Chemistry Reference
In-Depth Information
O
OR
O
O
∗
OR
N
N
N
N
N
N
N
N
N
N
N
N
H
H
H
H
H
H
H
H
RO
N
N
OR
H
H
H
H
H
H
N
N
N
H
N
N
N
N
H
N
N
N
N
N
OR
O
O
O
OR
OR
O
OR
O
O
(a)
(b)
Figure 2.34
1,4-Diaryl-1,2,3-triazole oligomers depicted in their chloride binding
conformations.
oligomer shown in Figure 2.34a has appreciable conformational freedom only around the
arene-triazole single bonds, but it is complexed in a helical fashion when it is bound with
an anion. The size of the binding cavity is complementary to chloride: indeed larger
anions give lower association constants, due to an improper fit within the cavity.
Meudtner and Hecht independently reported oligomers obtained by alternating aryl,
pyridyl and 1,2,3-triazole units [97]. In this case, the dipole-dipole repulsion between the
heterocycles, together with the dipole effect of triazole, forces the oligomers to adopt a
helical conformation in a water-acetonitrile mixture (Figure 2.34b). CD signals show the
most significant effect when the molecule binds with fluoride, while with chloride and
bromide there is a signal decrease and helicity inversion.
De Mendoza and coworkers reported the synthesis of an oligomer composed by four bicy-
clic guanidinium salts connected by a thioether spacer unit: in the presence of a sulfate
anion, two strands of it fold into a double-helical structure around it (Figure 2.35) [98].
Kruger and Martin reported a bispyridyl ligand that forms a helical dimer in the pres-
ence of hydrogen chloride (Figure 2.36a) [99]. The chloride ions are coordinated in a
pincer fashion by the two pyridinium moieties via ionic hydrogen bonds, as well as
N
N
Ph
2
t
BuSiO
S
S
N
N
N
N
2
H
H
H
H
(a)
(b)
Figure 2.35 (a) Tetraguanidinium salts. (b) Optimized model of a sulfate helicate from(
S,S
)-
guanidines.
Reprinted with permission from Ref. [98]. Copyright 1996 American Chemical
Society.
Search WWH ::
Custom Search