Cycloaddition versus Alkylation Reactions of 2-Vinylindoles with α,β-Unsaturated Carbonyl Compounds Under Gold Catalysis

The gold-catalyzed intermolecular Diels–Alder cycloaddition and competitive Michael addition reactions between 2-vinylindoles and enones/enals are reported. The reaction outcome strictly correlates with the electronic character of the heteroaromatic substrate. Thus, Diels–Alder cycloadducts are the sole products in the presence of less electron-rich heterocycles, whereas Michael addition adducts are observed with more electron-rich heterocycles. Plausible competitive reaction mechanisms are proposed and discussed as well.