Pd Nanoparticles on Solid Supports as Recyclable Catalysts for the Suzuki-Miyaura Cross-Coupling Reaction

Abstract

Suzuki-Miyaura cross-coupling reaction, palladium-catalyzed C-C bond coupling reaction using aryl halide and organoboranes, provides effective synthetic methods in wide variety of fields. Despite excellent activity and selectivity of homogeneous catalysts, drawbacks of costs, recyclability, and use of environmentally unfriendly substances disturbed the use of palladium catalysts in industries. The idea of immobilizing metal onto solid support as heterogeneous catalysts had drawn attention to resolve the drawbacks of homogeneous catalysts. By immobilizing metal onto insoluble solid supports, catalysts could easily be separated from reaction solution. However, due to less reactive site of heterogeneous catalysts, their low reactivity was shown compared to that of homogeneous catalysts. Modification of solid supports, development of nanosized metal catalysts, and change of solvent system has been studied to improve the reactivity of heterogeneous catalysts. In this research, two types of solid supports were synthesized and applied to Suzuki-Miyaura cross-coupling reaction. Both types of solid supports introduced alkyl chains to enhance hydrophobicity of solid supports. Introduction of alkyl chains improved reactivity of heterogeneous catalysts by occupying hydrophobic organic substances into catalysts. Among palladium immobilized TA solid supports, Pd(0)@TDTAT-TA showed the highest turnover frequency. In a similar manner, Pd(0)@TDTAT-TAm showed the highest turnover frequency over other palladium immobilized TAm solid supports. The change of building block from triazine to terephthalamide increased specific surface area and pore size of synthesized solid supports as bond angle was changed from 120o to 180o. Comparing two types of heterogeneous catalysts, increase of specific surface area and pore volume enhanced reactivity of Suzuki-Miyaura cross-coupling reaction. In addition, both the synthesized catalysts showed excellent recyclability without considering leaching of palladium metal.