RhIII-Catalyzed C(sp3)-H Bond Activation by an External Base Metalation/Deprotonation Mechanism: A Theoretical Study

The C(sp(3))-H bond activation of 8-methylquinoline followed by alkyne insertion catalyzed by a Rh-III complex has been studied by using density functional theory (DFT) calculations. Contrary to common belief, the C-H bond activation of methylquinoline does not occur by the traditional intramolecular concerted metalation/deprotonation (CMD) mechanism but by an external base CMD mechanism. The use of free acetate or copper(II) acetate as base permits the C-H activation step, as observed experimentally. However, the following insertion is possible only if copper(II) acetate is used. The insertion followed by metathesis occurs via a cationic Rh-III complex and is irreversible, which ensures the efficiency of the entire process. Therefore the use of copper is crucial for completing the catalytic cycle. The present work should help to rationalize the origins of the experimental results described in the literature.