Assessing the Cooperating Ability of 6‐Hydroxypicolinic Acid and Pyridyl‐Amide Ligands in Palladium‐Mediated C–H Activation

The behavior of 6‐hydroxypicolinic acid (pic‐6‐OH) and pyridyl‐amides as cooperating ligands in the C‐H activation of arenes has been studied experimentally. When deprotonated, both compounds are chelating ligands and bear either a pyridone moiety or and an N‐acyl substituent that can assist the C‐H cleavage in the same way as the successful bipyridone ligands and MPAAs do. In addition, they are easily available, commercially or via straightforward syntheses. Palladium complexes of formula (NBu4)[Pd(κ2‐O, N‐pic‐6‐O)(C6F5)py] (2) and [Pd(κ2‐N, N‐ py‐CH2N(COCF3)(C6F5)py] (6) have been synthesized and their decomposition in the presence of an arene gives the C6F5‐arene coupling product, showing that the ligands enable the C‐H activation of the arene (arene = pyridine, toluene, ethyl benzoate). The amount of C6F5‐arene products observed leads to the following trend in cooperating ability: pic‐6‐OH > pyridyl‐amide. DFT calculations on the pyridine activation are consistent with the experimental findings. The C‐H activation does not occur for the isomeric complex (NBu4)[Pd(κ2‐O, N‐pic‐4‐O)(C6F5)py] (4) with the pyridone oxygen far from the metal, showing the involvement of this moiety in the C‐H cleavage. The performance of these ligands in the direct arylation of arenes has been evaluated.