Is the tert-butyl group bulky enough to end-cap a pseudorotaxane with a 24-crown-8-ether wheel?

Article abstract of DOI:10.1021/ol048135n

(Chemical equation presented) Although rotaxane chemists have long believed that the tert-butyl group is bulkier than the cavity of dibenzo-24-crown-8- ether (DB24C8), it is essentially smaller than the cavity of DB24C8. The tert-butyl (or 4-tert-butylphenyl) group can actually function as an end-cap of DB24C8-based rotaxanes when the intercomponent interaction is effectively operative. When such attractive interaction is removed, deslippage occurs.

Full text of DOI:10.1021/ol048135n

ORGANIC
LETTERS
2
004
Vol. 6, No. 24
507-4509
Is the tert-Butyl Group Bulky Enough to
End-Cap a Pseudorotaxane with a
4
2
4-Crown-8-ether Wheel?
Yuya Tachibana, Nobuhiro Kihara, Yoshio Furusho, and Toshikazu Takata*,†
†
,‡
§
§,|
Department of Organic and Polymeric Materials, Tokyo Institute of Technology,
Ookayama, Meguro, Tokyo 152-8552, Japan, and Department of Applied Chemistry,
Graduate School of Engineering, Osaka Prefecture UniVersity, Sakai,
Osaka 599-8531, Japan
ttakata@polymer.titech.ac.jp
Received September 15, 2004
ABSTRACT
Although rotaxane chemists have long believed that the tert-butyl group is bulkier than the cavity of dibenzo-24-crown-8-ether (DB24C8), it is
essentially smaller than the cavity of DB24C8. The tert-butyl (or 4-tert-butylphenyl) group can actually function as an end-cap of DB24C8-
based rotaxanes when the intercomponent interaction is effectively operative. When such attractive interaction is removed, deslippage occurs.
2
Rotaxane chemists have long believed that the tert-butyl
group is sterically large enough to end-cap pseudorotaxane
consisting of dibenzo-24-crown-8-ether (DB24C8) and sec-
salt, a variety of rotaxanes have been prepared by researchers
1
e-g,3
including our group.
be bulkier than the cyclohexyl group, which possesses a
Basically, the end-cap group must
1
ammonium salt to suppress the deslippage of DB24C8.
However, this view has recently been challenged.
Since Stoddart et al. established that DB24C8 forms stable
threading complexes (pseudorotaxanes) with sec-ammonium
(2) (a) Ashton, P. R.; Campbell, P. J.; Chrystal, E. J. T.; Glink, P. T.;
Menzer, S.; Philp, D.; Spencer, N.; Stoddart, J. F.; Tasker, P. A.; Williams,
D. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 1865. (b) Ashton, P. R.;
Chrystal. E. J. T.; Glink, P. T.; Menzer, S.; Schiavo, C.; Spencer, N.;
Stoddart, J. F.; Tasker, P. A.; White, A. J. P.; Williams, D. J. Chem. Eur.
J. 1996, 2, 709.
(3) (a) Kolchinski, A. G.; Busch, D. H.; Alcock, N. W. J. Chem. Soc.,
Chem. Commun. 1995, 1289. (b) Ashton, P. R.; Glink, P. T.; Stoddart, J.
F.; Taasker, P. A.; White, A. J. P.; Williams, D. J. Chem. Eur. J. 1996, 2,
729. (c) Cantrill, S. J.; Fulton, D. A.; Heiss, A. M.; Rease, A. R.; Stoddart,
J. F.; White, A. J. P.; Williams, D. J. Chem. Eur. J. 2000, 6, 2274. (d)
Matthew, J. C.; Fyfe, M. C. T.; Stoddart, J. F. J. Org. Chem. 2000, 65,
1937. (e) Glink, P. T.; Oliva, A. I.; Stoddart, J. F.; White, A. J. P.; Williams,
D. J. Angew. Chem., Int. Ed. 2001, 40, 1870. (f) Kihara, N.; Shin, J.-I.;
Ohga, Y.; Takata, T. Chem. Lett. 2001, 30, 592. (g) Kihara, N.; Nakakoji,
N.; Takata, T.; Chem. Lett. 2002, 31, 924. (h) Furusho, Y.; Rajkumar, G.
