A Chiral Nanohoop as Highly Efficient Asymmetric Organocatalyst

Chiral phosphoric acids are privileged organocatalysts that have been shown to facilitate a large variety of asymmetric transformations. In recent years, the BINOL scaffold has been equipped with large aromatic groups and transformed into dimeric imidodiphosphates to im-prove both chiral induction and catalyst turnover by tuning pKa and creating a confined space around the catalytic center. In this work, we report an alternative approach for achieving such a confinement effect within the cavity of a chiral, shape-persistent “carbon nanohoop” mac-rocycle. We integrated a BINOL-derived phosphoric acid into the [9]cycloparaphenylene (CPP) scaffold and employed the nanohoop as organocatalyst for the asymmetric transfer hydrogenation of quinolines. We found that the chiral macrocycle shows excellent catalytic activity with near-quantitative yields and enantioselectivities up to 96% ee, which is far supe-rior to comparable non-cyclic reference catalysts. While the scope for quinolines bearing aromatic substituents is wide, we made the counterintuitive observation that the macrocyclic catalyst is not active for smaller alkyl-substituted substrates, which indicates that highly spe-cific non-covalent effects determine the reaction outcome within the nanohoop cavity. These results suggest that outstanding selectivities can be achieved by endowing organocatalysts not only with supramolecular binding sites but also with unusual topologies.