Bischler-Möhlau indole synthesis
- Bischler-Möhlau indole synthesis
The Bischler-Möhlau indole synthesis is a chemical reaction that forms a 2-aryl-indole from a α-bromo-acetophenone and excess aniline. [Bischler, A. "et al."; "Ber." 1892, "25", 2860.] [Bischler, A. "et al."; "Ber." 1893, "26", 1336.] [Möhlau, R.; "Ber." 1881, "14", 171.] [Möhlau, R.; "Ber." 1882, "15", 2480.] [Fischer, E.; Schmitt, T. "Ber." 1888, "21", 1071.]
In spite of its long history, this classical reaction has received relatively little attention in comparison with other methods for indole synthesis, perhaps owing to the harsh reaction conditions that it requires. Recently, milder methods have been developed, including the use of lithium bromide as a catalyst and an improved procedure involving the use of microwave irradiation. [Pchalek, K.; Jones, A. W.; Wekking, M. M. T.; Black, D. S. C. "Tetrahedron" 2005, "61", 77.] [Sridharan, V.; Perumal, S.; Avendaño, C.; Menéndez, J. C. "Synlett" 2006, 91.]
Reaction mechanism
For a reaction with such simple starting materials, the reaction mechanism is surprisingly complex. The first two step involve the reaction of the α-bromo-acetophenone with molecules of aniline to form intermediate 4. The charged aniline forms a decent enough leaving group for an electrophilic cyclization to form intermediate 5, which quickly aromatizes and tautomerizes to give the desired indole 7.
References
ee also
*Fischer indole synthesis
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