Cyclodextrin-based size-complementary [3]Rotaxanes: Selective synthesis and specific dissociation

Article abstract of DOI:10.1002/chem.201405005

α-Cyclodextrin (CD)-based size-complementary [3]rotaxanes with alkylene axles were prepared in one-pot by end-capping reactions with aryl isocyanates in water. The selective formation of [3]rotaxane with a head-to-head regularity was indicated by the X-ra

Full text of DOI:10.1002/chem.201405005

DOI: 10.1002/chem.201405005
Full Paper
&
Supramolecular Dynamics
Cyclodextrin-Based Size-Complementary [3]Rotaxanes:
Selective Synthesis and Specific Dissociation
Yosuke Akae,[a] Yasuhito Koyama,[a] Shigeki Kuwata,[b] and Toshikazu Takata*[a]
Abstract: a-Cyclodextrin (CD)-based size-complementary
[3]rotaxanes with alkylene axles were prepared in one-pot
by end-capping reactions with aryl isocyanates in water. The
selective formation of [3]rotaxane with a head-to-head regu-
larity was indicated by the X-ray structural analyses. Thermal
degradation of the [3]rotaxanes bearing appropriate end
groups proceeded by stepwise dissociation to yield not only
the original components but also [2]rotaxanes. From the ki-
netic profiles of the deslippage, it turned out that the maxi-
mum yield of [2]rotaxane was estimated to be 94%. Thermo-
dynamic studies and NOESY analyses of such rotaxanes re-
vealed that [2]rotaxanes are specially stabilized, and that the
dissociation capability of the [3]rotaxanes to the compo-
nents can be adjusted by controlling the structure of the
end groups, direction of the CD groups, and length of the
alkylene axle.
Introduction
appropriate end groups proceeds by stepwise dissociation to
yield not only the original components but also the corre-
sponding [2]rotaxanes. Notably, the dissociation capability of
[3]rotaxanes can be adjusted by controlling the structure of
the end groups, direction of the CD groups, and length of the
alkylene axle. Thermodynamic studies of the deslippage behav-
ior clarify the effect of the size-complementary end-groups on
the deslippage process. The transformation of these rotaxanes
also provides a new method for regulating the structure of
CD-based rotaxanes.
Size-complementary rotaxanes, a peculiar group of rotaxanes,
have substituents that are size-complementary to the wheel
cavity at the axle terminus.[1] These rotaxanes can be dissociat-
ed to their components using certain stimuli by deslippage of
the wheel without destruction of any covalent bonds. There-
fore, rotaxanes showing such unique dissociation behavior
have been exploited as key materials for the construction of
stimuli-responsive systems, for example, network polymers and
topology-switchable polymers.[2–5] Cyclodextrin (CD), as a repre-
sentative wheel in rotaxanes, is capable of incorporating a vari-
ety of linear axle components consisting of alkylene chains, ar- Results and Discussion
omatics, and polymers.[6,7] Although the synthesis and applica-
One-pot synthesis of a-CD-based rotaxanes
tions of CD-based rotaxanes have been extensively studied to
date,[8] there are few reports on CD-based size-complementary
rotaxanes[9] because of the challenges associated with the syn-
thesis of CD-based rotaxanes with exact components. We pre-
viously reported a new method for the synthesis of a-CD-
based size-complementary [3]rotaxanes by a one-pot pro-
cess.[10] Recently, we achieved the selective synthesis of various
a-CD-containing size-complementary [3]rotaxanes and evaluat-
ed their useful dissociation behavior, the results of which are
presented herein. Thermal degradation of [3]rotaxanes bearing
Syntheses of [3]rotaxanes were performed according to a one-
pot method.[10] After considerable efforts, we found that the
pseudorotaxanation reaction facilitates at temperature over
the melting points of the axle components (around 708C),
which is probably due to the insolubility of the axle compo-
nents in water. The threading complexation of 1,12-diaminodo-
decane or 1,10-diaminodecane with a-CD in refluxing water
for 1 h gave a heterogeneous mixture consisting of pseudo[3]r-
otaxanes, to which various phenyl isocyanate derivatives were
added at 08C. Standard workup afforded the corresponding
[3]rotaxanes in good yields without the formation of [2]rotax-
anes (Table 1, Scheme 1). It is worth mentioning that the pres-
ent end-capping method includes several advantageous condi-
tions to give high yields of CD-based rotaxanes such as the re-
action in water to make hydrophobic interactions of intermedi-
ary pseudo[3]rotaxane strong and the low-temperature reac-
tion to suppress entropy-driven decomposition of the
pseudorotaxane. Three remarkable points should be also em-
phasized about these results as shown in Table 1: 1) Less bulky
agents such as 2-methyl and 3-methoxy phenyl isocyanates
[a] Y. Akae, Dr. Y. Koyama, Prof. T. Takata
Department of Organic and Polymeric Materials
Tokyo Institute of Technology
2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552 (Japan)
Fax: (+81)3-5734-2888
E-mail: ttakata@polymer.titech.ac.jp
[b] Prof. S. Kuwata
Department of Applied Chemistry
Tokyo Institute of Technology
2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405005.
