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.