Highly effective and reversible control of the rocking rates of rotaxanes by changes to the size of stimulus-responsive ring components

Article abstract of DOI:10.1002/chem.200800257

We have designed and synthesized rotaxanes whose rates of rocking motion (pendular motion) were switched reversibly through changes to the size of the ring component in response to external stimuli. The ring molecules of the rotaxanes incorporate a metaph

Full text of DOI:10.1002/chem.200800257

FULL PAPER
DOI: 10.1002/chem.200800257
Highly Effective and Reversible Control of the Rocking Rates of Rotaxanes
by Changes to the Size of Stimulus-Responsive Ring Components
G
Keiji Hirose,* Kazuaki Ishibashi, Yoshinobu Shiba, Yasuko Doi, and Yoshito Tobe[a]
Abstract: We have designed and syn-
thesized rotaxanes whose rates of rock-
ing motion (pendular motion) were
switched reversibly through changes to
the size of the ring component in re-
sponse to external stimuli. The ring
molecules of the rotaxanes incorporate
a metaphenylene unit, which swings
like a pendulum, and a dianthrylethane
unit, which undergoes reversible iso-
merization in response to photo- and
thermal stimuli and changes the size of
the ring component. The rocking rates
were estimated quantitatively by vari-
able-temperature (VT) NMR spectros-
AHCTREUNG
copy and saturation transfer experi-
ments, which revealed substantial
changes in the rates between the open
and closed forms, particularly in the
case of rotaxanes with an isopropoxy
group attached to a phenylene unit.
Keywords: crown compounds
molecular devices rotaxanes
supramolecular chemistry
·
·
·
Introduction
stantially by changing the direction of the dipole moment of
helical peptides immobilized on a substrate.[11] The tunneling
Rotaxanes[1] are thought to be prime candidates for the con-
struction of artificial molecular machines[2] and fabrication
of molecular electronic devices,[3] since the ring and dumb-
bell components of rotaxanes are capable of exchanging the
position of one component relative to that of the other
through motions, such as shuttling[4] (linear motion), circum-
rotation[5] (rotary motion), and rocking[6] (pendular motion).
These motions can, in principle, be controlled by external
impetus, such as chemical,[7] electrical,[8] or photochemical[9]
stimuli. Of these motions, shuttling motions have been most
extensively studied for their applications in molecular devi-
ces. In contrast with controlling the relative position of the
ring component by the shuttling motion, control of its rock-
ing mobility (oscillation frequency) has not been studied.[10]
If a ring component of a rotaxane has a large dipole
moment and its direction or oscillation frequency is con-
trolled, then the rotaxane can be applied to switching devi-
ces on a molecular scale. For example, it is reported that the
efficiency of photoinduced electron transfer is changed sub-
current between an STM tip and an Au(111) substrate has
A
been shown to oscillate in response to the rotation of a di-
polar 9,9,10,10-tetrafluoro-9,10-dihydrophenanthrene rotor
unit of a molecular machine placed under the tip.[12]
A hypothetical scheme for an impetus-responsive dipole-
switching system based on the rocking motion of a rotaxane
is shown in Scheme 1. The ring component in the rotaxane
possesses a dipolar unit that undergoes pendular motion.
When the dipolar moiety flips rapidly, the net dipole
moment would be negligible (Scheme 1, situation A). By ap-
plying an external electric (E) or magnetic field (H) to this
system (Scheme 1, process a), the rocking motion can be
stopped or decelerated owing to the interaction of the
dipole with the external field (Scheme 1, situation B).[12] At
this stage, if the size of the ring component is contracted by
an external stimulus (Scheme 1, process b),[13] the rocking
motion would be frozen because of increased steric hin-
drance between the ring and axle components, thereby lock-
ing the dipole moment (Scheme 1, situation C). As a first
step in constructing such switching systems, we report herein
the reversible and effective switching of the rocking fre-
quency of rotaxanes in response to physical stimuli.
[a] Dr. K. Hirose, K. Ishibashi, Y. Shiba, Y. Doi, Prof. Dr. Y. Tobe
Division of Frontier Materials Science
Graduate School of Engineering Science
Osaka University, 1-3 Machikaneyama
Toyonaka, Osaka 560-8531 (Japan)
Results and Discussion
Fax : (+81)6-6850-6229
E-mail: hirose@chem.es.osaka-u.ac.jp.
