Remarkable effect of hydrogen bonding between ring and axle components on deslipping reactions of rotaxanes

Article abstract of DOI:10.1016/j.tetlet.2009.02.179

A series of rotaxanes 1·5, 2·5, 3·5, and 4·5 bearing a different substituent (X = NO₂, Br, H, and OMe, respectively) at para position on the phenol moiety of the ring component exhibit clear difference in deslipping behavior. The difference in

Full text of DOI:10.1016/j.tetlet.2009.02.179

Tetrahedron Letters 50 (2009) 3443–3445
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Remarkable effect of hydrogen bonding between ring and axle components
on deslipping reactions of rotaxanes
*
Keiji Hirose , Yamato Nakamura, Hirokazu Takano, Keiji Nishihara, Yoshito Tobe
Division of Frontier Materials Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-9531, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of rotaxanes 1Á5, 2Á5, 3Á5, and 4Á5 bearing a different substituent (X = NO2, Br, H, and OMe, respec-
tively) at para position on the phenol moiety of the ring component exhibit clear difference in deslipping
behavior. The difference in the deslipping rates is consistent with the difference in intercomponent
hydrogen bonding strength estimated from the O–H stretching vibration wavenumbers. The para substi-
tuent dictates the relative strength of the intra- and intermolecular hydrogen bonds. Thus the incorpora-
tion of an intramolecular hydrogen bond allows for tunability of the strength of the intercomponent
interaction.
Received 4 December 2008
Revised 21 February 2009
Accepted 24 February 2009
Available online 27 February 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Non-covalent interactions such as hydrogen bonding,1 aromatic
-stacking,2 hydrophobic interaction,3 electrostatic interaction,4
(X = NO2, Br, H, and OMe) at para position on the phenol unit.
The inner phenolic hydroxy group of the ring component is ex-
pected to act as a hydrogen bond donor, forming intercomponent
hydrogen bonding with the amide unit of the axle component.
The acidity or hydrogen bonding ability of the hydroxy group
should be highly sensitive to the electronic nature of the substitu-
ents due to the effective resonance between the substituents and
the hydroxy group.
The deslipping reactions of 1Á5, 2Á5, 3Á5, and 4Á5 were monitored
by 1H NMR spectroscopy in tetrachloroethane-d2 (TCE-d2) and
DMSO-d6. All deslipping reactions were confirmed to follow first-
order kinetics. From the rate constants determined at several tem-
peratures (Table S1 in Supplementary data), the kinetic parameters
and t1/2 (298 K) were estimated as summarized in Table 1. The
deslipping of rotaxane 1Á5 having a strong electron-withdrawing
nitro group on the ring took place at 90 °C in TCE-d2, whereas
the corresponding deslipping reactions of 2Á5 and 3Á5 were not ob-
served under the same conditions. At higher temperature of 110 °C,
thermal decomposition of 2Á5 and 3Á5 took place, giving compli-
cated NMR signals. On the contrary, deslipping reactions of 1Á5,
2Á5, and 3Á5 proceeded in DMSO-d6 at around 100 °C. The half-lives
of 2Á5 and 3Á5 were longer than that of 1Á5 up to 5 times. On the
other hand, rotaxane 4Á5 having an electron-donating methoxy
group on the ring component did not undergo deslipping reaction
in both solvents.
p
and metal coordination5 are intentionally incorporated into most
interlocked systems not only to allow template-directed syntheses
but also to control intercomponent interactions.6 Among the inter-
actions mentioned above, hydrogen bonding interaction is one of
the most important and attractive interactions, because its
strength, selectivity, and directionality would allow creation of
complex interlocked compounds with intriguing functions. There
are many reports on the control of shuttling motions of rotaxanes
by changing hydrogen bonding strength brought about by large
structural changes induced by external stimuli.1 However, there
are no reports on rotaxanes whose motions are controlled by mod-
ulation of intercomponent hydrogen bonding strength without
structural change. In this respect, it is interesting to see if the dif-
ference in hydrogen bonding strength would alter the relative ther-
modynamic stabilities of conformers of a rotaxane in which the
ring component locates at different stations and/or affects the ki-
netic mobility of the rotaxane.
In the present study, kinetics of deslipping reactions of rotax-
anes with different intercomponent hydrogen bonding abilities,7
by which the ring and axle components are disconnected without
breaking a covalent bond, were investigated. Deslipping reactions
are suitable for this purpose because the factors governing the
internal mobility of rotaxanes can be emphasized by the difference
in the reaction rates.7e,f,i Toward this end, four rotaxanes 1Á5, 2Á5,
3Á5, and 4Á5 were designed (Scheme 1).8 The rotaxanes are com-
posed of a common dumbbell component 5 and pseudo-crown
ether type ring components 1–4 with different substituents
Although half-life of 1Á5 at 100 °C in TCE-d2 is similar to that in
DMSO-d6, the activation parameters are different from each other.
The observed activation entropy in DMSO-d6 was more negative
than that in TCE-d2, while the activation enthalpy in DMSO-d6 was
smaller than that in TCE-d2. These results suggest that rotaxane 1Á5
suffers from greater solvation in DMSO-d6 than in TCE-d2 at the tran-
sition state of deslipping. In the case of the reactions of 2Á5 and 3Á5,
* Corresponding author. Tel.: +81 6 6850 6228; fax: +81 6 6850 6229.
E-mail address: hirose@chem.es.osaka-u.ac.jp (K. Hirose).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.179

