Asymmetric catalysis using titanium and palladium
thesisposted on 03.12.2012 by Simon J. Sesay
In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.
This thesis describes, in detail, the synthesis of novel heterobidentate ligands. These ligands were subsequently used in palladium catalysed allylic substitution reactions to synthesise enantiomerically enriched alkylated products. The thesis also describes novel approaches to asymmetric catalysis, in particular asymmetric epoxidation derived from Katsuki-Sharpless methodology. Chapter 1 - This chapter reviews the literature, discussing the significant synthetic advancements in asymmetric catalysis in the past 10-15 years. Chapter 2 - This chapter describes in detail the synthesis of new heterobidentate ligands containing nitrogen and phosphorus ligating atoms. These ligands are based on imines containing enantiomerically pure asymmetric centres in an alpha position to the nitrogen moiety. Other ligands that were synthesised were derived from C2- symmetric diamines, also containing an asymmetric centre alpha position to the nitrogen, that produce ligands with the nitrogen functionality contained in a ring. Chapter 3 - This chapter describes the use of the novel ligands synthesised in Chapter 2 in palladium catalysed allylic substitution reactions. The racemic substrate, 1 ,3-diphenyl-3-acetoxy-1-propene, was alkylated to produce an enantiomeric enriched alkylated product. The alkylated product was obtained with up to 77 % enantiomeric excess. The reaction was conducted with a palladium catalyst in the presence of a novel ligand using dimethyl malonate as a nucleophile. The development and optimisation of these ligands within this reaction is discussed. Chapter 4 - This chapter discusses some novel approaches to asymmetric epoxidation. The epoxidation is based on methodology developed by Katsuki and Sharpless. This epoxidation relies on the substrate containing an up-unsaturated alcohol. The chapter discusses the use of a reversible nucleophile in the form of cyanide. The nucleophile is designed to react with a substrate to provide an upunsaturated cyanohydrin, suitable to undergo a Katsuki-Sharpless epoxidation. Once the asymmetric epoxidation is complete, the nucleophile would be removed. This chapter describes the attempts to develop the principle further. An improvement to the system would be to provide an environment capable of sustaining a dynamic kinetic resolution. Chapter 5 - This chapter contains the experimental which provides the exact details of the reactions reported in the thesis.