Kolbe–Schmitt reaction
Carboxylation chemical reaction
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Through Carboxylic acid, Chemical reaction and Carbon dioxide
By using potassium hydroxide, 4-hydroxybenzoic acid is accessible, an important precursor for the versatile paraben class of biocides used e.g. in personal care products.
The methodology is also used in the industrial synthesis of 3-hydroxy-2-naphthoic acid; the regiochemistry of the carboxylation in this case is sensitive to temperature.
Reaction mechanism
The Kolbe–Schmitt reaction proceeds via the nucleophilic addition of a phenoxide, classically sodium phenoxide (NaOC6H5), to carbon dioxide to give the salicylate. The final step is the reaction (protonation) of the salicylate anion with an acid to form the desired salicylic acids (ortho- and para- isomers).
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Homogeneous Kolbe-Schmitt Reaction
A modification of the classical solid-phase Kolbe-Schmitt reaction, where the solvent play a role of active media dissolving reagents and solvating cations and anions. Typical solvents for homogeneous Kolbe-Schmitt reaction are DMSO, DMF, HMPTA. Carboxylation of sodium and potassium phenoxides in solution of DMSO proceeds under milder temperature (100 °C) with high para-regioselectivity (up to 97:3) producing 4-hydroxybenzoic acid (4HBA) instead of 2-hydroxybenzoic (salicylic) acid (SA). Addition into the reaction mixture of basic sodium salts like mesytolate, isopropylcarbonate, tert-butylcarbonate increase the chemical yield of a mixture 4HBA&SA up to two fold.
Screening of various metal cations, different substituted phenoxides, solvents (DMSO and DMF) in combination with activating additive (mesytolate) provide a general and effective method of homogeneous carboxylation.
Particularly, rubidium and cesium salts directed carboxylation into para-position of phenoxides with unprecedented regioselectivity. It is suggested that mechanism of the reaction involes molecular associates which responsible for decreasing of activation energy barrier of carboxylation. Carboxylation of potassium phenoxide was shown to occur rapidly under atmospheric CO₂ pressure at 113°C in a gasometric study.