Electrophilic cleavage of the iron-carbon sigma bond: Mechanism and the stereochemistry at iron

Electrophilic cleavage of the iron-carbon sigma bond in the resolved chiral complexes CpFE(CO)(PPh₃)R (where R = Me, Et, CH₂CO₂ (menthyl), and CH₂O (menthyl)) by I₂, ICl, and HgI₂ reveals that the reaction proceeds with net retention of configuration at iron, but stereospecifity is not high and is insensitive to the nature of the alkyl group. In every case, recovered starting material is partially racemized, but the source of this is not yet certain. Formation of CpFe(CO)(PPh₃)I from cleavages using ICl indicates that initial electrophilic attack is at the metal. A mechanism is discussed which accounts for net retention at metal in terms of steric and ion pairing rather than electronic effects. Cleavage using HO₂CCF₃ forms CpFe(CO)(PPh₃)O₂CCF₃ of ca. 50% enantiomeric excess as shown by the use of an optically active Eu NMR shift reagent, but net retention or inversion at iron cannot be inferred.