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Sulfonation of 2-lithio-indole via the Katritzky reaction; 2-toluenesulfonyl indole

SyntheticPage 16
DOI: 10.1039/SP16
Submitted Jun 18, 2001, published Jun 18, 2001
Stephen Caddick (s.caddick@sussex.ac.uk)


			Reaction Scheme: Sulfonation of 2-lithio-indole via the Katritzky reaction

Chemicals Used

BuLi (1.6 M)
Indole (10.8 mmol)
THF
t-BuLi (1.7 M)
Toluenesulfonyl fluoride

Procedure

BuLi (11.2 mmol) was added dropwise to a stirred solution of indole in THF (40 ml) at -78 o C. After about 30 minutes CO2 (gas - dried through CaCl2 tube) was passed through the solution which was then allowed to stand for 10 minutes and the solvent was evaporated at 0 oC (1 mm Hg- using a pump attached directly to the reaction vessel). This takes about an hour. The resulting material (white solid) was then dissolved in THF (40 ml) and cooled to -78 oC. t-BuLi (18.7 mmol) was then added dropwise and the solution stirred for about 20 minutes. TsF (1.91 11 mmol) in THF (8 ml) was then added at -78 oC and the mixture stirred at this temperature for one hour and then allowed to warm to r.t. The reaction was then stirred at room temperature for a couple of hours and the solkution then poured onto ice-brine solution and then extracted with ether. Purification was by chromatography using silica-gel, and petrol:ether as eluent (20:1, - 3:1, gradient) to give the product (1.04 g, 35%)

Author's Comments

Although the yield of this product is only moderate, its not a bad way of making this compound. The key point is to ensure that the temperature is maintained around 0oC when removing the solvent after the first lithiation. We have used this to prepare 2-tosyl-pyrrole - but that is not quite as reproducible.

Data

1 H NMR 8.82, 1H, br s; 7.89, 2H, d, J = 8.4 Hz; 7.67, 1H, d, J = 8.2 Hz; 7.42, 2H, d, J = 8.2 Hz; 7.34, 1H, d, 6.8 Hz; 7.2-7.14, 3H, m; 2.4, 3H, s

Lead Reference

S. Caddick, K. Aboutayab, K. Jenkins, R. I. West, J. Chem. Soc. Perkin Trans. 1, 1996, 675

Other References

A. R. Katritzky, K. Akutagava, tetrahedron Lett. 1985, 24, 5935

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Keywords: aromatics/arenes, heterocyclic compounds, indole, lithiation

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