The enantioselective hydrogenation of citral to (+)-citronellal is a key intermediate step in the synthesis of the valuable aroma compound (-)-menthol. In this study we developed a system based on a modified SCILL catalyst that allows this reaction. The chiral modifier (R)-(+)-2-(Diphenylmethyl)pyrrolidine and trifluoroacetic acid were dissolved in the Ionic Liquid [BMIM][BF4] and coated as a thin layer on a Pd catalyst (dPd = 2.5 nm). In the first step the role of each of the participating components was systematically evaluated. Only the combination of (R)-(+)-2-(Diphenylmethyl)pyrrolidine and trifluoroacetic acid leads to an enantiomeric excess, which can be further improved by the fixation of the modifier and trifluoroacetic acid on the catalyst via an Ionic Liquid. In a second step the effect of different reaction conditions (temperature, pressure and solvent) were investigated. The enantiomeric excess of (+)-citronellal was significantly increased by reducing the reaction pressure and using toluene as solvent, reaching a value of 61%, and (±)-citronellal was obtained at a yield of 76%.
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