Is Pd-II-Promoted sigma-Bond Metathesis Mechanism Operative for the Pd-PEPPSI Complex-Catalyzed Amination of Chlorobenzene with Aniline? Experiment and Theory

摘要

Reduction of the Pd-PEPPSI precatalyst to a Pd-0 species is generally thought to be essential to drive Buchwald-Hartwig amination reactions through the well-documented Pd-0/Pd-II catalytic cycle and little attention has been paid to other possible mechanisms. Considered here is the Pd-PEPPSI-catalyzed aryl amination of chlorobenzene with aniline. A neat reaction system was used in new experiments, from which the potentially reductive roles of the solvent and labile ligand of the PEPPSI complex in leading to Pd-0 species are ruled out. Computational results demonstrate that anilido-containing Pd-II intermediates involving sigma-bond metathesis in pathways leading to the diphenylamine product have relatively low barriers. Such pathways are more favorable energetically than the corresponding reductive elimination reactions resulting in Pd-0 species and other putative routes, such as the Pd-II/Pd-IV mechanism, single electron transfer mechanism, and halide atom transfer mechanism. In some special cases, if reactants/additives are inadequate to reduce a Pd-II precatalyst, a Pd-II-involved sigma-bond metathesis mechanism might be feasible to drive the Buchwald-Hartwig amination reactions.