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 i s the rate-determining and enantiodetermining step in the catalytic as ymmetric hydrogenation of prochiral enamides. The addition of H-2 to [ Ir(bisphosphine)(COD)](+) (COD = 1,5-cyclooctadiene) complexes is exam ined herein as a model for stereocontrol and dynamic processes related to catalytic hydrogenation. The diastereoselectivity of H-2 oxidative addition to form diastereomeric [Ir(chiral bisphosphine)(H)(2)(COD)]( +) complexes at -80 degrees C is kinetically controlled and varies sub stantially with the structure of the diphosphine. In one instance (chi ral bisphosphine = CHIRAPHOS), the kinetic and thermodynamic selectivi ties of H-2 addition are inverted; i.e., the dominant kinetic product is thermodynamically less stable than the minor kinetic product. For t he [IrH2(Me-DuPhos)(COD)](+) system, quantitative analysis of the 2D N OE data using two-dimensional conformer population analysis (2DCPA) es tablishes the absolute stereochemistry and the three-dimensional struc tures of the predominant conformers. Kinetic analysis of the interconv ersion of the [IrH2(Me-DuPhos)(COD)](+) diastereomers, and of their ex change with D-2, reveals a complex pathway for isotope scrambling and diastereomer interconversion that does not involve reductive eliminati on of H-2 to form [Ir(Me-DuPhos)(COD)](+). During the isotope scrambli ng, exquisite selectivity is seen for exchange in only two Ligand site s,one on the Me-DuPHOS ligand and one on the COD ligand.