(iso-bpy)Ni: Isomeric Bipyridine Ligands in Nickel-Catalyzed Coupling of Aryl Chlorides and Alcohols Under Thermal Conditions.

In search of more active and robust catalyst systems for the nickel-catalyzed coupling of aryl halides and heteroatom nucleophiles, a core scaffold modification of the conventional 2,2’-bipyridine ‘bpy’ ligand was explored. Transposition of one of the N-atom donors of bpy into the ligand backbone results in the isomeric bipyridine ligand ‘iso-bpy’, an N,Cdonor containing a highly electron-donating, strong-field 2-pyridylidene group. A series of (iso-bpy)Ni complexes were synthesized by a directed C–H metalation or C–Cl oxidative addition approach using N-(2-pyridyl)pyridinium salt ligand precursors. (iso-bpy)Ni complexes were investigated as precatalysts for the coupling of aryl chlorides and aliphatic alcohols under thermal conditions. Precatalysts of the general structure [(iso-bpy)Ni(Cl)(2-picoline)]BF4 efficiently catalyzed C–O coupling for a range of aryl chlorides, including those bearing electron-donating substituents, and were found to provide synthetically useful yields of product at lower catalyst loadings compared to their established (bpy)Ni counterparts. This work lays the foundation for further comparative studies of bpy and iso-bpy ligands within other transition metal catalyst systems.