Photo-Induced In Situ Generation of Brønsted Acid for Catalytic Friedel-Crafts Alkylation of Indoles.

A photo-induced indole 2-alkylation reaction was developed with unactivated stereo-congested alkenes used as the Friedel-Crafts alkylation reagent. The neutral, traceless and inexpensive perfluoroalkyl iodide C 4 F 9 I was used as the radical initiator in catalytic amount under irradiation by a blue LED light. Brønsted acids are in situ generated from the radical transfer reactions between the indole substrates and the perfluoroiodide catalyst. A variety of 2-branched alkylated indole molecules could be obtained from this approach in generally good efficiencies, with a broad scope of functional groups well tolerated. Mechanistic studies via both experimental and computational methods indicate that the reaction was accelerated by the protons generated from the crack of the indole-derived radical cation species.