Palladium Based-Polysaccharide Hydrogels as Catalysts in the Suzuki Cross-Coupling Reaction

A simple PdCl₂(TPPTS)₂ complex was successfully heterogenized in polysaccharide hydrogel beads by using two different methods: (1) dropping the complex and polysaccharide solution into aqueous or ethanol solution of 0.5 M calcium chloride, and (2) dropping the complex and polysaccharide solution into an aqueous solution of chitosan (C). FTIR analyses of the different beads revealed that the complex was heterogenized via interactions between the sulfonate group on the TPPTS and the hydroxyl groups on the polysaccharides. Additionally, the effects of preparation procedure and polysaccharide type on catalytic performance of the formed heterogeneous catalysts in Suzuki cross-coupling was also tested. It was found that the palladium-based hydrogels, which were prepared by the interactions of two polysaccharides, iota carageenan (I) and C, were more active and stable than those that precipitated in the CaCl₂ solution. In addition, the I-C-PdCl₂(TPPTS)₂ heterogeneous catalyst could be easily recycled with minor loss of activity. Moreover, the conversion rates of all heterogeneous systems employed with iodobenzene and phenylboronic acid were higher than those of their homogeneous analogues, a finding that is probably due to the presence of water, which accelerates the reaction, in the hydrogels. At last, SEM–EDS analysis of the hydrogels showed that the complex was immobilized in the hydrogels and that the I-C-PdCl₂(TPPTS)₂ beads were less condense than the beads that were produced with I-PdCl₂(TPPTS)₂ in CaCl₂ solution, which can explain the higher performance of the former.