Silica functionalised with PAMAM dendrimer chains has been investigated as a support for anchoring chiral salen complexes in both an axial and covalent fashion. It was found that using a high dendrimer chain density resulted in very low enantioselectivity in the epoxidation of styrene, 1-methyl-1-cyclohexene and α-methylstyrene. Through a thorough series of tests the poor performance was attributed to both interactions with the surface and with neighbouring dendrimer chains. It was found that the system could be improved by decreasing the dendrimer chain density and pacifying the surface by capping the remaining surface hydroxyl groups on the silica. This resulted in the epoxidation of α-methylstyrene with a considerably improved enantioselectivity. Dendritically functionalised silica and silica coated magnetic nanoparticles were also investigated as supports for the immobilisation of an achiral salen complex. High epoxide selectivity was achieved with α-pinene and 1-methyl-1-cyclohexene, while more moderate selectivity was achieved with cyclohexene and limonene as substrates. The heterogeneous catalysts could generally be used 3 times with no apparent loss in activity or selectivity. Both enantiomers of α-pinene and limonene were used to investigate immobilised chiral salen complexes. Results indicate that the diastereomeric excess produced is independent of the configuration or presence of stereogenic centres in the complex. Instead the stereoselectivity appears to be controlled by the nature of the substrate alone. These results call into question the use of such substrates for the investigation of immobilised chiral salen complexes.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:558611 |
| Date | January 2012 |
| Creators | McCue, Alan J. |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=185907 |
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