Gold for the Generation and Control of Fluxional Barbaralyl Cations

The frog prince with his two identities pales in comparison with the shape-shifting barbaralyl cation, which exists as a mixture of 181 400 degenerate forms. Gold-catalyzed cycloisomerizations of 7-alkynyl cyclohepta-1,3,5-trienes were found to proceed via fluxional barbaralyl intermediates (see scheme). The evolution of the intermediates into 1- or 2-substituted indenes could be controlled by the choice of gold complex.