3444
K. Hirose et al. / Tetrahedron Letters 50 (2009) 3443–3445
NO2
O
O
O
O
O
H
N
O
O
O
O2N
O
O
O
O2N
O
O
O
N
H
+
H
O
O
OH O
NO2
X
5
X
1-4
1 5 (X = NO2)
2 5 (X = Br)
3 5 (X = H)
4 5 (X = OMe)
Scheme 1. Deslipping reactions of rotaxanes 1Á5–4Á5.
Table 1
rotaxanes and the rings (DDmO–H
=
D
m
O–H (rotaxane)À
DmO–H(ring))
are also shown in Table 2.11
Kinetic parameters for deslipping reactions 1Á5, 2Á5, 4Á5, and 4Á5
The Dm
O–H value of the ring component 1 (260 cmÀ1) having ni-
Solvent
Rotaxane
D
Hà (kJ molÀ1
)
D
Sà (e.u).
D
Gàa (kJ molÀ1
)
t1/2 (h)
a
tro group was apparently larger than those of 2 (224 cmÀ1), 3
(218 cmÀ1), and 4 (206 cmÀ1), indicating that the intracomponent
hydrogen bonding interaction of the hydroxy group with ether
oxygen of the polyoxyethylene moiety of 1 is stronger than those
of 2–4. It has been demonstrated that the hydrogen donating abil-
ity of a hydroxy group correlates with the electronic nature of a
substituent on phenol.12 In the case of 1–4 too, a linear correlation
TCE-d2
1Á5
2Á5
3Á5
4Á5
1Á5
2Á5
3Á5
4Á5
110
À20
118
0.97
b
b
b
b
—
—
—
—
—
—
—
—
—
—
b
b
b
b
—
—
b
b
b
b
DMSO-d6
93
71
72
À67
118
124
124
0.96
5.3
À142
À140
6.4
b
b
b
b
—
—
—
—
between Dm
O–H versus rÀp was observed (Fig. 2): the electron-with-
At 373 K calculated from
Deslipping not observed up to 100 °C.
D
Hà and
D
Sà.
a
drawing nitro group reduces the electron density of the hydroxy
oxygen leading to the greater hydrogen donating ability.
b
Contrary to the intracomponent hydrogen bonding, the
DmO–H
values of rotaxanes 1Á5–4Á5 are similar to each other (234, 239,
235, and 242 cmÀ1, respectively). This indicates that the hydrogen
bonding abilities of the hydroxy group of 1Á5–4Á5 are similar to
each other in spite of the different hydrogen bonding strengths be-
tween the corresponding free ring components 1–4. In other
words, the hydrogen bonding strengths of the hydroxy group in
the rotaxane systems do not depend directly on the acidity of the
corresponding phenols, which is affected by the electronic nature
of the substituent on the phenol moiety.
Although there seems to be no relationship between the ease of
deslipping and the hydrogen bonding strengths of the rotaxanes,
the deslipping behavior should be understood by taking into ac-
count the intra- and intercomponent hydrogen bonding separately.
the significantly negative activation entropies (greater than
À140 e.u.) compared to 1Á5 in DMSO-d6 would originate from both
the solvationin the transitionstate and the cleavage of the intercom-
ponent hydrogen bonding present in the ground state.
To estimate the nature and strength of hydrogen bonding inter-
actions of rotaxanes, IR spectra of rotaxanes 1Á5–4Á5, ring compo-
nents 1–4, and para-substituted phenols 6–9 (Fig. 1) were
measured at 25 mM in tetrachloromethane at room temperature.
It is expected that the phenolic O–H group on the ring components
of rotaxanes 1Á5–4Á5 would form both inter- and intracomponent
hydrogen bonds with the amide group of the axle component
and the ether oxygen on the ring component, respectively.9 The
deslipping rates would be affected mainly by anchoring of the ring
component to the axle component by intercomponent hydrogen
bonding. Therefore, it is important to estimate each strength of in-
ter- and intracomponent hydrogen bondings of the O–H groups.10
To extract these hydrogen bonding strengths, the shifts of the O–H
For this purpose, we compare
D
m
O–H of rotaxanes and those of ring
components (DD
mO–H). The values of
D
mO–H for 2Á5, 3Á5, and 4Á5
Table 2
O–H stretching vibration bands for phenols 6–9, ring components 1–4, and rotaxanes
1Á5–4Á5 in tetrachloromethane
stretching vibration wavenumber (DmO–H) from the corresponding
para-substituted phenols and difference of
D
mO–H between the
a
b
Compound
X
mO–H (cmÀ1
)
D
mO–H (cmÀ1
)
DDmO–H (cmÀ1
)
6
1
1Á5
NO2
3595
3335
3361
260
234
NO2
O
À26
15
O
O
O
O
O
H
7
2
2Á5
8
3
3Á5
9
4
Br
3608
3384
3369
O
O
O
O
N
H
O2N
224
239
OH
X
O
O
O
O
O
H
H
3611
3393
3376
O
O
X
218
235
17
OMe
3617
3411
3375
6 (X = NO2)
7 (X = Br)
X
206
242
4Á5
36
1-4
8 (X = H)
1 5-4 5
a
The difference between mO–H of the ring components or rotaxanes and that of
9 (X = OMe)
the corresponding p-substituted phenol.
b
DDmO–H
=
D
mO–H (rotaxane)ÀDmO–H (ring).
Figure 1.

