An effective heterogeneous l-proline catalyst for the direct asymmetric aldol reaction using graphene oxide as support

Pristine l-proline was non-covalently loaded on the graphene oxide (GO) sheet in a simple route by mixing them in aqueous solution. Technologies of characterization well suggested that l-proline was efficiently loaded on the two sides and edge of the GO sheet through hydrogen-bonding or/and ionic interaction, giving the excellent l-proline/GO hybrid catalyst for the direct asymmetric aldol reaction. The unique multilayered structure of the GO carrier with sufficient interlayer space favored reagents’ diffusion toward l-proline chiral moiety and therefore resulted in the high catalytic efficiency of the heterogeneous l-proline. Excellent yield (96%) with high enantiomeric excess (79% ee) was obtained in the direct aldol reaction of 2-nitrobenzaldehyde with acetone catalyzed by l-proline/GO hybrid, which was comparable to that observed in the reactions promoted by l-proline itself. Furthermore, the l-proline/GO hybrid used as a heterogeneous catalyst could be easily recovered and recycled for seven times without significant loss of the reactivity.