Exploring Isotellurourea Catalysts: From Theory to Synthesis and Application

Since their development in 2006, asymmetric catalysis using isothiourea catalysts is an important topic of organic synthesis. Constant research over the past decades resulted in the development of various structures, including the well–known Okamoto and HyperBTM catalysts. The variation of the chalcogen, being part of the structure and important for the attack on the substrate, paved the way for even broader investigations.
It was proven in the group of Andy Smith, that a descend in the group of chalcogens and thereby by exchanging sulphur for selenium, catalytic activity and selectivity can be increased for various application reactions. By using oxygen however, catalytic activity is impaired. The explanation was among others found in the formation of chalcogen bonds between the substrate and the catalyst and increased nucleophilicity. Therefore, a tellurium derivative was synthesized in order to investigate, if known trends remain.
Within the course of this thesis, the focus was set on exactly this problem setting. In order to achieve more insight into the catalyst performance, NMR studies, application reactions and DFT calculations were conducted. Further, also the synthesis of the tellurium Okamoto and HyperBTM catalysts was set in focus, and improvements were made. Thereby, problems regarding the reproducibility of an intermediate cyclisation step as well as the purification of a ditelluride species could be solved.
The results of the NMR studies present, that the high nucleophilicity of the tellurium catalyst allows for excellent performance, when addition to substrates is regarded. However, it could also be shown, that a descend in their leaving group property in comparison to the other chalcogen analogues is most probable. This explains the drop in performance for the conducted acylation reactions as well as the Michael addition reaction to an a,b-unsaturated acyl ammonium acceptor. Further proof of this observation was achieved during DFT investigations, whereas the elimination of the catalyst became the rate determining step for the tellurium species.