Comparative Study of Graphite-Oxide and Graphene-Oxide Supported Proline Organocatalysts in Asymmetric Aldol Addition

Graphite oxide (GrO), graphene oxide (GnO) and their sulphated derivatives (GrOs, GrOo, GnOs, GnOo) were prepared from graphite (Gr) and graphene (Gn) as starting materials, using various methods. Previous experience suggests that in the course of sulphation relatively few functional groups get incorporated, which are mainly located along the edges of the layers. In the course of oxidation the layers are also converted to a significant extent, although aromatic regions are also retained, depending on the strength of the oxidative conditions. Proline impregnation of the functionalized catalyst supports obtained in this way enables the synthesis of Pro-GrO and Pro-GnO chiral catalysts. Proline interacts with the functional groups building up on the layers in a mostly reversible fashion and is relatively easily removed therefrom, whereas over sulphated materials groups forming on the edges allowed stronger bonding of proline. An asymmetric aldol reaction between 2-nitrobenzaldehyde and acetone with a selectivity of 90 % and an enantioselectivity of 77 % can be catalysed using these materials. The catalysts lifetime, however, is still limited at the present stage of our studies. Catalysts prepared from graphenes usually proved to be more active than those prepared from graphite, which was suggested to be due to the accessibility and amount of the functional groups created on the edges of the graphene sheets.