Gold(I)-Catalyzed Enantioselective Intermolecular Hydroarylation of Allenes with Indoles and Reaction Mechanism by Density Functional Theory Calculations

Chiral binuclear gold(I) phosphine complexes catalyze enantioselective intermolecular hydroarylation of allenes with indoles in high product yields (up to 90%) and with moderate enantioselectivities (up to 63% ee). Among the gold(I) complexes examined, better ee values were obtained with binuclear gold(I) complexes, which displayed intramolecular Au-I-Au-I interactions. The binuclear gold(I) complex 4c [(AuCl)(2)(L3)] with chiral biaryl phosphine ligand (S)-(-)-MeO-biphep (L3) is the most efficient catalyst and gives the best ee value of up to 63%. Substituents on the allene reactants have a slight effect on the enantioselectivity of the reaction. Electron-withdrawing groups on the indole substrates decrease the enantioselectivity of the reaction. The relative reaction rates of the hydroarylation of 4-X-substituted 1,3-diarylallenes with N-methylindole in the presence of catalyst 4c [(AuCl)(2)(L3)]/AgOTf [L3=(S)-(-)MeO-biphep], determined through competition experiments, correlate (r(2) = 0.996) with the substituent constants sigma. The slope value is -2.30, revealing both the build-up of positive charge at the allene and electrophilic nature of the reactive Au-I species. Two plausible reaction pathways were investigated by density functional theory calculations, one pathway involving intermolecular nucleophilic addition of free indole to aurated allene intermediate and another pathway involving intramolecular nucleophilic addition of aurated indole to allene via diaurated intermediate E2. Calculated results revealed that the reaction likely proceeds via the first pathway with a lower activation energy. The role of Au-I-Au-I interactions in affecting the enantioselectivity is discussed.