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While alkylborane oxidation constitutes one of the most widely utilized strategies to construct C–O bonds, asymmetric versions of this transformation remain elusive. Establishing such an asymmetric approach would unlock a strategically distinct disconnection to oxygen-bearing stereogenic centers, which are ubiquitous across biologically active scaffolds. Herein, we employ a Cu-catalyzed single-electron mechanism to achieve enantioconvergent alkylborane oxidation for the first time. The central challenge─suppressing the unselective carbocation pathway from alkyl radical oxidation by the Cu(II)–carboxylate─is addressed by (1) lowering the reaction temperature to attenuate the undesired radical–polar crossover while (2) leveraging photochemical activation to preserve the single-electron radical functionalization manifold. The reaction exhibits broad functional-group compatibility, engaging benzyl- and allylboronic esters; diverse carboxylic acids, including complex pharmaceutical substructures and heterocycle-containing substrates, are also well tolerated. The reported protocol is scalable to gram quantities and was utilized for the asymmetric synthesis of an immunosuppressant drug candidate. Mechanistic studies, including stoichiometric interrogation of elementary steps, rate law determination, radical trapping experiments, and density functional theory (DFT) computations, indicate that the reaction operates by balancing two light-driven processes: (1) rate-determining N–H bond homolysis followed by N-radical-mediated C–B bond activation and (2) enantioselective Cu-mediated radical functionalization via an inner-sphere pathway. The reactive Cu(II)–carboxylate intermediate was isolated and structurally characterized, permitting direct examination of its spectroscopic features and radical-trapping reactivity. Electron paramagnetic resonance (EPR) studies identified the Cu(II)–carboxylate species as the catalyst resting state, and stoichiometric reaction of the Cu(II)–carboxylate with a persistent trityl radical demonstrated its competency for C–O bond formation. Hammett analysis further revealed that the efficiency of C–O bond formation is governed by the electrophilicity of the Cu(II)–carboxylate intermediate. 

Abstract Phenoxazines are a successful class of organic photoredox catalysts (PCs) with tunable redox and photophysical properties. Originally, we aimed to realize more reducing phenoxazine PCs through heteroatom core substituted (HetCS) derivatives, while maintaining an efficiently oxidizing PC^·+. However, core modification with thioether or ether functionality to a PC that exhibits photoinduced intramolecular charge transfer (CT) negligibly alters the singlet excited state reduction potential (E_S1°*), while yielding a less oxidizing PC^·+(E_1/2) (E_1/2 = 0.50–0.64 V vs. SCE) compared to the noncore modified PC1(0.68 V vs. SCE). Photophysical characterization of HetCS PCs revealed that increasing electron density on the core of a CT exhibiting PC stabilizes the emissive state and PC^·+, resulting in a relatively unchangedE_S1°* compared to PC1. In contrast, modifying the core of a PC that does not exhibit CT yields a highly reducingE_S1°* (PC3= −2.48 V vs. SCE) compared to its CT equivalent (PC1d= −1.68 V vs. SCE). The impact of PC property on photocatalytic ability was evaluated through organocatalyzed atom transfer radical polymerization (O‐ATRP). HetCS PCs were able to yield poly(methyl methacrylate) with low dispersity and moderate targeted molecular weight as evaluated by initiator efficiency (I*) in DMAc (Ð= 1.20–1.26;I*= 47–57%). Ultimately, this work provides insight into how phenoxazine PC properties are altered through structural modification, which can inform future PC design.