Intermediates in Friedel‐Crafts Acylation of Fumaryl Halides

Abstract

Fumaryl chloride and fluoride were reacted with different Lewis acids to synthesize the intermediates of the Friedel‐Crafts acylation. The salt of the monoacyl cation [C4H2FO2]+[Sb3F16]− was obtained from the reaction of fumaryl fluoride with SbF5 in SO2ClF solutions. The reaction was repeated using fumaryl chloride as starting material, which reacted under halogen exchange to obtain the salt of the monoacyl cation [C4H2FO2]+[SbCl2F4]−. In addition, the reaction of fumaryl chloride with SbCl5 in SO2ClF was studied. The covalent donor‐acceptor complex C4H2Cl2O2 ⋅ 2 SbCl5 was formed, containing oxygen‐bonded Lewis acids. The compounds were characterized by low‐temperature vibrational spectroscopy. Single‐crystal X‐ray structure analyses were conducted for [C4H2FO2]+[Sb3F16]− as well as for C4H2Cl2O2 ⋅ 2 SbCl5. In the solid state of [C4H2FO2]+[Sb3F16]− C⋅⋅⋅O and C⋅⋅⋅F contacts are observed and the origin of these interactions is discussed by means of ESP maps and NBO analysis. The monoacyl cation is stabilized by electrostatic attraction and electron back‐donation from oxygen and fluorine ligands to the positive ring‐structured π‐hole at the oxocarbenium center. Besides, the formation of the diacyl cation is not observed, which is based on small distances between the positive charges involving charge‐charge repulsion. The great advantage of using fumaryl halides in Friedel‐Crafts acylation is featured by the possibility to synthesize ketones keeping an acyl fluoride moiety.