Photocatalyst-Dependent Enantioselectivity in the Light-Driven Deracemization of Cyclic α-Aryl Ketones

We report a photoredox-enabled deracemization of cyclic α-aryl ketones that occurs with high stereoselectivity and yield and proceeds by mechanistically distinct proton transfer reactions. This reaction is jointly mediated by a visible-light photocatalyst and a chiral phosphate base co-catalyst under blue light irradiation. Notably, the extent of deracemization for this reaction exhibits an unexpected dependence on the identity of the photocatalyst and the concentration of a chiral base co-catalyst, wherein the extent of deracemization can be increased by employing photocatalysts with more positive ground-state reduction potentials, raising the concentration of the chiral base co-catalyst, or by a combination of these factors. This effect is attributed to two competing processes, back electron transfer and deprotonation, that consume the same reaction intermediate, and we propose a kinetic model that rationalizes this behavior. We also demonstrate that the redox properties of the photocatalyst also impact the stereoselectivity of the product-forming step, which is the dominant stereoselective step in this transformation. Together, these mechanistic insights facilitate a deeper understanding of the complexity of light-driven deracemization reactions involving reversible electron transfer and suggest approaches by which the stereoselectivity of these processes may be increased.