Cyclization of 1-aryl-4,4,4-trichlorobut-2-en-1-ones into 3-trichloromethylindan-1-ones in triflic acid.

Trichloromethyl-substituted enones (1-aryl-4,4,4-trichlorobut-2-en-1-ones, ArCOCH=CHCCl, CCl-enones) undergo intramolecular transformation into 3-trichloromethylindan-1-ones (CCl-indanones) in Brønsted superacid CFSOH (triflic acid, TfOH) at 80 °C within 2-10 h in yields up to 92%. Protonation of the carbonyl oxygen of the starting CCl-enones by TfOH affords the key reactive intermediates, the O-protonated forms ArC(=OH)CH=CHCCl, which are then cyclized into the target CCl-indanones. These cations have been studied experimentally by means of NMR spectroscopy in TfOH and theoretically by DFT calculations.