Friedel–Crafts acyl rearrangements in the fluoranthene series

Friedel–Crafts monoacylation and diacylation of fluoranthene (FT) gave 3-acetyl-, 8-acetyl-, 3-benzoyl-, 8-benzoyl-, 3-(4-fluorobenzoyl)-, 8-(4-fluorobenzoyl)-, 3,9-diacetyl-, 3,9-dibenzoyl-, and 3,9-bis(4-fluorobenzoyl)fluoranthene (3-AcFT, 8-AcFT, 3-BzFT, 8-BzFT, 3-(4-FBz)FT, 8-(4-FBz)FT, 3,9-Ac₂FT, 3,9-Bz₂FT, and 3,9-(4-FBz)₂FT). The crystal and molecular structures of 8-AcFT, 3,9-Ac₂FT, 7,10-Ac₂FT, 3-BzFT, 8-BzFT, and 3-(4-FBz)FT were determined by X-ray crystallography. The structures of the fluoranthene derivatives, including 3,9-Ac₂FT were verified by ¹H-, ¹³C-, and ¹⁹F-NMR spectroscopy. The Friedel–Crafts acyl rearrangements in PPA of the above fluoranthene derivatives were studied at various temperatures and times. The kinetically controlled product 3-AcFT/3-BzFT rearranged to the thermodynamically-controlled product 8-AcFT/8-BzFT, not vice versa. 3,9-Ac₂FT, 3,9-Bz₂FT, and 3,9-(4-FBz)₂FT underwent deacylation in PPA to give 8-AcFT, 8-BzFT, and 8-(4-FBz)FT, respectively. Deacetylation of 3,9-Ac₂FT gave also 3-methyl-1H-benzo[cd]fluoranthene (3-MeBcdFT). The rich Friedel–Crafts acylation chemistry in PPA revealed in the fluoranthene series is characterized by regioselectivity. DFT calculations at B3LYP/6-31G(d) supported the regioselectivity including the formation of 3,9-Ac₂FT, and the win of kinetic control over thermodynamic control.