| dc.description.abstract | Gold complexes and salts have proven to be exceptional catalysts for the activation of carbon-carbon multiple bonds towards nucleophilic attack. Gold catalysts allow for the synthesis of complex products under mild conditions. The most common oxidation state for gold catalysts is Au(I), whereas Au(III), despite being catalytically active, has not received as much attention. This is especially evident in the field of asymmetric gold catalysis, where only a couple of notable examples of chiral Au(III) complexes affording high enantioselectivities have been reported. The goal of this Thesis was to synthesise new, chiral Au(III) complexes with the aim of utilising them as catalysts in asymmetric reactions. Other topics within gold catalysis were also touched upon.
The synthesis of spiroketals by dihydroalkoxylation reactions of alkynyl diols was found to be effectively catalysed by chiral Au(III) complexes with bisoxazoline and pyridine-oxazoline ligands. Despite the chiral nature of the catalysts, negligible enantioinduction was observed. Tunable conditions were developed for the Au(III) catalysed reaction between propargyl alcohols and aryl nucleophiles. By appropriate variation of the reaction conditions allenes, indenes, haloindenes as well as tetraaryl-allyl products could be obtained, all in one-pot reactions.
Chiral, alcohol functionalised NHC Au(I) and Au(III) complexes were synthesised and found to be effective catalysts in the alkoxycyclisation reaction of 1,6-enynes. The NHC Au(I) complexes were most efficiently synthesised in a two-phase system of water and dichloromethane. This led to the subsequent development of an efficient flow synthesis of the most commonly used NHC Au(I) catalysts, affording the complexes in high yields and short reaction times. A second flow step could be added, yielding NHC Au(III) complexes. A synthesis of NHC-oxazoline Au(III) complexes was developed, however the resulting complexes proved to be unstable. Nitrogen NMR was a vital tool in confirming that the target complex was synthesised, by showing a large coordination shift for the pendant oxazoline group.
Finally, the interaction between halogen bond donors and gold complexes was studied in depth. 13C, 31P and diffusion NMR experiments were used to assess the strength of the interaction taking place. An NMR titration experiment confirmed that a halogen bond was indeed being formed. The strongest halogen bond donors led to the most efficient activation of the gold catalysts. Bidentate, bisimidazolium halogen bond donors were the most effective activators. This activation method allows for the recycling of both the gold catalyst and the halogen bond donor. DFT calculations allowed us to propose a plausible mechanism to this activation method.
Overall, this Thesis contributes increased knowledge to various topics within gold catalysis but is also expected to have an impact on the wider field of transition metal catalysis and the study of halogen bonds. | en_US |
