Recyclable Magnetic Cu-MOF-74-Catalyzed C(sp²)-N Coupling and Cyclization under Microwave Irradiation: Synthesis of Imidazo[1,2-c]quinazolines and Their Analogues

Abstract

Magnetic Cu-MOF-74 (Fe 3 O 4 @SiO 2 @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 (Fe 3 O 4 @SiO 2 -COOH), which was prepared by coating core Fe 3 O 4 nanoparticles with hydrolyzed 2-(3-(triethoxysilyl)propyl)succinic anhydride and tetraethyl orthosilicate. The structure of Fe 3 O 4 @SiO 2 @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 Fe 3 O 4 @SiO 2 @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 Fe 3 O 4 @SiO 2 @Cu-MOF-74 along with a base to give imidazo[1,2- c ]quinazolines and imidazo[1,2- c ]pyrimidines, respectively, in good yields. The Fe 3 O 4 @SiO 2 @Cu-MOF-74 catalyst could be easily recovered by a super magnetic bar and recycled more than four times while almost maintaining catalytic activity.

Imidazopyridine Family: Versatile and Promising Heterocyclic Skeletons for Different Applications

“…Many of the developed synthetic approaches encountered certain limitations, often associated with the use of various catalysts such as Se(IV) [ 38 , 220 , 221 ], Pd(II) [ 222 , 223 ], Fe(II) and Fe(III) [ 224 , 225 ], Cu(II) ion [ 226 , 227 , 228 , 229 ], CuCl 2 [ 230 , 231 , 232 ], copper/iodine [ 233 ], Cu-MOF structures [ 234 , 235 ], boron trifluoride etherate [ 236 ], CBr 4 [ 237 ] or MnO 2 [ 53 ]. In addition, some catalysts require further reagents or co-catalysts such as selenium dioxide [ 238 ], dehydrating agents (e.g., molecular sieves [ 239 ] or propane phosphoric acid anhydride [ 213 ]), Lewis acids (BF 3 ), or oxidant agents (e.g., oxygen, iodine [ 240 , 241 ], elemental sulfur [ 242 ], or tert -butylperoxybenzoate) [ 243 ].…”

“…Other important examples of innovative synthetic approaches for particular imidazopyridines have been recently reported, focusing on the following: imidazo[1,2- a ]quinoline [ 222 , 244 , 245 ]; imidazo[1,2- c ]quinazoline [ 234 ]; imidazo[1,5- a ]quinoline [ 246 , 247 ]; imidazo[1,5- a ]pyridine [ 226 , 235 , 248 , 249 , 250 , 251 , 252 ]; imidazo[1,2- a ]isoquinoline [ 221 ]; imidazo[1,5- a ]isoquinoline [ 36 , 42 , 63 , 247 ]; imidazo[5,1- a ]isoquinoline [ 253 , 254 ]; imidazo[4,5- c ]pyridine [ 28 , 255 , 256 ]. …”