Cyclopropenylmethyl Cation: A Concealed Intermediate in Gold(I)-Catalyzed Reactions

The last years have witnessed many gold-catalyzed reactions of alkynes. One of the most prominent species in the reaction of two alkyne units is the vinyl-substituted gold vinylidene intermediate. Here, we were able to show that the reaction of a haloacetylene and an alkyne proceeds via a hitherto overlooked intermediate, namely the cyclopropenylmethyl cation. The existence and relative stability of this concealed intermediate is verified by quantum chemical calculations and ¹³ C-labeling experiments. A comparison between the cyclopropenylmethyl cation and the well-known vinylidene intermediate reveals that the latter is more stable only for smaller cycles. However, this stability reverses in larger cycles. In the case of the smallest representative of both species, the vinylidene cation is the transition state en route to the cyclopropenylmethyl cation. The discovery of this intermediate should help to get a deeper understanding for gold-catalyzed carbon-carbon bond-forming reactions of alkynes. Furthermore, since enynes can be formed from the cyclopropenylmethyl cation, the inclusion of this intermediate should enable the development of new synthetic methods for the construction of larger cyclic halogenated and non-halogenated conjugated enyne systems.