Supramolecular transition metal catalysts in two-phase systems

The concept of covalently connecting a catalytically active transition metal center with a water-soluble receptor (host molecule) makes a new type of supramolecular catalysis possible in which the features of molecular recognition, phase transfer catalysis and transition metal catalysis are combined in a single system. The first examples of this principle make use of the commercially available β-cyclodextrin (β-CD) as the receptor and rhodium complexes of diphosphanes as the catalytically active center, these being covalently connected to one another via a spacer. In competitive hydrogenation of certain olefins unusual degrees of substrate selectivity based on the molecular recognition are observed, not possible by conventional transition metal catalysts. The two-phase (H₂O/organic) hydrogenation of nitro-aromatics also is a smooth process with these supramolecular catalysts. They also constitute an unusually active catalyst system for the selective hydroformylation of higher olefins such as 1-octene in a two-phase system.