Discovery of Oxygen Induced Chemoselectivity in Pd-catalyzed C-H Functionalization: Cross Dehydrogenative Coupling vs C-H Amination

The functionalization of unactivated C-H bonds at will for the strategic introduction of bonds or functionalities have been a matter of extensive investigation for the last couple of decades. We have come across a substrate namely, 5-benzoyl-pyrrolo[2,1-a]isoquinoline in which three different functionalizable C-H bonds were identified that could be judiciously transformed site selectively for the generation of complex polyring fused N-heterocycles. At first, we attempted a Pd-catalyzed cross-dehydrogenative coupling in 5-benzoyl-pyrrolo[2,1-a]isoquinoline towards the synthesis of 8H-indeno-pyrrolo[2,1-a]isoquinolinones. Later, we identified a hitherto unknown oxygen induced palladium catalyzed selective C-H amination in 5-benzoyl-pyrrolo[2,1-a]isoquinoline towards a pentacene viz., 9H-indolo-pyrrolo[2,1-a]isoquinoline. Finally, we came across an unexpected site selective C-H amination towards the formation of multiring fused benzazepine which is believed to form due to its stability and to the higher electron density at the reaction centre on isoquinoline ring.