Reversible Friedel-Crafts Acylations of Anthracene: Rearrangements of Acetylanthracenes

Treatment of 1-acetylanthracene (1-AcAN) and 9-acetylanthracene (9-AcAN) with PPA at 80-120 °C leads to the nearly complete conversion of these isomers into 2-acetylanthracene (2-AcAN), an illustration of the Agranat-Gore rearrangement of polycyclic aromatic ketones (PAKs). Ab initio MP2/6-31(d) calculations predict the following order of stabilities: for σ-complexes: 9-AcAN>1-AcAN>2-AcAN; for ketones: 2-AcAN>1-AcAN>9-AcAN; for O-protonated ketones: 2-AcAN>1-AcAN>9-AcAN. Thus, 9-AcAN is the kinetically controlled product, whereas 2-AcAN is the thermodynamically controlled product. No reverse rearrangements of 2-AcAN to either 9-AcAN or 1-AcAN and of 1-AcAN to 9-AcAN were observed. The results strengthen the pattern of reversibility in Friedel-Crafts acylations of PAHs.