Supported palladium catalysts in Heck coupling reactions - Problems, potential and recent advances

The review reports the general approach, problems, potential and recent advances in heterogeneously catalyzed Heck reactions. It demonstrates the advantages and limitations for practical applications at present. A literature overview is given separately for substrates that are easier to activate (aryl iodides, activated aryl bromides) and for non-/ deactivated aryl bromides and aryl chlorides during the period between 2001 and 2005. New approaches and strategies for the activation of aryl bromides and especially chlorides by heterogeneous catalysts are discussed. The argumentations clearly exceed simple separation and reuse arguments. Particular attention is given to the relation between homogeneous and heterogeneous catalysis from the mechanistic point of view. Very recent reports show clear success in the understanding of corresponding reaction mechanisms. Palladium species dissolved from the support are proven to be responsible for high activity and selectivity in Heck reactions by supported catalysts (Pd on activated carbon, oxides, polymers and in zeolites). The careful choice of optimum catalyst and reaction conditions are crucial and allowed the development of simple heterogeneous catalysts that activate even deactivated aryl chlorides with high yields within few hours of reaction time.