the variation of the TR~ absorbance variation. Thus,
that (bCD) x É TR~ is less stable than its MO~ and TNS~
2
(bCD) x É TR~ is much more stable than bCD É TR~, which is
analogues.
Thus,
in
the
most
stable
complex,
2
discussed below, and the strength of cooperative binding
(bCD) ox É MO~, the interaction between the (bCD) ox
2
2
varies in the sequence (bCD) ur É TR~ [ (bCD) ox É TR~ [
recognition sites and the MO~ binding sites is maximized and
2
2
(bCD) su É TR~.
strain is minimized by comparison with the least stable
2
For the formation of bCD É TR~ and bCD É (TR) 2~
complex, (bCD) su É TR~, in which the combination of these
2
2
characteristics is less e†ective in stabilizing the complex.
K1
bCD ] TR~ A8B bCD É TR~
(7)
(8)
We gratefully acknowledge the award of an Australian Post-
graduate Research Award to C.A.H. and funding of this
research by the University of Adelaide and the Australian
Research Council. We thank Nihon Shokhuin Kako Co. Ltd.
for a gift of bCD.
K2
bCD É TR~ ] TR~ A8B bCD É (TR) 2~
2
K \ (7.1 ^ 0.7) ] 102 dm3 mol~1 and K \ (4 ^ 7) ] 106
1
2
dm3 mol~1, respectively, as shown by temperature-jump spec-
troscopy under identical conditions to those used in this
study.26 The uncertainty in K is very high, but the values of
2
K \ (4.18 ^ 1.47) ] 102 dm3 mol~1 and K \ (1.68 ^ 0.54)
References
1
2
] 106 dm3 mol~1 for the analogous cCD are better deter-
mined and show similar relative orders of magnitude for K
and K . The relatively high value of K , by comparison with
K , is attributed to the dimerization of TR~ (K
910 dm3 mol~1) being enhanced by cCD complexation.26
1
M. L. Bender and M. Komiyama, Cyclodextrin Chemistry,
Springer Verlag, New York, 1978.
1
2
2
2
3
W. Saenger, Inclusion Compounds, 1984, 2, 231.
R. J. Clarke, J. H. Coates and S. F. Lincoln, Adv. Carbohydr.
Chem. Biochem., 1989, 46, 205.
K. Harata, Inclusion Compounds, 1991, 5, 311.
S. E. Brown, J. H. Coates, C. J. Easton and S. F. Lincoln, J.
Chem. Soc., Faraday T rans., 1994, 90, 739.
\
1
dimerization
4
5
Discussion
6
7
8
9
S. E. Brown, C. A. Haskard, C. J. Easton and S. F. Lincoln, J.
Chem. Soc., Faraday T rans., 1995, 91, 1013, 4335.
K. Fujita, S. Ejima and T. Imoto, J. Chem. Soc., Chem. Commun.,
1984, 1277.
R. Breslow, N. Greenspoon, T. Guo and R. Zarzycki, J. Am.
Chem. Soc., 1989, 111, 8296.
J. H. Coates, C. J. Easton, S. J. van Eyk, S. F. Lincoln, B. L. May,
C. B. Walland and M. L. Williams, J. Chem. Soc., Perkin T rans.
1, 1990, 2619.
It is seen (Table 1) that K decreases in the sequence
1
(bCD) x É MO~ [ (bCD) x É TNS~ [ (bCD) x É TR~ for each
2
2
2
of the linkers, x, (Fig. 1) where TNS~ is 6-(p-toluidinyl)
naphthalene-2-sulfonate.20 [This discussion is conÐned to the
formation of (bCD) x É G as shown in eqn. (1) and (5) for the
2
reasons given above.] If the bCD moieties in (bCD) x could
2
act independently, (bCD) x É MO~1 should be twice as stable
2
as bCD É MO~ on a statistical basis and the same relationship
10 R. C. Petter, C. T. Sikorski and D. H. Waldeck, J. Am. Chem.
Soc., 1991, 113, 2325.
11 B. Zhang and R. Breslow, J. Am. Chem. Soc., 1992, 114, 5882.
12 B. Zhang and R. Breslow, J. Am. Chem. Soc., 1993, 115, 9353.
13 R. Breslow, Recl. T rav. Pay-Bas, 1994, 113, 493.
14 C. T. Sikorski and R. C. Petter, T retrahedron L ett., 1994, 35,
4275.
15 F. Venema, C. M. Baselier, E. van Dienst, B. H. M. Ruel, M. C.
Feiters, J. F. J. Engbersen, D. N. Reinhout and R. J. M. Nolte,
T etrahedron L ett., 1994, 35, 1773.
