Friedal-Crafts acylation of anisole over zeolite catalysts

The liquid phase Friedel-Crafts acylation of anisole with alkanoic acids, anhydrides and substituted benzoic acids, respectively, over zeolites was studied in detail. The acylation with substituted benzoic acids, as well as with alkanoic acids, could be successfully performed over zeolite catalysts. When carboxylic acids were used as acylating agents, the activity of the Y zeolite increased with its Lewis acidity, showing that the Lewis acid sites were more active than the Bronsted acid sites. The mechanism of the reaction was found to be similar to the case of homogeneous catalysis, that is, the electrophilic intermediate formed from the acylating agent over zeolite acid sites attacked at the aromatic ring of anisole. However, the structure of the electrophilic intermediate would depend upon the acylating agent and the nature of zeolite acid sites. The anhydride would transform into the acylium cation, but the carboxylic acid would lead to the protonated and coordinated carboxylic acid as the electrophiles over Bronsted and Lewis acid sites of the zeolites, respectively. The protonated carboxylic acid would preferentially attack at the oxygen atom of anisole and give phenyl carboxylic ester, while the coordinated carboxylic acid and the acylium cation would predominantly react with the aromatic ring, leading to the Friedel-Crafts acylation product 4-acyl anisole. (C) 1997 Elsevier Science B.V.