Beiträge zur Addition von Stickstoff- und Kohlenstoffnucleophilen an C-C-Doppelbindungen (2006)

The thesis at hand deals with the topic of palladium catalysts for the addition of nucelophiles to conjugated double bonds. The objective was the development of new ligands for the hydroamination of dienes and the creation of catalysts for homologous compounds. According to a model of the mechanism, a chemical lead was formulated and a small library of ligands was synthesized following a divergent synthetic strategy. With the addition of aniline to cyclohexadiene as a model reaction, different ligands were tested for their performance in catalysis. In order to optimise the reaction, the complex, the ligand and the metal/ligand ratio were varied. As an example, tertiary phosphines with carboxylic acid functions were tested as ligands for the palladium-catalysed hydroamination. One objective was to achieve a high activity without having to use an acid co-catalyst which would have a negative side-effect on the enantioselectivity of chiral ligands. The objective was to test whether the integration of a carboxylic group into the ligand backbone would show as much activity as if a co-catalyst was used. At first, omega-di-phenyl-phosphine-carboxylic acids and phosphine-substituted benzoic acids were used as ligands. 3-di-phenyl-phosphine benzoic acid proved to be suitable for this reaction when a ratio of two ligands per palladium atom was used. When [(eta5 Cp)Pd(eta3-allyl)] was used as a catalyst precursor, an average activity of the reaction at 70 h-1 could be obtained. This is approximately the same activity as of the known catalyst [(eta3-allyl)PdCl]2/PPh3, but it is still below the activity that can be obtained by using [Pd(PPh3)4] in combination with tri-fluoric-methane sulphone acid. In order to test the effect of different parameters such as the concentration of the reactant and the catalyst, the reaction was monitored by in situ IR. This was done by using a new fibre optical IR probe that allowed the tests to be carried out in the argon glove box. Thus, the initial activity of the catalyst under the given reaction parameters and the reaction mechanism could be determined. With [(eta3 allyl)PdCl]2, an active catalyst for the hydroamination could be obtained without the use of an external co-catalyst. This led to the assumption that it might be worth testing whether phosphines with carboxylic acid amide groups are suitable to carry out enantioselective reactions, whereby the substituent of the amide group would carry the chiral information of the ligand. A better enantioselectivity could partly be obtained with the carboxylic acid amide ligand than with the known ligand. Because strong donor ligands are able to the intermediate palladium hydride species in the catalyst cycle, N-hetero-cyclic carbenes were also tested as possible ligands for the addition of nucleophiles to dienes. In cooperation with the research group of Prof. Dr. Peter Wasserscheid, functionalised imidazolium compounds were synthesised. These not only proved as precursors for N-hetero-cyclic carbene ligands with a hemilabile side chain, but also as an easily obtainable ionic liquid because its melting point lies below room temperature. Using this new class of ionic liquids, transition-metal complexes with N hetero-cyclic carbene ligands were tested. The resulting complexes , however, did not show any catalytic activity in the model reaction.