Protic acid immobilized on solid support as an extremely eEfficient recyclable catalyst system for a direct and atom economical esterification of carboxylic acids with alcohols

Abstract

A convenient and clean procedure of esterification is reported by direct condensation of equimolar amounts of carboxylic acids with alcohols catalyzed by an easy to prepare catalyst system of perchloric acid immobilized on silica gel (HClO₄−SiO₂). The direct condensation of aryl, heteroaryl, styryl, aryl alkyl, alkyl, cycloalkyl, and long-chain aliphatic carboxylic acids with primary/secondary alkyl/cycloalkyl, allyl, propargyl, and long-chain aliphatic alcohols has been achieved to afford the corresponding esters in excellent yields. Chiral alcohol and N-t-Boc protected chiral amino acid also resulted in ester formation with the representative carboxylic acid or alcohol without competitive N-t-Boc deprotection and detrimental effect on the optical purity of the product demonstrating the mildness and chemoselectivity of the procedure. The esters of long-chain (>C₁₀) acids and alcohols are obtained in high yields. The catalyst is recovered and recycled without significant loss of activity. The industrial application of the esterification process is demonstrated by the synthesis of prodrugs of ibuprofen and a few commercial flavoring agents. Other protic acids such as H₂SO₄, HBr, TfOH, HBF₄, and TFA that were adsorbed on silica gel were less effective compared to HClO₄−SiO₂ following the order HClO₄−SiO₂ ≫ H₂SO₄−SiO₄ > HBr−SiO₂ > TfOH−SiO₂ ≫ HBF₄−SiO₂ ≈ TFA−SiO₂. When HClO₄ was immobilized on other solid supports the catalytic efficiency followed the order HClO₄−SiO₂ > HClO₄−K10 > HClO₄−Al₂O₃ (neutral) > HClO₄−Al₂O₃ (acidic) > HClO₄−Al₂O₃ (basic).