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

Divergent Fe-Mediated C-H Activation Paths Driven by Alkali Cations

The association of the ferrous complex FeIICl2(dmpe)2 (1) with alkali bases M(hmds) (M = Li, Na, K) proves to be an efficient platform for the activation of Ar-H bonds. Two mechanisms can be observed, leading to either Ar-FeII species by deprotonative ferration or hydrido species Ar-FeII-H by oxidative addition of transient Fe0(dmpe)2 generated by reduction of 1. Importantly, the nature of the alkali cation in M(hmds) has a strong influence on the preferred path. Starting from the same iron precursor, diverse catalytic applications can be explored by a simple modulation of the MI cation. Possible strategies enabling cross-coupling using arenes as pro-nucleophiles, reductive dehydrocoupling, or deuteration of B-H bonds are discussed.

Mechanistic Facets of the Competition between Cross-Coupling and Homocoupling in Supporting Ligand-Free Iron-Mediated Aryl–Aryl Bond Formations

In the context of cross-coupling chemistry, the competition between the cross-coupling path itself and the oxidative homocoupling of the nucleophile is a classic issue. In that case, the electrophilic partner acts as a sacrificial oxidant. We investigate in this report the factors governing the cross- versus homocoupling distribution using aryl nucleophiles ArMgBr and (hetero)aryl electrophiles Ar′Cl in the presence of an iron catalyst. When electron-deficient electrophiles are used, a key transient heteroleptic [Ar₂Ar′Fe^II]⁻ complex is formed. DFT calculations show that an asynchronous two-electron reductive elimination follows, which governs the selective evolution of the system toward either a cross- or homocoupling product. Proficiency of the cross-coupling reductive elimination strongly depends on both π-accepting and σ-donating effects of the Fe^II-ligated Ar′ ring. The reactivity trends discussed in this article rely on two-electron elementary steps, which are in contrast with the usually described tendencies in iron-mediated oxidative homocouplings which involve single-electron transfers. The results are probed by paramagnetic ¹H NMR spectroscopy, experimental kinetics data, and DFT calculations.

The crucial and multifaceted roles of main-group cations and their salts in iron-mediated cross-couplings

While a broad variety of iron-catalyzed cross-couplings involve 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, several beneficial effects of [M]ⁿ⁺ cations by themselves or involved in ionic salts used as additives have been observed in such procedures. At the molecular level, interaction of those [M]ⁿ⁺ cations with on-cycle organoiron intermediates can proceed in several ways. Intermolecular interactions can be observed, and also the implication of [M]ⁿ⁺ in the iron's first or second coordination sphere, e.g. by ambiphilic coordination of a [M]-X salt to an R-[Fe] bond. The use of [M]ⁿ⁺ cations in the reaction medium is also a powerful strategy enabling control of the distribution of iron oxidation states within the coupling process.