A structural chemistry practitioner: a fox rather than a hedgehog. Reversibility of Friedel–Crafts acyl rearrangements

Abstract

A retrospective review of reversibility revealed in Friedel–Crafts acyl rearrangements of polycyclic aromatic ketones derived from the polycyclic aromatic hydrocarbons naphthalene, phenanthrene, anthracene, fluorene, pyrene, and fluoranthene is presented. The reactions were carried out in polyphosphoric acid (PPA). A distinction is made between completely reversible reaction (equilibrium reaction) denoted by ⇌ and reaction, both directions denoted by ⇆. Reaction, both direction is considered reversible reaction, not completely reversible reaction. Gore’s 1955 proposition that Friedel–Craft acylation of reactive hydrocarbons is a reversible process has been verified. Complete reversibility has been revealed in ortho ⇌ para acyl rearrangements of fluorofluorenones (1FFL ⇌ 3FFL) and in dibenzofluorenones (DBahFL ⇌ DBbhFL). Both cases are intramolecular acyl rearrangements. The majority of reversible intramolecular and intermolecular Friedel–Crafts acyl rearrangements are formally “reaction, both directions,” e.g., dibenzoylpyrenes 1,6-Bz2PY ⇆ 1,8-Bz2PY. Acyl rearrangements and reversibility in Friedel–Crafts acylations are associated with thermodynamic control. However, sometimes kinetic control wins out over thermodynamic control. Polycyclic aromatic ketones in which the carbonyl group is coplanar with the aromatic nucleus may undergo Friedel–Crafts acyl rearrangements. The reversibility concept has been applied to the synthesis of new and known linearly annelated polycyclic aromatic ketones by intramolecular Friedel–Crafts rearrangements of their angularly annelated constitutional isomers. A linkage between reversible Friedel–Crafts acyl rearrangements in PPA and the Scholl reaction has been established in benzoylnaphthalenes and in dibenzoylpyrenes. As a structural chemistry practitioner, the scientific personality of the first author belongs to the foxes, rather than to the hedgehogs.