Palladium(II) Complexes with Anionic N-Heterocyclic Carbene–Borate Ligands as Catalysts for the Amination of Aryl Halides

Abstract

A series of (allyl)palladium(II) complexes of anionic N-heterocyclic carbenes that contain a weakly coordinating borate moiety (WCA-NHC) were prepared by deprotonation of the carbene IPr with n-butyllithium in the 4-position, which was followed by addition of B(C6F5)3 and reaction of the resulting lithium carbene–borate with [(η3-allyl)Pd(μ-Cl)]2 (allyl = 2-propenyl, 2-butenyl, 2-methyl-2-propenyl, 1-phenyl-2-propenyl). In polar solvents such as tetrahydrofuran, chloropalladate complexes of the type [Li(THF)3][(WCA-NHC)PdCl(η3-allyl)] were formed, whereas the reactions in toluene afforded neutral complexes of the type [(WCA-NHC)Pd(η3-allyl)], which exhibit an intramolecular interaction with the N-aryl groups of the carbene ligands. The allyl-palladium complexes were employed as catalysts for the Buchwald–Hartwig amination of various aryl halides with morpholine.