New Phosphine-Functionalized NHC Ligands: Discovery of an Effective Catalyst for the Room-Temperature Amination of Aryl Chlorides with Primary and Secondary Amines

We report convenient and high-yielding syntheses of new phosphine-functionalized dihydroimidazolium salts and demonstrate their utility as ligand precursors for Buchwald–Hartwig amination. Several examples of the general formula [1-Mes-3-{2-(PR2)phenyl}imidazolidin-2-ylium][BF4] have been prepared, where phosphines of varying steric and electronic properties (R = Ph (9), Cy (10), 1-Ad (11)) are tethered by an o-phenylene group. The synthesis was not adaptable to N-aryl groups other than mesityl, giving unexpected phosphonium salt species instead. The synthesis was adapted to flexible benzyl-linked variants of the formula [1-Ar-3-{2-(PCy2)benzyl}imidazolidin-2-ylium][BF4], which allowed more steric variation of the dihydroimidazolium N-aryl group (Ar = Mes (21), Dipp (22)). A preliminary study of these hybrid NHC/P ligands in Buchwald–Hartwig amination catalysis (in situ precatalyst formation) revealed 11 to be the most active of the series. Premixing the isolated free NHC ligand 1-Mes-3-{2-(PAd2)phenyl}imidazolidin-2-ylidene (23) with [Pd(cinnamyl)Cl]2 provided a highly active precatalyst that performed well at room temperature and 1 mol % catalyst loading. The system was shown to have an unprecedented ability to arylate both primary alkylamines (monoarylation) and secondary dialkylamines with aryl chlorides at room temperature. Electron-rich and -poor aryl and heteroaryl halides, as well as those featuring ortho substitution, were well tolerated, while substrates featuring both primary and secondary amine groups were selectively arylated at the NH2 position. Furthermore, a preliminary examination of performance in ammonia arylation and acetone α-arylation showed promising results, giving good conversion and high selectivity for monoarylation in both cases.