Immobilization of papain onto nitrogen graphene quantum dots and its use as an efficient catalyst for facile MW-assisted synthesis of polyhydroquinolines

A fast and environmentally benign procedure for the preparing of polyhydroquinolines using graphene quantum dots immobilized papain as an efficient catalyst was introduced. Papain, as a suitable known catalyst in organic synthetic chemistry, was immobilized onto nitrogen graphene quantum dots and used as a very powerful heterogeneous catalyst for the MW-assisted synthesis of various polyhydroquinoline derivatives. The Fourier transform infrared spectroscopy, X-ray diffraction pattern, scanning electron microscopy, and field emission scanning electron microscopy analysis were used for the characterization of the structure of newly prepared catalyst. These important organic compounds were prepared through the four-component fusion of dimedone, ethyl acetoacetate, and ammonia with different chosen arylaldehydes, under rapid, simple, and mild solvent-free conditions. This protocol offers some interesting benefits such as excellent yields (98–100%), very easy handling, simple workup procedure, economy, cleaner reaction profile, and shorter reaction times (2–3 min). All the products were characterized with 1 H-NMR, 13 C-NMR, and FT‐IR spectroscopy techniques. Graphical abstract