A.; Oku, T.; Takata, T. Tetrahedron 2002, 58, 6609. (i) Furusho, Y.; Sasabe,
H.; Natsui, D.; Murakawa, K.; Takata, T.; Harada, T. Bull. Chem. Soc.
Jpn. 2004, 77, 179.
†
Tokyo Institute of Technology.
Present address: Industrial Technology Center of Wakayama Prefecture,
‡
Ogura, Wakayama 649-6261, Japan.
§
Osaka Prefecture University.
Present address: JST, ERATO, Yashima Super-Structured Helix Project,
|
Moriyama, Nagoya 552-8555, Japan.
(1) (a) Schill, G. Catenanes, Rotaxanes, and Knots; Academic Press:
New York, 1971. (b) Sauvage, J.-P., Dietrich-Buchecker, C., Eds.; Molecular
Catenanes, Rotaxanes, and Knots; Wiley-VHC; Weinheim, 1999. (c)
Breault, G. A.; Hunter, A.; Mayers, P. C. Tetrahedron 1999, 55, 5265. (d)
Amabilino, D. B.; Stoddart, J. F. Chem. ReV. 1995, 95, 2725. (e) Hubin, T.
J.; Busch, D. H.; Coord. Chem. ReV. 2000, 200-202, 5. (f) Takata, T.;
Kihara, N. ReV. Heteroat. Chem. 2000, 22, 197. (g) Kihara, N.; Takata, T.
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1
0.1021/ol048135n CCC: $27.50
© 2004 American Chemical Society
Published on Web 11/04/2004

1
Figure 2. Partial H NMR (500 MHz) spectral change during the
deslippage of 2 to decompose to 3 and DB24C8 in DMSO-d at
18 K.
6
3
Rotaxane 1 having a DB24C8 wheel and a 4-tert-
butylphenyl end-cap on the axle was synthesized in 92%
yield based on the tributylphosphine-catalyzed acylative end-
capping protocol. When 1 was allowed to stand at 100 °C
6
Figure 1. Rotaxanes with tert-butyl groups and their components.
in DMSO-d
6
for 31 days, no decomposition was observed
1
(
H NMR). The intercomponent hydrogen bonding interac-
borderline bulkiness toward DB24C8, to fix the rotaxane
4
tion in 1 was then completely removed by acylative
neutralization of the ammonium group by treatment with
acetic anhydride in the presence of triethylamine at 0 °C for
structure. Rotaxanes with a 4-tert-butylphenyl group, which
7
5
is one of the most frequently used end-caps, are stable in
various solvents, even in DMSO, and it has thereby been
reported that the 4-tert-butylphenyl group is larger than the
cavity of DB24C8.4 Meanwhile, in our own CPK model
studies, we noticed that the 4-tert-butylphenyl group is
slightly smaller than the cavity of DB24C8. On the basis of
these observations, it seemed logical to assume that the CPK
model was unsuitable to estimate the bulkiness of the end-
cap. Recently, we have experimentally found that the tert-
butyl group is actually small enough to thread out from the
wheel. Here, we wish to discuss the “actual and apparent
bulkiness” of the tert-butyl end-cap along with the role of
intercomponent interaction between wheel and axle. Our
results reveal important aspects for the design of rotaxanes
with a robust interlocked structure.
4
8 h to give nonionic rotaxane 2 in 76% yield.
a,5b
Interestingly, 2 was isolated along with both DB24C8 and
(24% yield). Since no decomposition of the end-cap took
3
place under N-acylation conditions, the fact that DB24C8
and 3 were isolated from the reaction mixture suggested the
deslippage of the wheel DB24C8 over 4-tert-butylphenyl
end-cap, which was caused by the removal of the intercom-
ponent interaction.
1
In the H NMR spectrum of 2, the ester benzyl proton
signal (-CH
amide benzyl proton signals (-CH
2
OCO-) was observed at 5.89 ppm, and the
NAcCH -) were ob-
2
2
served as four peaks around 4.3 ppm due to the presence of
s-cis and s-trans conformers of the amide group. When a
DMSO-d
sluggishly appeared around 5.3 and 4.4 ppm in 10 h (Figure
). These signals were identified as the benzyl protons of
6
solution of 2 was heated at 45 °C, new signals
(4) (a) Ashton, P. R.; Baxter, I.; Fyfe, M. C. T.; Raymo, F. M.; Spencer,
N.; Stoddart, J. F. J. Am. Chem. Soc. 1998, 120, 2297. (b) Sohgawa, Y.-h.;
Fujimori, H.; Shoji, J.; Furusho, Y.; Kihara, N.; Takata, T. Chem. Lett.
