Palladium‐Nanoparticle‐Catalysed Ullmann Reactions in Ionic Liquids with Aldehydes as the Reductants: Scope and Mechanism

An efficient Ullmann‐type reductive homocoupling of aryl, vinyl and heteroaryl halides can be promoted by an aldehyde in tetraalkylammonium ionic liquids under very mild reaction conditions. This simple procedure generates symmetrical biaryls under relatively mild conditions. The ionic liquid is crucial for this process because it behaves simultaneously as a base, ligand and reaction medium. The role of the aldehyde is also discussed and a general mechanism for this unusual reaction is proposed. These results open the way to a new efficient method of Pd‐catalysed dehydrogenation of carbonyl compounds. An unusual reduction: A new method for synthesising symmetrical biaryls has been developed by making use of Pd colloids as the catalyst, an aldehyde as the reducing agent and an ionic liquid as the reaction medium (see scheme). Aryl bromides and iodides are activated without the use of special additives or ligands, under relatively mild conditions (reaction temperatures ranging from 40 to 90 °C).