Gold(I)-Catalyzed 1,3-O-Transposition of Ynones: Mechanism and Catalytic Acceleration with Electron-Rich Aldehydes

The gold-catalyzed 1,3-O-transposition of ynones occurs intermolecularly via a cyclic organo-gold acetal intermediate formed from the nucleophilic oxo attack of a second ynone, i.e. either starting material or product, on a gold-activated ynone. The combination of <sup>1</sup>H NMR monitored kinetic data, analyzed using variable time normalization analysis (VTNA) and kinetic modeling, and density functional theory (DFT) was used to elucidate the mechanism. A significant acceleration of the reaction rate could be achieved by the addition of a substoichiometric amount of electron-rich aldehyde as a mediator, allowing the gold-catalyzed 1,3-O-transposition of terminal ynones to ynaldehydes. The mechanism is further supported by NMR characterization of the acetal intermediate and <sup>18</sup>O labeling experiments. A model for predicting the reactivity from aldehyde frontier molecular orbital energies is also presented.