The Effect of β-Hydrogen Atoms on Iron Speciation in Cross-Couplings with Simple Iron Salts and Alkyl Grignard Reagents.

The effects of β-hydrogen-containing alkyl Grignard reagents in simple ferric salt cross-couplings have been elucidated. The reaction of FeCl₃ with EtMgBr in THF leads to the formation of the cluster species [Fe₈ Et₁₂ ]^2- , a rare example of a structurally characterized metal complex with bridging ethyl ligands. Analogous reactions in the presence of NMP, a key additive for effective cross-coupling with simple ferric salts and β-hydrogen-containing alkyl nucleophiles, result in the formation of [FeEt₃ ]^- . Reactivity studies demonstrate the effectiveness of [FeEt₃ ]^- in rapidly and selectively forming the cross-coupled product upon reaction with electrophiles. The identification of iron-ate species with EtMgBr analogous to those previously observed with MeMgBr is a critical insight, indicating that analogous iron species can be operative in catalysis for these two classes of alkyl nucleophiles.