Supramolecular stereoisomers

The highly selective processes of molecular recognition are, of course, of stereochemical nature. Thus may be defined a supramolecular stereochemistry that extends from supermolecules to polymolecular assemblies. Different spatial dispositions of the components of a supermolecule with respect to each other lead to supramolecular stereoisomers. Their eventual interconversion will depend on the properties of the interactions that hold them together, i.e. on the variation of the intermolecular interaction energy with distances and angles. There is thus an intermolecular conformational analysis like there is an intramolecular one. 

For each case in Figure 6.14, we have stereoisomers—structures with the same connectivities but differing arrangements of the atoms in space. They are not enantiomers, so they must be diastereomers. The novelty lies in the fact that these stereoisomers interconvert by a translation or reorientation of one component relative to the other. In some ways these structures resemble conformers or atropisomers, which involve stereoisomers that interconvert by rotation about a bond. For the supramolecular stereoisomers, however, interconversion involves rotation or translation of an entire molecular unit, rather than rotation around a bond. Note that for none of the situations of Figure 6.14 do we have topological stereoisomers. In each case we can interconvert stereoisomers without breaking and reforming bonds. 

When six coordination sites around octahedral metal ion are occupied by only bidentate ligands, stereoisomers around the metal ion are formed. However, the coordination of symmetric tridentate ligands to a six-coordinate metal ion leads to only one isomer. Furthermore, tridentate bridging ligands connect metals in a linear fashion, resulting in the formation of stereochemically well-defined supramolecular systems. The rigid structure of these systems is suitable for studies of electron or energy transfer events between the donor-acceptor dyads. 

In addition to the metallic species described above, TFA can initiate the polymerization of isocyanides.In the polymerization of stereoisomers of 103, a high stereospecificity was observed. Poly-LD-103 generated using TFA maintains polymerization activity and forms a homoblock copolymer on addition of ld-103, while copolymerization does not proceed in the presence of even 1% of dl-103. Also, poly-LD-103 species polymerized ll-103 but not dd-103. Thus, the TFA-initiated polymerizations are highly stereospecific. Because copolymerization of ld-103 with dl-103 and that of ld-103/ll-103 and LD-103/DD-103 readily take place with a Ni(II) catalyst, the stereospecificity observed with TFA may arise from a highly organized H-bonding supramolecular interaction. 

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