The Dual Effect of the Acetate Ligand on the Mechanism of the Pd‐Catalyzed C−H/C−H Coupling of Benzene

• Published in: ChemCatChem 2020-01, Vol.12 (1), p.90-94
• Main Authors: Beckers, Igor, Henrion, Mickaël, De Vos, Dirk E.
• Format: Article
• Language: English
• Subjects: Acetic acid; Activation; arenes; Aromatic compounds; Benzene; Carboxylates; Coupling; C−C coupling; C−H activation; Dissimilar metals; homogeneous catalysis; Hydrocarbons; Ligands; Oxidizing agents; Palladium; Substitution reactions
• ISSN: 1867-3880; 1867-3899
• DOI: 10.1002/cctc.201901238

Pd(II)carboxylates have emerged as potent catalysts for the direct coupling of (hetero)aromatics, enabling a two‐fold C−H activation with various oxidants. Yet, prior research on the mechanism of the C−H/C−H coupling without directing group(s) has led to contrasting insights. In this work, a direct effect of acetic acid (AcOH) concentration on the mechanism of Pd‐catalyzed benzene coupling to biphenyl is uncovered. The catalytic cycle was investigated via kinetic isotope effects, H/D exchange experiments and by assessing the reactivity of the Pd(aryl)‐intermediates. The study revealed that the catalytic reaction makes a transition between two dissimilar C−H activation steps: a “concerted metalation‐deprotonation” (CMD) mechanism followed by an electrophilic substitution (SEAr). A dual role of the acetate ligand explains the transition between these mechanisms, mediating the deprotonation via CMD and coordinating on the Pd(II) ion to preclude a SEAr mechanism for the first C−H activation. Dual Effect: The Pd(II)‐catalyzed C−H/C−H coupling of benzene was investigated by studying kinetic isotope effects, H/D exchange and the in situ formation of Pd(aryl)‐intermediates. These experiments convene into a new mechanistic view on the catalytic cycle, in which the dissociation of acetic acid mediates a transition between two dissimilar C−H activation mechanisms.