rocycle should be for threading to occur is a basic and
important question in threaded structure chemistry.3a For a
long time, it has been widely accepted that a macrocycle
needs at least 24 C, N, O, or S atoms for the threading of an
alkyl group into its cavity,1a,3a,4a,5 although Schill et al.6a,b
reported very low yields of rotaxanes by statistical threading
of 21-membered and 23-membered macrocycles more than
two decades ago, and more recent results suggest that 20-
membered macrocycles can be threaded6c and demonstrate
that some dibenzo-22- and 23-membered6d crown ethers
interact only weakly with secondary ammonium ions (but
without proof of threading). Dibenzo-24-crown-8 (DB24C8)
derivatives are the most widely used hosts for secondary
dialkylammonium salts.3,4 Crown ethers with less than 24
atoms in their macrorings have been observed to form face-
to-face complexes with secondary dialkylammonium salts.7
However, herein, we have found that secondary dialkylam-
monium salts can thread through the cavity of benzo-21-
crown-7 (B21C7) to form [2]pseudorotoaxane- and [2]-
rotaxane-type threaded structures.
plexed 1, and the complex between B21C7 and 1, indicating
1
slow-exchange complexation on the H NMR time scale.3a
This implied the threading of 1 through the cavity of B21C7
to form a pseudorotaxane. In the same way, complexations
of B21C7 with secondary ammonium salts 2 and 3 were
also found to be slow-exchange systems. From integrations
of all peaks, the stoichiometries of all three complexation
systems were determined to be 1:1. The association constants
(Ka) of 1:1 complexes,8 B21C7‚1, B21C7‚2, and B21C7‚3
in acetone-d6 are 527 ((4) M-1, 615 ((36) M-1, and 1062
((102) M-1, respectively. These values are higher than the
corresponding Ka values of 135 ((6) M-1,9 155 ((8)
M-1, and 261 ((13) M-1 for DB24C8-based complexes8
DB24C8‚1, DB24C8‚2, and DB24C8‚3 and the previously
reported Ka value, 360 M-1,3a for DB24C8‚4 in acetone-
d6, indicating that secondary dialkylammonium salts fit
the cavity of B21C7 better than the cavity of DB24C8
so more efficient hydrogen bonding interactions can form.
The Ka increase from B21C7‚1 to B21C7‚2 to B21C7‚3
is a result of the acidity increase of N-methylene and
ammonium hydrogens due to the increasing electron-
withdrawing ability from propyl to phenyl to p-cyanophenyl
substituents.
The 1H NMR spectrum (Figure 1) of an equimolar solution
(4) (a) Amabilino, D. B.; Stoddart, J. F. Chem. ReV. 1995, 95, 2725-
2828. (b) Glink, P. T.; Oliva, A. I.; Stoddart, J. F.; White, A. J. P.; Williams,
D. J. Angew. Chem., Int. Ed. 2001, 40, 1870-1875. (c) Tachibana, Y.;
Kihara, N.; Furusho, Y.; Takata, T. Org. Lett. 2004, 6, 4507-4509. (d)
Badjic, J. D.; Ronconi, C. M.; Stoddart, J. F.; Balzani, V.; Silvi, S.; Credi,
A. J. Am. Chem. Soc. 2006, 128, 1489-1499. (e) Chiu, C.-W.; Lai, C.-C.;
Chiu, S.-H. J. Am. Chem. Soc. 2007, 129, 3500-3501.
(5) Molecular Catenanes, Rotaxanes and Knots; Sauvage, J.-P., Dietrich-
Buchecker, C. O., Eds.; Wiley-VCH: Weinheim, Germany, 1999. Cantrill,
S. J.; Pease, A. R.; Stoddart, J. F. J. Chem. Soc., Dalton Trans. 2000, 3715-
3734. Hubin, T. J.; Busch, D. H. Coord. Chem. ReV. 2000, 200-202, 5-52.
Takata, T.; Kihara, N. ReV. Heteroatom Chem. 2000, 22, 197-218. Mahan,
E.; Gibson, H. W. In Cyclic Polymers, 2nd ed.; Semlyen, J. A., Ed.; Kluwer
Publishers: Dordrecht, The Netherlands, 2000; pp 415-560. Panova, I.
