Carboxylations of alkali metal phenoxides with carbon dioxide

Abstract

The reaction mechanism of the Kolbe–Schmitt reaction of phenol and 2-naphthol has been investigated. An alkali metal phenoxide–CO₂ complex is not an intermediate that can be easily transformed into a carboxylic acid, such as salicylic acid (SA) and p-hydroxybenzoic acid (pHBA). A direct carboxylation of phenoxide with CO₂ takes place even at room temperature, and is competitive with the formation of the CO₂ complex. The resulting complex decomposes thermally (above ca. 100 °C) to phenoxide, which then undergoes further competitive reactions. Experiments using a carbon-13 labeled complex support a mechanism of direct carboxylation, and not the mechanism via a CO₂ complex. The reactivity, C-13 NMR and MOPAC/PM3 calculations suggest a new carbonate-like structure for the CO₂ complex.