N,N′-Dioxide–scandium(III) complex catalyzed highly enantioselective Friedel–Crafts alkylation of indole to alkylidene malonates

A highly efficient enantioselective Friedel–Crafts alkylation of indoles with alkylidene malonates has been developed using chiral N,N′-dioxide L4–scandium(III) complex as the catalyst, giving the corresponding products in high yields with excellent enantioselectivities (up to 99% yield and 95% ee). The product 3a was facilely converted into several interesting compounds, such as tryptamines, indolepropionic acids and β-carbolines. It is noteworthy that the seven-membered β-carboline-like compound has been synthesized for the first time. Based on the crystal structure of the chiral N,N′-dioxide L10–scandium(III) complex, the proposed transition state and possible catalytic cycle were presented to elucidate the reaction mechanism.