Computational Exploration of Counterion Effects in Gold(I)-Catalyzed Cycloisomerization of ortho-(Alkynyl)styrenes

[Image: see text] A detailed theoretical analysis of the mechanism and chemoselectivity for gold(I)-catalyzed reaction of o-(alkynyl)styrene containing an isopropyl and a methyl at the terminal position of the alkene has been reported in this work. Two different counterions (SbF(6)(–) and OTs(–)) were studied as model catalysts. According to our calculation, for SbF(6)(–), the reaction pathway is more prone to direct 1,2-H shifts (isopropyl H) than the elimination and ring expansion pathway. However, an elimination pathway affords the indenyl derivative by forming p-toluenesulfonic acid (HOTs), which may be the main pathway in the presence of OTs(–). The chemoselectivity for the title reaction is mainly determined by the electronic effect of the counterion and the substituent rather than the steric effect. In other words, less basic SbF(6)(–) mainly provides the charge separation effect rather than assisted proton elimination. However, the more basic OTs(–) mainly assist proton elimination through the formation of HOTs. In addition to the good agreement with the experimental data, the density functional theory results also provide a significant contribution to the understanding of the reaction mechanism.