Strong-Acids-Promoted Protiodeacylation of Sterically Nonhindered Alkyl Aryl Ketones

Abstract

In the presence of strong acids in aqueous and anhydrous media alkyl aryl ketones bearing a strong electron-releasing group in the aromatic ring undergo protiodeacylation. In a reaction with triflic acid in aromatic solvents, the transfer of the liberated acyl group to aromatic solvent molecules is observed. The effects of the nature and the position of the activating group and the strength of the acid and the solvent, on the course of this ipso-substitution were investigated.

Key words

protiodeacylation - transacylation - ketones - retro-Friedel-Crafts reaction - ipso-electrophilic-aromatic substitution

References and Notes

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• 10 ¹H NMR: δ = 2.09 (q, 2 H, J = 6.2 Hz, 3-CH₂), 2.67 (t, 2 H, J = 6.2 Hz, 5-CH₂), 2.90 (t, 2 H, J = 6.1 Hz, 2-CH₂), 6.70 (dd, 1 H, J ₁ = 8.4 Hz, J ₂ = 0.9 Hz, 5-H), 6.79 (d, 1 H, J ₁ = 8.4 Hz, 7-H), 7.35 (t, 1 H, J = 8.4 Hz, 6-H), 12.43 (s, 1 H, 8-OH). GC-MS: [M⁺] = 162; [M] = 162.185 g/mol; spectrum is identical to the literature. See: Couche E. Fkyerat A. Tabacchi R. Helv. Chim. Acta  2003,  86:  210 
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8

General Procedure for the Reaction with Aqueous Acid
A ketone (100 mg) was dissolved in the appropriate cosolvent (toluene, xylene, 3 mL); an acid (3 mL of 48% or 62% HBr in H₂O or 1 mL of 50% TfOH in H₂O) was added, and refluxed for 48 h. The reaction mixture was cooled, diluted with H₂O (20 mL) and extracted with CH₂Cl₂ (4 × 5 mL); the combined organic phases were washed with a sat. aq solution of NaHCO₃, dried over anhyd Na₂SO₄, and evaporated. The products were identified by ¹H NMR spectroscopy and GC-MS.

12

General Procedure for the Reaction with Anhydrous Acid A ketone (100 mg) was dissolved in dry aromatic solvent (5 mL), TfOH (0.2 mL) was added, and the mixture was refluxed under argon. After 72 h the mixture was cooled, diluted with H₂O (15 mL), and the organic layer was separated. The water layer was extracted with CH₂Cl₂ (2 × 10 mL); combined organic phases were washed with a sat. aq solution of NaHCO₃, dried over anhyd Na₂SO₄, and evaporated. The mixture of compounds was analyzed by means of GC-MS.