Three-state molecular shuttles operated using acid/base stimuli with distinct outputs

Article abstract of DOI:10.1039/c1ob05236e

This paper describes the acid/base-mediated three-state translational isomerization of two [2]rotaxanes, each containing N-alkylaniline and N,N-dialkylamine centers as binding sites for threaded dibenzo[24]crown-8 units. Under neutral conditions, the dial

Full text of DOI:10.1039/c1ob05236e

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Three-state molecular shuttles operated using acid/base stimuli with distinct
outputs†
Yuji Tokunaga,* Masanori Kawabata and Naoki Matsubara
Received 15th February 2011, Accepted 6th April 2011
DOI: 10.1039/c1ob05236e
This paper describes the acid/base-mediated three-state translational isomerization of two [2]rotaxanes,
each containing N-alkylaniline and N,N-dialkylamine centers as binding sites for threaded
dibenzo[24]crown-8 units. Under neutral conditions, the dialkylamine unit predominantly recognized
the crown ether component through cooperative binding of a proton; when both amino units were
protonated under acidic conditions, both translational isomers were generated; the addition of a strong
base caused aniline–crown ether interactions to dominate. The three states of the [2]rotaxane featuring
the 3,5-diphenylaniline terminus in its dumbbell-shaped component were accompanied by distinct
absorptive outputs that were detectable using UV spectroscopy.
of protonation of an amine is related to its basicity and the
Introduction
acidity of the proton donor. A diamine featuring amino units
Rotaxane-based molecular shuttles that operate using various
having different basicities can exist in three states (amine/amine,
stimuli have attracted much attention1 because such switching
ammonium/amine, ammonium/ammonium) depending on the
systems might aid the development of molecular sensors,2 func-
pH. The strength of the hydrogen bonds formed between each
tionalized surfaces,2e,3 nanovalves,4 molecular devices,5 actuators,6
of these stations and a crown ether could be controlled by
and transporting systems.7 The dialkylammonium/crown ether
varying the degrees of protonation and deprotonation; when
recognition pair has been used widely in the synthesis of inter-
both amino groups are protonated under acidic conditions, the
locked molecules8 that function as acid/base-responsive molecular
more acidic ammonium center should bind predominantly to
shuttles.9–11 Because the strength of the hydrogen bonds between
the crown ether; under neutral conditions, the more basic amine
the ammonium (amine) and crown ether components can be
would be protonated selectively, causing the macrocycle to prefer
regulated by deprotonation (protonation) of the ammonium
this recognition site; under basic conditions, the more acidic
(amine) center, if the dumbbell-shaped component features an-
of the two amino groups might be recognized preferentially
other recognition site for the crown ether unit—one that is not
(Fig. 1). Here, we report the synthesis of two [2]rotaxanes in
affected by acid and base—the system will be capable of exhibiting
translational isomerism.
Alternative recognition sites for crown ether units in rotaxane
which the crown ether unit can be switched between two different
amino groups as binding sites under acidic, neutral, and basic
conditions. Moreover, UV spectroscopy revealed distinct signals
systems have been investigated widely. Loeb reported that the
from the [2]rotaxane featuring a 3,5-diphenylaniline terminus
N-alkylanilinium cation has an association constant for crown
upon varying the pH, allowing independent observation of the
shuttling process.14
ethers that is similar to that of the dialkylammonium cation.12
Leigh developed a [2]rotaxane featuring two different dialkylam-
monium recognition sites in the dumbbell-shaped component; the
difference in acidity between the two stations, which differed only
in the nature of their substituents, was sufficient to form the basis
of a switchable molecular shuttle that operated through changes
in the degree of protonation.13
Results and discussion
Synthesis of rotaxanes
We chose dialkylamine (ammonium) and aniline (anilinium) units
as our two different recognition sites because both protonated
forms are suitable recognition sites for crown ethers, but their
The strength of the hydrogen bonding interactions of an
ammonium group to a crown ether is based on the number
of hydrogen bonds and the acidity of the protons; the degree
+
+
basicities are quite different (pKa: Me2NH2 , 10.7; PhNH3 , 4.6).
