Asymmetric Syntheses by Use of Intercalation of Multi-nuclear Chiral Metal Complexes into a Clay

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The purpose of the present project is to develop of a new catalytic system for asymmetric syntheses by use of clay minerals. Our approach is to synthesize an intercalation compound of a clay with optically active metal complexes. The chiral metal complexes are expected to form a chiral reaction space within the clay layrs. If the metal complexes are capable to catalyze some reactions (e.g.oxidation, reduction, hydrogenation etc), they will achieve asymmetric syntheses with high efficiency.As a result, we have established the following reaction systems involving a clay. Two kinds of metal complexes were coadsorbed by a colloidally dispersed clay ; [Fe (terpy) ] ^<3+> terpy=2,2', 2''- terpyridyl) and [Ni (phen) _3]^<2+> (phen=1,10-phenanthroline). The Fe complex is capable of oxidizing an organic substance such as ascorbic acid or cystein, but it has no optical activity. The Ni complex has no redox properties but it is chiral due to the helical nature of three phenanthroline ligands. It was expected that the oxidation by the Fe complex proceeded under the chiral circumstances of the Ni complex when these two kinds of complexes were on the same clay surface. This was realized by comparing the oxidation rates of D- or L-cystein in the presence of a colloidally dispersed clay absorbing [Fe (terpy) ] ^<3+> and DELTA- [Ni (phen) _3] ^<2+>. L-Cystein was found to be oxidized at the higher rate than D-cystein, showing the stereoselectivity in the reaction.The structures of the present clay catalysis have been investigated with various spectroscopic methods : the measurements of induced CD,the electric dichroism spectra and the observations with atomic force microscopy.