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

Abstract

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)-6phenylpyridine) 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′diphenyl-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.

Nickel and Palladium Complexes of a PP(O)P Pincer Ligand Based upon a peri-Substituted Acenaphthyl Scaffold and a Secondary Phosphine Oxide

“…As such, the 1 H NMR spectra for the isolated complex show major solvent contaminations. Assignment of the aliphatic region was based on comparison with literature values for similar cyclooctenyl nickel (II) complexes. , …”

Cyclometalated Ni(II) Complexes [Ni(N∧C∧N)X] of the Tridentate 2,6-di(2-pyridyl)phen-ide Ligand

“…Cyclometalated organometallic nickel­(II) complexes have garnered enormous interest in recent years due to their role as catalysts or important intermediates in organometallic processes. In particular, complexes with a tridentate heteroaromatic ligand scaffold are considered robust and electronically flexible and represent a double directing group (DG) approach for C–H functionalizations, − C–C coupling reactions, and related ,− reactions. ,,, The simplest tridentate N ∧ N ∧ N heteroaromatic ligand scaffold is 2,2′;6′,2″-terpyridine (terpy) which has been used in Negishi-type C–C cross-coupling catalysis. ,,− Some years ago, we studied the first cyclometalated C ∧ N ∧ N Ni­(II) derivative using the anionic – Phbpy (HPhbpy = 6-phenyl-2,2′-bipyridine) ligand. Complex [Ni­(Phbpy)­Br] was initially obtained through oxidative addition of the brominated ligand precursor Br–Phbpy and [Ni­(COD) 2 ], ,, and we also reported a number of derivatives containing various substituents on the Phbpy ligand .…”

Advances in C(sp2)−H/C(sp2)−H Oxidative Coupling of (Hetero)arenes Using 3d Transition Metal Catalysts

“…The intramolecular cyclonickelation and reductive elimination resulted in Ni(I) species, which upon oxidation with Ag 2 CO 3 regenerates Ni(III) for the next catalytic cycle. Very recently, Anderson and co‐workers disclosed an example of Ni(0)‐mediated oxidative coupling of bulky phosphino‐pyridine [90] …”