Consecutive Friedel–Crafts acyl rearrangements and Scholl reactions of dinaphthyl ketones

Abstract

The three dinaphthylketone constitutional isomers, 1,1′-NA₂CO, 1,2′-NA₂CO, and 2,2′-NA₂CO, have been subjected to Friedel–Crafts Acylation (polyphosphoric acid (PPA), 90–300°C, 12 h) and Scholl reaction (AlCl₃/NaCl, 120–300°C, 4 h). The resulting product mixtures are analyzed by NMR and separated by column chromatography. The starting ketones and the products have been calculated with DFT, B3LYP/6-311G**, to support analysis of the reaction mechanisms. The three dinaphthyl ketones have three, four, and three E,Z-conformations, which may potentially give ten Scholl reaction products by ortho-ortho, ortho-peri, and peri-peri couplings. In PPA, the products formed at relatively low temperatures are due to Scholl reactions of the starting ketones. At higher temperatures, naphthalene is formed and Friedel Crafts acyl rearrangements (FCAcRs) 1,1′-NA₂CO → 1,2′-NA₂CO → 2,2′-NA₂CO are observed. At high temperatures, additional Scholl products show that complex multi-step reactions occur including FCAcRs in both directions and cyclic FCAcRs of Scholl products. The reactions in AlCl₃/NaCl are highly selective, giving only the 6-ring Scholl cyclization products 13H-dibenzo[a,i]fluoren-13-one and 7H-benzo[hi]chrysen-7-one (BhiCO) and 7H-benzo[de]naphthacen-7-one (BdeNCO). Up to 240°C 1,1′-NA₂CO forms BhiCO and up to 220°C 1,2′-NA₂CO forms BdeNCO. At higher temperatures, both polycyclic aromatic ketones are formed from each of the three dinaphthyl ketones indicating FCAcRs. The detailed analysis of the experimental data in combination with the DFT calculations shows that FCAcRs are reactions in both directions in PPA and AlCl₃/NaCl, substantiating Gore’s 1955 proposition that the Friedel–Crafts acylation reaction of reactive aromatic hydrocarbons is a reversible process. The lower onset temperatures and selectivity of cyclization products suggest lower activation energies for Scholl reactions as compared to FCAcRs in PPA and even more pronounced in AlCl₃/NaCl. The common network of reaction pathways underlying the reactions in both media highlights the linkage between Friedel–Crafts acyl rearrangements and Scholl reactions.

Graphical Abstract