Investigating Arylazoformamide Ligands in Palladium(II) Precatalysts through the Suzuki–Miyaura Cross-Coupling Reaction

Herein, is the reported synthesis and utilization of redox-active arylazoformamide (AAF) ligands in palladium(II) precatalysts for the Suzuki-Miyaura cross-coupling reaction. Complexes were formed from 2 equiv of an AAF ligand with Pd(II)Cl₂ in an appropriate solvent to create the square planar (AAF)₂PdCl₂ precatalyst. A thorough investigation of aryl bromides and arylboronic acids found that 1.0 mol % precatalyst with cesium carbonate (Cs₂CO₃) as base, 1,4-dioxane as solvent at 90 °C for 24 h allowed for excellent conversions to the biphenyl products (over 20 examples). To highlight the AAF ligand class, a set of comparison reactions were performed with redox-active arylazothioformamide ligands, i.e., an (ATF)₂PdCl₂ complex, other commercial palladium(II) complexes, and a Ni(II)Cl₂ arylazoformamide coordination complex₂. The (AAF)₂PdCl₂ complexes outperformed all of others tested. Mechanistically, it is proposed that the AAF ligand singly reduces to antiferromagnetically couple to the palladium(I) complex as a transmetalation intermediate. Ni-based precatalysts were found to be inactive for the studied Suzuki-Miyaura reaction. Overall, these ligand systems offer a unique look into redox-active palladium cross-coupling reactions as well as being phosphine-free and high yielding.