Asymmetric catalysis with octahedral stereogenic-at-metal complexes featuring chiral ligands

Abstract

The synthesis and the design of octahedral stereogenic-at-metal complexes featuring chiral ligands and their application as chiral catalysts in various asymmetric reactions are highlighted and discussed in the present review article. Owing to their ligand-located stereogenic elements, such complexes are conveniently accessible stereochemically-defined as Λ- and/or Δ-configured diastereomers, which can be viewed as ‘pseudoenantiomers’ when one primarily focuses on the stereogenic arrangement of multidentate ligands around the metal center. Their convenient synthesis poses a practical advantage over octahedral stereogenic-only-at-metal complexes, where the synthesis of the according Λ- and/or Δ-configured isomers, which are consequentially ‘true enantiomers’, is often somewhat cumbersome. However, octahedral stereogenic-at-metal complexes featuring chiral ligands offer the same favorable scaffold for the design of asymmetric catalysts as related octahedral stereogenic-only-at-metal complexes, namely a chiral, propeller-shaped arrangement of bidentate or tridentate ligands around a stereogenic octahedral metal center. These attractive features are the reason why we focus in the present review on the, from our perspective, highly capable and promising class of octahedral stereogenic-at-metal catalysts featuring chiral ligands. In the following paragraphs, we will discuss recent examples of such catalysts, which have so far been utilized as chiral Lewis acid catalysts, as chiral Brønsted acid catalysts, and as chiral hydrogen-bonding catalysts in various asymmetric reactions.