Design of rotaxanes: The structures of rotaxanes 1a,b and
2a,b are shown Scheme 2. We planned to switch the rates of
Supporting information for this article is available on the WWW
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unit. Because it is well known
that dianthrylethane derivatives
AHCTREUNG
undergo reversible photodime-
rization and the thermal reverse
reaction quantitatively,[14] the
size of the ring molecules can
be changed by employing this
protocol. Indeed, we have pre-
viously demonstrated switching
of the rates of slipping/deslip-
ping and shuttling motions by
using a size-variable ring com-
ponent.[15] A dibenzyl ammoni-
um cation was used as the sta-
tion that interacts more strong-
ly with the closed form of the
ring molecule than the open
form. Finally, a 3,5-bis(triiso-
propylsilyl)phenyl group was
employed as the stopper com-
ponent because it is bulky
enough to prevent dethreading
of the axle in the open form.[16]
Scheme 1. Schematic model of the switching system using rocking motion. a) Application of an external elec-
tric (E) or magnetic field (H) results in alignment of the dipolar unit with the field. b) The ring moiety con-
tracts to reduce the size of the ring, and thereby the rocking motion in the absence of the external field is
fixed. Red arrow: dipolar moiety in the ring component.
Preparation of rotaxanes 1a,b:
The synthetic route for the
preparation of rotaxanes 1a,b is
shown in Scheme 3. In accord-
ance with the literature,[17] 2,6-
bis(bromomethyl)-4-bromoani-
sole (5a) was prepared from 4-
bromophenol. The correspond-
ing isopropyl derivative (5b)
was prepared by using a similar
procedure with 3 as the starting
material, as shown in Scheme 3.
Condensation of these halides
(5a,b) with triethylene glycol
(2 equiv) in the presence of
NaOH gave 6a,b and subse-
quent tosylation gave 7a,b. On
the other hand, 1,2-bis[10-(tri-
G
methylsiloxyl)-9-anthryl]ethane
(8),[18] which was obtained from
the corresponding diol by using
N,O-bis(trimethylsilyl)aceta-
Scheme 2. Switching of the rocking rates (k) of rotaxanes 1a,b and 2a,b based on the change in the size of the
ring component.
mide (BSA), was deprotected
in situ and coupled with ditosyl-
ates 7a,b under high-dilution
the rocking motion of the metaphenylene unit by changing
the size of the ring component. We expected that the barrier
to the pseudorotation of the phenylene moiety would
change between the open (1a,b) and closed forms (2a,b) of
the rotaxanes. To induce an appropriate steric barrier for
switching the rocking frequencies between the open and
closed forms, a methoxy or an isopropoxy group was at-
tached at the para position of the bromometaphenylene
conditions to provide crown ethers 9a,b, in yields of 18 and
14% from 5a,b, respectively.
By photoirradiation of solutions of 9a,b in benzene with a
high-pressure mercury lamp, the corresponding closed forms
(10a,b) were formed. After the solvent was changed to a
mixture of dichloromethane and acetonitrile (10:1), pseudor-
otaxanes 12a,b were formed by complexation of 10a,b with
secondary ammonium salt 11[16] at À108C. The acylation
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Rocking Rates of Rotaxanes
FULL PAPER
Scheme 3. Syntheses of ring molecules 9a,b and rotaxanes 1a,b. TsCl=tosyl chloride.
capping reaction[19] of 12a,b with anhydride 13[16] catalyzed
by nBu3P afforded the corresponding rotaxanes 2a,b, which
then reverted to open-form rotaxanes 1a,b during the
workup and isolation procedures. The yields of rotaxanes
1a,b from crown ethers 9a,b (4 steps) were 46 and 50%, re-
spectively.
Interconversion between open and closed crown ethers: The
photoisomerization of the open ring molecule (9a) to the
corresponding closed form (10a) proceeded quantitatively.
A solution of 9a in CD3CN was placed in a Pyrex NMR
tube and degassed by bubbling with dry argon, before subse-
quent irradiation with a 500 W high-pressure mercury lamp
for 30 min in a water bath. After the photoirradiation proce-
dure, the solution was kept below 273 K to avoid significant
1
thermal conversion back to 9a. In the H NMR spectra, a
sharp singlet signal assigned to the benzylic protons (Ha) of
Figure 1. Partial 1H NMR spectra (270 MHz, CD3CN, 273 K) of 9a (top)
and 10a (bottom). The assignments of the protons refer to those indicat-
ed in Scheme 3.
the anthracene unit of 9a at d=4.10 ppm disappeared and a
singlet signal assigned to the aliphatic protons of 10a ap-
peared at d=2.95ppm after irradiation as shown in
Figure 1. In the aromatic region, the characteristic anthra-
cene signals at d=8.12 and 7.55 ppm (He and Hb, respective-
ly) shifted to d=7.23 and 7.10 ppm, respectively, after irradi-
ation. When the NMR sample of 10a was left at room tem-
perature overnight, the spectrum of 10a reverted to that of
9a, which implied that the thermal conversion of 10a to 9a
proceeded quantitatively. Reversible transformations of iso-
propyl derivatives 9b and 10b were also observed as shown
in Figure 2.
