Ring Opening of Diketene in Superacidic Media

The reactions of diketene were investigated in the HF/MF5 and DF/MF5 (M = As or Sb) binary superacidic systems. Depending on the stoichiometric ratio of the Lewis acids and diketene, monoprotonated acetoacetyl fluoride and diprotonated acetoacetyl fluoride were obtained. The salts were characterized by low-temperature vibrational spectroscopy, nuclear magnetic resonance spectroscopy, and single-crystal X-ray diffraction. [CH3C(OH)CH2COF][SbF6]·HF crystallizes in monoclinic space group P21/n with four formula units per unit cell, and [CH3C(OH)CH2C(OH)F][SbF6]2·HF in triclinic space group P1 with two formula units per unit cell. Ring opening is discussed together with quantum chemical pKa calculations. Monoprotonated acetoacetyl fluoride is characterized by a six-membered ring-like structure containing an intramolecular hydrogen bond, whereas the diprotonated species constitutes a 1,3-gitonic superelectrophile.