Synthesis of Indoles
Indole is a major constituent of coal-tar, and the 220–260 °C distillation fraction is the main industrial source of the material. Indole and its derivatives can also be synthesized by a variety of methods. The main industrial routes start from aniline.
Illustrative of such large-scale syntheses, indole (and substituted derivatives) form via vapor-phase reaction of aniline with ethylene glycol in the presence of catalysts:
In general, reactions are conducted between 200 and 500 °C. Yields can be as high as 60%. Other precursors to indole include formyltoluidine, 2-ethylaniline, and 2-(2-nitrophenyl)ethanol, all of which undergo cyclizations. Many other methods have been developed that are applicable.
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