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oxygen atoms of head faces
were around 2.7–2.9 ꢁ, indicat-
ing the presence of hydrogen
bonding between the head
faces (OÀH···O), and the bromo-
phenyl moieties as an end-cap-
ping group have no attractive
interaction with CD components.
Table 1. Synthesis of [3]rotaxanes.
End-cap agent
2-BrC6H4NCO
2-MeOC6H4NCO
3,5-Me2C6H3NCO
2,6-Me2C6H3NCO
n[a] =12 [3]rotaxane
[2]rotaxane
n[a] =10 [3]rotaxane
[2]rotaxane
1, 73%
2, –[b]
4, 64%
5, –[b]
6, 21%
7, –[b]
8, 60%
10, 13%
[b]
[b]
–
–
–
–
[b]
[b]
–
9, 28%
[b]
[b]
[b]
–
–
[a] n=number of alkylene carbon atoms in the axle (Scheme 1). [b] Not detected.
Thermodynamic analyses of rotaxanes
These [3]rotaxanes were sufficiently stable in [D6]DMSO for
1
room temperature H NMR analysis. To examine the size com-
plementarity of the [3]rotaxanes, solutions of [3]rotaxanes in
[D6]DMSO were heated at arbitrary temperatures. Figure 2
shows the time-dependent NMR spectral changes during the
deslippage reaction of 4. A clear spectral change was observed
Scheme 1. One-pot synthesis of [3]rotaxanes from a,w-diaminoalkanes and
a-CD by the urea end-capping method.
yield only dumbbell-shaped products derived from the axle;[11]
2) phenyl isocyanates with a bulky substituent at the ortho po-
sition such as 2-isopropyl-, 2-iodo-, and 2-ethyl phenyl isocya-
nate suppress urea formation, and thus no reaction occurs[11]
probably because of the steric hindrance around the isocya-
nate moieties; and 3) the use of 1,12-diaminododecane as
a longer axle affords higher yields of [3]rotaxanes because the
longer axle reduces the coverage of CD around the axle termi-
ni, thus increasing the nucleophilicity of the amine and the hy-
drophobic interactions between the components, which results
in stabilization of the intermediary pseudo[3]rotaxanes. Fur-
thermore, it should be noted that all [3]rotaxanes obtained
have a head-to-head regularity, which was determined by
1H NMR[11] and single crystal X-ray structural analyses (Figure 1).
The selective formation of this head-to-head structure is
a result of the effective formation of multipoint hydrogen
bonds between the two head faces, which indirectly supports
the preferential formation of [3]rotaxanes over [2]rotaxanes.
The X-ray crystal structure clearly exhibits the structural charac-
teristics of [3]rotaxane 1: The distances between the mutual
Figure 2. Time-dependent NMR spectral changes in the 4.5–8.0 ppm region
(400 MHz, [D6]DMSO, 298 K) during the degradation of 4 (8.5 mmolLÀ1 in
~
*
[D6]DMSO at 353 K). The diagnostic peaks are marked as 4 ( ), 5 ( ), and 11
&
( ).
after heating the solution at 808C that progressed as the heat-
ing time was prolonged, indicating the prompt thermal degra-
dation of 4 in a manner similar to that previously reported for
1 (Figure 2).[10] Structural analyses of the products supported
the formation of not only [2]rotaxane 5 but also dumbbell-
shaped molecule 11, both of which were easily purified by re-
verse-phase column chromatography (Scheme 2, Figure 3). This
result indicates that the thermal deslippage of a-CD from both
4 and 5 occurs. Furthermore, the detection of intermediary
[2]rotaxane 5 indicates that the deslippage from 5 to 11 is suf-
ficiently slower than that from 4 to 5. Kinetic profiles of the de-
slippage of both 4 and 5 were thus investigated to clarify the
detailed deslippage process.