We prepared the [2]rotaxanes 2 and 3 using a previously described
method (Scheme 1).15 The reaction of the aldehyde 1, DB24C8, and
3,5-dimethylaniline (3,5-diphenylaniline) though thermodynamic
covalent chemistry,16 followed by reduction of the corresponding
imine and subsequent spontaneous salt formation in air, gave the
Department of Materials Science and Engineering, Faculty of Engineering,
University of Fukui, Bunkyo, Fukui 910-8507, Japan. E-mail: tokunaga@
matse.u-fukui.ac.jp; Fax: +81-776-27-8758
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1ob05236e
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Fig. 2 1H NMR spectra (500 MHz, CD3CN) of the [2]rotaxanes 2a–c.
(a) 2a (X = HCO3-); (b) the sample in (a) after the addition of TfOH (5.0
eq.); (c) the sample in (b) after the addition of Et3N (5.0 eq). Capital letters
represent the signals of 2b; lower-case letters for 2c.
Fig. 1 Three-state acid/base responses of a molecular shuttle incorporat-
ing a dumbbell-shaped component featuring two different amino groups.
in the NOESY spectrum of 2a (ESI†). As expected, only the
more basic dialkylamine unit was protonated, forming the N,N-
dialkylammonium cationic center, which acted as the preferred
binding site under neutral conditions.17,18
Protonation-controlled molecular shuttling
The addition of five equivalents of trifluoromethanesulfonic acid
(TfOH) to a solution of 2a in CD3CN led to the presence
of two sets of signals related to the [2]rotaxane in the 1H
NMR spectrum. One new set of signals belonged to the doubly
protonated [2]rotaxane 2b, in which the DB24C8 unit encircles
the dialkylammonium center (Fig. 2b and Scheme 2); the other
represented its translational isomer 2c, in which the anilinium
center behaves as the recognition site for the DB24C8 unit. The
signals for the protons of 2c are all distinct from those of 2b.
The signals for the benzylic protons Hd and He of 2c are shifted
to significantly higher fields relative to those of 2b, attributable
to the loss of the deshielding effects of the DB24C8 unit. In
addition, the signal for Hh moved to lower field, a likely result
of the deshielding effect of the macrocycle. Shielding effects of
the protons of the dimethylbenzyl unit, responsible for the upfield
shifts in the signals of the aromatic (Hb and Hc) and methyl (Ha)
protons of 2b, were absent in 2c. These signals are characteristic
of crown ether/anilinium-type rotaxanes.12
Integration of the signals of the two species revealed a 7 : 3
mixture of 2b and 2c in CD3CN at 25 ◦C; although we might
have expected the DB24C8 unit to preferably encircle the anilin-
ium unit, because its protons are more acidic than those of the
dialkylammonium center, we suspect that the total factors, such
as p–p stacking of the catechol rings of the DB24C8 unit with the
aromatic units neighboring the dialkylammonium center and the
differences of steric repulsions between both components, caused
the equilibrium to favor 2b. A mixture of 2a and trifluoroacetic
acid (5 eq.) in CDCl3 afforded a 6 : 4 mixture of 2b and 2c.14
Scheme 1
[2]rotaxane 2a (3a), featuring a dimethylaniline (diphenylaniline)
stopper, in good yield. In the case of 3a, the hexafluorophosphate
salt was formed to facilitate isolation.
1H NMR spectroscopy revealed that each of the [2]rotaxanes
was isolated in its singly protonated form, with the DB24C8
component bound to the N,N-dialkylammonium moiety of
the dumbbell-shaped component (Fig. 2). The 1H NMR
spectrum of 2a features signals that are characteristic of a
crown ether/dibenzylammonium-type rotaxane: a concentration-
independent signal for the protons of the NH2 (Hq) unit at
7.52 ppm and signals for the pairs of benzylic protons (He,
Hd) at 4.42 and 4.56 ppm. Moreover, a correlation existed
between the signals of the DB24C8 unit’s protons Hm and Hn
and the protons Hc and Hf of the dumbbell-shaped component
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Fig. 3 1H NMR spectra (500 MHz, CD3CN) of [2]rotaxanes 2a and 2d.
(a) 2a (X = HCO3-); (b) the sample in (a) after the addition of tBuONa (1.2
eq.); (c–f) the sample in (b) after the addition of (c) phenol (1.5 eq.), (d)
4-nitrophenol (1.5 eq.), (e) 4-chloro-2-nitrophenol (1.5 eq.), and (f) AcOH
(1.5 eq.).
tBuONa were consistent with the formation of a new [2]rotaxane
2d, featuring two neutral amino groups. The signals of the benzylic
protons Hd and He of 2d had shifted significantly to higher fields
relative to those of 2a, attributable to the loss of the deshielding
effects associated with the ammonium center and the encircling
DB24C8 unit. In addition, the signal for the protons Hh moved
to lower field, presumably because of deshielding induced by the
encircling crown ether unit. Finally, we observed a correlation
between the signals of the DB24C8 protons Hl and the dumbbell-
shaped component’s protons Hh in the NOESY spectrum of
t
2a in the presence of BuONa (ESI†), confirming the structure
of 2d.
