Investigating Arylazoformamide\nLigands in Palladium(II)\nPrecatalysts through the Suzuki–Miyaura Cross-Coupling Reaction

Herein,\nis the reported synthesis and utilization of\nredox-active\narylazoformamide (AAF) ligands in palladium(II) precatalysts for the\nSuzuki–Miyaura cross-coupling reaction. Complexes were formed\nfrom 2 equiv of an AAF ligand with Pd(II)Cl<sub>2</sub> in an appropriate\nsolvent to create the square planar (AAF)<sub>2</sub>PdCl<sub>2</sub> precatalyst. A thorough investigation of aryl bromides and arylboronic\nacids found that 1.0 mol % precatalyst with cesium carbonate (Cs<sub>2</sub>CO<sub>3</sub>) as base, 1,4-dioxane as solvent at 90 °C\nfor 24 h allowed for excellent conversions to the biphenyl products\n(over 20 examples). To highlight the AAF ligand class, a set of comparison\nreactions were performed with redox-active arylazothioformamide ligands,\ni.e., an (ATF)<sub>2</sub>PdCl<sub>2</sub> complex, other commercial\npalladium(II) complexes, and a Ni(II)Cl<sub>2</sub> arylazoformamide\ncoordination complex<sub>2</sub>. The (AAF)<sub>2</sub>PdCl<sub>2</sub> complexes outperformed all of others tested. Mechanistically, it\nis proposed that the AAF ligand singly reduces to antiferromagnetically\ncouple to the palladium(I) complex as a transmetalation intermediate.\nNi-based precatalysts were found to be inactive for the studied Suzuki–Miyaura\nreaction. Overall, these ligand systems offer a unique look into redox-active\npalladium cross-coupling reactions as well as being phosphine-free\nand high yielding.