NMR Spectroscopic Investigations of Catalyzed Reactions. Mechanisms, Kinetics & Unexpected Intermediates

This thesis describes mechanistic studies by NMR spectroscopy in different areas of homogenous catalysis. Over the last years a wide range of organocatalysts were developed enabling highly diverse transformations with high enantioselectivities and excellent yields. In contrast to the high amount of publications on this field, a mechanistic understanding based on experimental evidences is often underinvestigated. Therefore the first two chapters of this thesis are focused on various (NMR-) mechanistic studies in the fields of enamine catalysis and Brønsted and Lewis acid catalysis. Besides the synthesis towards stable transition state analogues of proline catalyzed aldol reactions in order to detect the elusive and highly debated hydrogen bond interaction between the carboxylic acid moiety of the enamine and the electrophile in the enantiodetermining step, the influence of acid and base additives on the enamine formation in the α-benzylation of α-branched aldehydes was studied. Furthermore the course of the asymmetric Torgov cyclisation and the disulfonimide (DSI) catalyzed synthesis of β3-amino esters from N-Boc amino sulfones were investigated by kinetic NMR-measurements and the underlying mechanistic models were confirmed by the experimental results. Additionally, the silylation behavior of DSIs was studied by different NMR experiments. The last part of this dissertation investigates the mechanism of the catalytic trans-hydrogenation of internal alkynes with [RuCp*]-complexes by parahydrogen induced polarization (PHIP). During the studies different unexpected carbene species were observed and characterized. The role of the carbene as intermediate for several, mostly unwanted, side reactions was determined by OPSY-EXSY-experiments and extensive DFT calculations