Gold Catalysts with Polyaromatic-NHC ligands. Enhancement of Activity by Addition of Pyrene

Three Au­(I) complexes with N-heterocyclic carbene ligands fused to polycyclic aromatic hydrocarbons (pyrene and phenanthrophenazine) have been obtained and fully characterized. The complexes were tested as catalysts in the hydroamination of terminal alkynes, where they showed moderate to good activity. The addition of a catalytic amount of pyrene to the reaction mixtures produced an improvement in the performance of the catalysts. The study of the reaction order of the catalyst in this reaction indicated that the order is 0.5, indicating that an off-cycle inactive catalyst dimer is formed, as consequence of the self-association of the complex by π–π stacking interactions. The reaction order in catalyst when pyrene is added to the reaction mixture is 1 > n > 0.5, therefore being higher than that in the absence of pyrene. Studies made by 1H NMR spectroscopy demonstrate that an important self-association of the catalyst occurs at the concentrations used in the catalytic reactions. The association of pyrene with the catalyst was also observed and quantified by 1H NMR titrations. The studies allowed us to conclude that pyrene influences the activity of the Au­(I) catalysts due to reasons related to the modification of the reaction order in the catalyst rather than to allosteric issues.