Chemistry Reference
In-Depth Information
1. Ga(acac) 3 , KOH
2.
41 -H 4
K[( S )-Nic] 5 [Ga 2 ( 41 ) 3 ]
Figure 4.19 Enantiomerically pure helicate K[(S)-Nic] 5 [Ga 2 (41) 3 ] [(S)-NicĀ¼ (S)-N-methylni-
cotinium] via diastereoselective self-assembly by ion pair formation (X-ray crystal structure
analysis H atoms, solvent molecules, and potassium ion omitted).
inert cobalt(III). Following this approach or using more inert complexes from the begin-
ning, there are a few reports where helicates could indeed be resolved either by spontane-
ous resolution by crystallization as a conglomerate [43], or by chromatographic means
using enantiomerically pure stationary phases [44].
Much more common, however, are strategies where enantiomerically pure components,
especially ligand strands, are used to assemble a certain diastereomer in a stereoselective
fashion. In these cases the fact that any epimerization of metal centres results in the for-
mation of energetically different diastereomers rather than enantiomers (as in the example
in Section 4.2) prevents a loss of stereochemical integrity since the structural dynamics of
the self-assembly process will always lead back to the thermodynamically most favour-
able diastereomer.
Before we concentrate on the use of enantiomerically pure ligand strands, the only
example for the use of a chiral counterion should be mentioned. This was reported in 2001
by K.N. Raymond's group when they used enantiomerically pure ( S )- N -methylnicotinium
to induce the overall diastereoselective formation of a single optically pure anionic dinu-
clear triple-stranded helicate from an achiral bis(catecholate) ligand (Figure 4.19) [45].
In principle, there are two design strategies for introducing a chiral element into a lig-
and strand: either in the outer periphery, or in between two metal binding sites. In fact,
both strategies have been successfully realized in a number of examples using different
metal binding sites. In the following we will divide the individual approaches by sorting
them according to the metal (chelating) binding motifs.
2,2 0 -Bipyridine Ligands
4.7.1
The first example dates back to the year 1991 and was reported by the group of J.-M.
Lehn. By introducing methyl groups into the ether linkages of his well known
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