Enantioconvergent Chan-Evans-Lam C(sp³)-O Coupling: Cu-Catalyzed Asymmetric Benzyl- and Allylborane Oxidation

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.