Copper quinoline

Despite its synthetic importance, the mechanism of the copper-quinoline method has been studied very little, but it has been shown that the actual catalyst is cuprous ion. In fact, the reaction proceeds much faster if the acid is heated in quinoline with cuprous oxide instead of copper, provided that atmospheric oxygen is rigorously excluded. A mechanism has been suggested in which it is the cuprous salt of the acid that actually undergoes the decarboxylation. It has been shown that cuprous salts of aromatic acids are easily decarboxylated by heating in quinoline and that arylcopper compounds are intermediates that can be isolated in some cases. Metallic silver has been used in place of copper, with higher yields. ... 

Pyrrole-2-carboxylic acid easily loses the carboxylic group thermally. Pyrrole-3-carboxylic acid and furan-2- and -3-carboxylic acids also readily decarboxylate on heating to about 200°C. Thiophene-carboxylic acids require higher temperatures or a copper-quinoline catalyst. In furans, 2-carboxylic acid groups are lost more readily than 3-carboxylic acid groups (Scheme 64). 

Carbonation of lithiofurans is a useful method for obtaining these compounds. Furan-2-carboxylic acid (pKa 3.15) is a stronger acid than the 3-carboxylic acid (pKa 4.0) because of the inductive effect of the ring oxygen, and both are stronger than benzoic acid. Furancarboxylic acids can be decarboxylated by the copper-quinoline method or merely by heating. The 2-carboxylic acids are more easily decarboxylated than the 3-isomers, so furan-3-carboxylic acid can be obtained by stepwise decarboxylation of the tetracarboxylic acid via the 2,3,4-tricarboxylic acid and the 3,4-dicarboxylic acid. A more convenient source of the 3-carboxylic acid is by partial hydrolysis and decarboxylation of the readily available diethyl furan-3,4-dicarboxylate (71S545). 

Reductive decarboxylation of aryl and vinyl acids, not readily achieved by any of the methods described here, is best brought about by the classical copper/quinoline procedure. ... 

Grignard reagents were formed with difficulty from both 5-bromo- and 5,5 -dibromo-2,2 -dithienylmethane they gave acids (71 and 81%, respectively) on carbonation.16 A carboxyl group attached to a thiophene ring has been decarboxylated with copper-quinoline (80%)39 bis(2-thienyl)alkanoic acids of general formula 83 decarboxylate quite readily on heating.40,107 In one case (83, R1 = Me R2 = H) the expected decarboxylation product was accompanied by a little of the dimer 84. 

Syntheses of 3,4-Unsaturated Cyclic Compounds - Compound 30, available in three steps from 5-hydroxymethyl-l,6-anhydro-a-L-a//ro-hexopyranose was converted into ewMevoglucosenone by reaction with copper-quinoline (decarboxylation at C-5), zirconium oxide induced olefination at C-3 (reductive decarboxylation) then deacylation and oxidation. The 5-hydroxymethyl analogue of ent-levogluco-senone was also made from the same starting materials. ... 

In 1967, Wynberg et al. reported the first synthesis of air-sensitive thieno[3,4-b] thiophene [51]. The sulfoxide (131) was converted into anhydride 133 in refluxing acetic anhydride (132). Hydrolysis to form thieno[3,4-Z>]thiophene-2-carboxylic acid (134) then copper/quinoline decarboxylation gave 2, isolated initially as a picrate (Scheme 29). 

Copper quinoline Methylene compds. from /5-hydroxycarboxylic acids... 

Heat-triggered decarboxylation of aromatic carboxylic acids in quinoline solution in the presence of copper metal or copper salts (the copper-quinoline decarboxylation)... 

---