Friedel-Crafts acylation reaction using carboxylic acids as acylating agents

Dehydrative Friedel-Crafts acylation reaction of aromatic compounds with carboxylic acids as acylating agents was investigated in the presence of Lewis acid- or Brønsted acid-catalyst. Various metal triflates and bis(trifluoromethanesulfonyl)amides showed catalytic activity at high temperature, among which Eu(NTf₂)₃ proved to be the most effective and efficiently catalyzed the acylation reaction of alkyl- and alkoxybenzenes with aliphatic and aromatic carboxylic acids at 250 °C. Bi(NTf₂)₃ was more effective than Eu(NTf₂)₃ at lower temperature, but proved to be hydrolyzed in the presence of a small amount of water to give HNTf₂ and [Bi₆O₄(OH)₄(H₂O)₆](NTf₂)₆. The structure of the latter compound was confirmed by a single crystal X-ray analysis. Among five Brønsted acids, HOTf, HNTf₂, HCTf₃, TsOH, and Nafion^® SAC-13, HNTf₂ has proved to be the most efficient catalyst and more effective than Eu(NTf₂)₃ for the acylation of p-xylene with heptanoic acid at 220 °C or lower temperature. HNTf₂ catalyzed the acylation of anisole with carboxylic acids in high yields in refluxing toluene with azeotropic removal of water.