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 1H 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 18O labeling experiments. A model for predicting the reactivity from aldehyde frontier molecular orbital energies is also presented.