Visible light mediated activation of vinyl halides

This thesis starts with a brief introduction to vinyl radicals in organic chemistry. Beginning with early examples such as the combination of SnBu3H and AIBN in the 1980s, it later focuses on the generation of such intermediates through more benign visible light. After a short introduction to photoredox catalysis, it summarizes major contributions in this field from 2010 until the very recent date. The main part of this thesis deals with the activation of vinyl bromides and chlorides via visible light photoredox catalysis. The initial situation is based on previous investigations on α-bromo chalcones. Although these substrates readily undergo single electron reduction, this principle of activation could barely be transferred to less extended π-systems such as α-bromo cinnamates. The chapter “Preliminary studies with vinyl bromides” illustrates that the inert reactivity of α-bromo cinnamates is predominantly based on the more negative reduction potential. Several efforts to overcome this previously observed limitation are described. Unfortunately, all of these attempts were found to be unproductive and gave only mediocre results at best. The two subsequent chapters cover research results with vinyl chlorides as radical precursors. The chapter “Visible light mediated activation of α-chloro cinnamates” starts with a brief comparison of both vinyl halides and reveals that the chloro derivative surprisingly outcompetes the vinyl bromide in the visible light mediated coupling with enol acetates. The investigation of the reaction and its conditions demonstrates that α-chloro cinnamates represent excellent radical precursors and these substrates can be readily coupled with enol acetates in high yields. Subsequent studies regarding the substrate scope reveal the limitations of the described reaction. This coupling proves to be suitable for photochemical up-scaling and the corresponding products can be readily converted to biologically active γ-butyrolactones. These substrates are first accessed as racemates in a Luche reduction and subsequently enantioenriched in a CBS reduction. This chapter later focuses on solving this current halogen paradox. Numerous control experiments and theoretical calculations reveal that the major difference between both substrates does not rely on the initial bond fragmentation but is rather based on efficient catalyst deactivation in the case of the vinyl bromide. It provides evidence that, in direct comparison to the chloride, the vinyl bromide is consumed faster but nevertheless gives rise to lower yields due to poor conversion of the starting material. In the end, acetyl bromide which is formed as stoichiometric by-product in the described reaction is convicted as gradual but efficient catalyst poison for the photocatalyst.