Interconversion between open and closed forms of the ro-
taxanes: The photochemical ring closure and the thermal
conversion of the anthracene units took place reversibly be-
tween open-form rotaxanes 1a,b and closed forms 2a,b, re-
spectively. As shown in Figure 3, the photoreaction of rotax-
1
ane 1a in [D8]THF was monitored by H NMR spectroscopy
in a similar manner to that used for ring molecules 9a,b.
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K. Hirose et al.
ature overnight; this implied that the thermal conversion of
2a to 1a also proceeded quantitatively.
The reversible transformation of isopropyl derivatives 1b
and 2b was also observed, as shown in Figure 4. In contrast
with the photoisomerization of rotaxane 1a, photoirradia-
Figure 2. Partial 1H NMR spectra (270 MHz, CD3CN, 273 K) of 9b (top)
and 10b (bottom). The assignments of the protons refer to those indicat-
ed in Scheme 3.
Figure 4. Partial 1H NMR spectra (270 MHz, [D8]THF, 273 K) of 1b
(top) and 2b (bottom). The assignments of the protons refer to those in-
dicated in Scheme 2.
tion of corresponding isopropyl rotaxane 1b gave rise to ad-
1
ditional differences in the appearance of the H NMR spec-
tra. For example, the signals assigned to the protons of the
axle component of 2b appeared as a pairs of singlets (Hl:
d=8.26 and 8.24 ppm, Hk: 5.51 and 5.45 ppm). Moreover,
the benzylic protons (Hf) of the ring component appeared
as a double doublet at d=4.97 and 4.37 ppm. These results
indicate that the rate of the rocking motion of isopropyl de-
rivative 2b is slower than the NMR timescale.
1
Figure 3. Partial H NMR spectra (270 MHz, [D8]THF, 273 K) of 1a (top)
and 2a (bottom). The assignments of the protons refer to those indicated
in Scheme 2.
Rates of thermal conversions of crown ethers 10a,band ro-
taxanes 2a,b: The thermal conversion of closed forms 10a,
10b, 2a, and 2b into the corresponding open forms 9a, 9b,
1a, and 1b, respectively, gave rise to remarkable spectral
changes in the UV-visible spectra in CH3CN (Figure 5).
Based on the increase in the absorbance at l=383 nm, the
first-order rate constants of the thermal conversion were de-
termined. The rate constants of the thermal conversions of
10a, 10b, 2a, and 2b at 303 K were 1.36”10À3, 1.33”10À3,
4.97”10À4, and 2.85”10À4 sÀ1, respectively (the correspond-
ing half-lives were 9, 9, 23, and 41 min). As shown in
Table 1, the rates of conversion of 2a,b were slower than
those of the corresponding crown ethers 10a,b. These results
indicate that the closed-form rotaxanes 2a,b are more stabi-
lized than crown ethers 10a,b by the ion–dipole interactions
between the crown ether ring and the secondary ammonium
ion. In addition, it should be noted that the rates for 10a
and 10b were almost identical, whereas that of 2b was twice
Upon irradiation, the singlet peak at d=4.12 ppm as-
signed to the ethylene protons (Ha) of the dianthrylethane
unit of 1a disappeared and a characteristic signal of the cy-
clobutane protons of 2a appeared at d=2.94 ppm, which in-
dicated that ring closure proceeded efficiently. In contrast
with the photoisomerization of crown ether 9a, the signal of
the benzyl protons of the ring component (Hf) shifted down-
field and appeared as a broad singlet (from d=4.31 to
4.64 ppm). The signals assigned to the protons of the termi-
nal part of the axle component (Hl, Hk) did not shift signifi-
cantly by photoisomerization (from d=8.27 and 5.45 ppm to
8.25and .548 ppm, respectively), as shown in Figure 3.
These results indicate that ring contraction did not signifi-
cantly affect the magnetic environment of the terminal part
of the axle component. The spectrum of 2a reverted to that
of 1a when the NMR sample of 2a was left at room temper-
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Rocking Rates of Rotaxanes
FULL PAPER
bis(3,5-dimethylbenzyl)ammo-
nium hexafluorophosphate (14)
1
by using H NMR spectroscopy
in a mixture of CD2Cl2/CD3CN
(2:1), as shown in Scheme 4.
Because of the lability of the
closed form of the ring mole-
cules, the titration method
could not be used. Instead, the
complexation constants were
determined by the relative inte-
gration of the signals of the free
and complexed ring compo-
nents in the 1H NMR spectra.
Slightly bulky 14 was employed
as a guest molecule because the
slipping/deslipping rates be-
tween 10a,b and 14 were slow
enough to observe the respec-
tive signals of the compo-
nents.[20] As a result, compound
10b, which has an isopropoxy
substituent, exhibits complexa-
tion constants that are about
five times larger than those of
10a, which has a methoxy sub-
stituent.[21] This result is consis-
tent with the observed rate re-
duction of the thermal conver-
sion of closed-form rotaxanes
2a,b (Table 2).
