NHC-Catalyzed Radical Prozesses

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N-Heterocyclic carbenes (NHCs) have been intensively used in organic chemistry either as ligands for transition metals or as catalysts in the field of organocatalysis. Considering organocatalysis, in most cases these transformations proceed in an ionic mode via two-electron pathways (closed shell reactive intermediates). Hence, NHC organocatalysis operating via ionic bond formation is meanwhile very well established. However, NHC-based radical chemistry, where sigma-bond formation proceeds in a radical process, is not well explored to date. In this project, cooperative carbene/photoredox catalysis involving radical intermediates will be investigated. The single electron reduction (SET) of an acylazolium, readily generated upon reaction of an activated acid derivative with an NHC, leads to a persistent ketyl radical. Such ketyl radicals can engage in radical/radical cross coupling reactions with concomitantly oxidatively generated C-radicals, resulting in a novel approach towards ketones. SET-oxidation of the radical precursor and SET-reduction of the acylazolium will be conducted with a photoredox catalyst and the overall transformation represents a redox neutral process. The selective coupling is controlled by the Persistent Radical Effect (PRE). Various C-radical precursors will be tested in this ketone synthesis. Along with such two-component ketone syntheses, three-component processes will be studied as well, where the oxidatively generated radical first adds to an alkene prior to being trapped by the acylazolium-derived ketyl radical. Oxidatively generated heteroatom-centered radicals will also be included in these investigations. Finally, control of the stereoselectivity of the radical/radical coupling processes will be attempted.