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

Abstract

AbstractThe 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 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.