Studies On Novel Heterogeneous Catalysts For Suzuki Cross-Coupling Reactions

The Suzuki-Miyaura cross-coupling reaction, which is unique, requiring mild reaction conditions and showing a high degree of tolerance for a variety of functional groups, has become unarguably one of the most powerful synthetic methods for preparing biaryl compounds, such as natural products, pharmaceuticals, and polymers, etc.. Because of the great significance of this reaction, it was recognized by awarding the2010Nobel Prize in Chemistry. Most applications of the reaction involve homogeneous catalysts consisting of palladium complexes with a variety of ligands. These catalytic systems generally exhibit better activity and selectivity than heterogeneous systems, however, they suffer from the practical problems such as catalyst separation and recycling. To solve these problems and make the process more green, researchers have immobilized palladium complexes on various supports such as carbon, magnetic materials, silica, hydroxyapatite, zeolites, MOFs and organic polymers to create heterogeneous catalysts. Consequently, achieving a combination of the advantages of both the homogeneous and the heterogeneous processes is always the focus in catalysis.Accordingly, the main task of this PhD thesis is to develop some new heterogeneous palladium complex catalysts for Suzuki cross-coupling reaction under mild and environmental friendly conditions. Characterization of these heterogeneous catalysts was realized by scanning electron microscopy, transmission electron microscopy, X-Ray diffraction patterns, nitrogen adsorption/desorption isotherms and solid-state NMR techniques.Chapter I introduced the research background on Suzuki cross-coupling reaction and highlighted the research progress on heterogeneous catalysis systems.In Chapter II, a novel PdC12(py)2(py=pyridine) encaged in the interior space of silicalite-1hollow spheres, was successfully prepared through a "ship in bottle" method. This microcapsular reactor afforded fast conversions for the Suzuki-Miyaura cross-coupling reactions of various aryl halides and arylboronic acids (yield83-99%) even at catalyst loadings of0.0188mol%in aqueous media. The turnover frequency (TOF) could be up to63210h-1under mild conditions in air. Investigations on the reusability and stability of the heterogeneous palladium catalyst were also discussed herein.Chapter III described the synthesis of an inexpensive, stable, highly active, and recoverable palladium-phosphine heterogeneous catalyst with high surface area microporous knitting aryl network polymers via one-step’knitting’of rigid aromatic compounds blocks with an external crosslinker, i.e., knitting PPh3with benzene. This catalyst enabled fast Suzuki-Miyaura coupling reaction in an aqueous ethanol solution under mild conditions (80℃). Biaryl products were obtained consistently in excellent yields using various aryl chlorides and benzoboric acid. Moreover, this chapter demonstrated that the microporous polymer backbone knitting with phosphine ligand could efficiently disperse palladium to promote the catalytic activity, which was much higher than that of homogeneous catalysts under the similar conditions.In Chapter IV, the palladium-phosphine heterogeneous catalyst developed in Chapter III was used in the Suzuki cross-coupling reactions of benzyl halide and arylboric acid to get diarylmethane derivatives. This catalyst enabled fast convert within5min in ethanol and diarylmethane derivatives products were obtained consistently in excellent yields. Moreover, since ethanol served as hydrogen donor, which promoted the reaction to the direction of dehalogenation, so new reaction conditions was explored for4-nitrobenzyl chloride with phenylboronic acid and finally4-nitrobenzyl chloride afforded fast convert in toluene within30min (yield99%).In Chapter V, a new conjugated organic microporous polymers was synthesized through Scholl coupling reactions and two kinds of recoverable heterogeneous metal catalysts were prepared according different ligands:triphenylphosphine for palladium catalyzed Suzuki cross-coupling reactions and bipyridyl for copper catalyzed alcohol oxidation reactions. These catalysts afforded fast converts in the reactions with various substrates under mild conditions. In addition, the effect of conjugated structure of the support in the catalysis process was emphasized.