Application of laccase enzymes in organic synthesis

Abstract

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. Johannesburg, 2018.The use of enzymes as catalysts in various synthetic procedures appears to be an economical and profound way of providing selective processes in synthetic organic chemistry. Enzymes provide alternative and sustainable processes and have helped to avoid limitations encountered when using traditional heterogeneous and homogeneous catalysts; this includes the use of toxic substances, use of expensive heavy metals, extensive use of harmful organic solvents, harsh reaction conditions, and also poor selectivity of many catalysts. Laccases are oxidoreductase enzymes capable of catalysing oxidation reactions of several low molecular weight organic compounds such as polyphenols, aminophenols, methoxyphenols, polyamines, and lignin-related molecules. The catalytic process of these enzymes occurs though a one-electron oxidation and water is released as the only by-product. In this project we investigated the range and limitations of applications of laccase enzymes in organic synthesis. The project focus was on method development for cross-coupling reactions of Carbon, Nitrogen, Oxygen, and Sulphur substituted aromatic compounds. The laccase facilitated synthesis of five classes of compounds; biaryl compounds, benzoxazoles, benzimidazoles, benzothiazoles, and aminobenzoquinones, was investigated. The research explored the synthesis of biaryl compounds from simple substituted phenol substrates. The optimal reaction conditions for the synthesis of biaryl compounds from simple phenols were investigated. A condensation reaction between 2-aminophenol and aryl aldehyde derivatives was performed with the aim of synthesising 2-arylbenzoxazole derivatives; however various aminophenol derivatives were formed as the phenolic Schiff base failed to cyclise. Alternatively, when including the laccase-mediator ABTS, dimerization of 2-aminophenol to 2-amino-3H-phenoxazin-3-one (4) occurred. A chemo-selective method for the synthesis of 2-aryl-1H-benzimidazoles from condensation of 2-phenelynediamine and aryl aldehydes was developed using laccase as an oxidising catalyst. Optimal conditions for synthesising 2-aryl-1H-benzimidazoles were identified while using acetate buffer (0.1 M, pH 4.5), acetonitrile as a co-solvent and the commercial laccase preparation Novoprime base 268. A modern and practical laccase-catalysed route suitable for the synthesis of 2-arylbenzothiazoles was developed. To the best of our knowledge, the laccase catalysed method for preparation of 2-arylbenzothiazole derivatives derived from condensation–dehydration reaction of 2-aminothiophenol with aryl-aldehydes has not been reported before. The method described is green, effective and simply requires a facile work-up routine, utilising solvents such as acetonitrile and DMF as co-solvents. Finally, factors limiting yields for the synthesis of aminobenzoquinones were investigated by varying the reaction conditions. The laccase catalysed nuclear diamination of aromatic hydrobenzoquinones with aliphatic and aromatic amine molecules was investigated under mild reaction conditions using commercial laccases from Novozymes (Suberase®, Denilite® II Base, and Novoprime Base 268). Conducting the reactions under dilute conditions, sequential addition of enzyme and substrate over time and using Novoprime Base 268 as our laccase increased the yields to up to 100%.LG201