Parallel or classical kinetic resolution of a planar chiral ferrocenylketone through asymmetric reductions

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.

Highly Enantioselective Asymmetric Transfer Hydrogenation: A Practical and Scalable Method To Efficiently Access Planar Chiral [2.2]Paracyclophanes

We report herein a general, practical method based on asymmetric transfer hydrogenation (ATH) to control the planar chirality of a range of substituted [2.2]paracyclophanes (pCps). Our strategy enabled us to perform both the kinetic resolution (KR) of racemic compounds and the desymmetrization of centrosymmetric meso derivatives on synthetically useful scales. High selectivities (up to 99% ee) and good yields (up to 48% for the KRs and 74% for the desymmetrization reactions) could be observed for several poly-substituted paracyclophanes, including a series of bromine-containing molecules. The optimized processes could be run up to the gram scale without any loss in the reaction efficiencies. Because of its broad applicability, the ATH approach appears to be the method of choice to access planar chiral pCps in their enantiopure form.

Enantiorecognition of planar "metallocenic" chirality by a nitrile hydratase/amidase bienzymatic system

For the first time, a bacterial strain expressing a nitrile hydratase/amidase activity was able to recognize a planar element of chirality: Rhodococcus rhodochrous PA-34 whole cells catalysed with a high level of enantioselectivity the biotransformation of a novel nitrile ferrocene derivative into its corresponding amide and/or acid. An important parameter in the enzymatic recognition is the choice of the inducer selected for the bacterial growth phase.

Asymmetric synthesis of planar chiral ferrocenes by enantioselective intramolecular C-H arylation of N-(2-haloaryl)ferrocenecarboxamides

The palladium-catalyzed intramolecular C-H arylation reaction of N-(2-bromoaryl)ferrocenecarboxamides furnishes planar chiral ferrocene derivatives. TADDOL-derived phosphoramide ligands induce enantioselectivities ranging from 91:9 to 98:2 er.

Peptide-catalyzed kinetic resolution of planar-chiral metallocenes

Kinetic resolution of racemic planar-chiral metallocenes was performed by a resin-supported peptide catalyst, in which low-molecular-weight organocatalysts were not effective.