The linkage between reversible Friedel–Crafts acyl rearrangements and the Scholl reaction

Abstract

Friedel–Crafts acyl rearrangements in PPA (at 80–240 °C) and Scholl reactions in AlCl3/NaCl (at 140–220 °C) of benzoylnaphthalenes and fluorobenzoylnaphthalenes have been studied experimentally as a function of temperature and time and computationally. 1BzNA, 2BzNA, 1-4FBzNA, 2-4FBzNA, 1-3FBzNA, 2-3FBzNA, 1-2FBzNA, and 2-2FBzNA were synthesized by classical Friedel–Crafts acylations of naphthalene with benzoyl chloride, benzene with 2-naphthoyl chloride, fluorobenzene with 1- and 2-naphthoyl chlorides and of naphthalene with fluorobenzoyl chlorides, and served as substrates in the investigation. Their structures have been determined by X-ray crystallography and verified by their 1H-, 13C-, and 19F-NMR spectra. 1BzNA, 1-4FBzNA, 1-3FBzNA, and 2-2FBzNA crystallized as the E-diastereomers, whereas 2BzAN, 1-2FBzAN, 2-4FBzAN, and 2-3FBzAN crystallized as the Z-diastereomers. The deviations of the carbonyl group from the naphthyl plane were higher as compared with the deviations from the phenyl plane and were considerably higher in the α-naphthyl ketones than in the β-naphthyl ketones. Intermolecular interactions due to C–H···O and/or C–H···F contacts in the crystal structures of 1E-4FBzNA and 1E1′E-3FBzAN have been revealed. 1BzNA rearranged in PPA under argon to 2BzNA via deacylation to naphthalene (e.g., 140 °C, 10 h) and underwent a regioselective intramolecular cyclodehydrogenation at high temperatures to the Scholl reaction product 7H-benz[de]anthracen-7-one (BdeAN) (e.g., 200 °C, 6 h). At 80 °C, benzene was isolated. 2BzNA underwent in PPA deacylation to naphthalene (e.g., 160 °C, 6 h) and an intramolecular cyclodehydrogenation to BdeAN at high temperatures (e.g., 220 °C, 6 h), necessarily via the putative intermediate 1BzNA. Higher yields of the acyl rearrangement and the Scholl reaction products were obtained under oxygen. 1-4FBzNA and 2-4FBzNA reacted in PPA analogously to 1BzNA and 2BzNA, respectively, with the following exceptions: 2-4FBzNA underwent an acyl rearrangement in PPA to 1-4FBzNA at 260–300 °C, without any formation of the Scholl reaction product 10FBdeAN. 1-4FBzNA also did not yield 10FBdeAN. 1-2FBzNA and 2-2FBzNA behaved similarly. The formation of naphthalene and benzene in the deacylation steps indicated cleavages of both the 1- and 2-naphthyl–benzoyl bonds and the 1- and 2-naphthoyl–phenyl bonds to give naphthalene and benzoylium cation and benzene and 1- and 2-naphthoylium cation, respectively. At 80–100 °C, 1-2FBzNA, 1-3FBzNA, and 1-4FBzNA underwent deacylations to fluorobenzene in PPA, followed by reacylation, each giving a mixture of the three 1-fluorobenzoylnaphthalenes. 2FBzNA, 2-3FBzNA, and 2-4FBzNA behaved similarly, each giving a mixture of the three 2-fluorobenzoylnaphthalenes. The results taken together verified the reversibility of the 1-BzNA⇌2BzNA acyl rearrangements in PPA. The Scholl reaction (AlCl3/NaCl) of 1BzNA (e.g., at 140 °C) gave BdeAN and 2BzNA, whereas 2BzNA gave only BdeAN (at 200–220 °C). 1-4FBzNA and 2-4FBzNA gave (at 200–220 °C) only 2-4FBzNA and 1-4FBzNA, respectively. All the six FBzNA isomers failed to undergo Scholl reaction cyclodehydrogenations to give any FBdeAN isomer. A linkage between the Friedel–Crafts acyl rearrangements and the Scholl reaction has thus been established. A systematic DFT study at B3LYP/6-311(d,p)/PCM (formic acid)) substantiated the predicted mechanism and the reversibility of the acyl rearrangements of benzoylnaphthalenes, in which 1BzNA and 2BzNA are the kinetically controlled and the thermodynamically controlled products, respectively. The DFT-calculated Gibbs free-energy of the transition-state (1Z-1BzNH+ ➔➔ 2σ-2BzNA) in the Friedel–Crafts acyl rearrangement of 1Z-BzNA is considerably lower than the transition state of the cyclization step in the arenium-cation mechanism of the Scholl reaction, in line with experiment, which indicated higher temperatures for the Scholl reaction. DFT calculations of the dication pathway of the Scholl reaction of the E- and Z-diastereomers/conformers of 1BzNA confirmed the preference of the formation of BdeAN versus BaFL, consistently with experiment.