K. Hirose et al. / Tetrahedron Letters 50 (2009) 3443–3445
3445
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.02.179.
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Figure 2. Plots of rÀp versus
DmO–H: d, rotaxanes 1Á5–4Á5; Â, ring components 1–4:
rÀp (NO2) = 0.81, rÀp (Br) = 0.23, rpÀ (H) = 0, rÀp (OMe) = À0.27.
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are larger than those of the corresponding ring components
(DDmO–H > 0), indicating that the hydroxy group simultaneously
interacts with not only the polyoxyethylene oxygen of the ring
but also the amide group in the dumbbell. However, in the case
of 1Á5, the
DmO–H value is much smaller than that of the free ring
1 (DD O–H < 0), indicating that the insertion of the axle component
m
into the cavity of the ring disturbs the intracomponent hydrogen
bonding rather than to enforce it by the additional intercomponent
interaction. Therefore, the intercomponent hydrogen bonding of
1Á5 should be weaker than those of 2Á5–4Á5. The difference in the
hydrogen bonding strength between 1Á5, 2Á5, 3Á5, and 4Á5 esti-
mated from DDmO–H is consistent with the difference in the deslip-
ping rates. Namely, the deslipping reaction of 1Á5 took place more
easily than those of 2Á5–4Á5 because of the weaker intercomponent
hydrogen bonding. Regarding the solvent effect, the deslipping
reactions of 2Á5 and 3Á5 having stronger intercomponent hydrogen
bonding in TCE-d2 are accelerated in DMSO-d6 because solvation
by DMSO-d6 breaks the hydrogen bonds in the transition state of
deslipping, being consistent with the large negative activation
entropy.
In conclusion, we revealed that rotaxanes 1Á5, 2Á5, 3Á5, and 4Á5
bearing a different substituent at the para position on the phenol
unit of the ring component exhibit clear difference in deslipping
facility. The deslipping reaction of 1Á5 easily occurs compared to
those of 2Á5 and 3Á5. The reason for the low thermostability of
interlocked structure of 1Á5 is ascribed to the weaker hydrogen
bonding between the phenolic hydroxy group on the ring compo-
nent and the amide group in the dumbbell component than those
of 2Á5 and 3Á5, as indicated by the O–H stretching vibration wave-
number of these rotaxanes and corresponding free ring compo-
nents in the IR spectra. These results clearly demonstrate the
tunability of intermolecular interactions dictated by intra- and
intercomponent hydrogen bonds. These results could be exploited
in the rational design of molecular machines based upon hydrogen
bonding interactions.
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9. The phenolic OH/ether hydrogen bond (O–O 2.78 Å), phenolic OH/carbonyl
hydrogen bond (O–O 2.78 Å), and the amide/ether hydrogen bond (O–N length
3.02 Å) of rotaxane 2Á5 were confirmed as shown in Figure 1 by X-ray structural
analysis in our previous work.8
10. In the cases of rotaxanes 1Á5–4Á5, the stretching vibration frequencies (
mC@O) of
amide carbonyl groups of the axles in tetrachloroethane do not provide clear
difference (1667–1669 cmÀ1). The frequency can be affected by
intercomponent hydrogen bond between the amide NH group and ether
oxygen of the ring component in addition to the intercomponent hydrogen
bonding between the carbonyl oxygen and the hydroxyl group. The effect could
not be clearly estimated. Therefore, the stretching vibration frequency of the
carbonyl group cannot be used for the estimation of hydrogen bonding
strengths of rotaxanes 1Á5–4Á5.
Acknowledgments
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This work was supported by a Grant-in-Aid for Scientific Re-
search from the Ministry of Education, Culture, Sports, Science and
Technology, Japan. Y.N. expresses his special thanks to the Global
COE (Center of Excellence) Program ‘Global Education and Research
Center for Bio-Environmental Chemistry’ at Osaka University.