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.