Dynamic control of topological asymmetry:
Discussion and Summary

The X-ray structures of the complexes clearly show the predicted conformation; i.e., in all cases the substituent was oriented "anti" and the overall twist of the complex was determined by the absolute configuration od the chiral aplha-carbon. Only one conformer (anti) was observed in solid phase, for complexes derived from either racemic (e.g. [Zn((R,S)-a-MeTPA)Cl]ClO₄, [Zn((R,S)-a-PhTPA)Cl]ClO₄, or [Zn((R,S)-a-MeBQPA)Cl] ClO₄) or enantiomerically pure ligands (e.g. [Cu(R-a-MeBQPA)(CH₃CN)](ClO₄)₂). In general, the geometries predicted by computational methods correlates well with the solid phase results

While the solid phase data is encouraging, direct evidence of a similar behavior in solution is desired. The CD spectra are consistent with the presence of a propeller twist in solution. Two things are important to note from these spectra:

1. Only metal ions with trigonal bipyramidal geometry induce the asymmetry (i.e. Zn(II) and Cu(II) complexes), while the chiral ligand alone shows little or no signal. The Cd(II) complex, which is known to prefer octahedral geometry, also does not present the propeller asymmetry.
2. The Cotton effect for the chiral complex shows the expected sign as predicted by the exciton coupling [1] theory. As can be seen in the figure below (Figure 7), in the case in which the propeller is in a lambda conformation, the chromophores (pyridine or quinoline rings) are in a (+) relative orientation, which should give a positive Cotton effect as can be seen in the CD spectra of [Zn(R-a-MeTPA)Cl]ClO₄ or [Cu(R-a-MeTPA)Cl]ClO₄.

Figure 7: Expected Cotton effect based on the exciton coupling theory

The modeling of these dynamically controlled asymmetric complexes was successful, as judged by the physical studies of their complexes. Studies of possible application of these complexes in asymmetric reactions are underway.

We are grateful to the National Institutes of Health (GM 49170) for support of this research, and Dr. Nina Berova for fruitful discussions on the interpretation of the CD spectra.

1. Nakanishi, K.; Berova, N. In Circular Dichroism. Principles and Applications; Nakanishi,K., Berova, N. and Woody, R.W., Eds. VCH Publishers, Inc.; New York, 1994.

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