Zirconium(IV) Chloride as a New, Highly Efficient, and Reusable Catalyst for Acetylation of Phenols, Thiols, Amines, and Alcohols under Solvent-Free Conditions

Abstract

Zirconium(IV) chloride has been found to be a new, highly efficient, and reusable catalyst for acetylation of structurally diverse phenols, thiols, amines, and alcohols under solvent-free condtions. Acetylation of sterically hindered and electron deficient phenols is achieved in excellent yields with stoichiometric amounts of Ac₂O at room temperature. Acid-sensitive alcohols undergo acetylation with excellent chemoselectivity without competitive side reactions such as dehydration or rearrangement. The mild Lewis acid property of the catalyst enables the acetylation to be ­carried out with optically active substrates without any detrimental effect on the optical purity.

acetylation - zirconium compounds - catalysis - solvent-free reactions

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Typical procedure for acetylation: 2-Hydroxy-naphthalene(1) (1.44 g, 10 mmol) was treated with Ac₂O (0.96 mL, 10 mmol) under neat conditions at r.t. for 15 min under magnetic stirring in the presence of ZrCl₄ (23 mg, 0.1 mmol, 1 mol%). The reaction mixture was diluted with Et₂O (50 mL) and the supernatant solution decanted off to separate the catalyst. The filtrate was washed successively with 2% aqueous NaOH (15 mL), brine (15 mL), dried (MgSO₄) and concentrated to afford the product (1.77 g, 95%), which was in full agreement with the mp and spectral data (IR, ¹H and ¹³C NMR, and EIMS) of an authentic sample of 2-acetoxynaphthalene. The recovered catalyst was used for the acetylation of a fresh lot of 1 (1.44 g, 10 mmol) with Ac₂O (0.96 mL, 10 mmol) affording 2-acetoxynaphthalene (1.73 g, 93%).

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