Chemistry Reference
In-Depth Information
(ii) generation of higher order architectures such as a two-helix bundle or (iii) both. Illus-
trative examples of both approaches are discussed in the following sections.
11.4.1 Metal Coordination in Folded Aromatic Amide Oligomers
Recently, hydrogen-bonding interactions have been used in combination with aromatic
p-stacking interactions to induce folding in oligomer systems [1]. A common motif
throughout this research is the use of aryl amides [3,34]. These foldamers form stable
secondary structures in solution and solid state, therefore the incorporation of metal ions
is anticipated to further stabilize and/or generate higher-ordered structures. The group of
Parquette studied the impact of metal coordination on the conformational properties of
dendrimers using small foldamers [35]. Their first efforts were to coordinate multiple
Cu
2þ
centers to pyridine-2,6-dicarboxamide dendrons that adopt compact helical confor-
mations due to the syn-syn conformational preference of the pyridine-2,6-dicarboxamide
repeat unit [36]. In these systems, the helical antipodes experience a highly dynamic equi-
librium that interconverts the
M
and the
P
conformations rapidly with kinetic barriers too
small to be measured by NMR. Coordination of Cu
2þ
, however, produced a kinetically
stable, non-dynamic conformational state at room temperature. In a more recent work [37],
this group reported the structural consequences of coordinating a different foldamer (2,6-bis
{2-[(4S)-4,5-dihydro-1,3-oxazol-2-yl]phenyl}carbamoylpyridines,
7
) with divalent metal
ions such as Cu
2þ
,Ni
2þ
and Zn
2þ
(Figure 11.16). As shown by X-ray crystallography
Me
Me
O
O
N
N
O
N
O
N
H
H
N
H
H
X
X
N
N
N
N
O
N
O
O
O
Me
Me
2-P
2-M
X=Cl (
2
)
a
H
H
X=
(
3
)
S
b, c or d
Me
O
N
O
N
X=Cl, M = Cu (
2-Cu
)
X=Cl, M = Ni (
2-Ni
)
X
N
M
N
H
H
N
O
X=
, M = Zn (
3-Zn
)
S
O
Me
Figure 11.16 Synthesis of aromatic aryl amide foldamers and their metal complexation
(reported by the Parquette lab.). (a) NaH, PhCH
2
SH, THF, 72%. (b) Ni(OAc)
2
,CH
2
Cl
2
-MeOH
(9 : 1), (C
2
H
5
)
3
N, 69% (for 7-Cu); (c) Cu(OTf)
2
,CH
2
Cl
2
-MeOH (9 : 1), (C
2
H
5
)
3
N, 87% (for 7-Ni);
(d) (C
2
H
5
)Zn, toluene, (for 8-Zn). Reprinted with permission from Ref. [37]. Copyright 2006
American Chemical Society.
Search WWH ::
Custom Search