M.Sc. / The main objective of the research described in this dissertation was to prepare a range of triaryl phosphine ligands having systematically changing stereo-electronic properties using Grignard reagents and P-Cl derivatives. These ligands were utilised in the Rh-catalysed hydroformylation reaction in order to determine the effects of stereo-electronic properties of these ligands on the outcome of the reaction. Initially, a first series of tri-aryl phosphine ligands having methyl or ethyl groups substituted on the ortho or para position of the phenyl rings was successfully synthesised. The stereoelectronic properties of these ligands were probed by determining the P=Se coupling constants of the Se-phosphine derivatives and the CO vibrational frequencies of the Vaskatype complexes of these ligands. It was shown that the substitution of alkyl groups on the phenyl rings of the phosphine ligands resulted in the formation of more electron-rich ligands. In the Rh-catalysed hydroformylation reaction a systematic decrease in the yields of aldehydes were observed as the basicity of the phosphine ligand increased. Inspired by the outcome of the first series, a second series of tri-aryl phosphine ligands having both methyl and fluoro groups variously substituted (ortho, meta and para positions) on the phenyl rings was successfully synthesised. It was observed that the group substituted on the ortho position electronically dominated over the group substituted in the meta and para position of the phenyl rings. Substitution of fluorine atoms on the ortho position and methyl on the para position of the phenyl rings resulted in the formation of less electron-rich phosphine ligands, while the opposite substitution pattern (p-F-o-Me) resulted in the formation of more electron-rich ligands. The former caused an increase in the yields of aldehydes obtained in the Rh-catalysed hydroformylation reaction compared to triphenylphosphine. viii A third series of tri-aryl phosphine ligands having trifluoromethyl groups substituted on the ortho, meta or para positions of the phenyl rings was successfully synthesised. It was observed that substitution of the trifluoromethyl groups on the meta and para positions of the phenyl rings resulted in the formation of less electron-rich phosphine ligands, while substitution on the ortho position resulted in the formation of more electron-rich ligands. The hydroformylation reaction with these ligands followed the same trend as with the first and second series of tri-aryl phosphine ligands. A series of tri-aryl phosphine-borane adducts of the first series of phosphine ligands was also successfully synthesised using BH3·THF. These ligands were employed in the Rh-catalysed hydroformylation reaction with the aim to determine whether in situ deprotection of the phosphine-borane adducts with CO was possible. The hydroformylation results with these adducts provided yields of aldehydes similar to those of the free phosphines. This suggested that deprotection had occurred to release the free phosphine which complexed to the Rh catalyst. This indicated that phosphine-borane complexes can be directly applied to carbonylation reactions eliminating the use of common deprotection methods and provides an opening for more research in this regard.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:8359 |
| Date | 07 May 2009 |
| Creators | Kotze, Philippus Daniel Riekert |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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