Controlling the Reactivity of 1,3-Diacyl-2-gold Carbenes: [3+2] Annulation with External Imines over Wolff Rearrangement.

Novel [3+2] annulations between imines and 1,3-diacyl gold carbenes have been developed to provide 2-acyl-3-aminopyrrole derivatives efficiently. Mechanistic studies suggest that the reaction is initiated by nucleophilic addition of the imine to the 1,3-diacyl gold carbene, generating a gold carbene-imine intermediate that undergoes intramolecular carbocyclization involving the three-carbon 1,3-dicarbonyl unit and the C═N bond to furnish the pyrrole framework. A competitive reaction in this gold catalysis is the Wolff reaction of the 1,3-diacyl gold carbenes when ynone substrates comprise bulky ketones. Density functional theory (DFT) calculations for the reaction of species 1a with 2a reveal that the [3+2] annulation pathway is energetically more favorable than the competing Wolff rearrangement.