Cycloiridated Helicenes as Chiral Catalysts in the Asymmetric Transfer Hydrogenation of Imines

Abstract

The asymmetric synthesis of optically pure and conformationally locked oxabenzo[5]helicenes bearing pyridin-2-yl or isoquinolin-3-yl substituents and their transformation into the corresponding cycloiridated organometallics are described. These helically chiral Cp*IrIII(X)C,N-complexes (X = Cl, I) also contain a configurationally unstable pseudotetrahedral iridium center. This center undergoes epimerization at room temperature, and its relative stereochemistry, especially in the solid state, depends on the nature of the coordinated ligands. Cycloiridated helicenes were used in the asymmetric transfer hydrogenation of prochiral aromatic imines with formic acid/triethylamine to reach up to 96:4 er. It is assumed that the chirality transfer is controlled by the auxiliary helix rather than the IrIII stereogenic center of the chiral iridacycles.