- Fischer indole synthesis
The Fischer indole synthesis is a
chemical reaction that produces thearomatic heterocycleindole from a (substituted)phenylhydrazine and analdehyde orketone underacid ic conditions. [Fischer, E.; Jourdan, F. "Ber. " 1883, "16", 2241.] [Fischer, E.; Hess, O. "Ber. " 1884, "17", 559.] The reaction was discovered in1883 byHermann Emil Fischer . Today antimigraine drugs of thetriptan class are often synthesized by this method.The choice of acid catalyst is very important.
Bronsted acid s such asHCl , H2SO4, andpolyphosphoric acid have been used successfully.Lewis acid s such asboron trifluoride ,zinc chloride ,iron chloride , andaluminium chloride are also useful catalysts.Several reviews have been published. [Van Orden, R. B.; Lindwell, H. G. "
Chem. Rev. " 1942, "30", 69-96. ( [http://pubs3.acs.org/acs/journals/doilookup?in_doi=10.1021/cr60095a004 Review] )] [Robinson, B. "Chem. Rev. " 1963, "63", 373-401. ( [http://pubs3.acs.org/acs/journals/doilookup?in_doi=10.1021/cr60224a003 Review] )] [Robinson, B. "Chem. Rev. " 1969, "69", 227-250. ( [http://pubs3.acs.org/acs/journals/doilookup?in_doi=10.1021/cr60258a004 Review] )]Reaction mechanism
The reaction of a (substituted) phenyl
hydrazine with an aldehyde or ketone initially forms aphenylhydrazone which isomerizes to the respectiveenamine (or 'ene-hydrazine'). Afterprotonation , a cyclic [sigmatropic rearrangement| [3,3] -sigmatropic rearrangement] occurs producing animine . The resulting imine forms a cyclicaminoacetal (or "aminal"), which under acid catalysis that 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. [Allen, C. F. H.; Wilson, C. V. "
J. Am. Chem. Soc. " 1943, "65", 611.] [Clusius, K.; Weisser, H. R. "Helv. Chim. Acta " 1952, "35", 400.]Buchwald modification
Using
palladium chemistry developed atMIT byStephen Buchwald , the Fischer indole synthesis can be completed using aryl bromides as starting materials. [Buchwald, S. L. "et al." "J. Am. Chem. Soc. " 1998, "120", 6621-6622. (DOI|10.1021/ja981045r)] Note that this also gives evidence to the fact that hydrazones are potential intermediates in the classical Fischer indole synthesis.Methanesulfonic acid can be used as the acid catalyst in this reaction.References
ee also
*
Bartoli indole synthesis
*Japp-Klingemann indole synthesis
*Leimgruber-Batcho indole synthesis
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