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Title: Design and application of nanostructured flavin photocatalysts
Author: Crocker, Leander
ISNI:       0000 0004 9359 9146
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2020
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Flavin compounds are found in Nature within a range of flavin-containing enzymes (flavoenzymes) that are responsible for metabolic, antioxidant and photoreception processes across animal, plant and bacteria kingdoms. Their broad redox and photochemistry has seen the field of flavin-based catalysis emerge as a powerful addition to sustainable catalysis thanks to being cheap, non-toxic and highly active. Flavin photocatalysis in particular has shown great promise in a range of synthetic procedures such as benzylic oxidations, sulfoxidations, decarboxylative transformations, isomerisations and [2+2] cycloadditions to name a few. It has been shown that flavin photocatalysts can be tuned to a specific application either through chemical structure modification or by the design of advanced systems through heterogeneous or polymer carrier attachment. The latter has shown encouraging results to widen flavin photocatalyst applicability within organic synthesis but has lacked in displaying characteristics like flavoenzymes which enable highly efficient and selective catalysis of industrially relevant products with precise stereochemical control. This thesis details the design and application of novel flavin photocatalysts through the combination of polydopamine (PDA). It is shown that PDA not only acts as a carrier of the flavin, but actively engages in the catalytic mechanism and improves flavin photostability. In the first instance, copolymer flavin-polydopamine (FLPDA) nanoparticles were synthesised and their photocatalytic activity was characterised through model oxidation and reduction reactions. This study revealed enzyme-like kinetics of the catalysed reactions and improved photostability of the conjugated flavin moieties. Additionally, the biocompatibility of the nanoparticles was assessed through in vitro cell studies to ensure applicability to other fields such biomedicine or water remediation. Subsequently, the flavoenzyme specific oxidation of indole to indigo and indirubin dyes was explored using the FLPDA photocatalyst. The results showed that the nanoparticle system exhibited higher production of the valuable dyes over a homogeneous flavin photocatalyst. This increase in activity was investigated through reactive oxygen species (ROS) scavenging experiments which revealed that FLPDA’s mechanism of action in this reaction partially resembled natural flavoenzymes and therefore enhanced key product formation. Finally, reduction reactions catalysed by flavoenzymes were investigated using FLPDA and a chiral flavin-polydopamine system (RCPDA). The light-driven reduction of azobenzene dyes was first explored using FLPDA which provided evidence that hydride could be transferred from PDA-conjugated flavin moieties to a substrate upon irradiation in the presence of an electron donor reagent. Next, the reduction of C=C bonds within α,β-unsaturated ketones and aldehydes was explored by initially screening buffered electron donor reagents and homogeneous flavin photocatalysts with key substrates. The optimal conditions and compatible substrates were then applied to the nanoparticle RCPDA system which produced the saturated products in comparable yields to the homogeneous photocatalysts with very low catalyst loading (<1 mol% vs. 10 mol%). The inclusion of a chiral linkage between PDA and flavin and its effect on the stereochemical outcome of the reaction was also explored, with preliminary data showing that adopting such a strategy could enable some enantioselectivity over the product. In summary, this thesis provides new methodologies to design advanced flavin photocatalysts with enzyme-like characteristics that will help to further develop the field of sustainable catalysis.
Supervisor: Fruk, Ljiljana Sponsor: EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: photocatalysis ; nanoparticles