C-N bond formation employing palladium and nickel precatalysts containing anionic phosphinobenzimidazole ligands

Abstract

In this investigation, the catalytic utility of anionic, phosphinobenzimidzoles [Li(THF)2][1a] and [Li(THF)4][1b] in effecting the Buchwald-­Hartwig amination reaction was examined. Specifically, the coordination chemistry of ligands [Li(THF)2][1a] and [Li(THF)4][1b] with palladium was studied in an attempt to synthesize a distinct precatalyst containing either 1a or 1b that could be screened for catalytic activity in the reaction of interest. Additionally, the screening of a Pd/[Li(THF)4][1b] catalyst system for activity in the Buchwald-­Hartwig amination reaction was conducted concurrently. Complex 6, containing 1a, and complexes 7-­12, containing 1b, were successfully synthesized, and characterized using multi-­nuclear NMR spectroscopy. An optimized [Pd(cinnamyl)Cl]2/[Li(THF)4][1b] catalyst system was also developed, and performed the coupling of activated and deactivated aryl bromides with a selection of amines in moderate to excellent yields (36-­97%). A preference for primary vs. secondary amine coupling partners, as well as a high selectivity for the monoarylated product, was observed during the course of these studies. To highlight the remarkable potential of anionic ligands in catalysis, the catalytic efficiency of the [Pd(cinnamyl)Cl]2/[Li(THF)4][1b] catalyst system was compared with a corresponding system utilizing the related, neutral phosphinobenzimidazole 2, with the anionic system displaying superior catalytic activity in the Buchwald-­Hartwig amination reaction in almost all cases. This phenomenon was attributed to the more electron-­rich catalytic intermediates present in the anionic system, enabling more rapid oxidative addition, and thus more efficient catalysis. Further screening of the anionic precatalyst [PPh4][PdCl2(k2-­1b)] (10) and neutral precatalyst [PdCl2(k2-­2)] (13) corroborated these results. Finally, preliminary studies examining the potential of 1b as an ancillary ligand in Ni-­catalyzed C-­N bond formation was undertaken. The coordination chemistry of 1b with nickel was explored, resulting in the formation of complex 14, whose tentative structure was assigned on the basis of NMR spectroscopic evidence. Furthermore, initial screening of a NiCl2(DME)/[Li(THF)4][1b] catalyst system in the cross-­coupling of chlorobenzene and aniline did not result in observable product formation under various reaction conditions.