Photocatalysis is a promising way to efficiently convert solar energy into chemical one. Recently, more and more attention has been paid to the conservation of our planet, among the goals of sustainable development is also the widespread use of accessible and clean energy (e.g. solar energy). The natural emodin is a colourful compound with broad biological activity. The anthraquinone core of emodin also represents an interesting starting point for further modifications with different functional groups. As part of the master's thesis, we studied the use of emodin derivatives as photoredox catalysts for reduction of aryl halides, oxidation of sulfides and hydroxylation of arylboronic acids. We also investigated the influence of solvents and other parameters on reaction systems.
In the first part of the master's thesis, we investigated the arylation of 2-bromobenzonitrile with N-methylpyrrole. Emodin and its derivatives were used as photocatalysts. Unmodified emodin proved to be the most suitable photocatalyst. When replacing the heterocyclic reagent with pyrrole, N-phenylpyrrole, indole, furan, and thiophenone, we found that only pyrrole enabled simple product identification. We also switched to other bromo, chloro and iodo aryls and obtained good conversions with some substrates with electron acceptor substituents. The difficult isolation and purification of the products prevented us from obtaining pure final products.
Second part of the master's thesis was aimed at the synthesis of sulfoxides from sulfides using oxygen as oxidant and the photocatalyst emodin. For quantitative conversion, we needed only 0,1 mol% of photocatalyst in the case of sulfides with electron-donating substituents and 2 mol% of catalyst in the case of sulfides with electron-withdrawing substituents. Emodin derivatives proved to be suitable photocatalysts for this reaction system. The molecular oxygen is a green oxidant, as it does not cause the formation of large amounts of toxic products. We have shown that our oxidation method is selective for sulfoxide formation. Only in the oxidation of dibenzyl sulfide, only small amounts of benzaldehyde and benzoic acid were observed as side products, and in the case of other sulfides, only small amounts of sulfone were observed under certain reaction conditions.
In the last part of the master's thesis, we investigated the possibilities of photocatalytic hydroxylation of arylboronic acids with emodin. We optimized the synthesis of phenol from benzeneboronic acid using 5 mol% of photocatalyst. Molecular oxygen was also used as an oxidant during hydroxylation. The reaction conditions for hydroxylation can be applied to arylboronic acids with electron donating as well as arylboronic acids with electron withdrawing substituents. Better selectivity was observed for arylboronic acids with electron-withdrawing substituents (cyano, 4-(trifluoromethyl)).
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