Iron(iii) sulfate/H2O-catalysed, base-free esterification of carboxylic acids with dimethyl carbonate: the crucial role of water as a co-catalyst.

An eco-friendly, base-free protocol was developed for the esterification of carboxylic acids with dimethyl carbonate (DMC), which serves as both the reagent and solvent, using an iron(iii) sulfate/H2O catalytic system. The method combines high efficiency with a broad substrate scope under mild and environmentally benign conditions. Control experiments show that anhydrous Fe2(SO4)3 is inactive and that water is required for activity. A tentative mechanism is proposed, in which water acts both as a reagent and as a source of in situ Brønsted acidity, a contribution of Lewis acidity not being excluded. Furthermore, our results highlight the plausible dual role of DMC, which may serve both as a sustainable source of methanol and as a chemical desiccant, thereby potentially shifting the equilibrium towards complete conversion. Overall, this protocol provides a green and cost-effective alternative to conventional Fischer esterification.