The catalytic opportunities of near-critical water: a benign medium for conventionally acid and base catalyzed condensations for organic synthesis

Abstract

Near-critical water (NCW; 250–350 °C, 40–90 bar) provides an environmentally benign alternative medium for conducting organic synthesis. The Claisen–Schmidt condensation of benzaldehyde with 2-butanone was investigated and is used to demonstrate the ability to conduct conventionally acid or base catalyzed reactions homogeneously using NCW without the addition of a catalyst. Kinetic investigations of the Claisen–Schmidt condensation yielded activation energies of 24.7 and 22.6 kJ mol⁻¹ for the formation of products consistent with those formed under classical acidic and basic conditions, respectively. Investigations of other conventionally base catalyzed condensations were performed in uncatalyzed NCW for completeness, including the self-condensation reaction of butyraldehyde, the benzaldehyde–acetone cross-aldol condensation, the intramolecular Claisen condensations of ethyl-4-acetylbutyrate and ethyl levulinate, as well as the intramolecular Dieckmann condensation of diethyl adipate.