Mediator-Enabled Co-Catalyzed <i>Z</i> -Selective Semihydrogenation via Hydride-Free Multisite Proton-Coupled Electron Transfer

Redox mediators continue to emerge as powerful tools for driving electrochemical reactions, preventing electrode surface fouling and/or facilitating redox processes at milder operating potentials than direct electrolysis requires. In this study, we demonstrate a tandem electrocatalytic approach using a Cp₂Co redox mediator and (dppe)-CoCl₂ precatalyst to selectively hydrogenate a variety of alkynes to <i>cis-</i>alkenes. Controlled potential electrolysis (CPE) at the potential of the mediator (<i>E</i> _<i>app</i> = -1.45 V vs Fc^+/0) successfully generates a variety of terminal and internal alkenes in moderate to good yields. Notably, substrates with tethered Lewis bases further drive selectivity almost exclusively for the <i>Z</i> isomer. Mechanistic investigation of the tandem catalytic reaction via CV and DFT point toward a multisite proton-coupled electron transfer (MS-PCET) step involving a hydride-free pathway where Cp₂Co facilitates the reduction of alkyne- and acid-bound (dppe)-Co-(I)/(0). This developed methodology can be applied toward the synthesis of <i>cis</i>-stilbenoid natural products, as demonstrated with combretastatin A-4. It is also amenable to more practical gram-scale setup and proceeds with up to 99% Faradaic efficiency.