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

Abstract

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.