Cyclopropenimine-Enabled Redox Control in Copper-Catalyzed Radical Cyclization to 3,3-Disubstituted Oxindoles.

Radical-mediated construction of all-carbon quaternary centers offers a powerful approach, yet its application to tertiary alkyl halides under mild conditions remains challenging, often due to the need to orchestrate multiple redox events within a single catalytic cycle. Herein, we report a cyclopropenimine-enabled copper-catalyzed radical cyclization that addresses this challenge. A range of 3,3-disubstituted oxindoles bearing a quaternary carbon center at the C3 position, including those incorporating aryl substituents, was obtained in good yields under mild conditions. Mechanistic investigations, including radical trapping, EPR, kinetic isotope effect experiments, cyclic voltammetry, and DFT studies, reveal that productive catalysis arises from a balance between oxidative C-X bond activation and reductive catalyst regeneration. The phenylene-bridged cyclopropenimine scaffold effectively modulates the redox behavior of the copper center, thereby enabling efficient reactions of tertiary alkyl halides, including chlorides. These findings highlight how the structural features of the cyclopropenimine scaffold regulate competing redox processes within a catalytic cycle and provide a foundation for future radical transformations involving challenging alkyl electrophiles.