Synthesis of Aryl-Substituted Piperidines by Superacid Activation of Piperidones

Diarylpiperidines (8−12) may be prepared in good to excellent yields (80−99%) by the reaction of piperidones (3d−h) with benzene and the Bronsted superacid, trifluoromethanesulfonic acid (CF₃SO₃H, TfOH). Tropinone (6) and quinuclidone (7) also react in good yields with benzene in TfOH to give the condensation products (13 and 14). Ketal and acetal derivatives also give condensation products (8 and 24) upon reaction with C₆H₆ in TfOH. The conversion of 3g to 11 is sensitive to both acid quantity and acid strength; a mechanism is proposed for the conversion that invokes dicationic intermediates.