Figure 5. UV/Vis absorption spectra (in CH3CN, ꢀ0.1m, 303 K) of a) 10a, b) 10b, c) 2a, and d) 2b. UV spectra
were measured at intervals of four minutes. The top spectrum in each graph was measured when the thermal
conversion was completed.
Table 1. Reverse rate constants and half-lives of 10a,b and 2a,b.[a]
k [10À5 sÀ1
]
t1/2 [min]
Evaluation of the rates of rocking motions: The rates of the
rocking motions of 1b and 2b were determined by line-
shape analysis of their variable-temperature (VT) NMR
spectra in [D8]THF. Figure 6a shows partial experimental
1H NMR spectra of Hk on the axle component of 1b be-
tween 178 and 188 K and the simulated spectra assuming
the rate constants shown. Similarly, the rates of rocking of
2a were estimated on the basis of the line-shape analysis[22]
of the VT-NMR spectra for Hf as shown in Figure 6b. The
kinetic parameters were determined from the Eyring plots
and are listed in Table 3.
T=283 [K]
T=303 [K]
T=283 [K]
T=303 [K]
10a
10b
2a
9.50
9.33
3.00
1.47
136
133
49.7
28.5784
121
124
38523
9
9
2b
41
[a] Reactions performed in CH3CN.
as large as that of 2a in spite of the greater steric hindrance
in 2b. This result suggests that the ion–dipole interactions in
2b are stronger than those in 2a owing to the stronger basic-
ity of the isopropoxy group than that of the methoxy
group.[20]
In contrast with 1b, the room temperature spectrum of 1a
indicates that the rocking motion of the phenylene unit is
Complexation constants of
crown ethers 10a,bwith bis(3,5-
dimethylbenzyl)ammonium
hexafluorophosphate (14): The
relative strength of the ion–
dipole interactions of closed-
form crown ethers 10a,b with a
secondary ammonium ion were
estimated by determining their
complexation constants with Scheme 4. Complexation process of 10a,b with 14 (CD2Cl2/CD3CN 2:1).
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K. Hirose et al.
Table 2. Complexation constants of 10a,b with 14, respectively.[a]
K [mÀ1
nals, however, was not observed when the temperature was
elevated to 303 K, which indicated that the rocking rate of
2b is much slower than that of 2a.
]
Tc [K]
10a
10b
273
263
253
243
233
30
40
100
150
350
150
250
440
750
1200
The rate of rocking of closed-form rotaxane 2b was also
estimated by saturation transfer experiments.[23] The satura-
tion transfer experiment indicated that the rocking rate
(k303) of 2b was slower than 3.5s À1. Table 3 summarizes the
kinetic parameters and the rocking frequencies at 303 K of
rotaxanes 1a,b and 2a,b.[24]
[a] It was confirmed that the complexation abilities of open-form crown
ethers 9a,b were negligible by 1H NMR spectroscopy. Reactions per-
formed in CD2Cl2/CD3CN 2:1.
For rotaxanes 1a and 2a with a methoxy substituent, the
rate of rocking of open-form 1a (k303 >4.0”104 sÀ1) should
be at least four times faster than that of closed-form 2a
(k303 =1.0”104 sÀ1). On the other hand, for rotaxanes 1b and
2b, which contain the isopropoxy substituent, the difference
between the rocking rates should be more than 103 times
(1b: k303 =8.9”103 sÀ1, 2b: k303 <3.5s À1). These results clear-
ly demonstrate that the rocking rate is switched by the ex-
ternal stimuli. Moreover, the remarkable difference ob-
served in the ratio of the rocking frequencies between the
open and closed forms of the rotaxanes (1a/2a vs. 1b/2b) is
ascribed to the small rocking rate of 2b (Table 3). The
reason for the slow rocking rate can be attributed to the de-
stabilization of the transition state for rocking due to large
steric hindrance of the bulkier isopropoxy substituent. On
the other hand, it can also be attributed to the stabilization
of the ground state because the crown ether unit of 2b with
an isopropoxy substituent has a larger interaction with the
secondary ammonium cation than that of 1b with a methoxy
substituent due to the stronger electron-donating ability of
the isopropyl group. These two effects may contribute to in-
crease the activation energy of the rocking motion of 2b,
thereby decelerating the rocking rate substantially.
Figure 6. Experimental (left) and simulated (right) partial VT-NMR spec-
tra (270 MHz, [D8]THF) of a) Hk of 1b and b) Hf of 2a.
Conclusion
We synthesized rotaxanes with a dianthrylethane moiety in
the ring unit, of which the ring size was changed reversibly
by photochemical cycloaddition and thermal interconver-
sion. The rates of the rocking motion of the rotaxanes were
determined on the basis of NMR spectroscopy experiments.