Solutions of 4 in [D6]DMSO were heated at 40, 60, 80, and
1
1008C and subjected to H NMR analysis. Assuming that the
deslippage reaction obeys first-order kinetics, the rate con-
stants were determined from the time vs. ln([rotaxane]/[rotax-
1
ane]0) plots using the integral ratios from the H NMR spectra
(Table 2). The resultant time–yield curves for 4, 5, and 11 are
shown in Figure 4. The formation of [2]rotaxane 5 began soon
after heating was initiated, while 11 appeared more gradually.
Figure 1. ORTEP diagram of [3]rotaxane 1. Ellipsoids are set at 50% probabil-
ity; hydrogen atoms and solvating molecules are omitted for clarity.
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Scheme 2. Thermal degradation of [3]rotaxane 4 to [2]rotaxane 5 and dumb-
bell 11 by the deslippage of a-CD.
~
*
&
Figure 4. Time-yield curves for the degradation of 4 ( ) to 5 ( ) and 11 ( )
(8.5 mmolLÀ1 in [D6]DMSO at 353 K). Theoretical curves were drawn by using
the kinetic parameters of each compounds.[11]
[2]rotaxanes was expected to be faster than that of [3]rotax-
anes. However, contradictory results were observed. To evalu-
ate such extraordinary stability of [2]rotaxanes, NOESY NMR
study was carried out.[11] For 2, the NOE correlations indicated
the existence of an inclusion stabilization effect between the
CD moiety and the end group. On the other hand, in the
NOESY NMR spectrum of 5, no correlation was observed be-
tween protons of the two moieties, although the correlations
between the CD and alkylene chain protons were detected
(Figure 5). This difference in localized position of the CD must
depend on the electronic and/or steric properties of the end
groups. The methoxy end group may be polar enough to posi-
tively interact with the solvent and/or bulky enough to disfa-
vor the approach of CD.
Interestingly, [3]rotaxane 8 exhibited different deslippage
behavior compared with those of other [3]rotaxanes. Unlike
other [3]rotaxanes, for which deslippages occurred approxi-
mately at 608C, the deslippage of 8 required a temperature
above 1008C. Furthermore, no 1H NMR signals of the corre-
sponding [2]rotaxane were observed in the NMR spectrum of
8 under degradation conditions.[11] The result suggests that the
Figure 3. 1H NMR spectra of A) 4, B) 5, C) 11, and D) a-CD (400 MHz,
[D6]DMSO, 298 K).
The maximum yield of 5 was estimated to be 94% after 1.5 h.
The deslippage half-lives of 4 at all temperatures were approxi-
mately 100 times greater than those of 5. These results clearly
suggest the stepwise deslippage of 4 by the fast formation of
5 and subsequent slow decomposition of 5 to 11, thus facili-
tating the isolation of 5.
The CD component of [2]rotaxane can deslip from not only
the narrower rim (tail face), but also the wider rim (head face),
being different from [3]rotaxane. Therefore, the deslippage of
Figure 5. Difference of localization position of CD on [2]rotaxanes 2 and 5.
Table 2. Thermodynamic parameters for the deslippage of [2] and [3]rotaxanes.[a]
Rotaxane
Half-life t [h]
708C
Activation enthalpy DH°
[kJmolÀ1
Activation entropy DS°
[JmolÀ1·KÀ1
408C
508C
608C
808C
1008C
1208C
]
]
[b]
[b]
[b]
[b]
[3]rotaxane 1
[2]rotaxane 2
[3]rotaxane 4
[2]rotaxane 5
[3]rotaxane 6
[2]rotaxane 7
[3]rotaxane 8
–
–
–
–
–
–
9.2
80
1.3
–
–
–
–
3.0
29
0.39
3.7
–
–
78
85
96
120
73
À100
À99
À34
7.7
[b]
[b]
[b]
0.68
[b]
[b]
[b]
[b]
18
[b]
0.25
19
–
–
–
–
[b]
[b]
[b]
–
320
0.48
100
2.3
[b]
[b]
[b]
[b]
2.8
[b]
1.1
[b]
–
–
–
–
À91
À7.2
[b]
[b]
–
–
–
–
28
[b]
8.8
120
[b]
[b]
[c]
[c]
[b]
[b]
[b]
–
–
–
31
–
–
–
[a] Determined by 1H NMR analyses. The details of experiments and calculations are given in the Supporting Information. The reaction was performed
using a solution of each rotaxane (8.5 mmolLÀ1 in [D6]DMSO). [b] Not measured. [c] No reaction.
