Ionic liquid as dual-function catalyst and solvent for efficient synthesis of sucrose fatty acid esters

Abstract

Carbohydrate fatty acid esters are important bio-based, non-ionic surfactants. Their synthesis confronts the challenge of reacting substrates of vastly different polarity. Here, we show imidazolium-based ionic liquids with basic anions (dicyanamide, acetate) as dual-function catalysts and solvents for an efficient C4-C18 fatty acid esterification of sucrose. The conversion proceeds from the anhydride, the vinyl ester, and the free fatty acid in that order of reactivity. It benefits ≥ 3-fold from a moderately polar, protic co-solvent (2-methyl-2-butanol, tert‑butanol) in ∼1:1 vol ratio with the ionic liquid. Optimized synthesis from vinyl palmitate (C16; ≤ 3-fold excess over 50 mM sucrose; 60 °C) was unexpectedly selective, giving the 6-O-mono-acyl product (∼70%) in quantitative yield. The ionic liquid-promoted conversion gains efficiency from the imidazolium cation aiding in sucrose solubilization and the basic anion providing catalytic facilitation to the esterification. By virtue of high reactivity at mild conditions and useful product selectivity, it advances the current repertoire of synthetic methods, including biocatalytic transformations, for sucrose palmitate ester production.