Nickel-Mediated Dehydrogenative Aryl-Aryl Homocoupling of a Bulky Phosphino-Pyridine

Aryl-aryl bond formation through dehydrogenative coupling is an attractive transformation due to the atom and step economy of working with unfunctionalized C-H bonds. Pd catalysts are most common for this process, but Ni complexes have also been targeted as less expensive and more abundant alternatives. Here we report the ability of Ni-0 to activate a sterically encumbered phosphino-pyridine ligand with resulting dehydrogenative aryl-aryl homocoupling. The net H-2 equivalent is transferred to the coligand, resulting in hydrogenation of an olefin unit. We have investigated the mechanism of this process by stirring Ni(1,5-COD)(2) (COD = cyclooctadiene) and the PNPh ligand (PNPh = 2-((di-tert-butylphosphino)methyl)-6-phenylpyridine) at mild temperatures to afford a Ni-II complex. Isolation and characterization shows a PNPh ligand coordinated to Ni-II through an activated pyridine carbon and the directing phosphine. The coligand is an activated allylic cyclooctenyl fragment resulting from partial hydrogenation of 1,5-COD. We propose that this intermediate reacts intermolecularly with 1 equiv of itself, enabling the isolation of a bi-PNPh compound (bi-PNPh = 2,2'-bis((di-tert-butylphosphino)methyl)-6,6'-dipheny1-3,3'-bipyridine), coupled through the activated pyridyl position. This dehydrogenative coupling, although stoichiometric, demonstrates the potential of Ni-0-mediated C(sp(2))-H activation and homocoupling for synthetic applications.