Abstract: The AuCl_x (x=1, 3)-catalyzed benzannulation mechanisms between benzyne and 2-propynylhypnone were investigated using B3LYP, B3PW91, UB3LYP, and the second-order Møller-Plesset perturbation (MP2) methods with the LanL2DZ basis set for Au and the 6-31G^*, 6-311++G^** basis sets for C, H, O, Cl. For the AuCl or AuCl₃ catalysts the reaction occurs through both the [4 + 2] and the [3 + 2] benzannulation pathways to yield the product. With AuCl, the reaction occurs mainly through the [4 + 2] reaction pathway because of this pathway's low activation free energy. With AuCl₃, the reaction occurs by the [4+2] and the [3+2] reaction pathways. These two pathways are competitive because of their close activation free energies. An analysis of these results indicates that the gold oxidation states change the reaction mechanisms and greatly influence the reaction barriers. The calculated results indicate that the AuCl catalyst is more effective than AuCl₃ because in the reaction catalyzed by AuCl the activation free energy of the rate determining step is 11.18 kJ·mol^-1 lower than that of the reaction catalyzed by AuCl₃. These results are in good agreement with the experimental observations.

Key words: Gold catalyst, Density functional theory, Reaction mechanism, [4+2] Benzannulation, [3+2] Benzannulation