Regioselective Friedel–Crafts deacylations of polycyclic aromatic ketones in the pyrene series

1,3,6,8-tetrabenzoylpyrene (1,3,6,8-Bz₄PY) and 1,3,6-tribenzoylpyrene (1,3,6-Bz₃PY) were synthesized and their crystal structures were determined. The Friedel–Crafts deacylations in PPA of 1,3,6,8-Bz₄PY (at 120–200 °C) and of 1,3,6-Bz₃PY (at 80–160 °C) have been studied. The mono-deacylation of 1,3,6-Bz₃PY was regioselective and led to three dibenzoylpyrenes in the following order of relative amounts: 1,8-Bz₂PY > 1,6-Bz₂PY > 1,3-Bz₂PY. 1,3,6,8-Bz₄PY was resistant to deacylation at 120–160 °C. The deacylations of 1,3,6,8-Bz₄PY at 200 °C gave the polycyclic aromatic ketone (PAK) 8H-dibenzo[def,qr]chrysen-8-one (DBCO) via an intramolecular Scholl reaction. Two plausible pathways of the Friedel–Crafts deacylation of 1,3,6,8-Bz₄PY to give DBCO are proposed. A density functional theory (DFT) B3LYP/6-311(d,p) computational study of the conformational spaces of 1,3,6-Bz₃PY and 1,3,6,8-Bz₄PY was performed. The estimated energy barriers of formation of dibenzoylpyrenes by deacylation of 1,3,6-Bz₃PY increase in the following order: 1,8-Bz₂PY < 1,3-Bz₂PY < 1,6-Bz₂PY. A mechanism of the Friedel–Crafts deacylation of 1,3,6-Bz₃PY in PPA via the respective O-protonated ketone and σ-complexes is presented.