Catalytic specificity of linear polystyrene-stabilized Pd nanoparticles during Ullmann coupling reaction in water and the associated mechanism

In the present work, heterogeneous Pd nanoparticle catalysts supported on linear polystyrene (PS-PdNPs) were used to promote Ullmann coupling reactions in water. The coupling reaction of 4-bromotoluene proceeded readily in the presence of methanol as the external reductant to give 4,4'-dimethylbiphenyl in 85% yield. In contrast, only a trace of the coupling product was obtained when using ethanol or 2-propanol. Higher yields were also observed for aryl bromides than for aryl iodides. The difference in the yields obtained from 4-iodotoluene and 4-bromotoluene is attributed to variations in the rate of reduction to reform Pd(0) species. The leaching of Pd species into the reaction medium has a negative effect on the Ullmann coupling reaction. The unique reactivity of PS-PdNPs and mechanistic pathway are discussed herein.