Leimgruber–Batcho indole synthesis

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The Leimgruber–Batcho indole synthesis is a series of organic reactions that produce indoles from o-nitrotoluenes 1.[1][2][3] The first step is the formation of an enamine 2 using N,N-dimethylformamide dimethyl acetal and pyrrolidine.[4] The desired indole 3 is then formed in a second step by reductive cyclisation.

The Leimgruber-Batcho indole synthesis
The Leimgruber-Batcho indole synthesis

In the above scheme, the reductive cyclisation is effected by

stannous chloride, sodium hydrosulfite[5], or iron in acetic acid[6] are also effective reducing agents
.

Reaction mechanism

In the initial enamine formation,

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. The sequence can also be performed without pyrrolidine, via the N,N-dimethyl enamine, though reaction times may be much longer in some cases. 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 hydrogen is often generated in situ by the spontaneous decomposition of hydrazine hydrate to H2 and N2
in the presence of the nickel.

The reaction is a good example of a reaction that was widely used in industry before any procedures were published in the mainstream scientific literature. Many indoles are

chemical yield
under mild conditions.

The intermediate enamines are electronically related to

electron-withdrawing nitro group conjugated
to an electron-donating group. The extended conjugation means that these compounds are usually an intense red color.

Variations

Dinitrostyrene reductive cyclization

The reductive cyclization of dinitrostyrenes (2) has proven itself effective when other more common methods have failed.[7]

An example of a dinitrostyrene reductive cyclization
An example of a dinitrostyrene reductive cyclization

Most of the standard reduction methods listed above are successful with this reaction.

See also

References

  1. ^ Batcho, A. D.; Leimgruber, W. U.S. patent 3,732,245 & U.S. patent 3,976,639
  2. ^ Batcho, A. D.; Leimgruber, W. Organic Syntheses 1985, 63, 214–220. (Article)
  3. ^ Clark, R. D.; Repke, D. B. Heterocycles 1984, 22, 195–221. (Review)
  4. doi:10.1021/jo00321a053
    )
  5. ^ Garcia, E. E.; Fryer, R. I. J. Heterocycl. Chem. 1974, 11, 219.
  6. doi:10.1021/jo01336a065
    )
  7. ^ Chen, B.-C.; Hynes, Jr., J.; Randit, C. R.; Zhao, R.; Skoumbourdis, A. P.; Wu, H.; Sundeen, J. E.; Leftheris, K. Heterocycles 2001, 55, 951-960.
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