Rhodium-Catalyzed Copper-Assisted Intermolecular Domino C-H Annulation of 1,3-Diynes with Picolinamides: Access to Pentacyclic π-Extended Systems

A new mode of reactivity of 1,3-diynes in rhodium-catalyzed oxidative annulation reactions has enabled the rapid assembly of extended pi systems from readily available picolinamide derivatives. The process involves a double C-H bond activation and the iterative annulation of two 1,3-diyne units, with each alkyne moiety engaged in an orchestrated insertion sequence with high regiocontrol, leading to the formation of five new C-C bonds and the construction of four fused rings in a single operation. Either isoquinoline-1-carboxamides or fused polycyclic systems can be accessed by a switch in the regioselectivity of the second diyne insertion depending on the reaction conditions. DFT theoretical calculations have elucidated that the cooperative participation of both rhodium and copper in substrate activation, favored in the presence of excess of the copper(II) salt, is key to such a reversal of regioselectivity and subsequent multiple cyclization leading to fused polycyclic products. The role of copper was found to be essential in assisting both multiple insertion and rhodium-walking sequences, with the implication of intermediates with a Rh-Cu bond (2.60 angstrom).