Investigation of the Cycloisomerization of 1,6-Enynes Catalyzed by Gold Nanoparticles with First-Principles Calculations: Mechanism and Selectivity

The detailed mechanism of 1,6-enyne cycloisomerization catalyzed by the bare Au-38 cluster was investigated with density functional theory calculations. It was found that the single 5-exo-cleavage pathway is preferred on gold nanocluster because of the absence of unoccupied d orbitals of the surface Au atoms of the Au cluster, while the double 5-exo-cleavage pathway is favored for Au(I) complex-catalyzed 1,6-enyne cycloisomerization owing to the strong orbital interaction between the unoccupied d orbitals of the Au(I) complexes and the p orbitals of the substrate in the transition state. The regio-selectivity of 1,6-enyne cycloisomerization catalyzed by Au cluster is governed by the deformation of structure of the substrate fragment in the reaction process. However, the controlling factor for Au(I) complex-catalyzed 1,6-enyne cycloisomerization is the variation of the binding energies between the catalyst and the substrate fragments. The catalytic activity of Au-38(2+) is comparable with that of Au-38 but the catalytic activities of Au-38(+) and Au-38(3+) are lower than those of Au-38 and Au-38(2+). The cationic gold species could not effectively catalyze the 1,6-enyne cycloisomerization. The dichloroethane solvent could lower the energy barriers of the second step of the reaction pathway.