Asymmetric Hydrogenation with Iridium C,N and N,P Ligand Complexes: Characterization of Dihydride Intermediates with a Coordinated Alkene

Previously elusive iridium dihydride alkene complexes have been identified and characterized by NMR spectroscopy in solution. Reactivity studies demonstrated that these complexes are catalytically competent intermediates. Additional H₂ is required to convert the catalyst‐bound alkene into the hydrogenation product, supporting an Ir^III/Ir^V cycle via an [Ir^III(H)₂(alkene)(H₂)(L)]⁺ intermediate, as originally proposed based on DFT calculations. NMR analyses indicate a reaction pathway proceeding through rapidly equilibrating isomeric dihydride alkene intermediates with a subsequent slow enantioselectivity‐determining step. As in the classical example of asymmetric hydrogenation with rhodium diphosphine catalysts, it is a minor, less stable intermediate that is converted into the major product enantiomer.