Nakazono, K.; Koyama, Y.; Takata, T. Chem.
Commun. 2011, 47, 11739–11741. (d) Suzuki, S.; Ishiwari, F.; Nakazono,
K.; Takata, T. Chem. Commun. 2012, 48, 6478–6480.
Figure 1. TGA profile of 4 under nitrogen atmosphere (heating
rate of 10 °C minꢀ1).
(9) Kohsaka, Y.; Nakazono, K.; Koyama, Y.; Asai, S.; Takata, T.
Angew. Chem., Int. Ed. 2011, 50, 4872–4875.
3
(10) Kohsaka, Y.; Koyama, Y.; Takata, T. Angew. Chem., Int. Ed.
2011, 50, 10417–10420.
(11) (a) Clark, L. W. J. Am. Chem. Soc. 1955, 77, 3130–3131.
(b) Belsky, A. J; Maiella, P. G.; Brill, T. B. J. Phys. Chem. A 1999,
103, 4253–4260. (c) Sekiya, M.; Morimoto, T. Yuki Gosei Kagaku
Kyokaishi (Japanese) 1983, 41, 395–402.
1
Figure 2 shows the H NMR spectra of DB24C8 (2),
rotaxane 4, and nonionic rotaxane 6. In spectrum B,
the characteristic signals of N-benzyl protons (d and e,
δ= 4.45 and 4.63 ppm) in the axle component of 4appeared
as broad peaks because of geminal coupling, strongly
(12) See Supporting Information.
(13) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533–3539.
Org. Lett., Vol. 14, No. 16, 2012
4123

and 7.68 ppm) were downfield-shifted. The end group
methyl proton signals (b and j, δ = 2.16 and 2.38 ppm)
showed the opposite behavior to such signals, probably
because of shielding effects from DB24C8. These spectral
changes were most characteristic and led toward our
understanding of the translation of the wheel component
to a new position on the axle.6b
Treatment of 6with CCl3COOH yielded 4(seeScheme2),
suggesting that the shuttling system was established without
any residue accumulation.
Next, we investigated the thermal decomposition reac-
tions of various ammonium trichloroacetates, including
rotaxane 7, axle 9, and axle 11 (Scheme 3), to determine
why the rotaxane 4 ammonium trichloroacetate moiety
thermally decomposed at a much lower temperature
(59ꢀ84 °C) than CCl3COOH (167 °C). Progress of the
reactions was confirmed by TGA (Figure 3). Heating 7, 9,
and 11 to 200 °C resulted in the chemoselective degrada-
tion of the ammonium trichloroacetate moiety to give
quantitative yields of the free amines 8, 10, and 12,
respectively. The structures of 8, 10, and 12 were confirmed
by 1H NMR, 13C NMR, IR, and MS spectra.12
Figure 2. 1H NMR spectra (400 MHz, CDCl3, 298 K) of (A)
DB24C8 (2), (B) rotaxane 4, and (C) nonionic rotaxane 6.
Scheme 2. Shuttling System of 4 and 6, Driven by Heating and
Addition of CCl3COOH
Scheme 3. Thermal Decomposition Reactions of 7, 9, and 11
supporting the rotaxane structure, in accordance with the
literature.14 The N-benzyl proton signals (d and e) in the
spectrum of 6 were upfield-shifted, agreeing with a non-
ionic structure (see Figure 2C). In addition, some axle
signals in the spectrum of 6 were shifted by a ring current
effect from DB24C8; the O-benzyl proton signal (h, δ =
5.29 ppm) and aromatic proton signals (g and i, δ = 7.26
1
The H NMR spectra of rotaxanes 7 and 8 (Figure 4)
showed that heating tert-ammonium-type rotaxane 7 also
resulted in the wheel translation seen with rotaxane 4. In
spectrum B the N-benzyl proton signals (d and e) and the
N-Me proton signals were upfield-shifted compared to
spectrum A, suggesting a nonionic structure of 8. The
O-benzyl proton (h), aromatic proton (g and i), and methyl
proton (b and j) peak-shift patterns from the conversion
from 7 to 8 were similar to those seen from the conversion 4
to 6, indicating that wheel translation occurred during the
conversion of 7 to 8.5
The results of the thermal decomposition of 7, 9, and 11
are summarized in Table 1. The measured weight-loss
values of 7, 9, and 11 agree well with calculated values.