Next, we examined the reversibility of the interconversion
between 2a and 2d. Because a strong acid also promotes the
protonation of the aniline unit, here we used a weak acid to
ensure protonation only of the dibenzylammonium center. Fig. 3
displays the 1H NMR spectrum of a mixture of 2a (10 mM) and
tBuONa (1.2 eq.) in CD3CN and those obtained after the addition
of several acids. Phenol (pKa = 9.9) was not sufficiently acidic; the
spectrum recorded after the addition of a small excess of phenol
to 2d in CD3CN was not identical to that of 2a (Fig. 3c). Notably,
broad signals were present in this 1H NMR spectrum, suggesting
that protonation/deprotonation processes and/or interconversion
of the translational isomers had rates similar to the NMR
spectroscopic time scale at 500 MHz.19 It appears that when
hydrogen bonds exist, a p-phenylene group presents quite a steric
barrier for the passage of the DB24C8 unit, resulting in slow
interconversion of the components of the [2]rotaxane; as such
the exchange processes were rapid in the absence of hydrogen
bonding.
Scheme 2
The higher ratio of 2c in CDCl3 might reflect the greater role of
hydrogen bonding in this less polar solvent.
Next, we investigated the effect of neutralization of the mixture
of the [2]rotaxanes 2b and 2c on the relocation of the DB24C8 unit.
Fig. 2c displays the 1H NMR spectrum of the mixture of 2a
(10 mM) and TfOH (5 eq.) in CD3CN after the addition of
Et3N (5 eq.); it reveals that the interconversion was reversible.
Indeed, deprotonation of the anilinium units in 2b and 2c with
Et3N smoothly regenerated 2a, with the DB24C8 unit relocated at
the original dialkylammonium station.
Deprotonation-controlled molecular shuttling
After treating a solution of the [2]rotaxane 2a in CD3CN with
a strong base, sodium tert-butoxide (tBuONa, 1.2 eq.), and
sonicating the mixture for several hours, the resulting H NMR
1
spectrum revealed a new set of signals (Fig. 3b and Scheme 2).
The changes in chemical shift that occurred upon the addition of
On the other hand, protonation of 2b with a small excess of
4-nitrophenol (pKa = 7.2), 4-chloro-2-nitrophenol (pKa = 6.4),
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or AcOH (pKa = 4.8) smoothly regenerated 2a, with the
macrocyclic unit positioned around the dialkylammonium center.
Because the pKa of H2CO3 is 6.5, similar to that of 4-chloro-2-
nitrophenol, it is likely that we had isolated the [2]rotaxane 2a
in its mono-bicarbonate form after LiAlH4-mediated reduction
and exposure to the atmosphere. Although the addition of AcOH
might have generated some protonated aniline species (because
the pKa of 4.6 of the anilinium ion is close to that of AcOH), we
did not detect any clear signals representing anilinium ions in the
resulting 1H NMR spectrum.
Previously, we reported that selective N-acylation of the dialky-
lamine unit in the rotaxane 2a occurs in the presence of strong
bases.14 We proposed that the strong base (e.g., tBuONa) deproto-
nated the dialkylammonium center, causing the DB24C8 unit to
interact only weakly with the amino groups; as a result, the more
reactive dialkylamine in the deprotonated [2]rotaxane attacked the
electrophiles selectively. As a result, the selective N-acylation of
the [2]rotaxane 2a led to positioning of the DB24C8 unit around
the amino group of the aniline moiety.
Three-state molecular shuttling
Because this system features two reversible switching processes
(addition of strong acid followed by weak base; addition of strong
base followed by weak acid), we combined them to perform
three-state switching through repeated cycling involving sequential
t
addition of TfOH, Et3N, BuONa, and 4-chloro-2-nitrophenol.
First, we added TfOH (5 eq.) to protonate the aniline unit;
second, we added Et3N (5 eq.) to neutralize the mixture; third, to
deprotonate the dialkylammonium center in the dumbbell-shaped
t
component and the generated Et3NH·OTf, we added BuONa
Fig. 4 (a) UV absorption spectra of the [2]rotaxanes 3a, 3a + TfOH (10
eq.), and 3a + TfOH (10 eq.) + Et3N (10 eq.) in CH3CN–CH3OH (9 : 1).
