Importance of two-electron processes in Fe-catalyzed aryl-(hetero)aryl cross-couplings: evidence of Fe0 / FeII 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 FeII/FeIII 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/FeII 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.
Domaines
Chimie
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