16 F. Venema, C. M. Baselier, M. C. Feiters and R. J. M. Nolte,
T etrahedron L ett., 1994, 35, 8661.
17 R. Deschenaux, A. Greppi, T. Ruch, H. P. Kriemler, F. Raschdorf
and R. Ziessel, T etrahedron L ett., 1994, 35, 2165.
18 Y. Wang, A. Ueno and F. Toda, Chem. L ett., 1994, 167.
19 T. Jiang, D. K. Sukumaran, S. D. Soni and D. S. Lawrence, J.
Org. Chem., 1994, 59, 5149.
20 C. A. Haskard, C. J. Easton, B. L. May and S. F. Lincoln, J.
Phys. Chem., 1996, 100, 14457.
21 W. Broser, Z. Naturforsch., T eil B, 1953, 8, 722.
22 H. Zollinger, Azo and Diazo Chemistry: Aliphatic and Aromatic
Compounds, Interscience, New York, 1961.
23 J. Pitha and R. N. Jones, Can. J. Chem., 1966, 44, 3031.
24 F. Quadrifoglio and V. Crescenzi, J. Colloid Interface Sci., 1971,
35, 447.
25 R. L. Reeves, M. S. Maggio and S. A. Harkaway, J. Phys. Chem.,
1979, 83, 2359.
26 R. J. Clarke, J. H. Coates and S. F. Lincoln, J. Chem. Soc.,
Faraday T rans., 1984, 80, 3119.
27 R. J. Clarke, PhD Thesis, University of Adelaide, 1985.
28 I. Tabushi, Y. Kuroda and T. Mizutani, T etrahedron, 1984, 40,
545.
29 Y. Matsui and K. Mochida, Bull. Chem. Soc. Jpn., 1978, 51, 673.
30 A. Buvari and L. Barcza, J. Incl. Phenom., 1989, 7, 313.
31 H-J. Schneider and F. Xiao, J. Chem. Soc., Perkin T rans. 2, 1992,
387.
should exist for the analogous TNS~ and TR~ complexes.
However, in all cases, K for the (bCD) x complex ? 2K for
1
2
1
bCD, consistent with cooperative binding of the guest by the
linked bCD moieties being the dominant complex stabilizing
force. Accordingly, it is probable that variations in
(bCD) x É G complex stability with change in guest largely
2
reÑect di†erences in interaction of the two aromatic binding
groups of the guest with the linked bCD, and that changes in
complex stability for a given guest with change in (bCD) x
2
reÑect the extent to which the hostÈguest interactions
approach optimization as the length of the linker changes.
The most strongly complexed guest is linear MO~, whose
Ñexibility is restricted by conjugation through the diazo
linkage. This restriction may be a contributing cause of the
increase
in
complex
stability
in
the
sequence
(bCD) su É MO~ \ (bCD) ur É MO~ \ (bCD) ox É MO~1 .
2
2
2
Because (bCD) ur has the shortest and least Ñexible linker, the
2
two bCD moieties are probably less able to align their annuli
to accommodate linear MO~ than is the more Ñexible
(bCD) ox. However, while the longer linker in (bCD) su leads
2
2
to greater Ñexibility, the greater separation of the bCD moi-
eties apparently does not allow them to accommodate both
MO~ phenyl groups to maximize binding and complex stabil-
ity decreases as a result. The second most strongly complexed
guest, TNS~, has a more extended aromatic system because of
its naphthyl group and might be expected to interact more
extensively with the hydrophobic interior of the bCD annulus.
However, the rigidity of the naphthyl group seems to o†set
the Ñexibility gained from free rotation about the amine nitro-
gen of TNS~ so that it is less able to adapt to the steric
restraints imposed in (bCD) x É TNS~ which is consequently
32 R. I. Gelb and L. M. Schwartz, J. Incl. Phenom. Mol. Recogn.,
2
less stable than (bCD) x É MO~. The least strongly complexed
1989, 7, 537.
2
33 K. M. Tawarah and A. A. Wazwaz, Ber. Bunsen-Ges. Phys.
Chem., 1993, 97, 727.
34 R. L. Reeves and R. S. Kaiser, J. Org. Chem., 1970, 35, 3670.
guest, TR~, is also the most rigid and the most angular guest.
[In the largely hydrophobic environment of (bCD) x É TR~,
2
TR~ probably exists predominantly in the azo form shown in
Fig. 1].29,34 It appears that these properties render TR~ less
able to adapt to the stereochemical constraints of (bD) x so
Paper 6/03638D; Received 24th May, 1996
2
282
J. Chem. Soc., Faraday T rans., 1997, V ol. 93