Use of Electron-Transfer Mediators to Access Higher Electrochemical Current Densities in Ni-Catalyzed Cross-Electrophile Coupling

Ni-catalyzed cross-electrophile coupling (XEC) reactions are versatile methods for carbon-carbon bond formation, and they often use electrochemistry to supply the requisite electrons. These reactions, however, typically exhibit slow rates and operate at low current densities (e.g., 1-4 mA/cm²) to avoid formation of undesirable side products. Here, we show that much higher current densities and reaction selectivity can be accessed by using a homogeneous electron-transfer (ET) mediator. Bis(ethylcyclopentadienyl)cobalt(II), which has a redox potential similar to that of the bipyridyl-ligated Ni catalyst (-1.45 and -1.50 V vs Fc/Fc⁺, respectively), exhibits the best performance. This ET-mediator strategy is applied to numerous C(sp²)-C(sp³) coupling reactions and extended to a flow-based reaction on 10 mmol-scale that proceeds in 96% yield with 91% Faradaic efficiency at a current density of 18 mA/cm².