Elucidation of Concerted and Non-Concerted Mechanisms in Pd-Catalyzed Direct C–H Arylation of Heterocycles: A DFT Investigation

Direct C–H arylation has emerged as an efficient and sustainable strategy for the functionalization of heterocyclic compounds, avoiding the need for pre-functionalized substrates. However, the mechanistic aspects governing these transformations remain under debate, particularly regarding the nature of the C–H activation step. In this work, a density functional theory (DFT) study was conducted to investigate the reaction mechanism of Pd-catalyzed direct arylation of heterocycles. Several orientations of the catalytic system were initially explored to identify the most favorable reaction pathway. Subsequently, transition states and reaction intermediates involved in the catalytic cycle were characterized.

Special attention was devoted to comparing the concerted metalation–deprotonation (CMD) mechanism with alternative pathways proposed in the literature. The corresponding free energy profiles were determined, allowing the identification of the most energetically favorable route. Solvent effects were also considered through an implicit solvation model to evaluate their influence on the activation barriers and overall reaction energetics.

The results provide valuable insights into the origin of regioselectivity and the factors controlling the efficiency of Pd-catalyzed C–H activation processes. This study contributes to a deeper understanding of the mechanistic landscape of direct arylation reactions and may assist in the rational design of improved catalytic systems with potential pharmaceutical and industrial applications.