Assessing the cooperating ability of 6‐hydroxypicolinic acid and pyridyl‐amide ligands in palladium‐mediated C−H activation

Abstract

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 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(k2-O, N-pic-6-O)(C6F5)py] (2) and [Pd(k2-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 ben- zoate). 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 consis- tent with the experimental findings. The C-H activation does not occur for the isomeric complex (NBu4)[Pd(k2-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.