Solid base catalysts in organic synthesis

Abstract

The basic and catalytic properties of hydrotalcites, supported fluorides and mixed oxides are reviewed, with a special emphasis on C-C bond forming reactions. Relatively strong basicity can be induced on hydrotalcites by calcination followed by rehydration. In that case the shift from Lewis to Bronsted basicity has strong influence on the catalytic properties. The basicity is further enhanced by introduction of t-butoxide or fluoride anions in place of hydroxyls, while chloride anions inhibit activity. Excellent yields have been reported, by fine-tuning of basic sites of the hydrotaclites, supported fluorides and mixed oxides as exemplified in aldol, Henry, Knoevenagel, Michael, trans-esterification Wadsworth- Emmons and N-oxidation reactions. In many cases the catalysts can be recycled.