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 reduction of the precatalyst. In this case, the coupling mechanism relies on 2-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 electronwithdrawing 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 2-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, as well as by DFT calculations. KEYWORDS. iron catalysis • cross-coupling • mechanisms • two-electron processes • kinetics INTRODUCTION.Fe-catalyzed cross-coupling reactions have been intensely developed in the last decades, thanks to the pioneer work of Kochi, 1a-b Cahiez, 1c Fürstner, 1d-e Nakamura 1f-g and Bedford. 1h-i Use of this cheap and abundant metal led to a significant breakthrough in transition-metal catalysis; however, despite those significant ecologic and economic advantages, 2 reaching a fine understanding of the mechanistic facets of the corresponding transformations is still a challenging issue, especially due to the short lifetimes of the active species 3 and to the large panel of oxidation states formed under catalytically-relevant coupling conditions.From a mechanistic standpoint, numerous coupling systems involving aliphatic C sp3 ─X electrophiles (X = halide or pseudo-halide) proved to follow a radical pathway, with in-situ-generated organoiron(II) intermediates as active species. The key step of the coupling is in that case the activation of the electrophile by the latter in a single-electron transfer step (SET) leading to the generation of an organoiron(III) intermediate

While a broad variety of iron-catalyzed cross-couplings involves the use of main-group organometallics R-[M] as nucleophiles, the role of the [M]n+ cation in the coupling process is generally disregarded. However,...