Ligand-Controlled Nondirected meta- or para-C-H Olefination of Silyl-Protected Phenols

Recent advances in ligand design have enabled Pd(II)-catalyzed non-directed C-H functionalization using arenes as the limiting reagent, but achieving catalyst control over the site-selectivity in these transformations remains a significant challenge. Instead, selectivity is typically governed by the inherent steric and electronic properties of the arene substrates or directing effects. Consequently, it can be difficult to selectively functionalize para-position of electron-deficient arenes and meta-positions of electron-rich arenes respectively. In this report, we demonstrate that the choice of ligand in a Pd(II)-catalyzed olefination can switch selectivity between the activated para- and deactivated meta-C-H bonds of silyl-protected phenols, highly enabling site-selective functionalization of either position with broad substrate scopes. Specifically, monodentate 2-pyridone ligands enable high-yielding olefination with the conventional para-selectivity, largely governed by the intrinsic electronic bias of the substrate, whereas a dual-ligand system consisting of a bidentate pyridine-pyridone ligand and a monodentate pyridine ligand reversed the site selectivity to favor olefination of the relatively electron-deficient meta-position. Mechanistic studies indicate that the dual ligand system selectively renders para-C-H palladation reversible, but not the meta-C-H palladation, thereby favoring the meta-C-H olefination of electron-rich arenes.