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| Pschorr cyclization | |||||
|---|---|---|---|---|---|
| Named after | Robert Pschorr | ||||
| Reaction type | Ring forming reaction | ||||
| Reaction | |||||
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| Identifiers | |||||
| Organic Chemistry Portal | pschorr-reaction | ||||
| Conditions | |||||
| Catalyst | copper | ||||
ThePschorr cyclization is aname reaction inorganic chemistry, which was named after its discoverer, theGerman chemist Robert Pschorr (1868-1930). It describes the intramolecular substitution of aromatic compounds viaaryldiazonium salts as intermediates and iscatalyzed bycopper. The reaction is a variant of theGomberg-Bachmann reaction.[1] The following reaction scheme shows the Pschorr cyclization for the example ofphenanthrene:[2]
In the course of the Pschorr cyclization, a diazotization of the starting compound occurs, so that an aryldiazonium salt is formed as intermediate. For this, sodium nitrite is added tohydrochloric acid to obtainnitrous acid. The nitrous acid is protonated and reacts with another equivalent of nitrous acid to the intermediate1 which is later used for the diazotization of thearomatic amine:
The intermediate1 reacts in the following way with the starting compound:[3]
Intermediate1 replaces a hydrogen atom from the amino group of the starting compound. A nitroso group is introduced as new substituent, producing under the release of nitrous acid intermediate2. Intermediate2 then reacts via atautomerism anddehydration to the aryldiazonium cation3.
Nitrogen is then cleaved from the aryldiazonium cation3 by the use of the copper catalyst. The aryl radical thus formed reacts via ring closure to the intermediate stage4. Finally,rearomatization takes place using again the copper catalyst and phenanthrene is formed.
The Pschorr cyclization has a relatively goodatom economy, since essentially onlynitrogen is produced as a waste material. For thediazotization, two equivalents of nitrous acid are used, of which one equivalent is being re-formed in the course of the reaction. The copper is used in catalytic amounts only and does therefore not affect the atomic efficiency of the reaction. However, when considering the atom economy it has to be mentioned that the Pschorr cyclization has often only lowyields.[2][3]