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.