(d in Scheme 2). Eventually, we discovered that reduction
of 9NP‚4PF6 can be achieved by using the BH3‚2,6-lutidine
complex, followed by hydrolysis, to afford the “fixed” [2]-
rotaxane 10NP‚4PF6 in 40% yield.14
The research reported here has shown how [2]rotaxanes
with dynamic structures can be assembled under equilibrium
control. These dynamic species represent a halfway house
between rotaxanes and pseudorotaxanes. Although their
components are covalent in nature, under certain conditions,
e.g., in the presence of a small nucleophile and/or an acid
catalyst, they can equilibrate and exchange their components
as if they were their supramolecular counterparts, namely,
the pseudorotaxanes. In this respect, dynamic rotaxanes have
some of the hallmarks of the “rotaxanes” which have been
obtained using the slippage methodology15 where, in nonpolar
solvents at low temperatures, they behave like rotaxanes
while, at higher temperatures in more highly solvating polar
solvents, they assume the characteristics of pseudorotaxanes.
In the case of the dynamic rotaxanes, it is the presence of
the appropriate catalyst which imparts upon them their
pseudorotaxane characteristics. These new dynamic rotax-
anes, however, have the added advantage that they can be
transported out of a thermodynamic regime and transferred
into a kinetic one, simply by reduction of their imine bonds.
Figure 3. Partial 1H NMR (400 MHz, CD3CN) spectra of the t-Bu
region during the exchange reaction of 9NP·4PF6 with 9BZ·4PF6.
This reaction was carried out at 9.8 mM using 5% p-MeC6H4NH2/
1
NH4PF6 as catalysts. Time ) [0] indicates the H NMR spectrum
taken before any catalyst was added. The small peak observed just
upfield of the rotaxane peaks is believed to correspond to the
semirotaxane wherein one end of the “dumbbell” carries a 3,5-di-
tert-butylaryl group and the other end is occupied by either an
aldehyde or the p-methylaryl group.
Acknowledgment. The authors thank Stuart J. Cantrill
for producing the final version of this manuscript and UCLA
for generous financial support.
Supporting Information Available: Experimental pro-
cedures and characterization data for 1NP-10NP and 4BZ-
5BZ. 1H NMR spectra for 4NP-8NP, 10NP‚4PF6, 4BZ, and
equilibrium was reached after 14 h, i.e., the equilibrium takes
a lot longer to reach than in the previous experiments
involving only one dumbbell at a time. This difference in
time scale is not unexpected.12 The ratio of the dynamic
rotaxanes 9NP‚4PF6:9BZ‚4PF6, at equilibrium, is ca. 6:1,
reflecting the stronger binding of the naphthalene-derived
components over the benzene derivatives. Thus, this experi-
ment reveals all the subtlety and attractions of using
thermodynamic control13 exclusively in the self-assembly of
interlocked molecular compounds.
1
5BZ. H NMR spectra of the reaction mixtures formed in
the synthesis of 9NP‚4PF6 and 9BZ‚4PF6 and VT 1H NMR
of these two compounds. This material is available free of
OL991047W
(13) (a) Rowan, S. J.; Sanders, J. K. M. Chem. Commun. 1997, 1407-
1408. (b) Hamilton, D. G.; Feeder, N.; Teat, S. J.; Sanders, J. K. M. New.
J. Chem. 1998, 1019-1021. (c) Sanders, J. K. M. Chem. Eur. J. 1998, 4,
1378-1383. (d) Kidd, T. J.; Leigh, D. A.; Wilson, A. J. J. Am. Chem. Soc.
1999, 121, 1599-1600. (e) Tam-Chang, S.-W.; Stehouwer, J. S.; Hao, J.
J. Org. Chem. 1999, 64, 334-335. (f) Ipaktschi, J.; Hosseinzadeh, R.; Schalf,
P. Angew. Chem. Int. Ed. 1999, 38, 1658-1660. (g) Weck, M.; Mohr, B.;
Sauvage, J.-P.; Grubbs, R. H. J. Org. Chem. 1999, 64, 5463-5471. (h)
Fujita, M. Acc. Chem. Res. 1999, 32, 53-61. (i) Cousins, G. R. L.; Poulsen,
S.-A.; Sanders, J. K. M. Chem. Commun. 1999, 1575-1576.
(14) The yield is based on the assumption that the starting material
consists of only 90% 9NP‚4PF6.
One challenge remained. It was to show that the imine
functions in the dynamic [2]rotaxanes can be reduced without
losing the interlocking between their dumbbells and rings
(12) The increased reaction time reflects the fact that there are now an
increased number of reactions that are required to occur before 5NP can
exchange with 5BZ in the [2]rotaxane. The first step in the process is
presumably imine exchange (or hydrolysis) of 9BZ‚4PF6, resulting in a
semirotaxane (Figure 1) which can then undergo decomplexation of the
BZ thread component revealing “free” CBPQT‚4PF6. This “free” cyclophane
can then form 9NP‚4PF6 with 5NP by means of the process depicted by b
in Scheme 2.
(15) Raymo, F. M.; Houk, K. N.; Stoddart, J. F. J. Am. Chem. Soc. 1998,
120, 9318-9322.
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Org. Lett., Vol. 1, No. 12, 1999