B. O. Burek
Dissertationsschriften der Leibniz Universität Hannover doi: 10.15488/9994 (german)
The application of enzymes as catalysts in chemical synthesis allows for highly selective reaction steps and therefore offers shorter and more efficient routes with less waste leading to more sustainable processes. In particular, the enzyme class of oxidoreductases (EC 1) catalysing a variety of reactions are very promising for an application in organic synthesis. However, this is often limited by the need of expansive cofactors which have to be (re-)generated. Peroxygenases, like the unspecific peroxygenase from Agrocybe aegerita (AaeUPO), represent a special case as they utilize the cheap and easily producible hydrogen peroxide as a co-substrate to catalyse a variety of C-H-functionalizations. However, these enzymes suffer from their poor robustness against hydrogen peroxide. One strategy to overcome this issue is the in-situ hydrogen peroxide generation which can be realized for example via photocatalytic reduction of molecular oxygen. In the context of this thesis such a photoenzymatic catalysis process has been developed using titanium dioxide as a photocatalyst. Initially, the photocatalytic hydrogen peroxide generation has been examined separately. To identify optimized conditions the reaction parameters light intensity, catalyst amount, oxygen content, pH and temperature were studied for the oxidation of water as well as for the oxidation of propan-2-ol as a sacrificial electron donor. Furthermore, the oxidation kinetics of the alcohols methanol, ethanol, propan-2-ol and 2-Methylpropan-2-ol have been investigated in detail. Methanol turned out to be the most efficient and therefore suitable sacrificial reagent. These findings have been used for the combined system with the AaeUPO to study the enantioselective hydroxylation of ethyl benzene to (R)-1-phenylethanol. The use of methanol proved to be advantageous not only for a higher H2O2 production but also for a higher solubility of the substrate and a lower generation of enzyme damaging hydroxyl radicals. Therefore, the enzyme stability and the turnover numbers could be improved significantly. Finally, the scalability issue of photoreactors has been addressed utilizing inductively powered LEDs, so-called wireless light emitters, which were additionally coated with the photocatalyst. A proof-of-principle for all the investigated reactions has been demonstrated in this thesis.
