Photocatalysis with flavins

The thesis presents the use of flavin photo catalysis in organic synthesis. A short review of flavin chemistry emphasizing the redox chemistry and applications to organic synthesis is given in chapter 1. The results of mechanistic studies of the flavin mediated photo oxidation of 4-methoxybenzyl alcohol to 4-methoxybenzaldehyde are presented in chapter 2: The initial process of the oxidation reaction is an electron transfer from 4-methoxybenzyl alcohol to flavin in its triplet state. The quantum yield of the reaction decreased with increasing concentration of alcohol. It is reasoned that at higher concentrations the flavin singlet excited state is quenched, leading to an unproductive electron transfer from alcohol to flavin and back electron transfer. Diffusion is the limiting factor at low alcohol concentrations and the quantum yield of the oxidation reaction is dropping again. The quantum yields were determined using a newly developed actinometer, which was developed in collaboration with Prof. Dr. Eberhard Riedle and Uwe Megerle from the Ludwig-Maximilian-Universität, Munich. The application of flavins in photo catalysis is presented in chapter 3. It was shown that benzyl amines can be oxidized to the corresponding aldehydes in aqueous solution. This oxidation reaction was applied to the deprotection of benzyl protected primary amines and alcohols. Carboxybenzoyl protected amines were inert under these conditions. In chapter 4 the flavin-mediated photo oxidation has been extended to benzylic carbon atoms in hydrocarbons, alkenes, carboxylates. The electron density of the arene moiety was crucial for the rate of oxidation: electron poor and very electron rich arenes were not converted to the corresponding aldehydes, whereas 4-methoxy toluene could be oxidized in 58% to 4-methoxy benzaldehyde. The oxidative cleavage of stilbenes and cinnamic acids to benzaldehydes and the oxidation of tolane to benzil by flavin mediated oxidation were reported for the first time. Conditions to achieve quantitative decarboxylative oxidation of phenylacetic acids have been found. Based on literature evidence and the results presented, reaction mechanisms for the flavin photo catalyzed oxidation of methylbenzenes, stilbenes, tolane and phenyl acetic acids are proposed. In the closing chapter 5 the synthesis of a polymerizable flavin bearing a vinyl function is described. The monomer was copolymerized with acrylic acid and styrene yielding flavin copolymers that are water soluble or soluble in chloroform, respectively. Upon cross-linking with divinyl benzene insoluble polymers were obtained. The soluble copolymers were characterized by UV-Vis and fluorescence measurements