Systematic Variation of Bidentate Ligands Used in Aryl Halide Amination. Unexpected Effects of Steric, Electronic, and Geometric Perturbations

This paper presents effects of varying bidentate phosphine steric properties, electronic properties, and bite angle on product ratios in the amination of aryl bromides. Comparisons of the ratios of amine products to dehydrohalogenation products showed that catalysts containing electron rich, modestly hindered phosphines with small bite angles (∼90°) gave the best selectivities. Surprisingly, the arene side product formed from reaction of alkylamines deuterated in the N−H position or deuterated in the position α to the nitrogen showed low levels of deuterium incorporation in many examples. Steric properties and ligand bite angle had the greatest impact on the selectivity for monoarylation versus diarylation of primary amines; ligands with small bite angles gave higher monoarylation-to-diarylation ratios, as did ligands with increased steric bulk. Electron poor or sterically hindered bidentate phosphines reduced the amount of product resulting from aryl exchange of electron rich palladium-bound arenes with those of aryl groups on the phosphine ligands.