Electron donor-acceptor (EDA) complexes provide a means to initiate free-radical reactions under visible-light irradiation using molecules that do not absorb in this region on their own. However, implementation of this strategy in organic synthesis generally requires stoichiometric use of carefully designed substrates. One solution that remains underexplored is a catalytic approach to EDA complex formation. This method has the potential to greatly expand the scope of EDA photochemistry by utilizing a catalyst that exclusively forms an EDA complex for the photogeneration of radicals, thus eliminating the requirement for specific substrate functionality. Examples of such catalytic systems are limited, likely owing to the difficulty of implementing a suitable catalyst turn-over step, or because of issues arising from catalyst deactivation that result from radical addition to the catalyst. In this work, we present two distinct strategies for catalytic EDA complex formation. One strategy involves the use of substituted hydroquinones as donor catalysts for the activation carbon-halogen bonds in radical (perfluoro)alkylation reactions. These reactions are broad in scope and proceed through the in-situ generation of a halogen-bonding complex between the hydroquinone catalyst and the (perfluoro)alkyl halide radical precursor. An alternative strategy we are exploring for catalytic EDA complex formation involves the interaction between a substrate and the surface of an inorganic semiconductor, such as titanium dioxide (TiO2). These EDA complexes can be described as a ligand-to-metal charge-transfer (LMCT) interaction. Our progress towards a generalized approach of employing a heterogeneous TiO2 catalyst for visible-light generation of carbon-centered radicals will also be discussed.

Catalytically generated electron donor-acceptor complexes in visible-light-mediated free-radical reactions

ACS Spring 2022

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