Mild Aromatic Palladium-Catalyzed Protodecarboxylation: Kinetic Assessment of the Decarboxylative Palladation and the Protodepalladation Steps

Mechanism studies of a mild palladium-catalyzed decarboxylation of aromatic carboxylic acids are described. In particular, reaction orders and activation parameters for the two stages of the transformation were determined. These studies guided development of a catalytic system capable of turnover. Further evidence reinforces that the second stage, protonation of the arylpalladium intermediate, is the rate-determining step of the reaction. The first step, decarboxylative palladation, is proposed to occur through an intramolecular electrophilic palladation pathway, which is supported by computational and mechanism studies. In contrast to the reverse reaction (C-H insertion), the data support an electrophilic aromatic substitution mechanism involving a stepwise intramolecular protonation sequence for the protodepalladation portion of the reaction.