Reaction Kolbe-Schmitt

Since the cyanide anion is an ambident nucleophile, isonitriles R—NC may be obtained as by-products. The reaction pathway to either nitrile or isonitrile can be controlled by proper choice of the counter cation for the cyanide anion. 

With alkali cyanides, a reaction via a SN2-mechanism takes place the aUcyl halide is attacked by cyanide with the more nucleophilic carbon center rather than the nitrogen center, and the alkylnitrile is formed. In contrast, with silver cyanide the reaction proceeds by a SnI-mechanism, and an isonitrile is formed, since the carbenium intermediate reacts preferentially with the more electronegative center of the cyanide—i.e. the nitrogen (Komblum s rule, HSAB concept)  

The reaction works well with primary alkyl halides, especially with allylic and benzylic halides, as well as other alkyl derivatives with good leaving groups. Secondary alkyl halides give poor yields. Tertiary alkyl halides react under the usual reaction conditions by elimination of HX only. Nitriles from tertiary alkyl halides can however be obtained by reaction with trimethylsilyl cyanide 4  

This variant gives good to very good yields, and it is chemoselective the primary alkyl halide function in 3 is left unaffected under these reaction conditions. 

Carbon dioxide reacts with phenolates 1 to yield salicylate 2 with less reactive mono-phenolates, the application of high pressure may be necessary in order to obtain high yields. This reaction, which is of importance for the large scale synthesis of salicylic acid, is called the Kolbe-Schmitt reaction  

Carboxylation of sodium phenoxides with carbon dioxide, to give salicylic acid, the precursor to the synthesis of aspirin. 

Example 2, the Marasse modification of the Kolbe-Schmitt reaction uses excess of anhydrous potassium carbonate in place of carbon dioxide  

Erlenmeyer, H. Prijs, B. Sorkin, E. Suter, E. Helv. Chim. Acta 1948, 31, 988. Marasse, S. German Patent 73,279,1893. 

Kosugi, Y. Rahim, M. A. Takahashi, K. Imaoka, Y. Kitayama, M. Appl. Or-ganomet. Chem. 2000,14, 841. 

Ortho- or para-substituted aromatic hydroxy acid 

O-alkyl, NR2 base alkali metal hydroxides (e.g., NaOH, KOH, CsOH), K2CO3, KHCO3 

The mechanism of the Kolbe-Schmitt reaction was investigated since the late 1800s, but the mechanism of the carboxylation could not be elucidated for more than 100 years. For a long time, the accepted mechanism was that the carbon dioxide initially forms an alkali metal phenoxide-C02 complex, which is then converted to the aromatic carboxylate at elevated temperature. The detailed mechanistic study conducted by Y. Kosugi et al. revealed that this complex is actually not an intermediate in the reaction, since the carefully prepared phenoxide-C02 complex started to decompose to afford phenoxide above 90 °C. They also demonstrated that the carboxylated products were thermally stable even at around 200 °C. The CO2 electrophile attacks the ring directly to afford the corresponding ortho- or para-substituted products. (When the counterion is large (e.g., cesium) the attack of CO2 at the ortho-position is hindered therefore, the para-substituted product is the major product.) 

Phenols that have more than one hydroxyl group may be carboxylated with CO2 at atmospheric pressure under basic conditions. The research team of Y.-C. Gao synthesized 3,5-di-fert-butyl-y-resorcylic acid from 4,6-di-fert-butyl resorcinol using the Kolbe-Schmitt reaction under these conditions. The resorcylic acid derivative was needed in order to prepare ternary complexes of lanthanide(lll)-3,5-di-fert-butyl-Y-resorcylate with substituted pyridine-A/-oxide. 

Ternary complexes of lanthanide (lll)-3,5-di-fert-butyl-y-resorcylate with substituted pyridine-A/-oxide 

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Knoevenagel reaction Knorr pyrrole synthesis. Kolbe>Schmitt reaction Leuckart reaction Mannich reaction... 

CARBOXYLATION OF PHENOLS ASPIRIN AND THE KOLBE-SCHMITT REACTION... 

The key compound m the synthesis of aspirin salicylic acid is prepared from phe nol by a process discovered m the nineteenth century by the German chemist Hermann Kolbe In the Kolbe synthesis also known as the Kolbe—Schmitt reaction, sodium phen oxide IS heated with carbon dioxide under pressure and the reaction mixture is subse quently acidified to yield salicylic acid... 

Carboxylation of Phenols Aspirin and the Kolbe-Schmitt Reaction... 

The Kolbe-Schmitt reaction is an equilibrium process governed by thermodynamic control The position of equilibrium favors formation of the weaker base (salicylate ion) at the expense of the stronger one (phenoxide ion) Thermodynamic control is also responsible for the pronounced bias toward ortho over para substitution Salicylate anion IS a weaker base than p hydroxybenzoate and predominates at equilibrium... 

The Kolbe-Schmitt reaction has been applied to the preparation of other o hydroxy benzoic acids Alkyl derivatives of phenol behave very much like phenol itself... 

Kolbe-Schmitt reaction (Section 24 10) The high pressure re action of the sodium salt of a phenol with carbon dioxide to give an o hydroxybenzoic acid The Kolbe-Schmitt reac tion IS used to prepare salicylic acid in the synthesis of as pinn... 

The Kolbe-Schmitt reaction is limited to phenol, substituted phenols and certain heteroaromatics. The classical procedure is carried out by application of high pressure using carbon dioxide without solvent yields are often only moderate. In contrast to the minor importance on laboratory scale, the large scale process for the synthesis of salicylic acid is of great importance in the pharmaceutical industry. 

Sodium phenoxides can be carboxylated, mostly in the ortho position, by carbon dioxide (the Kolbe-Schmitt reaction). The mechanism is not clearly understood, but... 

Knoevenagel reaction Knorr p3rrrole synthesis. Kolbe-Schmitt reaction Leuckart reaction Mannich reaction Mccrwein-Ponndorf-Verley reduction Michael reaction Oppenauer oxidation... 

Salicylic acid. The preparation of salicylic acid by passing carbon dioxide into dry sodium phenoxide at 170-190° is the classical example of the Kolbe-Schmitt reaction. The latter is a method for introducing a carboxyl group directly into a phenol nucleus. 

The BASF route started from hydroquinone, which was converted to 2,5-dihydroterephthalic acid by a Kolbe-Schmitt reaction. One mole of this acid was treated with two moles of an arylamine, both components being in the form of a suspension in aqueous methanol. This was added to a small amount of a solution of vanadium(III) chloride and sodium chlorate. Gentle heating gave a 95% yield of 2,5-bis(arylamino)benzo-l,4-quinone-3,6-dicarboxylic acid. Ring closure to the trans-quinacridonequinone took place in the presence of concentrated sulphuric acid at 60-80 °C. This was then reduced to the required crude pigment by zinc or aluminium powder in caustic soda under pressure,in an aluminium chloride/urea melt or by the use of a sulphuric acid/polyphosphoric acid mixture. 

Example 3, the Jones modification of the Kolbe-Schmitt reaction employs sodium ethyl carbonate ... 

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