Preparation of Palladium Colloids in Block Copolymer Micelles and Their Use for the Catalysis of the Heck Reaction

Abstract

Colloidal dispersions of nanometer sized palladium colloids with very high stability were prepared in block copolymer micelles of polystyrene-b-poly-4-vinylpyridine and analyzed by electron microscopy and X-ray analysis. The resulting polymer/metal hybrids can easily be dissolved and handled in standard organic solvents such as toluene, tetrahydrofuran, and cyclohexane. They were successfully used for the Pd-catalyzed carbon−carbon coupling of aryl halides with alkenes (Heck reaction). Such block copolymer stabilized palladium colloids exhibit about the same reactivity as low molecular weight Pd complexes classically used for the Heck reaction, but show a much higher stability: in most reactions, the hybrids remain catalytically active even after 50000 turn-over cycles. Reaction rates were significantly controlled by the reactivity of the educts, but also respond to micelle architecture and dispersity of the palladium. Other advantages of the block copolymer stabilizer are that they are more simple and readily accessible than the phosphor-containing chelates, and that they dissolve even in “simple” solvents such as toluene (instead of amidic solvents).