Лако-красочные материалы — производство

In most indigo syntheses the indole structure is built up by ring closure of appro­priate benzene derivatives by C-N or C-C bond formation. Examples of C-N bond formation include von Baeyer’s 1878 synthesis from phenylacetic acid (3) via oxindole (4).

In the Heumann I synthesis, which is the basis of modern synthetic indigo production (see Section 3.5), ring closure takes place by C-C bond formation. Aniline is first treated with a chloroacetate salt to give phenylglycine salt 5, which is converted under alkaline conditions to indoxyl salt 6. Ring closure of the alkali metal salt of phenylglycine 5 gives particularly high yields in the presence of stoi­chiometric quantities of sodium amide. This suggests that in the first reaction step the phenylglycine salt is deprotonated to give the dianion, followed by a C-C bond formation and a 1,3-H shift to yield the indoxylate dianion 6. The mecha­nism is similar to that of an intramolecular (aza)-Kolbe-Schmitt synthesis. Hydrolysis, oxidation, and dimerization of the indoxylate dianion (6) then result in indigo (1) (Scheme 2.5).