Palladium-Catalyzed Synthesis of Fluorescent Benzo[4,5]imidazo[1,2-a]pyridines through Annulation Reaction of Benzimidazoles and Alkynyl Bromides with Internal Alkynes

An efficient synthesis of fused azapolycycles based on (benz)imidazole and pyridine scaffolds has been developed. In all cases, the first nucleophilic addition of (benz)imidazoles to alkynyl bromides in tert-pentyl alcohol can proceed in a stereoselective manner to provide (Z)-N-(1-bromo-1-alken-2-yl)benzimidazoles at 110 degrees C. Sequentially, these adducts containing alkenyl bromide can undergo Pd-catalyzed intermolecular C-H annulation in the presence of internal alkynes in dimethylacetamide, affording fluorescent (benz)imidazole-fused pyridines in good to high yields. These compounds generally exhibit blue or green fluorescences (454-503 nm for solution states and 472-506 nm for solid states), and the fluorescence quantum yields remained in 0.19-0.89 and 0.02-0.74 for solution and solid states, respectively.