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Ligand synthesis and Pd-catalysed C-C bond-forming reactions

  • Authors: Hughes, Tanya
  • Date: 2012-08-17
  • Subjects: Ligands , Catalysis , Catalysts - Synthesis , Phosphine
  • Type: Thesis
  • Identifier: uj:2667 , http://hdl.handle.net/10210/6110
  • Description: Ph.D. , The focus of this study was placed on developing stable recyclable catalyst systems that are both active and selective in Pd-catalysed reactions such as the Suzuki-Miyaura cross-coupling reaction, and the methoxycarbonylation reaction. The aim was therefore to prepare ligands with improved efficiencies as catalyst precursors, which are stable towards oxidation and/or hydrolysis, that are soluble in aqueous media and that can be recycled a number of times without significant loss in activity. This study initially focused on the preparation of functionalised phosphine ligands to be loaded onto the peripheries of different generations of PAMAM and PPI dendrimers. These functionalised ligands were then to be used in the Pd catalysed Suzuki-Miyaura cross-coupling of phenylboronic acid and aryl halides, as well as the Pd catalysed carbonylation of phenylacetylene. It was thought that loading of these ligands onto the dendrimers would result in a soluble support for subsequent catalyst systems to allow catalyst recycling. This would then potentially bridge the gap between heterogeneous and homogeneous catalysis. There has been a lot of progress made over the last decade or so in the preparation and use of dendritic phosphines and the positive effects seen are summarised in the first chapter. Water-soluble and thus environmentally friendly ligands have also gained widespread interest as these ligands offer economical and environmental advantages for many reactions. One of the aims of this study was therefore to prepare ligands that show greater solubility in aqueous solvents that can be successfully recycled and that show retention of activity upon reuse. Initially covalent loading of the prepared phosphine ligands onto the peripheries of the dendrimers were attempted. However based on all the problems experienced with the covalent bonding, as discussed in Chapter 2 of this document, a simple acid base titration allowed loading of the prepared phosphine ligands onto the peripheries of the dendrimers via electrostatic interactions, to afford a stable phosphine ligand-dendrimer salt. Electrostatic interactions contribute much more to the solubility enhancement of poorly soluble compounds, thus the formation of the non-covalent phosphine-dendrimer salts was expected to result in greater potential for catalysis in biphasic systems as well as catalyst recycling. Only low generation dendrimers were utilised as previous reports indicated that an increase of the dendrimer generation leads to observation of negative dendritic effects due to the increased bulk that inhibits the approach of the catalytic metal centre.
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Knoevenagel and Heck catalytic studies with Metal Organic Frameworks (MOFs)

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