The Fischer indole synthesis is a chemical reaction that produces the aromatic heterocycle indole from a (substituted) phenylhydrazine and an aldehyde or ketone under acidic conditions.^[1][2] The reaction was discovered in 1883 by Emil Fischer. Today antimigraine drugs of the triptan class are often synthesized by this method.

This reaction can be catalyzed by Brønsted acids such as HCl, H₂SO₄, polyphosphoric acid and p-toluenesulfonic acid or Lewis acids such as boron trifluoride, zinc chloride, and aluminium chloride.

Several reviews have been published.^[3][4][5]

Reaction mechanism

The reaction of a (substituted) phenylhydrazine with a carbonyl (aldehyde or ketone) initially forms a phenylhydrazone which isomerizes to the respective enamine (or 'ene-hydrazine'). After protonation, a cyclic [3,3]-sigmatropic rearrangement occurs producing a diimine. The resulting diimine forms a cyclic aminoacetal (or aminal), which under acid catalysis eliminates NH₃, resulting in the energetically favorable aromatic indole.

Isotopic labelling studies show that the aryl nitrogen (N1) of the starting phenylhydrazine is incorporated into the resulting indole.^[6][7]

Buchwald modification

Via a palladium-catalyzed reaction, the Fischer indole synthesis can be effected by cross-coupling aryl bromides and hydrazones.^[8] This result supports the previously proposed intermediacy as hydrazone intermediates in the classical Fischer indole synthesis. These N-arylhydrazones undergo exchange with other ketones, expanding the scope of this method.

Application

• Indometacin preparation.
• Triptan synthesis
• Iprindole synthesis (phenylhydrazine + suberone → 2,3-Cycloheptenoindole).