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20.2.10.1.1.2 Variation 2: Kolbe–Schmitt Reaction

DOI: 10.1055/sos-SD-020-00506

Yoon, T. P.; Jacobsen, E. N.Science of Synthesis, (200720534.

The direct carboxylation of alkali and alkaline earth phenoxides by carbon dioxide has been used commercially in the synthesis of aromatic hydroxy acids for well over a century (Scheme 2).[‌5‌] Sodium phenoxides undergo predominantly ortho carboxylation, e.g. in the synthesis of 2-hydroxy-3-isopropyl-6-methylbenzoic acid (2), while the reactions of phenoxides with larger cations such as rubidium or cesium result in selective para carboxylation.[‌6‌] Potassium phenoxides afford mixtures of para- and ortho-potassium salicylates that rearrange upon heating to the para-isomers.[‌7‌] The reaction of simple phenols proceeds most efficiently under high pressures of carbon dioxide,[‌8‌] but catechols and higher polyhydric phenols can undergo efficient carboxylation at atmospheric pressures.[‌9‌]

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