Studies on the Intermolecular Hydroarylation of N ‐Ts‐ or N ‐Ac‐Protected Indoles and 2,3‐Allenoates

An operationally simple, TFA‐promoted regioselective hydroarylation reaction of 2,3‐allenoates with N ‐Ts‐ or N ‐Ac‐indoles to afford 4‐indolyl‐4‐arylbut‐2‐enoates is described. A series of substrates was tested, and the E / Z selectivity was found to depend on the reaction temperature and time. A mechanism involving the formation of E and Z allylic carbocations generated in situ from the reaction of 2,3‐allenoate with TFA and subsequent Friedel–Crafts attack at the 3‐position of the indoles was proposed to explain the results.