The 50% decarboxylation temperature estimated by TGA
analysis (Td1/2) of rotaxane 7 was higher (100 °C) than that
of 4 (71 °C), indicating decreased decarboxylation cap-
ability of the counteranion of 7, which we attribute to the
(14) (a) Takata, T.; Kawasaki, H.; Asai, S.; Furusho, Y.; Kihara, N.
Chem. Lett. 1999, 28, 223–224. (b) Kawasaki, H.; Kihara, N.; Takata, T.
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A.; Kihara, N.; Takata, T. Chem. Lett. 2000, 29, 18–19. (d) Watanabe,
N.; Kihara, N.; Takata, T. Chem. Commun. 2002, 38, 2720–2721.
(e) Kihara, N.; Nakakoji, N.; Takata, T. Chem. Lett. 2002, 31, 924–
925. (f) Furusho, Y.; Rajkumar, G. A.; Oku, T.; Takata, T. Tetrahedron
2002, 58, 6609–6613. (g) Furusho, Y.; Oku, T.; Hasegawa, T.; Tsuboi,
A.; Kihara, N.; Takata, T. Chem.;Eur. J. 2003, 9, 2895–2903. (h) Oku,
T.; Furusho, Y.; Takata, T. Org. Lett. 2003, 5, 4923–4925. (i) Furusho,
Y.; Sasabe, H.; Natsui, D.; Murakawa, K.; Harada, T.; Takata, T.
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Takata, T. Tetrahedron Lett. 2005, 46, 3851–3853. (n) Makita, Y.;
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Org. Lett., Vol. 14, No. 16, 2012

Table 1. Effect of Ammonium Structure on Thermal Decom-
position of Ammonium Trichloroacetates
ammonium
Td1/2
°Ca
/
weight loss
(measured)/%b
weight loss
(calculated)/%c
trichloroacetate
rotaxane 4
rotaxane 7
axle 9
71
100
158
169
15.7
19.0
26.2
29.2
16.3
16.1
29.7
28.9
axle 11
a 50% Decarboxylation temperature determined by TGA analysis
(under nitrogen atmosphere, heating rate of 10 °C minꢀ1) on the basis of
the weight-loss value at the sharp inclination region. b Measured by TGA
analysis. c Calculated weight loss of the substrate.
Figure 3. TGA profiles of 7, 9, and 11 under nitrogen atmo-
sphere (heating rate of 10 °C minꢀ1).
3
werehigher thanthe Td1/2’sof the corresponding rotaxanes
4 and 7. It is particularly interesting that the threading
structure of DB24C8 with the ammonium cation facilitates
decarboxylation of the counteranion without any residue
accumulation, in contrast to the decarboxylation of an
alkali metal trichloroacetate with 18-crown-6-ether, which
leads to inorganic salt residues.15
In conclusion, in this study, we present the construction
of an accumulation-free thermoresponsive rotaxane shut-
tling system driven by the chemoselective thermal decom-
position of ammonium trichloroacetate, which could
prove ideal for repeated use of rotaxane-incorporated
polymer systems. This is a new synthetic method for pro-
ducing a nonionic rotaxane and a new molecular switch.
Further studies on the application of this system to poly-
rotaxane and stimuli-responsive polymer systems are cur-
rently underway.
Acknowledgment. This work was financially supported
by a Grant-in-Aid for Scientific Research from MEXT,
Japan (No. 23245031).
Figure 4. 1H NMR spectra (400 MHz, CDCl3, 298 K) of (A)
rotaxane 7 and (B) nonionic rotaxane 8.
Supporting Information Available. Full experimental
details for all new compounds are provided, including 1H
NMR, 13C NMR, and IR spectra and TGA profiles of
ammonium trichloroacetates. This material is available
free of charge via the Internet at http://pubs.acs.org.
introduction of a substituent on the nitrogen atom. Steric
factorsin7 will reducethekineticacidity and leadtopartial
generation of CCl3COOH in an equilibrium mixture. The
Td1/2’s of DB24C8-free axles 9 (158 °C) and 11 (169 °C)
(15) Idemori, K.; Takagi, M.; Matsuda, T. Bull. Chem. Soc. Jpn.
1977, 50, 1355–1356.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 16, 2012
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