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

Recent advances in ligand design have enabled Pd(II)-catalyzed nondirected 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, the 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 the 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 the selectivity between the activated para- and deactivated meta-C–H bonds of silyl-protected phenols, thereby 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.