Palladium-Mediated Site-Selective C–H Bond Activation and Arylation of 9(10<i>H</i>)-Acridinone and Mechanistic Investigation: Stoichiometric and Catalytic Approaches

Palladium-mediated site-selective C–H bond activations of 9(10H)-acridinone, resulting in the formation of 4- and 1-arylated 9(10H)-acridinones, are presented through the use of both stoichiometric and catalytic experiments. In the stoichiometric reactions, two distinct classes of acridinone palladacycles are generated via C (4)–H and C (1)–H bond activations, with the pyridine and ketone groups serving as the directing group, respectively. The constitutional conformation and thermal stability of these palladacycles were elucidated through both X-ray crystallography and NMR spectroscopy. Additionally, an investigation was conducted into the interconversion of C (4 and 1)–H bond-activated palladacycles in trifluoroacetic acid and acetic acid. During the course of C (1)–H bond arylation, a serendipitous discovery led to the isolation of a chromeno [4,3,2-kl] acridinone compound as a byproduct, showcasing good fluorescence properties. Controlled experiments, kinetic isotope effect study, key palladacycles, and the formation of corresponding products support the proposed mechanisms for these presented reactions. Finally, the pyridinyl group can serve as a removable directing group and can be readily eliminated from both 4- and 1-arylated 9(10H)-acridinones.