Active Species and Mechanistic Pathways in Iron-Catalyzed C–C Bond-Forming Cross-Coupling Reactions

Abstract

Over the past decade, considerable progress on iron-catalyzed C–C bond-forming cross-coupling reactions has been made, leading to the successful development of several new catalytic systems. This perspective presents the proposed mechanistic pathways of iron-mediated cross-coupling reactions of organohalides and Grignard reagents and discusses the evidence documented in the literature that distinguishes whether such pathways proceed via single- or double-electron processes. When cross-coupling reactions are conducted in the presence of ligands, there is still much discussion in the literature as to whether the lowest iron oxidation state responsible for catalytic activity is Fe(I) or Fe(II). However, when ligand-free conditions are employed, it has been shown that iron reaches an Fe(I) oxidation state, allowing an Fe(I)/Fe(III) catalytic cycle. Moreover, for cross-couplings using alkyl halide electrophiles, evidence suggests that the reaction proceeds through single-electron steps, with the generation of ...