Observing the Agostic Hydrogen in Pd(II)-Catalyzed Aromatic C–H Activation

Abstract

Direct C–H activation and functionalization offer a convenient protocol for pharmaceutical and material syntheses. Although versatile mechanisms have been proposed to depict transition-metal-catalyzed C–H activation, to date, the shared key agostic hydrogen intermediate in several major mechanisms has not been observed yet, which apparently puzzles the mechanism-based catalyst design. This work reports the direct observations of this intermediate in Pd(II)/Sc(III)-catalyzed C–H activation of acetanilides, and its stability and reactivity in C–H activation are investigated. Remarkably, this intermediate is only observed in electron-rich acetanilides, and the meta-substituent with increased σm constant generally accelerates C–H activation, a characteristic of the base-assisted C–H activation mechanism. This study has unveiled the masks of this intermediate with an understanding of its first-hand physicochemical properties, shedding new light on mechanism-based catalyst design.