Superacid‐Promoted Synthesis of CF3‐Indenes Using Brominated CF3‐Enones

The reactions of 3-bromo- and 3,4-dibromo-CF3-enones with superacids were studied. Protonation of these CF3- enones with FSO3H resulted in the formation of cationic species that subsequently cyclized at -60 degrees C to give the corresponding mono-and dibromoindenols in up to 98 % yield. Protonation of the indenol products at room temperature with triflic acid (CF3SO3H) provided highly electrophilic trifluoromethylated indenyl cations. Subsequent reaction with various arenes gave access to 1-CF3-substituted indenes in up to 84 % yields. DFT calculations were carried out to obtain data about the electronic structure and electrophilicity of the cationic intermediates. The mechanisms of these multistep transformations of highly electrophilic cationic intermediates are discussed.