Palladium leaching dependent on reaction parameters in Suzuki-Miyaura coupling reactions catalyzed by palladium supported on alumina under mild reaction conditions

Suzuki couplings of aryl bromides as well as activated aryl chlorides can be efficiently performed by palladium supported on alumina under mild reaction conditions (T=65 degrees C). The catalyst prepared by controlled precipitation of palladium hydroxide shows a high activity and represents a perfect and robust alternative to other complex and expensive preparation procedures. Palladium leaching in the Suzuki couplings by supported palladium catalysts has been investigated in detail for the first time for such mild reaction conditions. Catalytic activity is due to palladium species dissolved from the solid particles. The active palladium species in solution are re-deposited onto the support with the completion of the reaction. Catalytic activity correlates with the amount of palladium dissolved. Palladium leaching is a prerequisite for highly efficient catalysis in particular for more demanding substrates. Palladium leaching depends on a variety of parameters (temperature, solvent, base, substrates, and additives). Transmission electron microscopic investigations show that - depending on the parameters - the efficient palladium re-deposition process can leave the catalyst with an unchanged dispersion and particle size distribution after the reaction. (C) 2010 Elsevier Inc. All rights reserved.