Reaction Parameters and Mechanistic Studies of the Kolbe-Schmitt Synthesis

Reaction Parameters and Mechanistic Studies of the Kolbe-Schmitt Synthesis 

4-hydroxy-benzoic add, which is actually also isolated and finds applications mostly in the polymer industry. In order to understand the general mechanism of the Kolbe-Schmitt reaction and to fine-tune the reaction parameters, many experimental and spectroscopic investigations have been undertaken. In general, these have been tedious, due mainly to the moisture-sensitivity of the starting products and the easy contamination by side products such as inorganic alkali carbonates and hydrogen-carbonates. 

These studies, which employed density functional theory (DFT) methods (B3LYP/LANL2DZ/Gaussian 98) proposed that the reactions of all alkali metal phenoxides with C02 followed a similar ground mechanism that comprised three intermediates and three transition states. In step 1, C02 must first be activated by an alkali metal phenoxide. In the case of the sodium phenoxide [24a], C02 can only attack at the polarized O-Na bond to form a Ph0Na/C02 complex as the first intermediate (structure 4). The calculation definitely rules out a direct C-C bond formation at the aromatic ring. 

It appears that, under the conditions of the Kolbe-Schmitt reaction (high pressure and temperature), the labeled potassium phenoxide- C02 complex of Kosugi undergoes a scrambling of labeled and unlabeled C02 molecules  

Recent Applications of the Kolbe-Schmitt Carboxylation Synthesis of l,3-Dialkylimidazolium-2-Carboxylates 

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