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.