Fair to substantial differences between the rocking rates of
the open and closed forms of the rotaxanes were observed,
and demonstrated that the rocking motions were switched
by the external stimuli. Moreover, it was found that the dif-
ference of the rocking frequencies between the open and
closed forms of the rotaxanes varies considerably depending
on the steric and electronic properties of the substituent at-
tached to the ring component.
Table 3. Rocking rates and kinetic parameters of 1a,b and 2a,b.[a]
[b]
Tc [K]
DH [kJmolÀ1
]
DS [JKÀ1 molÀ1
]
k303 [sÀ1
]
1a
1b
2a
2b
n.d.
186
241
n.d.
n.d.
17Æ3.0
32Æ5.0
n.d.
n.d.
>4.0”104[c]
3[d]
À113Æ158.9”10
À59Æ21
1.0”104[d]
n.d.
<3.5[e]
[a] Reactions performed in [D8]THF; n.d.=not determined. [b] Rocking
rate at 303 K. [c] Minimum estimate (see text). [d] Estimated by extrapo-
lation of the Eyring plot. [e] Maximum value estimated on the basis of
the saturation transfer experiment.
rapid on the NMR timescale and the spectrum did not
change even when the solution was cooled to 165K. There-
fore, the minimum rate of rocking (k303) of 1a was estimated
to be 4.0”104 sÀ1 by assuming that the rocking frequency of
1a at the coalescence temperature is same as that of 1b, and
that the temperature dependence of their rocking frequen-
Experimental Section
General procedure: 1H (270, 300, or 400 MHz) and 13C NMR (67.5, 75.0,
or 100 MHz) spectra were recorded on a JEOL JNM-AL-400, a Varian
Mercury 300, or a JEOL JNM-GSX-270 spectrometer. The chemical
1
cies is also identical. In contrast, the H NMR spectrum of
2b at 273 K indicates that the rocking motion is slow on the
NMR timescale. In contrast with 2a, coalescence of the sig-
1
shifts of H and 13C NMR signals are quoted relative to tetramethylsilane
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Rocking Rates of Rotaxanes
FULL PAPER
or residual solvent. IR spectra were recorded as KBr disks on a JASCO
FTIR-410 spectrometer. Mass spectral analyses were performed on a
664, 554 cmÀ1; elemental analysis calcd (%) for C37H51O13S2Br: C 52.42,
H 6.06; found: C 52.22, H 6.03.
JEOL JMS-DX303HF. Elemental analyses were carried out with
Perkin–Elmer 2400II analyzer. UV/Vis spectra were recorded on a Hita-
chi U-3310 spectrometer in acetonitrile. Preparative HPLC separation
a
Crown ether 9b. A solution of 13 (2.20 g, 3.94 mmol) and 12b (3.36 g,
3.96 mmol) in degassed THF (80 mL) through a Hershberg dropping
funnel over 18.5h was added to a solution of KOH (40 g, 71 mmol) in
water (14 mL) and THF (480 mL) at reflux. After stirring for an addi-
tional 0.5h under reflux, the reaction mixture was cooled in an ice bath.
The resulting reddish suspension was neutralized with 6n HCl in an ice
bath. The solvent was removed by evaporation and the residue was ex-
tracted with CH2Cl2. The extract was washed with brine and dried over
anhydrous MgSO4. After evaporation of the solvent, chromatography on
silica gel (eluent: hexane/ethyl acetate 1/1) followed by preparative
was undertaken with
a JAI LC-908 chromatograph using 600 mm”
20 mm JAIGEL-1H and 2H GPC columns with CHCl3 as the eluent. Sol-
vents were dried (drying agent in parentheses) and distilled prior to use:
THF (sodium benzophenone ketyl), CH3CN (CaH2), CH2Cl2 (CaH2).
Compounds 8, 11, and 13 were prepared according to literature proce-
dures.[16, 17]
Compound 4b: A solution of 3b[17] (crude product obtained from p-bro-
mophenol; 20.0 g, 85.8 mmol), K2CO3 (15.4 g, 112 mmol), and 2-iodopro-
pane (9.40 mL, 94.4 mmol) in acetone (420 mL) was stirred at reflux for
43 h. The mixture was cooled to RT and diluted with H2O (120 mL). Ace-
tone was removed under reduced pressure. The precipitate was collected
by filtration and washed with a small amount of water. This solid (4b:
16.3 g, 69%) was used for subsequent reactions without any purification.
