Effect of steric barrier on the shuttling of rotaxane having crown ether wheel

Article abstract of DOI:10.1246/cl.2007.208

Rotaxanes comprising dibenzo-24-crown-8 and a symmetrical ammonium salt were desymmetrized by N-acylation. The shuttling behavior was investigated in detail. For the N,N-di-n-alkyl-type axle, the propionyl group was necessary to stop shuttling, whereas th

Full text of DOI:10.1246/cl.2007.208

208
Chemistry Letters Vol.36, No.2 (2007)
Effect of Steric Barrier on the Shuttling of Rotaxane Having Crown Ether Wheel
Nobuhiro Kihara,ꢀ1 Yoshifumi Koike,2 and Toshikazu Takataꢀ3
1Department of Chemistry, Faculty of Science, Kanagawa University, Tsuchiya, Hiratsuka 259-1293
2Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Gakuencho, Sakai, Osaka 599-8531
3Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552
(Received October 16, 2006; CL-061219; E-mail: kihara@kanagawa-u.ac.jp)
Rotaxanes comprising dibenzo-24-crown-8 and a symmetri-
cal ammonium salt were desymmetrized by N-acylation. The
CO O
, TfOH
HO
R
N
H
R
OH
shuttling behavior was investigated in detail. For the N,N-di-n-
alkyl-type axle, the propionyl group was necessary to stop shut-
tling, whereas the acetyl group was bulky enough for the N,N-di-
benzyl-type axle. According to the ꢀSz term of the shuttling, the
N-formyl and N-acetyl rotaxanes with an N,N-di-n-alkyl-type
axle showed similar shuttling frequencies at room temperature.
2
+
CH2Cl2
DB24C8
O
O
O
O
O
O
TfO-
OCO
+
R'COX, Et3N
N
R
COO
R
H2
Rotaxanes exhibit the interesting property of to-and-fro mo-
tion of the wheel on the axle, which is termed as shuttling.1 On
the basis of the shuttling behavior of the rotaxane, various mo-
lecular devices such as molecular switches have been proposed.2
The rate of shuttling and stability of the on–off states of the mo-
lecular switch can be regulated by the structure and bulkiness of
the barrier on the axle that determines the activation energy
(ꢀGz) of shuttling (Figure 1).3 Although various rotaxanes have
been prepared using dibenzo-24-crown-8 (DB24C8) as the
wheel component,4 the steric effect of the axle on the cavity of
DB24C8 has been studied only for the system with strong inter-
component interaction.3f We studied the quantitative analyses to
investigate how the bulkiness of the barrier on the axle attenu-
ates the shuttling speed of the DB24C8 wheel using intercompo-
nent interaction-free system and observed the unusual effect of
the barrier on the shuttling behavior of DB24C8.
Symmetrical rotaxanes 1 and 2 were prepared from the
corresponding aminoalcohols via the trifluoromethanesulfonic
acid-catalyzed acylation method (Scheme 1).5 The desymmetri-
zation of 1 and 2 was carried out by N-acylation using acid
anhydride in the presence of triethylamine to obtain rotaxanes
3 and 4, respectively.6 For the formylation reaction, mixed
anhydride 5 was used as the acylation agent.7
O
O
O
O
O
1, 2
R'
O
R
O
O
R
OCO
O
COO
R
N
a
b
c
H
CH3
CH2CH3
none
1, 3
2, 4
R'
O
O
3, 4
O
O
H
O
5
Scheme 1.
at one of the two split sides of the axle to desymmetrize the axle
component. Therefore, every 1H NMR signal of the axle compo-
nent was observed as a pair of signals with equal integration at
lower temperature.8 Above the coalescence temperature (Tc),
the split signals coalesced with each other; this is because the
two sides of the axle could not be distinguished from each other
due to rapid shuttling. The ꢀGz values for 3 and 4 were estimat-
ed from the Tc value of the methyl group at the axle terminal.9
The results are listed in Table 1. Although Tc values were ob-
served for 3a, 3b, and 4a, no coalescence or peak broadening
was observed for 3c and 4b, indicating that the propionyl
group in 3c and acetyl group in 4b strongly prevent shuttling
(ꢀGz > 23 kcal/mol). The value of ꢀGz depends not only on
the bulkiness of the amide group but also on the structure of
Since the system becomes unstable when the wheel crown
ether is situated on the central amide group, the wheel is located
Table 1. Thermodynamic parameters of the shuttling of 3 and 4
1
evaluated by H NMR using peak coalescence methoda
∆E
∆
G‡
270 MHz
Tc/K
400 MHz
Tc/K
Rotaxane
ꢀGz/kcal molꢁ1
ꢀGz/kcal molꢁ1
3a
3b
3c
4a
4b
419
420
none
391
21.8
21.5
—
19.8
—
431
432
none
402
22.4
21.7
—
20.1
—
none
none
: stopper
: barrier on the axle
aCoalescence behavior was observed for the terminal methyl group in
