Asymmetric Catalysis: Ligand Design and Conformational Studies

This thesis deals with the design of ligands for efficient asymmetric catalysis and studies of the conformation of the ligands in the catalytically active complexes. All ligands developed contain chiral oxazoline heterocycles. The conformations of hydroxyand methoxy-substituted pyridinooxazolines and bis(oxazolines) during Pd-catalysed allylic alkylations were investigated using crystallography, 2D-NMR techniques and DFT calculations. A stabilising OH-Pd interaction was discovered which might explain the difference in reactivity between the hydroxyand methoxy-containing ligands. The conformational change in the ligands due to this interaction may explain the different selectivities observed in the catalytic reaction. Polymer-bound pyridinooxazolines and bis(oxazolines) were synthesised and employed in Pd-catalysed allylic alkylations with results similar to those of monomeric analogues; enantioselectivities up to 95% were obtained. One polymer-bound ligand could be re-used several times after removal of Pd(0). The polymer-bound bis(oxazoline) was also used in Zn-catalysed Diels-Alder reactions, but the heterogenised catalyst gave lower selectivities than a monomeric analogue. A series of chiral dendron-containing pyridinooxazolines and bis(oxazolines) were synthesised and evaluated in Pd-catalysed allylic alkylations. The dendrons did not seem to have any influence on the selectivity and little influence on the yield when introduced in the pyridinooxazoline ligands. In the bis(oxazoline) series lower generation dendrimers had a postive on the selectivity, but the selectivity and the activity decreased with increasing generation. Crown ether-containing ligands were investigated in palladium-catalysed alkylations. No evidence of a possible interaction between the metal in the crown ether and the nucleophile was discovered. A new type of catalyst, an oxazoline-containing palladacycle was found to be very active in oxidations of secondary alcohols to the corresponding aldehydes or ketones. The reactions were performed with air as the re-oxidant. Therefore, this is an enviromentally friendly oxidation method.