An analytical sample of 4b was obtained after isolation by chromatogra-
phy on silica gel (eluent: CHCl3/EtOH 9:1) and subsequent recrystalliza-
tion from ethyl acetate/hexane. M.p. 125–1268C; 1H NMR (270 MHz,
[D6]DMSO): d=7.44 (s, 2H), 5.20 (t, J=5.7 Hz, 2H), 4.48 (d, J=5.7 Hz,
4H), 4.12 (septet, J=5.9 Hz, 1H), 1.18 ppm (d, J=5.9 Hz, 6H);
13C NMR (67.5MHz, [D 6]DMSO): d=150.5, 137.9, 128.8, 115.3, 75.7,
57.6, 22.1 ppm; MS (EI): m/z: 274 [M+]; IR (KBr): n˜ =3265, 3167, 2971,
2930, 1443, 1385, 1351, 1196, 1103, 1067, 935, 867, 827, 734 cmÀ1; elemen-
tal analysis calcd (%) for C11H15O3Br: C 48.02, H 5.50; found: C 48.29, H
5.33.
1
HPLC gave 9b as a yellow foam (713 mg, 18%). M.p. 44–458C; H NMR
(270 MHz, CDCl3): d=8.15(d, J=8.7 Hz, 4H), 7.55 (d, J=8.9 Hz, 4H),
7.54 (s, 2H), 7.20–7.15 (m, 4H), 7.00–6.94 (m, 4H), 4.55 (s, 4H), 4.17–
4.03 (m, 9H), 3.98–3.94 (m, 4H), 3.83–3.63 (m, 16H), 1.18 ppm (d, J=
6.1 Hz, 6H); 13C NMR (100 MHz, CDCl3): d=152.4, 150.0, 134.1, 131.1,
130.7, 129.1, 124.7, 124.1, 124.0, 123.9, 122.6, 116.8, 76.9, 74.9, 71.2, 71.1,
70.83, 70.81, 70.0, 67.6, 27.6, 22.4 ppm; MS (FAB): m/z: 917 [M++H]; IR
(KBr): n˜ =3067, 2921, 2870, 1439, 1349, 1281, 1199, 1100, 933, 772, 683,
618 cmÀ1; elemental analysis calcd (%) for C53H57O9Br: C 69.35, H 6.26;
found: C 69.24, H 6.24.
Closed crown ether 10b: A solution of 9b (2.7 mmol) in CD3CN (750 mL)
in a Pyrex NMR tube was thoroughly degassed by bubbling dry argon for
30 min. Then, the solution was irradiated by using a 500 W high-pressure
mercury lamp for 30 min in a water bath. After irradiation the solution
was maintained below 08C to avoid thermal reversion. The structure of
10b was confirmed by 1H NMR spectroscopy. 1H NMR (270 MHz,
CD3CN, 08C): d=7.50 (s, 2H), 7.17–7.13 (m, 4H), 7.03–6.98 (m, 4H),
6.83–6.79 (m, 8H), 4.51 (s, 4H), 4.10 (septet, J=6.1 Hz, 1H), 3.66–3.24
(m, 24H), 2.86 (s, 4H), 1.21 ppm (d, J=6.1 Hz, 6H).
Compound 5b: A suspended solution of 4b (16.0 g, 58.2 mmol) in 48%
hydrobromic acid (450 mL) was stirred at 708C for 3 h. The mixture was
cooled to RT and the precipitate was collected by filtration. The precipi-
tate was dissolved in CHCl3 and the solution was washed with water and
dried over anhydrous MgSO4. After evaporation of the solvent, chroma-
tography on silica gel (eluent: CHCl3) gave 5b as a pale yellow solid
(7.00 g, 26% in 3 steps from p-bromophenol). M.p. 88–898C; 1H NMR
(270 MHz, CDCl3): d=7.51 (s, 2H), 4.49 (septet, J=5.9 Hz, 1H), 4.47 (s,
4H), 1.37 ppm (d, J=5.9 Hz, 6H); 13C NMR (67.5MHz, CDCl 3): d=
152.7, 134.8, 134.0, 116.7, 77.3, 27.3, 22.6 ppm; MS (FAB): m/z: 400 [M+];
IR (KBr): n˜ =3052, 2968, 2927, 1451, 1385, 1371, 1240, 1210, 1194, 1171,
1099, 934, 874, 829, 780, 613, 543 cmÀ1; elemental analysis calcd (%) for
C11H13Br3: C 32.95, H 3.27; found: C 33.18, H 3.28.
