Development of an Aryl Amination Catalyst with Broad Scope Guided by Consideration of Catalyst Stability.

We have developed a new dialkylbiaryl monophosphine ligand, GPhos, that supports a palladium catalyst capable of promoting carbon-nitrogen cross-coupling reactions between a variety of primary amines and aryl halides; in many cases, these reactions can be carried out at room temperature. The reaction development was guided by the idea that the productivity of catalysts employing BrettPhos-like ligands is limited by their lack of stability at room temperature. Specifically, it was hypothesized that primary amine and N-heteroaromatic substrates can displace the phosphine ligand, leading to the formation of catalytically dormant palladium complexes that reactivate only upon heating. This notion was supported by the synthesis and kinetic study of a putative off-cycle Pd complex. Consideration of this off-cycle species, together with the identification of substrate classes that are not effectively coupled at room temperature using previous catalysts, led to the design of a new dialkylbiaryl monophosphine ligand. An Ot-Bu substituent was added ortho to the dialkylphosphino group of the ligand framework to improve the stability of the most active catalyst conformer. To offset the increased size of this substituent, we also removed the para i-Pr group of the non-phosphorus-containing ring, which allowed the catalyst to accommodate binding of even very large α-tertiary primary amine nucleophiles. In comparison to previous catalysts, the GPhos-supported catalyst exhibits better reactivity both under ambient conditions and at elevated temperatures. Its use allows for the coupling of a range of amine nucleophiles, including (1) unhindered, (2) five-membered-ring N-heterocycle-containing, and (3) α-tertiary primary amines, each of which previously required a different catalyst to achieve optimal results.

Biaryl Phosphine Based Pd(II) Amido Complexes: The Effect of Ligand Structure on Reductive Elimination.

Kinetic studies conducted under both catalytic and stoichiometric conditions were employed to investigate the reductive elimination of RuPhos (2-dicyclohexylphosphino-2',6'-diisopropoxybiphenyl) based palladium amido complexes. These complexes were found to be the resting state in Pd-catalyzed cross-coupling reactions for a range of aryl halides and diarylamines. Hammett plots demonstrated that Pd(II) amido complexes derived from electron-deficient aryl halides or electron-rich diarylamines undergo faster rates of reductive elimination. A Hammett study employing SPhos (2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl) and analogues of SPhos demonstrated that electron donation of the "lower" aryl group is key to the stability of the amido complex with respect to reductive elimination. The rate of reductive elimination of an amido complex based on a BrettPhos-RuPhos hybrid ligand (2-(dicyclohexylphosphino)-3,6-dimethoxy-2',6'-diisopropoxybiphenyl) demonstrated that the presence of the 3-methoxy substituent on the "upper" ring of the ligand slows the rate of reductive elimination. These studies indicate that reductive elimination occurs readily for more nucleophilic amines such as N-alkyl anilines, N,N-dialkyl amines, and primary aliphatic amines using this class of ligands.