One-Electron Approach for Trans-Selective Alkyne Semi-Reduction via Cobalt Catalysis

The diastereoselective semireduction of alkynes to alkenes is a powerful transformation in synthetic chemistry, yet catalytic methods for trans-selective (E) alkyne reduction remain limited. Herein, we introduce a fundamentally new approach for the highly selective trans-semireduction of internal alkynes, enabled by a cobalt-catalyzed electrochemical radical pathway. This method offers a broad substrate scope, accommodating alkynes with diverse electronic and steric profiles, and displays exceptional chemoselectivity and functional group tolerance. The methodology was extended to isotopically labeled trans-deuteration and demonstrated excellent chemoselectivity in substrates containing multiple alkyne motifs. Mechanistic studies, including cyclic voltammetry, UV-vis spectroelectrochemistry, and DFT calculations, support a dual catalytic cycle involving electrochemical Co-H formation and a subsequent organometallic radical pathway. Insights from this mechanism guided the development of a complementary chemical oxidative protocol, enabling access to <i>E</i>-alkenes from substrates that are otherwise unreactive under electroreductive conditions. This work introduces a fundamentally new and general strategy for accessing <i>trans</i>-alkenes from alkynes via cobalt catalysis while opening a new avenue for radical-based alkyne functionalization.