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.

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.

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.

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