The olefination of aryl halides (Heck reaction), one of the most important CC-coupling reactions in organic synthesis, is mostly catalysed by palladium complexes in homogeneous solution. Important advantages of this reaction are the broad availability of arylbromides and chlorides and the tolerance of the reaction for a wide variety of functional groups. During the last years, the development of new highly active Pd complexes allowed the activation and conversion even of aryl chlorides to considerable extent. However, homogeneous catalysts suffer from the need of ligands (usually phosphine ligands) which are eventually not easy to handle (e.g. air sensitive basic phosphines) and from a difficult removal of the catalyst from the product. This includes the ligand as well as the precious metal.

There have been several attempts to overcome these obstacles, e.g. by the use of Pd/C. However, according to the prior state of the art, Pd/C catalysts did not reach the excellent activities that are observed with homogeneous catalysts. In addition, supported Pd catalysts often gave rise to unwanted dehalogenation of the haloaromatics, converted only in some instances interesting arylbromides, and often suffered from substantial Pd leaching.

By screening of reaction conditions and the optimisation of the Pd/C catalyst we managed to develop a Pd/C catalyst (E 105 CA/W 5% Pd), which exhibits the highest activity of heterogeneous catalysts reported up to now for the Heck reaction of aryl bromides with olefins. In addition, we took a very close look into the Pd leaching issue which made us understand what influences Pd leaching. These findings make it possible to tune the Pd concentration in solution after the reaction down to 0.05 ppm Pd and opens new insights into mechanism of Pd/C catalysed Heck reactions.