A Highly Active Water-Soluble Cross-Coupling Catalyst Based on Dendritic Polyglycerol N-Heterocyclic Carbene Palladium Complexes

A new water-soluble polyglycerol derivative functionalized with N-heterocyclic carbene palladium complexes was prepared and applied as catalyst for Suzuki cross-coupling reactions in water. The complex displays a metal loading of around 65 metal centers per dendrimeric molecule, which is estimated to contain 130 chelating groups and thus corresponds approximately to the formation of 2:1 NHC/metal complexes. Monomeric analogues were also synthesized to volidate the reactivity of the dendritic catalyst. Both types of catalysts were tested with various aryl bromides and arylboronic acids. Turnover frequencies of up to 2586 h(-1) at 80 degrees C were observed with the dendritic catalyst along with turnover numbers of up to 59000, which are among the highest turnover numbers reported for polymer-supported catalysts in neat water. The dendritic catalyst could be used (reused) in five consecutive reactions without loss in activity.