The Design and Application of Self-assembled Cooperative Catalysts
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This thesis describes the application of self-assembly and cooperativity in the design of efficient catalytic systems. In particular, we are looking at chemical reactions which require two or more catalytic units working cooperatively to achieve productive catalysis. In such systems, it is advantages to bring the catalytic units together into close proximity, and much research has been performed to achieve this using direct covalent linking of catalysts or immobilisation onto solid surfaces. In this thesis, we aim to use the concept of self-assembly to bring catalytic units together, creating more efficient catalytic systems. In particular, we designed a number of amphiphilic catalysts based on the amino acids L-leucine and L-proline and investigated their catalytic activity in the formation of tetrahydroxanthenones. This reaction involves a reaction between benzaldehyde and cyclohexenone, where each substrate is activated by the two amino acid-based catalysts. While we were able to achieve a rate acceleration with our amphiphilic catalyst system, the amount of acceleration was not sufficiently greater than the control conditions to effectively demonstrate our concept. We also determined the enantiomeric excess of our catalysed reaction to be 30%, which was not high enough to be synthetic useful. In the second part of this thesis, we investigated a cooperative bimetallic catalyst featuring amphiphiles terminating in 18-crown-6 ethers which bound Ba2+ ions. These complexes have been found to act cooperatively in the ethanolysis of esters and anilides. We investigated our amphiphilic system with a number of substrates, including esters, anilides, phosphate esters and activated esters and found that they were effective for the cleavage of the ester pnitrobenzoate. We were able to correlate the onset of catalysis with the onset of structure formation, which were determined by UV and fluorescence spectroscopy, respectively. Further work is still needed to fully demonstrate that cooperative effects are the cause of the rate accelerations observed. The next steps are to visualise and characterise the supramolecular structures formed by our amphiphiles. In this thesis, we demonstrate that it is possible to create a self-assembled system capable of cooperative catalysis. In the future, we want to use this study as a foundation for the design of cooperative catalytic systems which have synthetic utility. This can decrease the amount of catalyst required for a given chemical reaction and lead to benefits in terms of the cost and sustainability of a chemical process. The dynamic nature of the systems described in this thesis also allows the incorporation of stimuli-responsive units which can lead to the production of smart and intelligent materials.