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deslippage from 8 to [2]rotaxane is much slower than the con-
version of [2]rotaxane to the dumbbell-shaped molecule. This
behavior is clearly different from that of [3]rotaxanes other
than 8 (Scheme 3). Furthermore, the NMR analyses of com-
Scheme 3. Thermal degradation of [3]rotaxane 8 to the dumbbell and a-CD.
pounds 9 and 10 suggested that the deslippage of two CDs of
9 and 10 may be competitive and occur by a mechanism simi-
lar to that of 8. Although the precise thermodynamic analyses
of 9 and 10 could not be achieved owing to much less solubil-
ity of 9 and 10 and its derivatives, it was found that two types
of the decompositions were operated depending on the axle
terminal structure.
The kinetic and thermodynamic parameters for the deslip-
page processes of several [2]- and [3]rotaxanes are summarized
in Table 2. The activation enthalpy (DH°) and activation entro-
py (DS°) were obtained by the Eyring plots. A comparison of
the half-lives of 1, 2, 4, and 5 at 608C revealed that the deslip-
page of 4 was faster than that of 1, whereas the deslippage of
5 was slower than that of 2. Furthermore, the enthalpy term
(DH°), rather than the entropy term (TDS°), was observed to
be dominant in these systems. Moreover, the deslippage pro-
cesses for two rotaxanes with methoxy groups (4 and 5) were
found to have larger DH° and smaller DS° than those of the
rotaxanes with bromo substituents (1 and 2). Figure 6a pres-
ents illustrations to explain these results. Based on the NOESY
analyses mentioned above, the mobility of the two CDs in
[3]rotaxane 1 is expected to be greater than that of 4, proba-
bly because the methoxy substituents of 4 suppress the ap-
proach of the CDs to the end groups. Assuming that the flexi-
bility of CD is restricted during the deslippage transition state,
DS° for the entire deslippage process in 1 is greater than that
of 4. Based on the enthalpy–entropy compensation rule,[12] the
deslippage system in 4 is expected to have a larger DH° than
1. Meanwhile, the enthalpy gain by the inclusion complex for-
mation of 2 results in a smaller DH° for the deslippage process
compared with that of 5, leading to the faster deslippage of 2.
On the other hand, deslippage rates for the shorter axle-con-
taining rotaxanes (6 and 7) were approximately three times
faster than those of the longer-axle containing rotaxanes (4
and 5).
Figure 6. a) Illustration of the difference in DH° according to the enthalpy–
entropy compensation rule using [3]rotaxanes 1 and 4; b) illustration of the
inhibitory capacity of the deslippage depending on the end group struc-
tures using [3]rotaxane 8 and [2]rotaxanes 2 and 5; c) energy profile differ-
ence of the stepwise degradation of 1 (top) and 8 (bottom).
their original structures can somewhat resemble that of the
transition-state structure. Furthermore, the deslippage of 8
was very slow even compared with those of deslippage for 2
and 5, which have special stabilizing factors. This result sug-
gests that steric effect of the narrower rim (tail face) on 8
during the deslippage is larger than such stabilization energy
(Figure 6b). Because the CD in the corresponding [2]rotaxane
of 8 can readily deslip from a wider rim after the first deslip-
page step,[9a] the first deslippage in 8 is energetically large
enough to be the rate-determining step for the entire process
(Figure 6c).
Conclusion
In conclusion, a practical synthetic method for a-CD-based
size-complementary [3]rotaxanes was developed. This method
consists of quantitative formation of pseudo[3]rotaxanes fol-
lowed by a rapid end-capping reaction between terminal
amine and phenyl isocyanate in one pot, enabling the effective
conversion to [3]rotaxanes. The deslippage rate of the compo-
This difference may be attributed to the smaller DH° values
of 6 and 7 compared to those of 4 and 5, which originates
from the higher probability that the CDs in the shorter axle-
containing rotaxanes exist near the bulky end groups. Because
of the dynamic movement of the CD groups, it is possible that
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Acknowledgements
This work was supported by JSPS KAKENHI Grant Number
23245031. A JSPS Fellowship for Young Scientists (Y.A.) is grate-
fully acknowledged.
Keywords: cyclodextrin
groups · supramolecular chemistry · thermodynamics
·
rotaxane
· size-complementary
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Received: August 26, 2014
Published online on October 28, 2014
Chem. Eur. J. 2014, 20, 17132 – 17136
www.chemeurj.org
17136
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