Photoinduced electron transfer reaction for synthesis of tetrahydroquinoline derivatives with conjugated structure using chlorophyll b‐modified magnetic titanium dioxide photocatalyst

Abstract

Photocatalytic conversion of organic compounds has recently emerged as a cost‐effective, safe, and easy‐to‐operate procedure to synthesize value‐added materials. In this study, the magnetic titanium dioxide‐based (Fe3O4/SiO2/TiO2) photocatalyst was designed to synthesize conjugated derivatives of tetrahydroquinoline through a photoinduced electron transfer (PET) reaction. Chlorophyll b was immobilized on the surface of magnetic titanium dioxide, as a natural visible‐light‐sensitive compound using 3‐aminopropyltriethoxysilane (APTES) as a coupling agent containing terminal amine (Fe3O4/SiO2/TiO2‐NH2‐Chb) to improve light harvesting ability. Fourier transform infrared (FT‐IR), powder X‐ray diffraction (PXRD), thermogravimetric analysis (TGA), energy‐dispersive X‐ray spectrometer (EDS), and vibrating sample magnetometer (VSM) results confirmed the successful synthesis of Fe3O4/SiO2/TiO2‐NH2‐Chb. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images displayed the preserved spherical morphology of Fe3O4/SiO2/TiO2‐NH2‐Chb. Chlorophyll b‐modified magnetic titanium dioxide was highly active under visible‐light irradiation toward the cyclization of (E)‐3‐[4‐(dimethylamino)phenyl)]‐1‐arylprop‐2‐en‐1‐one with 1‐aryl‐1H‐pyrrole‐2,5‐dione to achieve new tetrahydroquinoline derivatives with conjugated structure in high yields at ambient temperature, in air. The incorporation of chlorophyll b in the photocatalyst plays an important role in the photocatalytic mechanism, facilitating photoinduced electron transfer to the conduction band of TiO2. Moreover, the magnetic property enabled easy recovery of the photocatalyst and improved its reusability up to three runs. The characteristics of tetrahydroquinoline derivatives were studied by FT‐IR, CHN, 1H NMR, and 13C NMR analyses.