Graphene-based recyclable and bifunctional heterogeneous chiral catalyst for direct asymmetric aldol reaction

Application of metal-free heterogeneous asymmetric catalysis is of great interest in the field of asymmetric synthesis. Herein, two novel graphene-supported praline-based bifunctional carbocatalysts were prepared by post-grafting c-proline onto graphene oxide sheets via organoamine-functIonalized graphene oxide (GO-NH2) and reduced graphene oxide (GO-NH2). Different methods such as Fourier-transform infrared spectroscopy (FT-IR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron spectroscopy (TEM) were employed to examine the surface functionalization of GO. The catalytic properties of these materials were investigated in the direct asymmetric aldol reaction. r-proline grafted heterogeneous catalyst, GO-Pro, was found to be an efficient and powerful organocatalyst for the enantioselective aldol reaction under mild conditions in hexane and afforded the corresponding aldol adducts with satisfactory isolated yields (up to 88%) and enantiomeric excesses (up to 85%). Furthermore, graphene-based carbocatalyst could be recovered and recycled easily by a simple washing procedure and re-used for several times without substantial reduction in its catalytic activity.