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

Abstract

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 (SbF6- and OTs-) were studied as model catalysts. According to our calculation, for SbF6-, 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 SbF6- 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.