16- and 17-Electron Intermediates in the MeCN/RNC Exchange in Chiral-at-Metal [CpFe(Prophos)NCMe]X (X = I, PF6)

The compounds (RFe,RC)-/(SFe,RC)-[CpFe­(Prophos)­NCMe]­X (X = I, PF6), configurationally labile at the metal center, were used in the MeCN/ligand exchange reactions with cyclohexyl isocyanide (CyNC) and tert-butyl isocyanide (tBuNC). Kinetic measurements showed that the MeCN/CyNC exchange in diastereomerically pure (SFe,RC)-[CpFe­(Prophos)­NCMe]­X proceeded via the slow SN1-type dissociation of the Fe–NCMe bond, already observed in the MeCN/phosphite exchange reactions. The product (RFe,RC)-/(SFe,RC)-[CpFe­(Prophos)­CNCy]­X (X = I, PF6) was formed in diastereomer ratios between 40:60 and 60:40. However, specific for the MeCN/CyNC exchange in (SFe,RC)-[CpFe­(Prophos)­NCMe]­PF6, in some of the samples a fast initial reaction interfered, initiated by traces of oxygen, which oxidized the cation in (SFe,RC)-[CpFe­(Prophos)­NCMe]­PF6 to (SFe,RC)-[CpFe­(Prophos)­NCMe]2+. This dipositive cation started an electrocatalytic chain reaction, producing (RFe,RC)-/(SFe,RC)-[CpFe­(Prophos)­CNCy]­PF6 with a high stereoselectivity of 2:98 in favor of (SFe,RC)-[CpFe­(Prophos)­CNCy]­PF6. Deactivation processes terminated the chain reaction, depending on the varying amounts of (SFe,RC)-[CpFe­(Prophos)­NCMe]2+ present in the system. Larger amounts of oxygen or oxidants, such as I2 and AgPF6, caused immediate complete conversion to (RFe,RC)/(SFe,RC)-[CpFe­(Prophos)­CNR]­PF6 in a diastereomer ratio of 2:98. In contrast to the hexafluorophosphate salt, addition of a crystal of iodine did not initiate the chain reaction in the iodide salt [CpFe­(Prophos)­NCMe]­I, because I2 added to I– to form I3–, which did not oxidize the cation of [CpFe­(Prophos)­NCMe]­I. Instead, there was slow conversion according to the dissociative pathway. The correlation between the configuration of (RFe,RC)- and (SFe,RC)-[CpFe­(Prophos)­CNCy]­X and the conformation of the Fe-Prophos chelate ring on the one hand and the correlation with the P–P coupling constants of the Prophos ligand on the other hand was corroborated.