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 3 , CCl 3 -enones) undergo intramolecular transformation into 3-trichloromethylindan-1-ones (CCl 3 -indanones) in Brønsted superacid CF 3 SO 3 H (triflic acid, TfOH) at 80 °C within 2-10 h in yields up to 92%. Protonation of the carbonyl oxygen of the starting CCl 3 -enones by TfOH affords the key reactive intermediates, the O-protonated forms ArC(=OH + )CH=CHCCl 3 , which are then cyclized into the target CCl 3 -indanones. These cations have been studied experimentally by means of NMR spectroscopy in TfOH and theoretically by DFT calculations. Under the same superacidic conditions in TfOH, CCl 3 -hydroxy ketones (1-aryl-4,4,4-trichloro-3-hydroxybutan-1-ones; ArCOCH 2 CH(OH)CCl 3 ) undergo dehydration to the corresponding CCl 3 -enones, which are further cyclized into CCl 3 -indanones. The yields of CCl 3 -indanones starting from CCl 3 -hydroxy ketones are up to 86% in TfOH at 80 °C within 3-18 h.