The Importance of Metal–Ligand Cooperativity in the Phosphorus–Nitrogen PN³P Platform: A Computational Study on Mn-Catalyzed Pyrrole Synthesis

We studied the mechanism of the pyrrole synthesis catalyzed by a PN³P–Mn pincer ligand system and show that a bifunctional double hydrogen transfer mechanism is favored over the alternative β-hydride elimination. Both dehydrogenation and hydrogen-formation steps benefit from proton shuttles, with alcohol-mediated processes being consistently favored, leading to energy barriers that, in good agreement with the experimental results, are similar to those for the previously reported corresponding iridium-catalyzed process. We also show that the coordination of one potassium ion to the ligand lowers the energy barriers for the key steps. The overall rate-determining step is the regeneration of the catalyst with an energy barrier of 30.7 kcal/mol with potassium and 31.1 kcal/mol without potassium. Our results support the involvement and the importance of the aromatization/dearomatization paradigm in the reaction.