Computational Study on Cycloisomerization/Oxidative Dimerization of Aryl Propargyl Ethers Catalyzed by Gold Nanoclusters: Mechanism and Selectivity

A theoretical analysis of the cycloisomerization and oxidative dimerization of phenyl propargyl ether catalyzed by the Au₃₈ cluster is performed by means of density functional theory. The role of the cationic gold species is also clarified. The substituent effect at the para site of phenyl is studied to explore the selectivity of cycloisomerization and oxidative dimerization. Phenyl propargyl ether preferred to adsorb at the T1 site of the Au₃₈ surface with an adsorption energy of −10.61 kcal/mol. The 6-<i>endo</i> pathway to give 2<i>H</i>-chromene is the most feasible pathway, with an energy barrier of 20.50 kcal/mol in dichloroethane solvent. The energy barriers of the 5-<i>exo</i> and oxidative dimerization pathways in dichloroethane solvent are 25.81 and 30.14 kcal/mol, respectively. 2<i>H</i>-Chromene is the main product of the cycloisomerization of phenyl propargyl ether catalyzed by the gold cluster. The presence of the cationic gold species can increase the yield of dimeric 2<i>H</i>-chromene, which is in agreement with experiment results. The binding strength between the active sites and 2<i>H</i>-chromen-3-yl is crucial for oxidative dimerization. Substituents at the para site of phenyl have only a slight influence on the 6-<i>endo</i> pathway, except for the methoxyl group. The differences in the energy barriers between cycloisomerization and oxidative dimerization are in agreement with the ratio of 2<i>H</i>-chromenes and 2<i>H</i>,2′<i>H</i>-4,4′-bichromenes obtained in experiments. The selectivity for the 6-<i>endo</i>/dimeric pathways is sensitive to the substituents of the substrates and the electronic prosperities of the active site of the catalysts. Our theoretical results are in agreement with the product distribution and phenomena in experiments