DMSO-d6. none: no coalescence was observed below 452 K.
Figure 1. Energy diagram of shuttling.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.2 (2007)
209
Table 2. Thermodynamic parameters of shuttling of 3 and 4
1
J. Am. Chem. Soc. 1991, 113, 5131. d) K. Nørgaard, B. W.
Laursen, S. Nygaard, K. Kjaer, H.-R. Tseng, A. H. Flood,
J. F. Stoddart, T. Bjørnholm, Angew. Chem., Int. Ed. 2005,
evaluated by H NMR using SPT-SIR methoda
Rotaxane ꢀHz/kcal molꢁ1 ꢀSz/cal Kꢁ1 molꢁ1 ꢀGz298/kcal molꢁ1
´
44, 7035. e) S. Garaudee, S. Silvi, M. Venturi, A. Credi,
3a
3b
3c
4a
4b
2.84
12.9
ꢁ49:1
ꢁ18:6
17.5
18.4
A. H. Flood, J. F. Stoddart, ChemPhysChem 2005, 6, 2145.
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b
b
b
—
7.60
—
—
16.9
ꢁ31:2
b
b
b
—
—
—
aSpin polarization of the terminal methyl groups was observed in
DMSO-d6. bNo shuttling was detected.
2
3
4
a) V. Balzani, M. Venturi, A. Credi, in Molecular Devices and
Machines, Wiley-VCH, Weinheim, 2003. b) Molecular
Switches, ed. by B. K. Feringa, Wiley-VCH, Weinheim,
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the axle. For example, the acetamide group was bulkier in 4b
than in 3b. Interestingly, the ꢀGz value of 3b is slightly smaller
than that of 3a in spite of the fact that the acetamide group is
evidently bulkier than the formamide group. This observation
indicates that thermodynamic analyses are necessary over a wide
range of the temperatures.
To determine the thermodynamic parameters of shuttling at
the desired temperature, we employed the SPT-SIR method10
that has recently been used to examine the thermodynamic
behavior of supramolecular systems.11 The rate of shuttling
was determined at a specific temperature, and the ꢀGz value
was calculated from Eyring’s equation. The ꢀHz and ꢀSz val-
ues were evaluated from the relationship between the tempera-
ture and ꢀGz. The results are listed in Table 2. The ꢀGz values
calculated at 298 K are also listed in Table 2.
No shuttling was detected for 3c and 4b even at 180 ꢂC be-
cause the ꢀGz values are too large for these rotaxanes to bring
about shuttling.12 When the N,N-di-n-alkyl-type axle is used,
the propionyl group is necessary to stop the shuttling, whereas
the acetyl group is bulky enough when the N,N-dibenzyl-type
axle is used. The term ꢀHz reflects the bulkiness of the barrier:
the ꢀHz value of 4a is greater than that of 3a, and the ꢀHz value
of 3b is considerably greater than that of 3a (Table 2). However,
the ꢀSz values for the formamide barrier are surprisingly small,
resulting in a small difference in the ꢀGz values among 3a, 3b,
and 4a. Although the difference increases at lower temperatures,
the difference in the ꢀGz values between 3a and 3b is less than
1 kcal/mol at 298 K. At higher temperatures, the formamide
group acts as a bulkier barrier than the acetamide group accord-
ing to the contribution of ꢀSz. At present, the reason for such
small ꢀSz values of formamide is unclear.
In summary, we have demonstrated how the bulky barriers
introduced on the axle component attenuate the frequency of
shuttling. The attenuation was affected by not only the steric
bulkiness of the barrier but also the structure of the axle. Further,
the bulky substituent on the axle did not always effectively
attenuate shuttling because the ꢀSz term played an important
role in the rate of switching.
´
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When acetic formic anhydride, the most common formylation
reagent, was used, a mixture of formamide 3a and acetamide
3b was obtained.
At low temperature, 1H NMR spectra became very complex
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6
7
8
9
´
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We acknowledge the financial support from the Ministry
of Education, Culture, Sports, Science and Technology (Grant-
in-Aid for Scientific Research (C)) and the Yazaki Memorial
Foundation for Science and Technology.
References and Notes
12 3,5-Dimethylphenyl group is bulky enough to prevent the
dethreading, see: Y. Tachibana, N. Kihara, Y. Furusho, T.
Takata, Org. Lett. 2004, 6, 4507.
13 Supporting Information is available electronically on the
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1
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Published on the web (Advance View) December 26, 2006; doi:10.1246/cl.2007.208