Development of new biomimetic non-heme iron epoxidation catalysts and new pyridine-derived light absorbing systems

Language
en
Document Type
Doctoral Thesis
Issue Date
2017-07-03
Issue Year
2017
Authors
Fingerhut, Anja
Editor
Abstract

The thesis consists of three independent research topics:

Chapter 1: Non-heme Iron Catalyzed Epoxidation of Olefins

Chiral terminal epoxides are valuable subunits in bioactive compounds and versatile building blocks in the synthesis of fine chemicals and pharmaceuticals. Within the framework of this work a new biomimetic enantioselective non-heme iron catalyzed epoxidation system for terminal olefins was developed and investigated. The epoxidation of 2 vinylnaphthalene was chosen as model reaction applying in situ generated complexes as catalysts originated from a ligand and FeCl3 x 6 H2O. A screening of different imidazole-based peptide-like ligands for the in situ generated chiral iron(III) catalysts took the imidazole substitution pattern, coordinating motif and further important functionalities into account. Among all synthesized ligands the L-tert-leucine-derived ligand providing a [NN]-binding motif, a 1,2 substituted imidazole and beneficial tert-butyl groups was identified as the most promising one. The catalyst system was tested for different terminal and non-terminal substrates. Moreover, the catalyst was applied in aziridination reaction wherein it was found to be inactive. However, the new catalytic epoxidation system could be combined in a one-pot process with a following aminolysis reaction towards pharmaceutically relevant 2 aminoalcohols. Besides, a SiO2 catalyzed Meinwald rearrangement of the obtained terminal epoxides was observed and suggested as promising method for an easy access to acetaldehyde derivatives.

Chapter 2: Synthesis of Pyridine-derived Ligands for Light Absorbing Metal Complexes

Since pyridine-based compounds were found to be versatile structures for sensitizer in DSSC, DS-PEC or NLO, a variety of different 2,2’-bipyridines were designed during the last decades, providing different linkers and different end-capping moieties. In the scope of this chapter three new 4,4’ π conjugated 2,2’-bipyridine-based compounds containing a dimethoxyphenyl linker were synthesized by conventional approach starting from 4,4’ dimethyl 2,2’-bipyridine. These two ligands with electron-accepting end-capping moieties contain either a cyanoacrylic acid or a cyanoacrylic acid ester unit. This ligands were investigated in regard to their adsorption behavior on TiO2, photophysical and electrochemical properties. Further coordination experiments forming ruthenium(II) complexes demonstrated this to be a challenging task. The third ligand, which was end-capped with an electron-donating propenyl thiophene unit, was made synthetically accessible, even though the ligand was not obtained in fully pure form because it was suspected to undergo light induced E/Z isomerism in solution. A new one-pot procedure towards pyridinyl quinoline-derived ligands was attempted to be developed, but was found to be infeasible under the chosen conditions.

Chapter 3: Investigations of 1,4-Dihydropyridine-based Systems in Photocatalysis

Since 1,4-dihydropyridines such as the Hantzsch ester or BNAH were known to be efficient NAD(P)H mimics, these compounds found a variety of applications as reductive equivalents in enantioselective and non-enantioselective catalysis. Although 1,4-dihydropyridines are important compounds, they are cost-intensive as well when they have to be used in equimolar amounts. Thus, there is a high demand for the development of an efficient in situ recycling system. With regard to a sustainable recycling system the utilization of solar power might be a promising option. Thus, the photocatalytic regeneration of the Hantzsch ester was investigated by screening of different proton sources, electron donors and reaction media in combination with eosin Y as a photosensitizer and CoIII(dmgH)2pyCl as mediating transition metal catalyst. However, within the framework of this study no photocatalytic recycling of the Hantzsch ester was enabled. The different solubilization properties of all compounds were identified as the biggest challenge to overcome. Moreover, a two-phase reaction system which combines photoinduced transfer hydrogenation of an imine with recycling of catalytic amounts of a N-methylated Hantzsch ester by reduction with sodium dithionite was investigated. However, just when an equimolar amount of the synthesized N-methylated pyridinium salt was applied, conversion from the imine towards the amine could be observed. While searching for an explanation for this behaviour, a breakdown of the N methylated Hantzsch ester recycling system was observed in the aqueous phase in the presence of light.

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