Application of a Ferrocene‐Based Palladacycle Precatalyst to Enantioselective Aryl‐Aryl Kumada Coupling

Redox-Active Triazole-Derived Mesoionic Imines with Ferrocenyl Substituents and their Metal Complexes: Directed Hydrogen-Bonding, Unusual C-H Activation and Ion-Pair Formation

We present herein the synthesis, characterization and complexation of ferrocenyl-substituted MIIs (mesoionic imines) and their metal complexes. In the free MIIs, strong hydrogen bonding interactions are observed between the imine-N and the C-H bonds of the ferrocenyl substituents both in the solid state and in solution. The influence of this hydrogen bonding is so strong that complexation of the MIIs with [IrCp*Cl2]2 yields unique six-membered iridacycles via C-H-activation of the corresponding C-H-site at the Fc-substituent and not the Ph-substituent. This result is in contrast to previous reports in which always a preferential C-H activation at the phenyl substituent is observed in competitive reactions in the presence of ferrocenyl substituents. The corresponding Ir complexes formed after in-situ halide exchange reaction exist in either [Ir-I] contact or as [Ir]+I- solvent separated ion-pairs depending on the solvent polarity. The iodide coordinated and solvent separated ion-pairs display drastically different physical properties. The TEP (Tolman-electronic-parameter) of these ligands was determined and lines up with previously reported MII-ligands. The redox properties were investigated by a combination of electrochemical and spectroelectrochemical methods. We show here how non-covalent interactions can have a drastic influence on the physical and chemical properties of these new class of compounds.

Ferrocene derivatives with planar chirality and their enantioseparation by liquid-phase techniques

In the last decade, planar chiral ferrocenes have attracted a growing interest in several fields, particularly in asymmetric catalysis, medicinal chemistry, chiroptical spectroscopy and electrochemistry. In this frame, the access to pure or enriched enantiomers of planar chiral ferrocenes has become essential, relying on the availability of efficient asymmetric synthesis procedures and enantioseparation methods. Despite this, in enantioseparation science, these metallocenes were not comprehensively explored, and very few systematic analytical studies were reported in this field so far. On the other hand, enantioselective high-performance liquid chromatography has been frequently used by organic and organometallic chemists in order to measure the enantiomeric purity of planar chiral ferrocenes prepared by asymmetric synthesis. On these bases, this review aims to provide the reader with a comprehensive overview on the enantioseparation of planar chiral ferrocenes by discussing liquid-phase enantioseparation methods developed over time, integrating this main topic with the most relevant aspects of ferrocene chemistry. Thus, the main structural features of ferrocenes and the methods to model this class of metallocenes will be briefly summarized. In addition, planar chiral ferrocenes of applicative interest as well as the limits of asymmetric synthesis for the preparation of some classes of planar chiral ferrocenes will also be discussed with the aim to orient analytical scientists towards 'hot topics' and issues which are still open for accessing enantiomers of ferrocenes featured by planar chirality.