The Friedel-Crafts Reaction of Indoles with Michael Acceptors Catalyzed by Magnesium and Calcium Salts

Friedel–Crafts alkylation of indole and its derivatives with a variety of electron-deficient alkenes catalyzed by Mg and Ca salts has been studied. The dependence of the results on the nature of the starting olefins, substituents on indole, and Michael acceptors, as well as on the composition of the Lewis acid is discussed. High yields of the addition products were achieved in the addition of indole to β,γ-unsaturated α-keto esters and coumarin derivatives, some nitroolefins, and arylidenemalonates. Reactions involving arylidenemalonates were found to be the most versatile and smooth, the best yields reached 92%. Among the Mg and Ca salts tested, magnesium iodide (MgI2) proved to be the most appropriate catalyst in the addition to various unsaturated carbonyl compounds, while calcium triflimide [Ca(NTf2)2] efficiently catalyzed the addition to nitroolefins.