|Title:||Asymmetric homogeneous hydrogenations catalyzed by transition metal complexes containing chiral diphosphine ligands in a protein cavity|
Transition metal catalysts
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Applied Biology and Chemical Technology|
|Pages:||xi, 158 leaves : ill. ; 30 cm|
|Abstract:||Catalytic enantioselective reactions have received much attention during the past decade and have played a crucial role in modern synthetic chemistry. Transition-metal catalysts containing various chiral ligands have been extensively studied for this purpose. In addition to the use of chiral metal complexes, one strategy used to design selective catalysts is to incorporate nonspecific achiral catalytic groups into chiral cavities. The general method is to modify a known protein or enzyme at a defined site with a cofactor or new functional group to create a semisynthetic system with novel properties. The significant advantage offered by this strategy is the obviation of the arduous synthesis of the chiral ligands. However, most of these systems are usually limited to those substrates which specifically bind to the native enzymes. Only few systems developed recently based on protein cavity can accommodate a variety of substrates. Since transition metal complexes catalyze a large variety of chemical reactions, the development of new catalysts by introducing a chiral catalytic functionality into the tertiary restricted environment of a protein cavity is of great interest. For the convenient test of this concept, we decided to use a carrier which binds strongly to specific sites of the target protein to introduce the catalyst to the protein cavity in a manner like a "guided missile". If the combination is properly optimized, it may possess not only better compatibility with different substrates but also a multiplicity of various catalytic reactions. Moreover, the protein-based catalysts may offer the possibility of easier catalyst recovery and reuse. In this study, we examined the effect of the interaction of a chiral catalyst with a protein cavity by converting three kinds of diphosphine ligands including Pyrphos, PPM and 5-amino-BINAP to their corresponding biotinylated derivatives and attaching the biotinylated diphosphine Rhodium(I) complexes to the specific binding site of avidin. The catalytic hydrogenations of various olefinic substrates with this new type of catalysts were investigated. The details about the preparation of the rhodium(I) complexes with several biotinylated diphosphine ligands attached to the binding site in avidin, and their applications in aqueous asymmetric hydrogenations have been depicted in this thesis. By introducing the chiral diphosphine-rhodium(I) moiety into the constrained environment of the protein cavity it was found that the enantioselectivity of the system was mainly dominated by the tertiary conformation with the avidin cavity rather than by the coordinated chiral ligands. Besides, the reaction conditions such as temperature, hydrogen pressure, and the pH of the buffered solution were observed to have a significant influence on the resulting enantioselectivity. In summary, the binding of the biotinylated chiral metallic complexes to avidin and the subsequent influence of the avidin environment on the catalyst systems are established. All the results and discussion on this subject in this study gave positive albeit preliminary indications for the potential of the combination of chiral transition-metal complexes and protein cavities.|
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