Visible Light-Promoted Functionalization of Coal-Tar Dyes: Mechanism and Synthetic Utility

Halogenated organic dyes have been used as photoredox catalysts due to the increased oxidizing and reducing ability of their long-lived excited states. The reactivity of these excited states, however, has not been exploited to functionalize the dye itself. This proposal describes a mechanism for the functionalization of halogenated dyes initiated by visible-light photoinduced electron transfer (PET). Literature evidence and preliminary data suggest that visible light can induce hydrodehalogenation in the presence of mild reductants via a reactive sp2 radical. We propose that first-row transition metal complexes could act as redox catalysts to trap this intermediate and perform useful bond-forming reactions. With the energy provided by light, formal oxidative addition is expected to occur for complexes that would not engage the ground-state electrophile. We will explore the reactivity of the resulting organometallic intermediates with substrates that are traditionally challenging partners for cross coupling. Moreover, we will parametrize the catalytic reaction for a range of dyes and catalysts to devise a predictive model for optimal dye/catalyst combinations. Mechanistic understanding of this reactivity will open the door to the discovery of new visible light-mediated reactions.