Recyclable Magnetic Cu-MOF-74-Catalyzed C(sp)-N Coupling and Cyclization under Microwave Irradiation: Synthesis of Imidazo[1,2-]quinazolines and Their Analogues.

Magnetic Cu-MOF-74 (FeO@SiO@Cu-MOF-74) was synthesized for the first time by grafting MOF-74 (copper as the metal center) on the surface of core-shell magnetic carboxyl-functionalized silica gel (FeO@SiO-COOH), which was prepared by coating core FeO nanoparticles with hydrolyzed 2-(3-(triethoxysilyl)propyl)succinic anhydride and tetraethyl orthosilicate. The structure of FeO@SiO@Cu-MOF-74 nanoparticles was characterized by Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM). The prepared FeO@SiO@Cu-MOF-74 nanoparticles could be applied as a recyclable catalyst to the synthesis of N-fused hybrid scaffolds. 2-(2-Bromoaryl)imidazoles and 2-(2-bromovinyl)imidazoles were coupled and cyclized with cyanamide in DMF in the presence of a catalytic amount of FeO@SiO@Cu-MOF-74 along with a base to give imidazo[1,2-]quinazolines and imidazo[1,2-]pyrimidines, respectively, in good yields. The FeO@SiO@Cu-MOF-74 catalyst could be easily recovered by a super magnetic bar and recycled more than four times while almost maintaining catalytic activity.