Abstract
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.