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

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.