Rotaxane 1b. A solution of 9b (200 mg, 218 mmol) in benzene (20 mL)
was placed in a Pyrex glass tube and thoroughly degassed by bubbling
dry argon for 30 min. The solution was irradiated with a 500 W high-pres-
sure mercury lamp for 15min in an ice bath. After irradiation, the sol-
vent was evaporated with cooling to give 10b. A solution of 5 (88 mg,
218 mmol) in dry CH3CN (250 mL), a solution of 7 (377 mg, 442 mmol) in
dry CH3CN (1.9 mL), and nBu3P (27 mL, 109 mmol) were added to the
residue. The mixture was stirred in an ice/salt bath for 3 h and at RT for
20 min. Evaporation of the solvent followed with isolation by preparative
HPLC separation gave 1b as a yellow foam (235mg, 50%). M.p. 86–
1
878C. H NMR (400 MHz, CDCl3): d=8.23 (d, J=1.0 Hz, 4H), 7.90–7.84
Compound 6b: Compound 5b (3.00 g, 7.48 mmol) was added to a solu-
tion of sodium hydroxide (761 mg, 19.0 mmol) in triethylene glycol
(21.7 g, 144 mmol) and the mixture was stirred at 1108C for 40 min. The
solution was cooled to RT and diluted with water. The mixture was ex-
tracted with CHCl3 and the extract was washed with brine and dried over
anhydrous MgSO4. After evaporation of the solvent, chromatography on
silica gel (eluent: CHCl3/EtOH=9/1) gave 6b as a colorless oil (3.49 g,
86%). 1H NMR (300 MHz, CDCl3): d=7.52 (s, 2H), 4.55 (s, 4H), 4.13
(septet, J=6.0 Hz 1H), 3.72–3.59 (m, 24H), 2.63 (brs, 2H), 1.25 ppm (d,
J=6.0 Hz, 6H); 13C NMR (75.5 MHz, CDCl3): d=152.3, 133.8, 131.2,
116.6, 77.0, 72.5, 70.7, 70.6, 70.4, 69.9, 67.7, 61.7, 22.4 ppm; MS (FAB):
m/z: 541, 539 [M++H]; IR (neat): n˜ =3302 (br), 2988, 2941, 2788, 2710,
1560, 1414, 1041, 840, 809 561 cmÀ1; elemental analysis calcd (%) for
C23H39O9Br: C 51.21, H 7.29; found: C 51.24, H 7.43.
(m, 8H), 7.53–7.49 (m, 8H), 7.34–7.32 (m, 6H), 7.10–7.07 (m, 4H), 6.90–
6.86 (m, 4H), 5.47 (s, 4H), 4.37 (s, 4H), 4.07 (brs, 4H), 4.02 (septet, J=
6.1 Hz, 1H), 3.72 (brs, 8H), 3.62–3.57 (m, 8H), 3.46 (brs, 8H), 1.43
(septet, J=7.3 Hz, 12H), 1.17 (d, J=6.1 Hz, 6H), 1.07 ppm (d, J=7.3 Hz,
72H); 13C NMR (100 MHz, CDCl3): d=167.0, 153.0, 149.0, 147.1, 138.4,
136.4, 134.2, 133.8, 132.7, 130.7, 129.8, 129.5, 129.1, 128.4, 128.0, 124.7,
124.5, 124.1, 123.6, 122.0, 116.4, 77.2, 74.7, 71.3, 71.0, 70.5, 70.4, 70.1, 68.7,
65.8, 52.3, 27.2, 22.4, 18.6, 10.8 ppm; MS (FAB): m/z: 2010 [M+ÀPF6À];
IR (KBr): n˜ =3054, 2921, 2870, 1439, 1349, 1281, 1199, 1100, 933,
772 cmÀ1
66.33, H 7.72, N 0.65; found: C 66.29, H 7.80, N 0.79.
Closed form rotaxane 2b: solution of 1b (3.5 mmol) in CD3CN
; elemental analysis calcd (%) for C119H165O13NF6Si4PBr: C
A
(750 mL) was placed in a Pyrex NMR tube and thoroughly degassed by
bubbling dry argon for 30 min. Then, the solution was irradiated by using
a 500 W high-pressure mercury lamp for 30 min in an ice bath. After irra-
diation, the solution was maintained below 08C to avoid the thermal re-
verse reaction. The structures of 2b were confirmed by 1H NMR spec-
troscopy. 1H NMR (270 MHz, [D8]THF, 08C): d=8.25(d, J=7.3 Hz,
6H), 7.95(d, J=7.9 Hz, 2H), 7.85(d, J=4.5Hz, 2H), 7.67–7.62 (m, 4H),
7.55 (s, 4H), 7.28–7.19 (m, 4H), 7.30 (d, J=7.1 Hz, 4H), 6.80–6.69 (m,
8H), 5.48 (d, J=15Hz, 4H), 4.97 (d, J=9.7 Hz, 2H), 4.38–4.35(m, 5H),
3.79 (brs, 4H), 3.57–3.23 (m, 24H), 2.95 (s, 4H), 1.48–1.39 (m, 12H), 1.33
(d, J=5.9 Hz, 4H), 1.08–1.04 ppm (m, 72H).
