Design and Characterization of Heterogeneous Dirhodium Catalysts – Application in Asymmetric Cyclopropanation Reaction

Due to their high performance in both activity and selectivity, dirhodium catalysts have been widely employed in syntheses of important bioactive substances or pharmaceuticals, and have been recognized as one of the indispensable organometallic catalysts. However, rhodium is a kind of rare and precious metal element on earth, and thus catalysts based on this element are very expensive. Furthermore, metal contamination of pharmaceuticals in homogeneous reactions is feasible owing to the unsatisfactory homogeneous catalyst recovery and recycling. Accordingly, the development of heterogeneous dirhodium catalysts, which can be easily separated from a reaction mixture and used multiple times, is considered to be an effective way to overcome these intrinsic problems of homogeneous catalysts. This thesis presents two different approaches to prepare heterogeneous chiral dirhodium catalysts. One approach is the immobilization of dirhodium catalysts based on self-assembled coordination polymers via ligand exchange. The second approach is to immobilize dirhodium catalysts on mesoporous silica material SBA-15 via Cu(I)-catalyzed azide-alkyne click reaction. 1. A facile approach is reported for the preparation of dirhodium coordination polymers via ligand exchange between dirhodium trifluoroacetate Rh2(TFA)4 and differently sized chiral dicarboxylic acids derived from L-phenylalanine or L-tert-leucine. Multiple techniques including FT-IR, XPS, and solid-state NMR spectroscopy reveal the formation of the coordination polymers via ligand exchange. Although the quantitative 19F CP MAS NMR spectra demonstrate incomplete ligand exchange in the resulting coordination polymers, these dirhodium polymers show excellent catalytic activity in the asymmetric cyclopropanation reaction of diazooxindole and aryl alkenes. SEM images indicate that the dirhodium polymers possess a lamellar structure. XPS data demonstrate that the state of rhodium in the dirhodium nodes is maintained during the synthesis of the polymers. In particular, the enantioselectivity of the dirhodium coordination polymers, which were synthesized from aromatic dicarboxylic acids derived from L-tert-leucine, showed significant improvement compared to the dirhodium polymers carrying dicarboxylic acids derived from L-phenylalanine. Importantly, the dirhodium polymers can be easily recycled and reused five times without significant decrease in their catalytic performance. 2. In the second approach a novel heterogeneous chiral dirhodium catalyst, namely Rh2(S-PTTL)3(S-PTTL-linker)~SBA-15, has been prepared via click reaction of azide-functionalized SBA-15 with the dirhodium complex Rh2(S-PTTL)3(S-PTTL-alkyne) containing an alkyne moiety. During the preparation of this catalyst, an analogous chiral ligand system containing an alkyne moiety (S-PTTL-alkyne) is employed to exchange one chiral ligand of the parent Rh2(S-PTTL)4 catalyst, which could largely preserve the intrinsic catalytic performance of the catalyst. The resulting heterogeneous dirhodium catalyst is characterized by FT-IR and 13C solid-state NMR to validate the successful immobilization. The catalytic performance of the heterogeneous catalyst Rh2(S-PTTL)3(S-PTTL-linker)~SBA-15 is explored in the asymmetric cyclopropanation of 3-diazooxindole with different aryl alkenes that form spiro-cyclopropyloxindoles which serve as precursors for pharmaceuticals. The novel catalyst shows excellent catalytic activity with high levels of enantioselectivity. Meanwhile, it can be readily recovered and reused at least four times without significant loss of its catalytic performance