G.; Topchieva, I. N. Russ. Chem. ReV. 2001, 70, 23-44.
(6) (a) Schill, G.; Beckmann, W.; Vetter, W. Chem. Ber. 1980, 113, 941-
54. (b) Schill, G.; Beckmann, W.; Schweickert, N.; Fritz, H. Chem. Ber.
1986, 119, 2647-2655. (c) Gibson, H. W.; Nagvekar, D. S.; Yamaguchi,
N.; Bhattarcharjee, S.; Wang, H.; Vergne, M.; Hercules, D. M. Macromol-
ecules 2004, 37, 7514-7529. (d) Tokunaga, Y.; Yoshioka, M.; Nakamura,
T.; Goda, T.; Nakata, R.; Kakuchi, S.; Shimomura, Y. Bull. Chem. Soc.
Jpn. 2007, 80, 1377-1382.
(7) Metcalfe, J. C.; Stoddart, J. F.; Jones, G. J. Am. Chem. Soc. 1977,
99, 8317-8319. Metcalfe, J. C.; Stoddart, J. F.; Jones, G.; Atkinson, A.;
Kerr, I. S.; Williams, D. J. J. Chem. Soc., Chem. Commun. 1980, 540-
543. Abed-Ali, S. S.; Brisdon, B. J.; England, R. J. Chem. Soc., Chem.
Commun. 1987, 1565-1566. Cantrill, S. J.; Pease, A. R.; Stoddart, J. F. J.
Chem. Soc., Dalton Trans. 2000, 3715-3734.
(8) The Ka values of B21C7-based complexes, slow-exchange complex-
ation systems, were calculated from integrations of complexed and
uncomplexed peaks. The Ka values of DB24C8-based complexes, fast-
exchange complexation systems, were calculated from chemical shift
changes. All of these Ka values are at 1.00 mM host and guest in ace-
tone. Though DB24C8‚1, DB24C8‚2, and DB24C8‚3 are fast-
exchange complexation systems, DB24C8‚4 is a slow exchange complex-
ation system. These were also observed by Stoddart et al.3a From these,
we can know whether a complexation system is fast-exchange or slow-
exchange is mainly dependent on the relative sizes of the end groups of
the guest.
Figure 1. Partial 1H NMR spectra (500 MHz, acetone-d6, 22 °C)
of 1.00 mM secondary dialkylammonium salt 1 (a), 1.00 mM
B21C7 and 1 (b), and 1.00 mM B21C7 (c). Complexed and
uncomplexed species are denoted by “c” and “uc”, respectively.
of B21C7 and dibutylammonium salt 1 in acetone-d6 shows
three sets of resonances for uncomplexed B21C7, uncom-
(9) The Ka values reported in ref 3a for DB24C8‚1 is 50 M-1 by proton
NMR titration and 70 M-1 by proton NMR dilution in acetonitrile.
(10) From Figure 3, it is obvious that the benzene ring of B21C7 is not
a required part for the threading of secondary ammonium salts through the
caviety of B21C7. Therefore, 21-crown-7, the corresponding crown ether
without a benzene ring, should also be able to form threaded structures
with secondary ammonium salts. Previously, Loeb et al. found that both of
DB24C8 and 24-crown-8 can complex N-benzylanilinium salts (Loeb, S.
J.; Tiburcio, J.; Vella. S. J. Org. Lett. 2005, 7, 4923-4926).
(3) (a) Ashton, P. R.; Chrystal, E. J. T.; Glink, P.; Menzer, T. S.; Schiavo,
C.; Spencer, N.; Stoddart, J. F.; Tasker, P. A. White, A. J. P.; Williams, D.
J. Chem. Eur. J. 1996, 2, 709-728. (b) 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. (c) Cantrill, S. J.; Fyfe, M. C. T.; Heiss, A.
M.; Stoddart, J. F.; White, A. J. P.; Williams, D. J. Org. Lett. 2000, 2,
61-64. (d) Tachibana, Y.; Kawasaki, H.; Kihara, N.; Takata, T. J. Org.
Chem. 2006, 71, 5093-5104.
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