Homogeneous/heterogeneous palladium based catalytic system for Heck reaction. The reversible transfer of palladium between solution and support

Abstract

Pd(II)/Al2O3 and Pd(0)/Al2O3, containing 0.6% of palladium were characterized by EDX, SEM and XRD methods and used as catalysts of the Heck coupling of bromobenzene with butyl acrylate at 140 °C in molten [Bu4N]Br salt. Monoarylated (trans-PhCH = CHC(O)OBu) (1) and diarylated (Ph2C = CHC(O)OBu) (2) products were obtained in amounts dependent on kind of base present in the system (Scheme 1). During the reaction palladium was partially leached from the support forming [Bu4N][PdBr4] complex that catalyze Heck reaction or undergo readsorption on Al2O3. These soluble palladium complexes are partially reduced to Pd soluble nanoparticles which can also be anchored on Al2O3 giving active catalyst of Pd(0)/Al2O3 type. Reduction of Pd(II) to Pd(0) during catalytic process at the presence of bases (NaHCO2, NaHCO3, NaOAc, Cs2CO3) was studied by XPS method and the total reduction was observed in reactions of PdCl2(PhCN)2 with NaHCO2 or with NaHCO3 and [Bu4N]Br. It was experimentally proved that heterogenized catalyst, Pd(0)/Al2O3, after oxidative addition of aryl halides, serve as a source of soluble palladium species and colloidal nanoparticles that are active as homogeneous catalysts.