(b) Absorption spectra of the [2]rotaxanes 3a, 3a + BuOK (8 eq.), and
3a + tBuOK (8 eq.) + AcOH (10 eq.) in CH3CN–CH3OH (9 : 1).
(7 eq.); finally, we added 4-chloro-2-nitrophenol (7 eq.) to the
mixture. Using the same sequence, we required larger excesses of
the additives to complete the second three-state switching cycle.
We achieved up to four switching cycles, which we monitored using
1H NMR spectroscopy (ESI†).
t
Using the same approach, we examined the three-state trans-
lational isomerization of the [2]rotaxane 3a, bearing a 3,5-
diphenylaniline moiety as a terminus. We performed the reversible
transformations from 3a to a mixture of 3b and 3c (process C) and
back (process D) and from 3a to 3d (process A) and back (process
B) using the corresponding acids and bases, with the existence of
each state confirmed using 1H NMR spectroscopy (Scheme 3 and
the ESI†).
the p–p* transitions of aniline and anilinium species in water
are 1430 and 160, respectively. Addition of Et3N led to the
recovery of the intensity of this signal. In contrast, the absorption
t
maximum of the [2]rotaxane 3a in the presence of BuOK (1.5
eq.) appeared at 347 nm, presumably because of the interaction
between the 3,5-diphenylaniline unit and the DB24C8 unit; this
value of lmax is close to those of similar rotaxanes featuring
3,5-diphenylaniline and [24]crown-8 units.15 Protonation of the
dialkylamine with AcOH led to spontaneous switching of the
DB24C8 unit back to its original position and regenerated the
broad band with its absorption maximum at 333 nm. Remarkably,
the acid/base-mediated shuttling of the macrocyclic component
could be repeated many times without obvious degradation. The
spectral variations resulting from the interconversion of the three
states were separately distinguishable by the absorptions at 330 and
360 nm. We can consider the intensities of the absorbances at 330
and 360 nm in the UV spectra as outputs, and the acidic, neutral,
and basic conditions as inputs. Fig. 5 displays the operation of
the rotaxane-based molecular switch, with detection based on
absorbance at 330 or 360 nm (state 1: absorbance > 0.1; state
0: absorbance < 0.1). The three different inputs clearly reflect
the operation of a logic gate; indeed, this multistate [2]rotaxane
mimics three-input AND and XOR logic gates.21
Scheme 3
The transformations of the [2]rotaxane 3 between its three
states led to changes in its absorption properties (Fig. 4).
For the [2]rotaxane 3a, we observed a broad band having its
maximum (lmax) at approximately 333 nm, representative of a
typical 3,5-diphenylaniline unit.20 Addition of TfOH (10 eq.)
decreased the intensity of the band as a result of protonation
of the 3,5-diphenylaniline unit; note that the values of e for
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the aniline terminus, making it possible for this rotaxane to act as
a molecular storage device.
Experimental
General methods
Infrared spectra were recorded using a Shimadzu FTIR-8600PC
instrument. 1H NMR spectra were recorded using JEOL AL-300
and LA-500 spectrometers, with TMS as the internal standard.
Mass spectra were recorded using a JMS-700T instrument. UV
spectra were recorded using a HITACHI U-3900H spectrometer.
All reactions were performed under a positive atmosphere of
dry N2, unless otherwise indicated. All solvents were removed
through rotary evaporation under reduced pressure. Silica gel
column chromatography was performed using Kanto Chemical
silica gel 60N. Thin-layer chromatography was performed using
Merck Kieselgel 60PF254
.
