Asymmetric alkene epoxidation by Mn(III)salen catalyst in ionic liquids

Abstract

The enantioselective epoxidation of 6-cyano-2,2-dimethylchromene (Chrom) catalysed by the Jacobsen catalyst, using sodium hypochlorite (NaOCl) as oxygen source, at room temperature, was performed in a series of 1,3-dialkylimidazolium and tetra-alkyl-dimethylguanidium based ionic liquids. All the room temperature ionic liquids (RTILs) could be used as reaction media for the enantioselective epoxidation of the alkene giving, generally, moderate to good epoxide yields and enantiomeric excesses ( ee%). For the series of ionic liquids derived from the 1,3-dialkylimidazolium cation, it was observed some relationship between the RTILs physical properties and the catalytic reaction parameters, exemplified by linear correlations between (i) the ee% and the α Kamlet–Taft parameter (hydrogen bond acidity of the solvent) for CH 2Cl 2 and [C 4m n im][BF 4] ionic liquids ( n = 1 or 2), and (ii) the ee% and the β Kamlet–Taft parameter (hydrogen bond basicity of the solvent) for CH 2Cl 2 and [C 4mim][X] ionic liquids (X = PF 6, NTf 2 or BF 4). All the RTILs could be reused in further catalytic cycles, with the exception of [C 8mim][PF 6]. The reutilisation of the Jacobsen catalyst for four times generally led to a decrease in the epoxide yield and to a slight decrease in the enantioselectivity. The recycling of the catalyst could be improved by imparting an ionic character to the complex through abstraction of the axially coordinated chloride anion (Cat 2). Other oxygen sources, such as iodosylbenzene, hydrogen peroxide and urea–hydrogen peroxide adduct, were also tested coupled with Jacobsen catalyst, but the best results were achieved with NaOCl.