Oxazaborolidines in Organocatalysis - The Importance of Noncovalent Interactions in the Corey-Bakshi-Shibata Reduction

Abstract

Part 1: Oxazaborolidines in Organocatalysis The major part of this thesis deals with oxazaborolidines (OXB) as chiral organocatalysts for asymmetric Diels-Alder reactions and Corey-Bakshi-Shibata reductions. In order to expand the library of catalytically active OXB, the first two chapters describe the synthesis of new types of chiral OXB, which were then tested in catalysis. In the third chapter we investigated the noncovalent interactions (NCIs) in the CBS reduction and provide a novel explanation for the high selectivity of proline-based CBS catalysts. Part 2: Lewis Acid Enhancement in the House-Meinwald Rearrangement of Epoxides The second part of this thesis deals with titanium Lewis acids, the catalytic activity of which can be enhanced by coordination to thioureas or BINOL ligands. We developed a protocol for the in situ formation of the complexes and verified their structure via NMR spectroscopy. The complexes were employed in the catalytic House-Meinwald rearrangement of epoxides. For complexes consisting of ureas or thioureas, we observed a dependency of the catalytic activity on the pKA value. Therefore, the best results are obtained with Schreiner’s thiourea. Squaramides and thiosquaramides generally deliver lower conversion. The use of chiral BINOL ligands leads to a kinetic resolution of the epoxides. Substitution in the 3,3‘-position of the BINOL ligands determines the quantitative enantioselectivity of the rearrangement, as well as the absolute configuration of the rearrangement products.