Poly(ethylene-glycol)-supported proline:: a recyclable aminocatalyst for the enantioselective synthesis of γ-nitroketones by conjugate addition

A poly(ethylene-glycol)-supported proline was used as the catalyst for two enantioselective conjugate addition reactions leading to gamma-nitroketones. In both cases the chemical yield and the stereochemical efficiency of the processes greatly depended on the reaction conditions. In the additions of ketones (cyclohexanone, cyclopentanone, and acetone) to 2-nitrostyrene, fair yields (up to 60%) and good diastereoselectivity (up to 95/5 synlanti ratios) were observed. The enantiomeric excesses (up to 40%) were lower than those obtained with non-supported proline (e.e. up to 57%). In the addition of 2-nitropropane to cyclohexenone the use of the sodium salt of supported proline allowed to almost match the e.e. of the adduct obtained with rubidium prolinate as the catalyst (50% e.e. versus 59% e.e.) in comparable yields. Examples of recovery and recycling of the supported catalysts in both types of processes were also reported. (C) 2003 Elsevier Science B.V. All fights reserved.