Enantiopure Ferrocene-Based Planar-Chiral Iridacycles: Stereospecific Control of Iridium-Centred Chirality

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.

Copper-Catalyzed Single C-H Amination of 8-Aminoquinoline-Directed Ferrocenes

An unprecedented copper-catalyzed C-H monoamination of ferrocenes directed by an 8-aminoquinoline amide directing group is described. This reaction proceeds in the presence of a catalytic amount of copper catalyst with both cyclic and acyclic amines to afford the various aminoferrocenes. The C-H amination of ortho-substituted ferroceneamides was also achieved, enabling rapid access to multisubstituted ferrocenes that are useful for developing new functional molecules.

Recent Development in the Synthesis and Catalytic Application of Iridacycles

Cyclometallated complexes are well-known and have found many applications. This article provides a short review on the progress made in the synthesis and application to catalysis of cyclometallated half-sandwich Cp*Ir(III) complexes (Cp*: pentamethylcyclopentadienyl) since 2017. Covered in the review are iridacycles featuring conventional C,N chelates and less common metallocene and carbene-derived C,N and C,C ligands. This is followed by an overview of the studies of their applications in catalysis ranging from asymmetric hydrogenation, transfer hydrogenation, hydrosilylation to dehydrogenation.

Chiral cyclometalated iridium complexes for asymmetric reduction reactions

A series of chiral cyclometalated iridium complexes have been synthesised by cyclometalating chiral 2-aryl-oxazoline and imidazoline ligands with [Cp*IrCl2]2. These iridacycles were studied for asymmetric transfer hydrogenation reactions with formic acid as the hydrogen source and were found to display various activities and enantioselectivities, with the most effective ones affording up to 63% ee in the asymmetric reductive amination of ketones and 77% ee in the reduction of pyridinium ions.

Challenges in cyclometalation: steric effects leading to competing pathways and η1,η2-cyclometalated iridium(iii) complexes

The iridation of (R)-N,N-dimethyl-1-(1-naphthyl)ethylamine in the presence of a base afforded an assortment of products ranging from organic molecules to coordinated systems and cyclometalated complexes. The transformation affirmed the postulation where steric effects within the coordination sphere favor a β-hydride elimination-like decomposition pathway, competing alongside ortho-metalation, thus leading to iminium intermediates. The same procedure also generated an unprecedented carbocyclic η¹,η²-cycloiridated species that could not be attained from the direct cyclometalation of its organic ligand.

Electrophilic Carboxamidation of Ferrocenes with Isocyanates

Ferrocenes undergo one-step carboxamidation by reaction with isocyanates in CF₃SO₃H solution. The chemistry is most efficient in excess superacid, and it has been accomplished with aryl and aliphatic isocyanates. In conversions with ferrocene carboxylic acids, isocyanates provide imides in good yields. A mechanism for this conversion is suggested involving carbamic acid anhydride formation and subsequent intramolecular reaction at the substituted cyclopentadienyl ring.

Iridacycles for hydrogenation and dehydrogenation reactions

Iridacycles are a group of cyclometalated metal complexes, which have recently been shown to be versatile catalysts for a range of reactions. This Feature Article provides an account of the work carried out by our groups. We start with an introduction to the variety of iridacycles and how they entered into catalysis. The following sections provide an overview of the discovery and applications in catalysis of the iridacycles originated from our labs, including transfer hydrogenation with formic acid, hydrogenation with H₂, dehydrogenation and borrowing-hydrogen reactions. Where possible, mechanistic insight is also presented. A notable advantage of these iridacycles is their ease of preparation, stability to air and water and high modularity. With only one coordination site available for substrate activation, the iridacycles differ from most homogeneous catalysts structure-wise.

Deuterium as a Stereochemically Invisible Blocking Group for Chiral Ligand Synthesis

Highly diastereoselective lithiation (s-BuLi/TMEDA in Et₂O, -78 °C, 2 h) of (S)-2-ferrocenyl-4-(substituted)oxazolines followed by addition of MeOH-d₄ gave up to 95% D incorporation. Subsequent application of alternative lithiation conditions (n-BuLi in THF, -78 °C, 2 h), followed by addition of an electrophile, resulted in a reversal of diastereoselectivity controlled primarily by the high k_H/k_D value for lithiation (isomer ratio typically between 10:1 and 20:1).

Metallocene to metallocene conversion. Synthesis of an oxazoline-substituted pentamethyliridocenium cation from a ferrocenyloxazoline

A novel metallocene to metallocene conversion is reported in which an air-stable oxazoline-substituted ferrocene is converted into an air-stable oxazoline-substituted iridocenium ion.