Polyethyleneglycol as scaffold and solvent for reusable C-C coupling homogeneous Pd catalysts

An oxime carbapalladacycle was covalently anchored on a soluble polyethyleneglycol (PEG) (averaged MW 6000 Da) scaffold, and the resulting polymer was used as soluble catalyst for the Suzuki, Sonogashira, and Heck reactions. The reactions were conveniently carried out in PEG as solvent. The catalytic system reported here is among the most active, reusable phosphine-free Pd catalysts. Thus, for low demanding C-C couplings, such as the Suzuki and Sonogashira couplings of aryl bromides, the catalytic system could be reused 10 times with complete substrate conversion and without Pd depletion. However, although the PEG-anchored carbapalladacyle was stable on heating in PEG over extended periods, it decomposes during catalysis to form catalytically less active palladium nanoparticles (averaged particle size 6 nm, sigma +/- 2). For this reason, the initial high catalytic activity of pegylated Pd complex, decreases on the first run, completely or partially losing the catalytic activity for highly demanding reactions, such as those of aryl chloride couplings. (c) 2006 Elsevier Inc. All rights reserved.