Abstracts Chemistry

Highly enantioselective epoxidation with hydrogen peroxide and biologically inspired iron and manganese catalysts

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Asymmetric epoxidation is one of the most important reactions in synthetic organic chemistry, because chiral epoxides act as versatile electrophiles which can be converted in a number of interesting and useful chiral products. It’s known that current methodologies for obtaining excellent enantioselectivities need to be improved in terms of reaction times, use of not expensive or toxic metals, and avoidance of chlorinated solvents and oxidants that produce undesired wastes. Natural systems, such as iron enzymes are capable of perform this chemistry with high selectivity and efficiency under very mild conditions. Inspired by oxidations taking place at oxygenases, the combination of iron- and manganese-based catalysts and hydrogen peroxide is an attractive approach for developing oxidation methods because of availability, low cost and low toxicity considerations. One of the main objectives of this thesis is the development of iron and manganese complexes that mimics the structure and functions of these natural enzymes and the finding of new methodologies for enantioselective epoxidations using environmentally not aggressive conditions. One of the strategies developed in this thesis consists in investigating iron and manganese chiral coordination complexes with different electronic and steric properties on the ligand. Furthermore, it was envisioned that the study of these compounds could give useful information about the oxidation mechanism operating in oxygenase enzymes and may help to provide a basis for a rational design for future catalyst developments. The results obtained in this thesis show that the electronic properties of the ligand play an important role in order to get excellent enantioselectivities and different carboxylic acid additives can extent the substrate scope of the system. Finally, three new iron and manganese complexes are described as an efficient and stereoselective catalysts capable of performing highly enantioselective epoxidation of different kind of olefins, such as, cis-aromatic, trans-aromatic and also challenging substrates such as steroids, cyclic aliphatic enones and terminals olefins.