Rotaxane 2a. A suspension of the ammonium salt 1 (1.00 g,
2.50 mmol), DB24C8 (2.25 g, 5.01 mmol), and 3,5-dimethylaniline
(0.34 g, 2.76 mmol) in dichloromethane (10 ml) and acetonitrile
(10 ml) was stirred at rt for 1.5 days. Evaporation of the solvent
gave the corresponding crude imine, which was dissolved in THF
(40 ml) and added to a suspension of LiAlH4 (0.36 g, 14.7 mmol)
in THF (20 ml) at 0 ◦C. The resulting mixture was stirred at
rt for 2 h and then treated with water, stirred under air, and
filtered. The filtrate was concentrated and the residue purified
chromatographically (SiO2; CHCl3/MeOH, 10 : 1) to yield crude
2a, which was washed with hexane and dried to afford 2a (1.42 g,
61% from 1) as a solid. IR (KBr) nmax 3146, 3020, 2916, 1599,
1503, 1450, 1251, 1210, 1120, 1056, 952, 840, 749 cm-1. 1H NMR
(500 MHz, CD3CN) d 2.07 (s, 6H), 2.12 (s, 6H), 3.45–3.54 (m,
8H), 3.67–3.76 (m, 8H), 3.97–4.06 (m, 8H), 4.19 (d, J = 5.2 Hz,
2H), 4.52–4.56 (m, 2H), 4.58–4.64 (m, 2H), 4.71–4.77 (m, 1H),
6.20 (br s, 2H), 6.25 (br s, 1H), 6.77–6.90 (m, 11H), 7.14–7.18
(m, 2H), 7.28–7.32 (m, 2H), 7.38–7.57 (br s, 2H). 13C NMR
(125 MHz, CD3CN) d 26.8, 31.2, 34.6, 52.2, 52.4, 68.2, 70.2, 70.6,
112.7, 121.8, 125.5, 126.3, 127.3, 127.9, 128.3, 128.7, 129.0, 129.1,
129.3, 130.5, 137.6, 137.7, 143.0, 147.5, 152.5. HR MS (FAB)
Fig. 5 Monitoring the three-state molecular switching of the [2]rotaxane 3
using UV spectroscopy through the absorbance at (a) 330 and (b) 360 nm.
Processes A–D are defined in Scheme 3; 1–4 represent cycle numbers.
Dashed lines indicate the detection limits. First line: no additive; A1: +
tBuOK (10 eq.); B1: + AcOH (10 eq.); C1: + TfOH (30 eq.); D1: + Et3N
t
(15 eq.); A2: + BuOK (40 eq.); B2: + AcOH (10 eq.); C2: + TfOH (85
t
eq.); D2: + Et3N (10 eq.); A3: + BuOK (90 eq.); B3: + AcOH (10 eq.);
C3: + TfOH (320 eq.); D3: + Et3N (10 eq.); A4: + tBuOK (310 eq.); B4: +
AcOH (15 eq.); C4: + TfOH (640 eq.); D4: + Et3N (260 eq.).
+
calcd for C49H63N2O8 [M - HCO3]+: m/z 807.4569; found: m/z
807.4584.
Summary
Rotaxane 3a. Following the procedure outlined above, but
using 3,5-diphenylaniline as the amine, we obtained 3a as a solid in
60% yield. IR (neat) nmax 3444, 3151, 3059, 2931, 1597, 1504, 1454,
1254, 1215, 1122, 1057, 953, 845, 756, 559. 1H NMR (500 MHz,
CD3CN) d 2.00 (s, 3H), 2.14 (s, 3H), 3.36–3.38 (m, 8H), 3.57–3.62
(m, 8H), 3.84–3.96 (m, 8H), 4.40 (d, J = 6.4 Hz, 2H), 4.48–4.51 (m,
2H), 4.54–4.58 (m, 2H), 5.29 (br t, J = 6.4 Hz, 2H), 6.71–6.77 (m,
4H), 6.80–7.69 (m, 20H), 7.11 (t, J = 1.5 Hz, 1H), 7.23–7.36 (m,
6H), 7.36–7.45 (m, 4H), 7.59–7.64 (m, 4H). 13C NMR (125 MHz,
CD3CN) d 21.18, 47.40, 53.18, 53.34, 68.96, 71.00, 71.43, 111.75,
113.51, 115.46, 122.27, 127.83, 127.95, 128.42, 128.47, 129.81,
130.76, 131.09, 131.54, 132.86, 139.15, 142.34, 142.47, 143.28,
148.50, 150.13. Anal. calcd for C59H67F6N2O8P: C, 65.79; H, 6.27;
N, 2.60%; found: C, 65.59; H, 6.40; N, 2.52%. MS (FAB) calcd for
We have examined the acid/base-driven three-state molecular
shuttling of a [2]rotaxane comprising DB24C8 as the macrocycle
and dialkylamine (ammonium) and aniline (anilinium) units
as stations in the dumbbell-shaped component. Under basic
conditions, the DB24C8 unit encircled the aniline moiety; under
neutral conditions, the macrocycle encircled the dialkylammo-
nium station; under acidic conditions, both protonated amino
groups acted as stations. Furthermore, UV spectroscopy revealed
different absorption maxima for the three states of the [2]rotaxane
possessing a 3,5-diphenylaniline terminus, with the intensity of the
absorption of the aniline moiety being affected by the presence of
the encircling DB24C8 species and the degree of protonation. The
reversible shuttling of the macrocyclic component was, therefore,
accompanied by reversible changes in the absorption intensity of
+
C59H67N2O8 [M - PF6]+: m/z 931.5; found: m/z 931.5.
