Recyclable Dirhodium(II) Catalyst Rh₂(esp)₂ for the Allylic Oxidation of Δ⁵-Steroids

The chelating dirhodium(II) catalyst Rh₂(esp)₂ was shown to efficiently catalyze the allylic oxidation of Δ⁵-steroids using T-HYDRO (70% tert-butyl hydroperoxide in water) as oxidant. Reaction yields were affected by the coordination ability of the solvent. The noncoordinating solvent n-heptane was determined to be an optimal solvent. At gram scale, the product, Δ⁵-en-7-one steroid, precipitated from the reaction mixture. The Rh₂(esp)₂ complex did not undergo catalytic degradation and was recycled using Merrifield-pyridine resin for further allylic oxidation cycles. The results of ultraviolet/visible spectral analysis suggested that the Rh₂(II,II) species, rather than the Rh₂(II,III) species, was in the catalyst resting state during the reaction, which helps to explain the high durability of the catalyst.