Design of heterogeneous metal catalysts for organic synthesis

Abstract

The chemical industry is the world’s largest industry and produces a variety of chemical products - from fuels and plastics to life-supporting pharmaceuticals. In this industry, catalysts play an indisputable role as it is estimated that up to 90% of all processes today utilize catalysts to a certain extent. The most frequently used type is heterogeneous catalysts where their inherent properties provide cheaper and more sustainable setups. Yet, classical heterogeneous catalysts are still not that common in the synthesis of fine-chemicals as pharmaceuticals due to their lack of selectivity commonly connected with this type of catalysis. Thus, designing and synthesizing novel heterogeneous catalysts with high selectivity are very attractive in organic chemistry. This dissertation includes five different projects aiming to improve heterogeneous catalysis by designing, synthesizing, and/or optimizing (novel) catalytic systems and evaluating them in organic synthesis relevant for the industry. A short introduction has been made for each topic in order to present the industrial relevance and some of the potential challenges. For all projects, the heterogeneous nature of the catalysts has been assessed and possible limitations have been investigated and presented. The first project focused on the synthesis of cobalt/nickel alloy nanoparticles encapsulated in nitrogen-doped carbon. The material was made by using ZIF-67 as a sacrificial precursor that after impregnation with nickel and carbonization created the alloyed Co/Ni nanoparticles. The material was tested as a catalyst in the hydrosilylation of ketones where the catalyst was easily reused due to its magnetic properties. However, the alloyed nanoparticles did agglomerate during the recycling, which decreased the catalytic activity. Nevertheless, the alloy material was able to hydrosilylate a variety of ketones with moderate success. In the second project, the α-alkylation of ketones with alcohols using a commercially available palladium on carbon as heterogeneous catalyst was investigated. The simple and easy accessible palladium catalyst showed excellent performance compared to other heterogeneous palladium systems as the reaction could be carried out under mild reaction conditions and with high substrate tolerance. The reaction pathway was elucidated by in-situ IR spectroscopy and stoichiometric tests where the results matched the hydrogen borrowing mechanism seen for many homogeneous catalytic systems. In the third project, PP-based POPs were synthesized, which acted as heterogeneous ligands that were capable of hydrogenating CO2 to formic acid (FA) in the presence of iridium. The process is highly attractive as it can enable the use of H2 as a renewable energy source. A POP incorporated with bpy ligands showed the most promising results yielding a TON value above 20,000. The origin of the catalytic activity was evaluated by a filtration experiment, using homogeneous iridium complexes or nanoparticles of iridium as the active catalyst, and other relevant control experiments. The tests indicated that the incorporation of the bpy ligand into the POP enhanced the catalytic performance beyond making recovery and recycling easier. The fourth project described POP-based catalytic systems as well. However, the application was very different as the POPs were polystyrene (PS)-based providing swelling properties to the materials. The POPs’ ability to swell were exploited to mimic homogeneous catalytic systems in the challenging asymmetric C(sp3)-H functionalization of 3-arylpropanamides using an auxiliary methodology. The enantioselectivity was achieved by incorporating chiral phosphoramidite into the POP, which in the presence of palladium showed to have good catalytic performance providing up to 70% yield and 86% enantiomeric excess (ee). Even though no metal leaching was observed, the system showed diminishing catalytic performance during reuse due to the formation of metal nanoparticles and ligand oxidation. In the fifth project, the borylation of arenes using a POP-based system was examined. The borylation reaction represents a strong and widely used tool to gain access to key building blocks in organic synthesis. The catalytic system was made in-situ by mixing [Ir(cod)Cl]2 with a bpy-based PS-POP using the arene as solvent. The system was capable of borylating two times for each B2pin2 making the transformation highly atom efficient. Various arenes were tested and showed high influence on steric effects in term of both activity and regioselectivity. The reaction pathway was examined using competition studies, isotope labeling, and other relevant test. The results from the different arenes and mechanistic tests matched strongly with an Ir(III)/Ir(V) catalytic cycle.