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(
S
,
S
)-
42
Figure 4.20 Enantiomerically pure ligand (S,S)-42 that gave rise to optically pure helicate
{Cu
3
[(S,S)-42]
2
}
3þ
via diastereoselective self-assembly.
oligo(bipyridine) he was able to prepare an enantiomerically pure ligand (
S,S
)-
42
(Figure 4.20) which gave rise to enantiomerically pure double-stranded helicates upon
coordination to silver(I) or copper(I) ions, as revealed by NMR and CD spectroscopy
although the absolute configuration of the metal centres could not be assigned unambigu-
ously based on these experimental data [46].
A few years later J.S. Siegel's group came up with systems where two ligand strands
were actually covalently coupled to C
2
-symmetric building blocks. In this way these cen-
tral building blocks (
R
)-
43
,(
S
)-
44
,or(
R
)-
45
acted as templates to ensure diastereoselec-
tive formation of the helicate structures {e.g. (
P
)-[Cu
2
{(
R
)-
46
}]
2þ
} by translating their
stereogenic information into the helix as proven by NMR and CD spectroscopic means
(Figure 4.21) [47].
O
O
OH HO
O
O
(
R
)-
43
N
N
N
N
Cu
+
N
N
2 Cu
+
N
N
O
O
O
O
HOOC
COOH
(
S
)-
44
N
N
N
N
Cu
+
N
N
N
N
HOOC
COOH
(
P
)-[Cu
2
(
R
)-
46
]
2+
(
R
)-
45
(
R
)-
46
Figure 4.21 Enantiomerically pure building blocks (R)-43, (S)-44, and (R)-45 that were used
to translate their stereogenic information into optically pure helicate structures like, for exam-
ple, (P)-[Cu
2
(R)-46]
2þ
via diastereoselective self-assembly.
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