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9 For examples of ammonium/crown ether-based rotaxanes used as
molecular switches, see: (a) M.-V. Mart´ınez-D´ıaz, N. Spencer and
J. F. Stoddart, Angew. Chem., Int. Ed. Engl., 1997, 36, 1904–1907;
(b) P. R. Ashton, R. Ballardini, V. Balzani, I. Baxter, A. Credi, M. C. T.
Fyfe, M. T. Gandolfi, M. Go´mez-Lo´pez, M.-V. Mart´ınez-D´ıaz, A.
Piersanti, N. Spencer, J. F. Stoddart, M. Venturi, A. J. P. White and
D. J. Williams, J. Am. Chem. Soc., 1998, 120, 11932–11942; (c) V.
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Stoddart and D. J. Williams, Chem.–Eur. J., 2003, 9, 5348–5360; (d) S.
Garaude´e, S. Silvi, M. Venturi, A. Credi, A. H. Flood and J. F. Stoddart,
ChemPhysChem, 2005, 6, 2145–2152; (e) J. D. Badjic´, C. M. Ronconi,
J. F. Stoddart, V. Balzani, S. Silvi and A. Credi, J. Am. Chem. Soc., 2006,
128, 1489–1499; (f) Y. Tokunaga, T. Nakamura, M. Yoshioka and Y.
Shimomura, Tetrahedron Lett., 2006, 47, 5901–5904; (g) C.-L. Lin, C.-
C. Lai, Y.-H. Liu, S.-M. Peng and S.-H. Chiu, Chem.–Eur. J., 2007, 13,
4350–4355; (h) G. Rogez, B. F. Ribera, A. Credi, R. Ballardini, M. T.
Gandolfi, V. Balzani, Y. Liu, B. H. Northrop and J. F. Stoddart, J. Am.
Chem. Soc., 2007, 129, 4633–4642; (i) F. Coutrot, C. Romuald and E.
Busseron, Org. Lett., 2008, 10, 3741–3744; (j) K. C.-F. Leung, C.-P.
Chak, C.-M. Lo, W.-Y. Wong, S. Xuan and C. H. K. Cheng, Chem.–
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Lett., 2010, 12, 4248–4251; (l) M. Baroncini, S. Silvi, M. Venturi and
A. Credi, Chem.–Eur. J., 2010, 16, 11580–11587.
10 For examples of anilinium/crown ether-based rotaxane systems used
as molecular switches, see: (a) E. Busseron, C. Romuald and F. Coutrot,
Chem.–Eur. J., 2010, 16, 10062–10073; (b) D. A. Leigh and A. R.
Thomson, Org. Lett., 2006, 8, 5377–5379.
11 For an example of a tertiary ammonium center used as a station, see:
S. Suzuki, K. Nakazono and T. Takata, Org. Lett., 2010, 12, 712–715.
12 (a) S. J. Loeb, J. Tiburcio and S. J. Vella, Org. Lett., 2005, 7, 4923–
4926; (b) S. J. Vella, J. Tiburcio and S. J. Loeb, Chem. Commun., 2007,
4752–4754 .
13 D. A. Leigh and A. R. Thomson, Tetrahedron, 2008, 64, 8411–8416.
14 We have reported the protonation-controlled molecular shuttle 2a in
a preliminary communication; see: Y. Tokunaga, M. Kawabata, Y.
Yamauchi and N. Harada, Heterocycles, 2010, 81, 67–72.
15 (a) Y. Tokunaga, S. Nakashima, T. Iwamoto, K. Gambayashi, K.
Hisada and T. Hoshi, Heterocycles, 2010, 80, 819–823; (b) Y. Tokunaga,
T. Iwamoto, S. Nakashima, E. Shoji and R. Nakata, Tetrahedron Lett.,
2011, 52, 240–243.
Notes and references
1 (a) For selected reviews on rotaxanes, see: Molecular Catenanes,
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