Importance of Two-Electron Processes in Fe-Catalyzed Aryl-(hetero)aryl Cross-Couplings: Evidence of Fe⁰/Fe^II Couple Implication

We demonstrate in this work that two drastically distinct mechanisms can be involved in aryl-(hetero)­aryl Fe-mediated cross-couplings between Grignard reagents and organic halides, depending on the nature of the latter. (Hetero)­aryl electrophiles, which easily undergo one-electron reduction, can be involved in a Fe^II/Fe^III coupling sequence featuring an in situ generated organoiron­(II) species, akin to their aliphatic analogues. On the other hand, less easily reduced substrates can be activated by transient Fe⁰ species formed by the reduction of the precatalyst. In this case, the coupling mechanism relies on two-electron elementary steps involving the Fe⁰/Fe^II redox couple and proceeds by an oxidative addition/reductive elimination sequence. Hammett analysis shows that both those elementary steps are faster for electrophiles substituted by electron-withdrawing groups. The two mechanisms discussed herein can be involved concomitantly for electrophiles displaying an average oxidative power. Attesting to the feasibility of the aforementioned bielectronic mechanism, high-spin organoiron­(II) intermediates formed by two-electron oxidative addition onto (hetero)­aryl halides in catalytically relevant conditions were also characterized for the first time. Those results are sustained by paramagnetic ¹H NMR, kinetics monitoring, and density functional theory (DFT) calculations.