It Did Open Our Eyes. for the Past 40 Years, a New Field in Catalysis Came to Light: The
A lot of research is performed in the field of catalysis. More and more catalysts are developed and they are all better than any catalyst reported earlier, if we may believe the publishers. The field of homogeneous catalysis is responsible the majority of published work in chemistry magazines, while heterogeneous catalysis seem to be less popular. However, the facts are clear: industry uses far more heterogeneous catalysts than homogeneous ones. This should open our eyes.
It did open our eyes. For the past 40 years, a new field in catalysis came to light: the heterogenisation of homogeneous catalysis. This field was born because industry demands recyclable catalysts. It is important to be able to retrieve the catalyst from the reaction mixture, because catalysts often contain transition metals, which are poisonous and polluting and mostly very expensive. All of this answers to the trend of “green” chemistry.
In theory, heterogenized homogeneous catalysts are the ideal catalysts, because they possess the qualities of both fields, while circumventing the negative aspects. In practice, this turns out to be more complicated.
N-Heterocyclic Carbenes (NHCs) are a type of stable carbenes that are found to be good ligands for transition-metal catalysts. This is because of their high tunability and their electron-donating character, which stabilizes and activates the metal centre during catalysis. In order to heterogenize these NHCs, a linker in the backbone was searched and found in the form of histidine, a natural amino acid that possesses an imidazole functionality. Imidazoles are precursors for NHCs.
In this thesis, the road to a heterogenizationable histidine-derived NHC-palladium catalysts is described, including catalysis and a crystal structure.
Starting from histidine, a Pd(II) and a Pd(0) catalyst have been synthesized as analogues of well-established homogeneous NHC catalysts. These Pd(II) and Pd(0) catalysts are synthesized in two types of catalysts: one variant with a Boc-protecting group attached to the amine functionality in the amino-acid tail and one variant with a free amine. The Boc-group acts as a mimic for a solid support, so the effect of the solid support in catalysis can be studied. Three of these variants were successfully synthesized. The unprotected Pd(II) catalysts proved to undergo side-reactions.
All three complexes that were obtained pure were tested in catalysis for the Z-selective transfer semi-hydrogenation of 1-phenyl-1-propyne. The Boc-Pd(II) catalysts proved to be the fastest and most selective one. The Boc-Pd(0) catalysts was much slower than the normal NHC one, published by this group in 2010. The Pd(0) catalyst containing the free amine was faster than its Boc-protected analogue, but much less selective due to the non-innocent amine moiety.
No heterogenization experiments have been performed.
Having accomplished this, future research may be devoted to the actual heterogenization process. Also, investigation towards recyclability and activity of the immobilized catalysts need to be performed.