Asymmetric photocatalysis over robust covalent organic frameworks with tetrahydroquinoline linkage

The asymmetric photocatalytic organic synthesis (APOS) process is a sustainable and environmentally benign method for the production of optically active chemicals with sunlight as an energy source. However, it still lacks efficient semiconductors with tunable band structures and has a low recycling stability. Herein, we report the synthesis of tetrahydroquinoline-linked covalent organic frameworks (QH-COFs) with irreversible tetrahydroquinoline linkage as efficient semiconductors for the visible-light-driven asymmetric α-alkylation of aldehydes by merging with a chiral secondary amine. Up to 94% ee was obtained over QH-COFs, and the activity of QH-COFs was significantly higher than those of inorganic semiconductors (e.g., TiO2, BiVO4, and WO3) under similar conditions, which is mainly attributed to their narrow band gap and suitable band edge. As far as we know, QH-COFs are the most active semiconductors for asymmetric α-alkylation of aldehydes ever reported. The QH-COFs were prepared via a one-pot Povarov cascade imine formation and cycloaddition reaction using Sc(OTf)3/Yb(OTf)3 as Lewis acid catalysts. Attributed to the tetrahydroquinoline linkage, QH-COFs showed extremely high recycling stability, which made practicals application possible. This work not only opens up a new avenue for asymmetric photocatalysis but also provides an efficient and general method for the construction of robust COFs.