2
2
001, 30, 774.
5) (a) Rowan, J. S.; Cantrill, S. J.; Stoddart, J. F. Org. Lett. 1999, 1,
29. (b) Chiu, S.-H.; Rowan, S. J.; Cantrill, S. J.; Glink, P. T.; Garrell, R.
free axle 3, which was actually isolated from the solution.
After 5 days, the signals of 2 disappeared completely, and
both DB24C8 and 3 were isolated quantitatively. In the same
manner, the thermal stability of rotaxane 4 with a 3,5-
dimethylphenyl end-cap at both termini was examined. A
(
1
L.; Stoddart, J. F. Org. Lett. 2000, 2, 3631. (c) Rowan, S. J.; Stoddart, J. F.
J. Am. Chem. Soc. 2000, 122, 164. (d) Chiu, S.-H.; Rowan, S. J.; Cantrill,
S. J.; Stoddart, J. F.; White, A. J. P.; Williams, D. J. Chem. Eur. 2002, 8,
7
5
170. (e) Chiu, S.-H.; Elizarov, A. M.; Glink, P. T.; Stoddart, J. F. Org.
Lett. 2002, 4, 3561. (f) Takata, T.; Kawasaki, H.; Asai, S.; Furusho, Y.;
Kihara, N. Chem. Lett. 1999, 28, 223. (g) Furusho, Y.; Oku, T.; Hasegawa,
T.; Tsuboi, A.; Kihara, N.; Takata, T. Chem. Eur. J. 2003, 9, 2895. (h)
Loeb, S. J.; Wisner, J. A. Chem. Commun. 1998, 2757. (i) Loeb, S. J.;
Wisner, J. A. Chem. Commun. 2000, 845. (j) Loeb, S. J.; Wisner, J. A.
Chem. Commun. 2000, 1939. (k) Davidson, G. J. E.; Loeb, S. J.; Parekh,
N. A.; Wisner, J. J. Chem. Soc., Dalton Trans. 2001, 3135. (l) Davidson,
G. J. E.; Loeb, S. J. Dalton Trans. 2003, 4319. (m) Asakawa, M.; Ikeda,
T.; Yui, N.; Shimizu, T. Chem. Lett. 2002, 31, 174. (n) Ikeda, T.; Asakawa,
M.; Goto, M.; Nagawa, Y.; Shimizu, T. Eur. J. Org. Chem. 2003, 3744.
6
DMSO-d solution of 4 was heated at 100 °C for 30 days,
1
but no decomposition product was detected in the H NMR
spectra. Therefore, it was found that the tert-butyl (i.e., 4-tert-
butylphenyl) group is smaller than the cavity of DB24C8
and also smaller than the 3,5-dimethylphenyl group.
Since the decomposition of 2 followed first-order kinetics,
the rate constant k , half-life time τ1/2, and thermodynamic
d
(
o) Tokunaga, Y.; Kakuchi, S.; Akasaka, K.; Nishikawa, N.; Shimomura,
Y.; Isa, K.; Seo, T. Chem. Lett. 2002, 31, 810. (p) Tokunaga, Y.; Kakuchi,
S.; Akasaka, K.; Hisada, K.; Shimomura, Y.; Suzuka, K. Chem. Commun.
(6) Kawasaki, H.; Kihara, N.; Takata, T. Chem. Lett. 1999, 28, 1015.
(7) Kihara, N.; Tachibana, Y.; Kawasaki, H.; Takata, T. Chem. Lett. 2000,
2
003, 2250.
29, 506.
