Erbium-Organic Framework as Heterogeneous Lewis Acid Catalysis for Hantzsch Coupling and Tetrahydro-4H-Chromene Synthesis

An Erbium-organic framework was prepared by hydrothermal reaction. The prepared framework was characterized by Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM), and X-ray powder diffraction (XRD). The framework has open metal sites at Er(III) centers, thus providing an accessible Lewis acid center for electrophile activation. Accordingly, the synthesized framework was used as Lewis acid heterogeneous catalyst for Hantzsch coupling reaction and tetrahydro-4H-chromene synthesis. The reaction condition has been optimized by variation of the reaction time, temperature, solvent and catalyst concentration. A variety of tetrahydro-4H-chromenes was synthesized and characterized by FT-IR and 1 H NMR spectroscopy. Er-MOF, as a Lewis acid heterogeneous catalyst, showed excellent selectivity and high yield for these transformations. Graphical Abstract