Zinc oxide aromatics acylation

The preparation of ketones from aromatic hydrocarbons and acyl chlorides in the presence of zinc metal or zinc oxide was reported by Grucareviz and Merz in the same year, and by Doebner and Stackman in 1876. Despite all authors having recognized the presence of zinc chloride in the final reaction mixture, no mention was made of the possible role of a metal halide as catalyst in the reaction. 

Table 4.14 Acylation of aromatics with acyl chlorides in the presence of zinc oxide...

Zinc oxide, an inexpensive and commercially available inorganic solid, can be utilized as an efficient catalyst in the Friedel-Crafts acylation of activated and unactivated aromatic compounds with acyl chlorides at room temperature for 5 to 120 min (Table 4.14). Acylation is claimed to occur exclusively at the para-position of the monosubstituted aromatic compounds. The catalyst can be recovered and reused, after washing with methylene chloride, for at least two further cycles, showing quite similar high yield (-90%) in the model benzoylation of anisole. Mechanistically, it seems that zinc chloride can be the true catalyst, generated in situ by the reaction of zinc oxide with hydrogen chloride. 

The Friedel-Crafts reaction involves an electrophilic aromatic substitution that facilitates the alkylation or acylation of arenes (135) and heterocyclic compounds catalyzed by acidic catalysts. Zinc oxide has been found to be an effective catalyst for the Friedel-Crafts acylation of activated and nonactivated aromatic compounds (135) (Hosseini-Sarvari and Sharghi 2004) under solvent-free and room temperature conditions (Scheme 9.44). The catalyst provides a large surface area for the reaction. This Friedel-Crafts reaction is a safe and environmentally benign method which requires simple workup, mild reaction conditions and a short reaction time. 

Keywords Aromatic compounds, acid chlorides, zinc oxide powder, solvent-free, room temperature, Friedel-Crafts acylation, acyl derivatives... 

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