Pd-catalyzed annulation of imidazo[1,2-]pyridines with coumarins and indoles: synthesis of benzofuran and indole fused heterocycles.

Palladium-catalyzed annulation of imidazo[1,2-]pyridines with coumarins provided benzofuran fused transannulated products in good to excellent yields a decarbonylative approach, while imidazo[1,2-]pyridines with -methyl indoles in the presence of palladium and base yielded conjugated imidazopyridine fused indole derivatives. Additional experiments revealed that the presence of the phenyl ring at the C-2 position of imidazo[1,2-]pyridines is essential for the annulation than the alkyl groups. Both transformations follow the ionic mechanism by Pd-catalyzed double C-H bond activation. All the annulated products were shown to exhibit high fluorescence characteristics and their photophysical properties were evaluated.