Oxidative Addition of Dihydrogen to [Ir(bisphosphine)(1,5-cyclooctadiene)]BF₄ Complexes:  Kinetic and Thermodynamic Selectivity

The activation of dihydrogen by cationic diphosphine complexes of Rh is the rate-determining and enantiodetermining step in the catalytic asymmetric hydrogenation of prochiral enamides. The addition of H₂ to [Ir(bisphosphine)(COD)]⁺ (COD = 1,5-cyclooctadiene) complexes is examined herein as a model for stereocontrol and dynamic processes related to catalytic hydrogenation. The diastereoselectivity of H₂ oxidative addition to form diastereomeric [Ir(chiral bisphosphine)(H)₂(COD)]⁺ complexes at −80 °C is kinetically controlled and varies substantially with the structure of the diphosphine. In one instance (chiral bisphosphine = CHIRAPHOS), the kinetic and thermodynamic selectivities of H₂ addition are inverted; i.e., the dominant kinetic product is thermodynamically less stable than the minor kinetic product. For the [IrH₂(Me-DuPhos)(COD)]⁺ system, quantitative analysis of the 2D NOE data using two-dimensional conformer population analysis (2DCPA) establishes the absolute stereochemistry and the three-dimensional structures of the predominant conformers. Kinetic analysis of the interconversion of the [IrH₂(Me-DuPhos)(COD)]⁺ diastereomers, and of their exchange with D₂, reveals a complex pathway for isotope scrambling and diastereomer interconversion that does not involve reductive elimination of H₂ to form [Ir(Me-DuPhos)(COD)]⁺. During the isotope scrambling, exquisite selectivity is seen for exchange in only two ligand sites, one on the Me-DuPHOS ligand and one on the COD ligand.