4508
Org. Lett., Vol. 6, No. 24, 2004

Table 1. Kinetic Data for the Decomposition of Rotaxanes 2 and 5
kd, 313 K (sec-1)
t1/2, 313 K (h)
∆G313 K (kJ mol-1)
q
∆H (kJ mol-1)
q
∆S (J mol K-1
)
q
-1
rotaxane
2
solvent
-
6
6
6
5
5
5
DMSO-d6
(4.6 ( 0.2) × 10
(2.0 ( 0.1) × 10
(1.4 ( 0.2) × 10
(4.1 ( 0.2) × 10
(5.2 ( 0.1) × 10
(3.1 ( 0.2) × 10
42
97
143
4.7
3.7
7.0
108.8 ( 0.1
111.2 ( 0.1
112.0 ( 0.1
103.1 ( 0.1
102.5 ( 0.1
103.8 ( 0.1
84.1 ( 1.6
86.8 ( 1.1
84.6 ( 1.0
100.3 ( 0.3
96.7 ( 0.4
99.3 ( 0.6
-79 ( 5
-77 ( 3
-75 ( 8
-9 ( 1
-19 ( 1
-15 ( 2
-
-
-
-
-
chloroform-d1
benzene-d6
DMSO-d6
chloroform-d1
benzene-d6
5
parameters were calculated.5b The results are summarized
in Table 1.
The large ∆H indicates that the disadvantageous confor-
mation change is necessary for the deslippage of DB24C8
over the 4-tert-butylphenyl group. The decomposition rate
is dependent on the solvent, although the reasons behind the
solvent effect are not clear at the present time.
The bulkiness of the 4-tert-butylphenyl group can be
attributed to the tert-butyl group. The van der Waals diameter
of the tert-butyl group is larger than that of the phenyl group,
and DB24C8 can actually freely slip over the benzene ring.
The thermal stability of rotaxane 5 with a tert-butyl end-
cap was then investigated. Although 5 could be prepared
similarly to 2 and 4, 5 was always obtained as a mixture
with a small amount of axle 6 and DB24C8 even by repeated
purifications, suggesting the decomposition of 5 via deslip-
page. 5 with a purity of ca. 90% was used for the
decomposition experiment, in which the kinetic parameters
were determined by taking purity into consideration. The
results are summarized in Table 1.
of rotaxane consisting of DB24C8 as a wheel. The introduc-
tion of a phenylene group changing to a 4-tert-butylphenyl
group suppresses the activation entropy of the deslippage
due to the decrease of freedom of the tert-butyl group by
the adjacent rigid benzene ring, eventually leading to more
effective end-capping. Furthermore, it was confirmed that
any group can only be declared as a true end-cap of rotaxane
after the complete removal of the intercomponent interaction.
The intercomponent hydrogen-bonding in 1, which did not
completely disappear even in DMSO, made the end-cap look
more bulky. N-Acylative neutralization of 1 to 2 to eliminate
the intercomponent interaction made the bulkiness of the end-
cap obvious.
q
In summary, the following points are deemed important:
(1) the tert-butyl group is not large enough to function as
an end-cap group of DB24C8-based rotaxanes, (2) the
attractive interaction between the rotaxane components
enhances the apparent bulkiness of the end-cap group, and
(3) end-capping ability depends on both the bulkiness and
8
the structural rigidity of the end-cap group.
Although the decomposition of 5 was faster than that of
q
2
, ∆H of 5 was larger than that of 2 in all solvents tested,
Acknowledgment. N.K. is thankful for the financial
support from Yazaki Memorial Foundation for Science and
Technology.
probably because of the dipole-dipole repulsion between
the crown ether and the carbonyl group. The total decrease
q
q
of ∆G in 5 was attributed to the extremely large ∆S of 5.
This result indicates a more suppressed molecular freedom
at the transition state of 2 than that of 5, due to the
introduction of a p-phenylene moiety. The bulkiness in this
case may include not only bulkiness but also flexibility, both
of which are considered important factors for the end-cap
group of rotaxane.
Supporting Information Available: Experimental pro-
cedures, spectroscopic data of all compounds, and H NMR
spectral changes of the decomposition of 5. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
OL048135N
In conclusion, the present study demonstrated that the tert-
butyl group is not large enough to act as an end-cap group
(
8) (a) Felder, T.; Shalley, C. A. Angew. Chem., Int. Ed. 2003, 42, 2258.
(b) Linnartz, P.; Bitter, S.; Schalley C. A. Eur. J. Org. Chem. 2003, 4819.
Org. Lett., Vol. 6, No. 24, 2004
4509