 Reductive coupling is a coupling reaction of aryl halides with a stoichiometric equivalent of copper metal that occurs in the Ullmann reaction.In an example of a present-day cross coupling reaction called decarboxylative coupling, a catalytic amount of Cu(I) displaces a carboxyl group forming the arylcopper (ArCu) intermediate.Simultaneously, a palladium catalyst converts an aryl bromide to the organopalladium intermediate (Ar'PdBr), and on transmetallation the biaryl is formed from ArPdAr'.