Compound 7b: Tosyl chloride (4.25g, 22.3 mmol) was added portionwise
to a solution of 6b (4.00 g, 7.41 mmol) in pyridine (60 mL) with stirring
at 08C. After stirring at 08C for 10 h, the solution was diluted with 6n
HCl. The reaction mixture was extracted with CHCl3 and the extract was
washed with brine and dried over anhydrous MgSO4. After evaporation
of the solvent, chromatography on silica gel (eluent: CHCl3) gave 7b as a
pale yellow oil (5.60 g, 89%). 1H NMR (270 MHz, CDCl3): d=7.79 (d,
J=8.4 Hz, 4H), 7.49 (s, 2H), 7.33 (d, J=8.4 Hz, 4H), 4.54 (s, 4H), 4.18–
4.06 (m, 5H), 3.71–3.59 (m, 20H), 2.43 (s, 6H), 1.24 ppm (d, J=5.9 Hz,
6H); 13C NMR (100 MHz, CDCl3): d=152.4, 144.6, 134.0, 133.0, 131.2,
129.7, 127.8, 116.6, 76.9, 70.7, 70.6, 70.5, 69.9, 69.2, 68.7, 67.6, 22.3,
21.6 ppm; MS (FAB): m/z: 869 [M++Na]; IR (neat): n˜ =3066, 2972,
2872, 1598, 1451, 1357, 1292, 1246, 1190, 1177, 1101, 1018, 925, 817, 775,
Determination of complexation constants:
A solution of 9a or 9b
(8.0 mmol) in CD2Cl2 (0.60 mL) and a solution of 8 (8.0 mmol) in CD3CN
(0.30 mL) in an NMR tube was degassed by bubbling argon in an EtOH-
Chem. Eur. J. 2008, 14, 5803 – 5811
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K. Hirose et al.
dry ice bath for 30 min, before it was irradiated using a 500 W high-pres-
sure mercury lamp. A small amount of CD3CN was added to this solution
to readjust the volume back to the original level owing to evaporation of
the solvent as a result of bubbling argon through it. The H NMR spectra
were measured at 108C intervals from 0 to À408C.
Determination of rates of thermal reversion: A solution of an open-form
compound (ca. 0.1 mm) in CH3CN was charged into a UV/Vis cuvette
equipped with a side neck for degassing. The solution was degassed by
bubbling argon, followed by five freeze-pump-thaw cycles, and then
sealed. The solution was irradiated for 15min with a 500 W high-pressure
mercury lamp in a water bath. After irradiation, the UV spectral change
was followed at 10 and 308C for 6a,b and 2a,b, from which the rates of
the reverse reaction and the half-lives were determined.
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1
Acknowledgements
This work was supported by the Izumi Science and Technology Founda-
tion and a Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science and Technology of Japan. K.I. ex-
presses his special thanks to the Global COE (Center of Excellence) Pro-
gram “Global Education and Research Center for Bio-Environmental
Chemistry” at Osaka University.
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Rocking Rates of Rotaxanes
FULL PAPER
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[23] One of the signals of Hk which appeared at higher field was irradiat-
ed at 303 K. However, no change in the signal intensity of the other
Hk signal was observed, which suggests that the relaxation time (T1)
of Hk is longer than the lifetime of the conformation. T1 was deter-
mined by the null point method. Details of the saturation transfer
experiment are given in the Supporting Information.
[24] From the VT-NMR spectroscopy study of open-form rotaxanes
1a,b, it turned out that the shape of the anthracene protons, espe-
cially Hc and Hd, changed dramatically as shown in Figures S4 and
S5in the Supporting Information. This spectral change can be re-
sulted from restriction of the quasi-rotation of the dianthrylethane
unit shown below. The rates of this twist motion of the dianthryl-
ethane unit were also determined by using VT-NMR spectroscopy.
From the coalescence temperature and the rough estimate of Dn for
anthracene proton Hc or Hd, the twisting frequency was estimated to
be ca. 103 Hz at 213–203 K. The rates of rocking motion of 1b at the
same temperature range vary from 270 to 510 Hz, which indicates
that the rocking motion of the metaphenylene unit and the twisting
motion of the dianthrylethane unit are independent.
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alkyl group due to both inductive and stereoelectronic effects: E. M.
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[22] Line-shape analyses were carried out by using the following pro-
gram: a) P. H. M. Budzelaar, gNMR, Program for Simulation of
One-Dimensional NMR Spectra, Adept Scientific, Letchworth
(United Kingdom), 1999; and a spreadsheet program for determina-
tion of the rocking rates: b) Microsoft Excel 2002, Microsoft Corpo-
ration, Tokyo (Japan), 2001; we also referred to the following arti-
cles: c) T. Mizutani, S. Yagi, A. Honmaru, T. Goldacker, S. Kitaga-
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Received: February 11, 2008
Published online: May 19, 2008
Chem. Eur. J. 2008, 14, 5803 – 5811
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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