Mechanistic Studies on a Facile Ring-Flipping Process in Planar Chiral Ferrocenes under Ambient and High Pressure and Its Relevance to Asymmetric Catalysis

The bis-planar chiral ferrocenyldiphosphine bis(1-(diphenylphosphino)-η5-indenyl)iron(II) is observed to undergo an isomerization from the meso isomer to the rac isomer in THF solvent at ambient temperature. This process requires ring flipping of one of the indenyl ligands. The isomerization is slowed by the addition of the noncoordinating solvent chloroform and is accelerated by addition of salts such as LiCl and LiClO4. Rate and activation parameters have been determined for the spontaneous isomerization: k obs = 1.6 × 10-5 s-1 at 23 °C, ΔH ⧧ = 58 ± 4 kJ mol-1, ΔS ⧧ = −140 ± 15 J mol-1 K-1, and ΔV ⧧ = −12.9 ± 0.8 cm3 mol-1. Labeling of the ferrocene with deuterium in the 3- and 3‘-positions followed by isomerization showed no incorporation of deuterium into the 2- or 2‘-positions, thus ruling out 1,2-hydrogen-shift mechanisms. Attempted crossover experiments with dideuterated and nondeuterated ferrocenes gave no monodeuterated products of crossover, thus ruling out ligand dissociative mechanisms. The proposed isomerization mechanism involves coordination of two THF ligands with ring slippage of one indenide and then displacement of that indenide and coordination through the phosphine. Coordination of the indenide by the other face in the reverse process then leads to the other isomer.