Importance of two-electron processes in Fe-catalyzed Aryl-(hetero)aryl cross-couplings: evidence of Fe 0 /Fe II couple implication
Résumé
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 Fe$^0$ species formed by the reduction of the precatalyst. In this case, the coupling mechanism relies on two-electron elementary steps involving the Fe$^0$/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 $^1$H NMR, kinetics monitoring, and density functional theory (DFT) calculations.