Leimgruber–Batcho indole synthesis

Palladium-on-carbon and hydrogen, stannous chloride, sodium hydrosulfite[5], or iron in acetic acid[6] are also effective reducing agents.

In the initial enamine formation, dimethylamine (a gas) is displaced by pyrrolidine from the dimethylformamide dimethylacetal, producing a more reactive reagent.

The mildly acidic hydrogens of the methyl group in the nitrotoluene can be deprotonated under the basic conditions, and the resultant carbanion attacks to produce the enamine shown, with loss of methanol.

In the second step the nitro group is reduced to -NH2 using hydrogen and a Raney nickel catalyst, followed by cyclisation then elimination of the pyrrolidine.

The process has become a popular alternative to the Fischer indole synthesis because many starting ortho-nitrotoluenes are commercially available or easily made.