Asymmetric Morita-Baylis-Hillmann reaction : catalyst development and mechanistic insights based on mass spectrometric back reaction screening

The aim of the research project described in chapter 2 of this thesis was to develop an efficient bifunctional phosphine catalyst that outperforms literature-known catalysts in the Morita-Baylis-Hillman reaction of methyl acrylate with aldehydes. For the evaluation of the chiral organocatalysts a mass spectrometric back reaction screening protocol of quasi-enantiomeric substrates was applied. Based on this technique, a multi-catalyst screening was developed which allowed the simultaneous determination of the intrinsic enantioselectivities of phosphines in a crude catalyst mixture. Finally, based on the data from the back reaction screening in hand together with kinetic measurements, the rate- and enantioselectivity-determining step in the catalytic cycle were identified.
In chapter 3, the synthesis of new morpholine- and piperidine-based triazolium salts and their use as NHC catalysts for the asymmetric cross-benzoin reaction is discussed. In particular, the cross-benzoin reaction between benzaldehyde and hydrocinnamaldehyde was studied. The aim of this project was to design a catalyst that could produce the desired cross-benzoin product with high enantiomeric excess and chemoselectivity.
Chapter 4 deals with the development of chiral NHC-phosphine ligands for the asymmetric iridium-catalyzed hydrogenation of different model substrates. This project was inspired by the previous work of Nanchen, a former member of the Pfaltz group. He synthesized a small library of different NHC-phosphine ligands for the hydrogenation of various substrates, however only moderate results in terms of reactivity and enantioselectivity were achieved. The aim of this project was to investigate NHC-based catalysts with conformationally more rigid structures.