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OAc
S
S
O
NH
AcO
OAc
AcO
AcO
OAc
OAc
OAc
O
S
S
10
NH
OA c
S
S
OA c
NH
HN
A
cO
NH
HN
O
NH
HN
OAc
NH
HN
OAc
S
S
O
OAc
O
AcO
A
cO
OAc
OAc
OAc
NH
HN
AcO
AcO
AcO
O
NH
HN
AcO
S
S
OAc
11
12
Fig. 5 Structures of anion receptors 10-12.
The m-xylene linkers in 11 and 12 were chosen to allow a cooperative
interaction between the corresponding binding sites without building up
of substantial strain upon anion binding.
The binding properties of derivatives 10-12 were studied with model
dicarboxylate - glutarate dianion in CDCl
3
or in DMSO-d
6
. Titration ex-
periments for receptor 10 in DMSO-d
6
showed a 1 : 1 model of binding
with moderate association constant (K
ass
=
391 M
1
) which is much
smaller than that reported for a similar binding motif bearing n-butyl
substituents at the nitrogen atoms (10 000 M
1
).
20
In less demanding
CDCl
3
, however, the titration data and Job plot were consistent with a
2 : 1 host/guest binding model with binding constants K
ass1
=
235 and
K
as2
=
247 M
1
. Well preorganized receptor 11 displayed, on the other
hand, a 1/1 stoichiometry in DMSO-d
6
as well as in CDCl
3
. The binding
constants in CDCl
3
and in DMSO-d
6
were too high to be accurately
measured. Increasing water content in DMSO to 10% allowed for de-
termination of binding constant (1000 M
1
). In the case of 12 analysis of
titration data showed more complex equilibria. In CDCl
3
a 2 : 1 host/guest
model of binding was found (K
ass1
=
104 M
1
, K
ass2
=
103 M
1
) whereas in
DMSO-d
6
1 : 2 model (K
ass1
=
10
5
, K
ass2
=
10
2
M
1
) was observed. The
suggested modes of anion binding by receptors 11 and 12 are shown in
Fig. 6. These results show that proper hydrogen-bond-donor topology
and appropriate preorganization should be considered in constructing
conformationally flexible anion receptors such as 10-12.
Sugar-thiourea receptors were also studied to probe possibility of long-
range hydrogen bonds in supramolecular helix formation.
21
This type of
foldamers were shown to form intramolecular 15-membered hydrogen
bonds between thiourea NH and sugar oxygen atom (Fig. 7).
The existence of this intramolecular hydrogen bond and defined folding
in 15 was confirmed by an analysis of rotameric populations of urea
moieties in 13-15.Receptors13 and 14 were shown to exist as a mixture of
Z,E and Z,Z rotamers, whereas 15 exists as a single Z,Z conformer.
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