over the slippage stoppering group of the dumbbell. If the
rodlike part of the dumbbell contains a recognition site for
the macrocyclic componentsi.e., if attractive noncovalent
interactions are presentsthen a thermodynamic trap exists
and a rotaxane-like complex can be isolated as a kinetically
stable entity upon cooling to ambient temperature from a
mixture that has been equilibrated at elevated temperatures.
If the stoppers are too small, only pseudorotaxane formations
with fast rates of slipping on and offswill ensue and the
complex will not be interlocked. If the stoppers are too large,
slippage cannot occur and a rotaxane-like complex will not
be formed.
Scheme 1
A rotaxane-like complex [DB24C8‚1-H][PF
6
] has been
synthesized in 98% yield, after more than 20 days, in
refluxing CH Cl by the slippage of dibenzo[24]crown-8
DB24C8) over the cyclohexyl termini of dumbbell 1-H‚
7
2
2
(
6
PF . The need for long reaction times may make such
synthetic approaches toward rotaxane-like complexes im-
practical, especially when trying to uncover other crown
ether/slippage stopper pairs. To expedite the synthesis, we
have madesand now reportsa new rotaxane-like complex
by a more traditional threading-followed-by-stoppering ap-
8
proach. This protocol first allows a crown ether and a
dialkylammonium ion thread containing a slippage stopper
on one end and an electrophilic unit on the other to form
the corresponding [2]pseudorotaxane and then attaches a
bulky stopper via nucleophilic substitution to the reactive
end of the thread at low temperature.
thread 2-H‚PF6 with BMP25C8 in MeNO at ambient
2
10
temperature and then added PPh , the rotaxane-like complex
3
[BMP25C8‚3-H][PF ] was generated and isolated in 40%
6 2
yield in less than 1 day. The lower yield, compared to the
Although the p-(tert-butyl)phenyl group has been shown8a
to be a sterically impassible stopper for DB24C8, it may
not be so for crown ethers with larger cavities. We have
found (Scheme 1) that it is not a true stopper, but rather a
6
slippage synthesis of complex [DB24C8‚1-H][PF ], is a
1
1
consequence of the weaker binding between dialkylam-
monium ion-containing threads and this larger crown ether.
An interesting phenomenon was observed relating to the
polarity of the eluent used for column chromatography on
slippage stopper, for benzometaphenylene[25]crown-8
9
(
BMP25C8). When we mixed the bromo-functionalized
silica gel: when using CH
rotaxane-like complex was isolated. It always contained some
decomplexation product. CH Cl /MeCN (9:1), however, gave
the pure complex intact, even on a gram scale. This
observation implies that the more polar mixture of CH Cl
MeOH/SiO decreases the free energy of activation for the
slipping off process relative to that in a less polar mixture
of CH Cl /MeCN/SiO and, in so doing, increases the rate
2 2
Cl /MeOH (19:1), no pure
(5) The “slippage” methodology was first exploited for rotaxane syntheses
2
2
that were conducted in a statistical manner. In the absence of mutual
recognition interactions between the ring and thread, formation of a rotaxane-
like complex relied simply upon chance encounters between the components.
For examples, see: (a) Harrison, I. T. J. Chem. Soc., Chem. Commun. 1972,
2
2
/
2
2
31-232. (b) Agam, G.; Gravier, D.; Zilkha, A. J. Am. Chem. Soc. 1976,
9
8, 5206-5214. (c) Schill, G.; Beckmann, W.; Schweikert, N.; Fritz, H.
Chem. Ber. 1986, 119, 2647-2655. The successful marriagesfirst reported
in 1993 (see ref 3a)sof template-directed approaches with slippage resulted
in efficient noncovalent syntheses of rotaxane-like complexes. See also:
ref 3b-d, and (d) Ashton, P. R.; Ballardini, R.; Balzani, V.; Belohradsky,
M.; Gandolfi, M. T.; Philp, D.; Prodi, L.; Raymo, F. M.; Reddington, M.
V.; Spencer, N.; Stoddart, J. F.; Venturi, M.; Williams, D. J. J. Am. Chem.
Soc. 1996, 118, 4931-4951. (e) Asakawa, M.; Ashton, P. R.; Ballardini,
R.; Balzani, V.; Belohradsky, M.; Gandolfi, M. T.; Kocian, O.; Prodi, L.;
Raymo, F. M.; Stoddart, J. F.; Venturi, M. J. Am. Chem. Soc. 1997, 119,
2
2
2
of dissociation such that it occurs on the time-scale of flash
chromatography. Other factors that can affect hydrogen
bonding between the crown ether and the dialkylammonium
ion-containing thread should affect the dissociation of
complex [BMP25C8‚3-H][PF
PF ] dissociates completely in less than 18 h at ambient
temperature when dissolved in CD SOCD ; dissociation was
6 2
] . Complex [DB24C8‚1-H]-
[
6
3
02-310. (f) Ashton, P. R.; Fyfe, M. C. T.; Schiavo, C.; Stoddart, J. F.;
White, A. J. P.; Williams, D. J. Tetrahedron Lett. 1998, 39, 5455-5458.
g) Heim, C.; Affeld, A.; Nieger, M.; V o¨ gtle, F. HelV. Chim. Acta 1999,
3
3
(
not studied in other solvents, at other temperatures, or in
the presence of added bases. We have, however, now
conducted investigations of the dissociation behavior of the
8
2, 746-759. (h) Fyfe, M. C. T.; Raymo, F. M.; Stoddart, J. F. In
Stimulating Concepts in Chemistry; Shibasaki, M., Stoddart, J. F., V o¨ gtle,
F., Eds.; VCH-Wiley: Weinheim, 2000, pp 211-220.
(6) Raymo, F. M.; Houk, K. N.; Stoddart, J. F. J. Am. Chem. Soc. 1998,
1
20, 9318-9322.
7) Ashton, P. R.; Baxter, I.; Fyfe, M. C. T.; Raymo, F. M.; Spencer,
N.; Stoddart, J. F.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1998,
20, 2297-2307.
8) For recent examples, see: (a) Rowan, S. J.; Stoddart, J. F. J. Am.
(
(9) Cantrill, S. J.; Fulton, D. A.; Heiss, A. M.; Pease, A. R.; Stoddart, J.
F.; White, A. J. P.; Williams, D. J. Chem. Eur. J. 2000, 6, 2274-2287.
(10) See for example: (a) Rowan, S. J.; Cantrill, S. J.; Stoddart, J. F.
Org. Lett. 1999, 1, 129-132. (b) Rowan, S. J.; Cantrill, S. J.; Stoddart, J.
F.; White, A. J. P.; Williams, D. J. Org. Lett. 2000, 2, 759-762.
(11) Ashton, P. R.; Bartsch, R. A.; Cantrill, S. J.; Hanes, R. E., Jr.;
Hickingbottom, S. K.; Lowe, J. N.; Preece, J. A.; Stoddart, J. F.; Talanov,
V. S.; Wang, Z.-H. Tetrahedron Lett. 1999, 40, 3661-3664.
1
(
Chem. Soc. 2000, 122, 164-165. (b) Loeb, S. J.; Wisner, J. A. Chem.
Commun. 2000, 845-846. (c) Seel, C.; V o¨ gtle, F. Chem. Eur. J. 2000, 6,
2
2
1-24. (d) Buston, J. E. H.; Young, J. R.; Anderson, H. L. Chem. Commun.
000, 905-906.
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Org. Lett., Vol. 2, No. 23, 2000