A Voltage-Controlled Redox Mediator Strategy Enables Electrocatalytic Z-Selective Semi-Hydrogenation

Selective semi-hydrogenation to generate alkenes from alkynes is an invaluable method to access olefins for materials, pharmaceuticals, and nat-ural products. Electrochemical transition-metal-catalyzed hydrogenation is an attractive alternative enhancing functional group tolerance while employing milder conditions; however, limited access to functionally diverse alkenes remain challenging. In this study, we demonstrate a tan-dem redox-mediated electrocatalytic approach to selectively hydrogenate a variety of alkynes to cis-alkenes via a cobaltocene redox mediator and bisphosphine cobalt co-catalyst under voltage control. Controlled potential electrolysis (CPE) at the potential of the mediator (Eapp = -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 Z isomer. Mechanistic investigations of the tandem catalytic reaction via CV and DFT point towards a unique mechanism involving a hydride-free pathway where the cobaltocene mediator facilitates concerted insertion/protonation via coordinated Co-alkyne complex in the presence of bound carboxylic acid. This developed methodology can be applied toward the synthesis of cis-stilbenoid natural products, as demonstrated through the synthesis of combretastatin A-4.