Zinc perchlorate hexahydrate [Zn(ClO₄)₂·6H₂O] as acylation catalyst for poor nucleophilic phenols, alcohols and amines: scope and limitations

Abstract

Commercially available zinc perchlorate hexahydrate [Zn(ClO₄)₂·6H₂O] was found to be a highly efficient catalyst for acylation of electron deficient phenols, sterically hindered alcohols and electron deficient and sterically hindered amines. The acetylated derivatives were obtained in excellent yields under solvent-free conditions and at room temperature. However, the reaction of highly electron deficient and sterically hindered phenol, e.g., 2,4-dinitrophenol required heating at 80 °C. In case of reactions with alkoxy or dialkoxy benzylic alcohols, a complex product mixtures were formed containing the corresponding dibenzylic ethers. The catalyst was found to be of general use with respect to other acylating agents such as propionic, iso-butyric, pivalic, benzoic and chloroacetic anhydrides. However, the rate of acylation was influenced by the steric and electronic factors of the anhydrides and followed the order Ac₂O > (PhCO)₂O > (EtCO)₂O > (ⁱPrCO)₂O > (^tBuCO)₂O ≫ (ClCH₂CO)₂O. Benzoylation and pivalation of phenols (including electron deficient phenols), dihydric phenol and sterically hindered alcohol afforded excellent yields. The catalytic activity of various zinc compounds was found to be of the order Zn(ClO₄)₂·6H₂O > ZnI₂ ∼ ZnBr₂ > ZnCl₂ ≫ Zn(OAc)₂ > ZnCO₃ and was parallel to the acidic strength of the corresponding protic acids.