The Japp–Klingemann reaction is a chemical reaction used to synthesize hydrazones from β-keto-acids (or β-keto-esters) and aryl diazonium salts. The reaction is named after the chemists Francis Robert Japp and Felix Klingemann.
:500px|The Japp-Klingemann reaction
The hydrazone products of the Japp–Klingemann reaction are most often used as intermediates in syntheses of more complex organic molecules. For example, a phenylhydrazone product can be heated in the presence of strong acid to produce an indole via the Fischer indole synthesis.
:550px|The Japp-Klingemann reaction used to synthesize indoles
If there is a leaving group elsewhere in the Japp–Klingemann product, the hydrazone instead can cyclize at that site via a substitution reaction to give a pyrazole. This process is a key part of the synthesis of and related compounds:
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Reaction mechanism
To illustrate the mechanism, the Japp-Klingemann ester variation will be considered. The first step is the deprotonation of the β-keto-ester. The nucleophilic addition of the enolate anion 2 to the diazonium salt produces the azo compound 3. Intermediate 3 has been isolated in rare cases. However, in most cases, the hydrolysis of intermediate 3 produces a tetrahedral intermediate 4, which quickly decomposes to release the carboxylic acid 6. After hydrogen exchange, the final hydrazone 7 is produced.
center|1440px|The Japp-Klingemann reaction mechanism
Applications
Indole synthesis: Hydrazones formed in this reaction undergo Fischer Indole synthesis.
Pyrazole synthesis: On cyclization, Hydrazones yield Pyrazoles.
Preparation of Azo Derivatives.
Widely used in Medicinal Chemistry for Heterocyclic frameworks.