Photoactive EDA complexes in visible light driven aerobic oxidations
Abstract: Organic synthesis driven by photochemical activation has been proven very promising in a range of scenarios within academia and industry. The emergence of photoredox catalysis has fueled an ever-growing interest in the use of visible light as a convenient energy source for a variety of radical reactions otherwise difficult to achieve. However, the use of homogeneous catalysts, based on transition metals or complicated organic dyes as the light harvesting moiety, does not come without drawbacks. Problems include the price and availability of ruthenium and iridium, and the separation and re-use of the catalysts from the products. Photochemical transformations that use simpler systems, such as catalyst-free versions or small organic molecules as photocatalysts, are therefore attractive to develop. For certain systems, one potential solution to these problems is the use of electron donor-acceptor (EDA) complexes. These complexes can form between a molecule of high electron affinity (acceptor) and a molecule of low ionization potential (donor). A key feature of the EDA complex is that it can be excited with light of a lower energy than that needed to excite the reactants on their own. The excitation of the complex can result in the formation of reactive radical species that can be harvested for further chemical reactions. EDA complexes can, with other words, be used as a convenient light-absorbing moiety to drive chemical reactions without the need of an external photocatalyst. Oxidation reactions are a universal part of organic chemistry. The oxidants commonly used are however associated with certain drawbacks such as troublesome waste production. Oxygen obtained from the air around us is a potentially ideal alternative. The use of aerobic conditions in the combination with photoactive EDA complexes poses an interesting and underdeveloped part of photochemical transformations. In this thesis, an EDA complex approach to the synthesis of N-heterocycles has been investigated. The developed methods use alkylated anilines as donors and activated alkenes as acceptors to form different EDA complexes. Visible light is used to activate the complexes, and aerobic oxygen is used as the terminal oxidant to furnish the target compounds. In the first part of the thesis, new EDA complexes between dialkyl anilines and 1,2-dibenzoyl ethylenes are identified. The results show that upon excitation of the EDA complexes, 3,4-disubstituted tetrahydroquinolines can be formed in excellent diastereoselectivity and in high yields. In the second part, the identified EDA complexes are used as part of a catalytic system for the generation of α-amino alkyl radicals under aerobic conditions and under visible light irradiation. Lastly, in the third part, new EDA complexes between arylated amino acids and maleimides are identified. The photoactivation of these complexes was shown to be an efficient way of generating secondary α-amino alkyl radicals to furnish N-H-tetrahydroquinolines.
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