Conjugate Addition of Indoles to Electron-Deficient Olefins Catalyzed by CuFe2O4@SO3H under Mild Conditions

Over the past few years a variety of methods has been reported for the preparation of 3-substituted indoles. Addition reactions of indoles to electron deficient olefins have receivedmuch interest because a number of their derivatives occur in nature and possess a variety ofbiological activities. Since the 3-position of indole is the preferred site for electrophilic substitutionreactions, 3-alkyl indoles are versatile intermediates for the synthesis of a wide range ofindole derivatives. A simple and direct method for the synthesis of 3-alkylated indoles involvesthe conjugate addition of indoles to α,β-unsaturated compounds in the presence of eitherprotic or Lewis acids. However, the acid-catalyzed conjugate addition of indoles requirescareful control of acidity to prevent side reactions such as dimerization or polymerization [1-3]. In continuation of our work on the development of useful synthetic methodologies we recentlyobserved that CuFe2O4@SO3H catalyzes the conjugate addition of indoles to electrondeficientolefins to form the corresponding Michael adducts under mild conditions. Initial researchwas focused on the reaction of indole with chalcone, and different solvents and differentamounts of the catalyst were tested to find the optimized conditions.