Project Summary/Abstract. Deracemization reactions are a rare but powerful means of obtaining enantioenriched material from racemic or scalemic mixtures. The limited number of known deracemization methods are primarily due to challenges that arise from thermodynamic and kinetic considerations. The conversion of a racemic mixture to an enantiopure one causes a loss in entropy and is endergonic. Moreover, because enantiomers have identical potential energies, the barriers for enantiomer interconversion in a thermal reaction are equal for both the forward and reverse steps, which is a consequence of microscopic reversibility. Thus, the formation of enantioenriched material from a racemate is both thermodynamically unfavorable and kinetically difficult. However, a light-driven deracemization protocol can surmount these challenges. The input of light overcomes the endergonic penalty of this transformation, and as photochemical reactions traverse multiple potential energy surfaces, the forward and reverse reactions of a light-driven deracemization can be judiciously engineered to have unequal kinetic barriers to circumvent microscopic reversibility. Herein, we describe a photocatalytic strategy to deracemize organic compounds containing benzylic moieties commonly found in pharmaceuticals and studies on its mechanism. This reaction deracemizes α-aryl carbonyl compounds and involves a photocatalytic stereoablation process to generate an achiral intermediate that undergoes asymmetric protonation by a proton transfer catalyst. This proposed pathway involves oxidation of the aromatic substrate to the aryl radical cation by the excited state of the photocatalyst. The benzylic C–H bonds of the aryl radical cation are significantly acidified and engage in proton transfer with a chiral base co-catalyst to furnish an α-carbonyl, benzylic radical. This open- shell species undergoes reduction to the corresponding enolate by the reduced state of the photocatalyst, and the enolate reacts with the conjugate acid of the chiral base co-catalyst to preferentially generate one enantiomer of product. Continued iterations of these steps through catalysis leads to enantioenrichment, and we demonstrate the feasibility of this proposal by describing a preliminary substrate scope. The degree of enantioenrichment (up to 97:3 er) and mass balance (≥90%) is high. We also provide preliminary mechanistic data on this system, and this investigation is ongoing. We have found that the rate of deprotonation of the radical cation intermediate and the rate of back electron transfer (BET) from the oxidized substrate to the reduced photocatalyst affect the overall degree of enantioenrichment obtained, and we provide a rationale for this behavior. Should ongoing studies support our hypotheses, this study will demonstrate how the ground state reduction potential of a catalyst, which governs the rate of BET, and the concentration of a chiral proton transfer catalyst modulate the degree of en...