GB819438A - Process for the rearrangement of salts of heterocyclic carboxylic acids - Google Patents

Abstract

Salts of polybasic heterocyclic carboxylic acids are prepared by heating above 275 DEG C. a salt of a different mono- or polycarboxylic acid in which the carboxyl groups are attached to a heterocyclic ring having an aromatic structure. Suitable heterocyclic systems include furan, thiophen, pyrrole, a -pyran, a -thiopyran, quinoline, isoquinoline, indole, benzotriazole and benzimidazole. The preferred salts are those of the alkali metals, especially sodium and potassium. Instead of the salts themselves, mixtures from which these are formed may be treated, e.g. the free acids, their chlorides or their anhydrides with metal carbonates. The salts should be as dry as possible. Aqueous solutions are preferably dried by spray drying followed by further drying if necessary. Temperatures of 300-500 DEG C. are preferred, the optimum generally being 20-40 DEG C. below the fusion temperature of the salt at normal pressure. Inert fillers such as sand, finely-divided coke or other forms of carbon, metal powders, turnings or fragments or metal carbonates or bicarbonates may be present. It is preferred to carry out the reaction under pressure, e.g. 5-150 atm. in the cold rising to 15-500, preferably 100-250 atm. on heating. The pressure is advantageously produced by an inert gas such as nitrogen or CO2 or by adding the corresponding heterocyclic compound free from carboxyl groups. Catalysts may be used especially the oxides, hydroxides and the inorganic and organic salts of metals whose oxides are capable of conducting surplus electrons, e.g. lead, cadmium, zinc and mercury. These metals may be used in the form of their salts with the heterocyclic carboxylic acids to be treated when the total cations necessary to neutralize the carboxyl group may be provided by the catalyst metal. The free metals may also be used, e.g. as a lining to the reactor. The quantity of catalytic metal is generally 0-15 per cent by weight referred to the reaction mixture. The catalysts may be particularly finely dispersed in the reaction mixture by drying an aqueous solution of the heterocyclic acid salts containing the catalysts dissolved or suspended therein, e.g. by atomization or on drum driers. The free acid may be obtained from the reaction mixture by acidifying an aqueous solution and crystallizing or extracting the acid which may be converted to derivatives, e.g. esters, especially methyl and ethyl esters, or the acid chloride. These may also be obtained from the crude reaction product especially from the product obtained by spray-drying an aqueous solution. In examples: isocinchomeric acid and its dimethyl ester are prepared from potassium or sodium nicotinate, potassium isonicotinate, potassium quinolate or potassium picolinate with cadmium fluoride as catalyst or with no catalyst; dehydromucic acid, its dimethyl ester and its chloride are prepared from sodium or potassium pyromucate with cadmium fluoride or chloride as catalyst; thiophene-2,5-dicarboxylic acid and its dimethyl and diethyl esters are prepared from potassium thiophene-2-carboxylate using cadmium oxide as catalyst; pyrrole-2,5-dicarboxylic acid is prepared from potassium pyrrole-2-carboxylate using cadmium fluoride as catalyst; pyrazine-2,5-dicarboxylic acid is prepared from dipotassium pyrazine-2,3-carboxylate using cadmium chloride as catalyst; quinoline-2,4-dicarboxylic acid is prepared from potassium quinaldate using cadmium fluoride as catalyst; the monopotassium salt of pyridine-2,4,6-tricarboxylic acid is obtained from potassium isonicotinate or potassium quinolinate using cadmium fluoride as catalyst, and converted to the trimethyl ester; pyridine carboxylic acid mixtures of acid number 601 to 660 are prepared from barium or calcium nicotinate using cadmium fluoride as catalyst; CO2 under pressure is present in all examples.