M.Sc. / This study initially focused on the synthesis of a set of triaryl phosphine ligands, encompassing a broad range of electron withdrawing functionalities on the ortho-position of one of the aryl rings. These varying moieties were readily incorporated into diphenylphosphino benzaldehyde as starting material through both Wittig and Knoevenagel chemistry. The ligands produced were tested in the Pd-catalysed Suzuki reaction. The electronic as well as the steric nature of the alkene ligands largely dictated the activities observed: the more electron poor or the bulkier the ligand, the higher the activity observed in the Suzuki reaction. This is in contrast to much work in the literature stating that highly active Suzuki catalysts require a very electron rich system. At the same time, the literature indicates that co-ordinatively unsaturated Pd-complexes are also active catalysts. The activities observed were ascribed to the ability of these electron poor bulky phosphine alkene ligands to stabilise or promote the reductive elimination step of the Suzuki mechanism in preference to the oxidative addition step, which is the typical rate determining step. The study then investigated carbonylation reactions, specifically the methoxycarbonylation and hydrocarboxylation reactions, which are typically Brønsted acid co-catalysed. The alternative was the first time use of metal-triflate based Lewis acids as co-catalysts in these types of reactions. Thus, a systematic study was performed. It was found that metal trifluoromethane sulfonate (hereafter referred to as triflate) based Lewis acid co-catalysts outperformed the typical Brønsted acid co-catalysts by between one and a half to two and a half times on the rate of the methoxycarbonylation reaction, depending on the substrate used. The system was tested with Pd loadings in the region 2–0.03 mol%. A competing heat-induced styrene polymerisation reaction ultimately affected the results at such low Pd loadings. A low level kinetic analysis was performed indicating zero order kinetics on the alkene concentration of the reaction, with a fractional order dependence on the Lewis acid concentration. There was little to no effect on the linear/branched ratio of the product in response to the use of the Lewis acid. The nature of the metal within the metal triflate based co-catalyst also seemed to be critical to the reaction, with the 4+ based Zr and Hf ultimately providing the highest obtainable turn over frequencies, the 1+ and 2+ based metals providing no conversion, and reactivity in the presence of the 3+ metals depended on the specifics of the reaction. In addition to a great deal of work being performed on styrene and ethylene as substrates, reactions using phenylacetylene were also optimised. Here, it was found that the bidentate BINAP ligand and the literature preferred ligand, PyPPh2 afforded good catalyst activity. Both of these ligands offered much faster catalyst systems than PPh3 and various other bidentate ligands tested.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:2250 |
| Date | 03 May 2012 |
| Creators | Shaw, Megan Lorraine |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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