Use of "Homeopathic" Ligand-Free Palladium as Catalyst for Aryl-Aryl Coupling Reactions

We have previously shown that the use of ligand‐free palladium employing Pd(OAc)2 as catalyst precursor in the Heck reaction of aryl bromides is possible if low catalyst loadings, typically between 0.01–0.1 mol % are used. We have now tested this phenomenon, which we have dubbed “homeopathic” palladium, in biaryl formation using the Suzuki, the Negishi and the Kumada cross‐coupling reactions. The Suzuki reaction of aryl bromides, both activated and deactivated, is possible using 0.02–0.05 mol % of Pd(OAc)2. In this reaction turnover frequencies up to 30,000 have been reached with activated substrates. Even aryl chlorides could be reacted if strongly electron‐withdrawing substituents were present. The Negishi coupling with a variety of arylzinc halides was possible on aryl bromides containing electron‐withdrawing substituents. The Kumada reaction only gave low yields of products under “homeopathic' conditions.