Phosphorus-containing hybrid calixphyrins: promising mixed-donor ligands for visible and efficient palladium catalysts.
Phosphorus-sulfur-containing hybrid calixphyrins were prepared by the BF3-promoted dehydrative condensation between sigma4-2,5-bis[(pyrrol-2-yl)methyl]phosphole and 2,5-bis[hydroxy(phenyl)methyl]thiophene. X-ray crystallographic analysis of the Pd-P,N2,S-hybrid calixphyrin complex revealed that the Pd center was coordinated by the four heteroatoms to adopt a distorted square planar geometry. The Pd complex, displaying a characteristic reddish purple color in solution, catalyzed the Heck reaction of bromoarenes with n-butyl acrylate with high efficiency at elevated temperatures.