Dissecting Anion Effects in Gold(I)-Catalyzed Intermolecular Cycloadditions

From a series of gold complexes of the type [t-BuXPhosAu(MeCN)]X (X=anion), the best results in intermolecular gold(I)-catalyzed reactions are obtained with the complex with the bulky and soft anion BAr4F- [BAr4F-=3,5-bis(trifluoromethyl)phenylborate] improving the original protocols by 10-30% yield. A kinetic study on the [2+2]cycloaddition reaction of alkynes with alkenes is consistent with an scenario in which the rate-determining step is the ligand exchange to generate the ((2)-phenylacetylene)gold(I) complex. We have studied in detail the subtle differences that can be attributed to the anion in this formation, which result in a substantial decrease in the formation of unproductive sigma,-(alkyne)digold(I) complexes by destabilizing the conjugated acid formed.