View source for Chemistry:Pschorr cyclization

{{Reactionbox
| Name = Pschorr cyclization
| Type = Ring forming reaction
| NamedAfter = Robert Pschorr
| Reaction = {{Reactionbox Reaction
  | Reactant = biaryl with diazonium group
  | Product = tricyclic biaryl
 }}
| Section1 = {{Reactionbox Identifiers
  | OrganicChemistryNamed = pschorr-reaction
 }}
| Section2 = {{Reactionbox Conditions
  | Catalyst = copper
 }}
}}
The '''Pschorr cyclization''' is a [[Chemistry:Name reaction|name reaction]] in [[Chemistry:Organic chemistry|organic chemistry]], which was named after its discoverer, the {{wipe|Germany}} chemist Robert Pschorr (1868-1930). It describes the intramolecular substitution of aromatic compounds via aryldiazonium salts as intermediates and is [[Chemistry:Catalysis|catalyzed]] by [[Chemistry:Copper|copper]]. The reaction is a variant of the Gomberg-Bachmann reaction.<ref>{{citation|first1=Robert |last1=Pschorr|periodical=Berichte der deutschen chemischen Gesellschaft|title=Neue Synthese des Phenanthrens und seiner Derivate|volume=29|issue=1|pages=496–501|date=1896|language=German|doi=10.1002/cber.18960290198
|url=https://zenodo.org/record/1425836}}</ref> The following reaction scheme shows the Pschorr cyclization for the example of [[Chemistry:Phenanthrene|phenanthrene]]:<ref name="Cepanec">{{citation|first1=Ivica |last1=Cepanec |title=Synthesis of biaryls|edition=1st|publisher=Elsevier|location=Amsterdam|pages=25–27|isbn=9780080444123|date=2004
}}</ref>

[[File:Pschorr cyclization ÜV1.svg|center|450px|Pschorr-Cyclisierung]]

== Reaction mechanism ==
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 to [[Chemistry:Hydrochloric acid|hydrochloric acid]] to obtain [[Chemistry:Nitrous acid|nitrous acid]]. The nitrous acid is protonated and reacts with another equivalent of nitrous acid to the intermediate '''1''' which is later used for the diazotization of the [[Chemistry:Aromatic amine|aromatic amine]]:

[[File:Pschorr cyclization part 1 MV1.svg|center|600px|Erster Teil des Reaktionsmechanismus der Pschorr-Cyclisierung]]

The intermediate '''1''' reacts in the following way with the starting compound:<ref name="Li">{{citation|first1=Jie Jack |last1=Li|title=Name reactions: A collection of detailed mechanisms and synthetic applications|edition=5th|publisher=Springer|location=Cham|pages=499–500|isbn=978-3-319-03979-4|date=2014|doi=10.1007/978-3-319-03979-4
|s2cid=93616494}}</ref>

[[File:Pschorr cyclization part 2 MV1.svg|center|550px|Zweiter Teil des Reaktionsmechanismus der Pschorr-Cyclisierung]]

Intermediate '''1''' 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 intermediate '''2'''. Intermediate '''2''' then reacts via a [[Chemistry:Tautomer|tautomer]]ism and [[Chemistry:Dehydration reaction|dehydration]] to the aryldiazonium cation '''3'''.

[[File:Pschorr cyclization part 3 MV1.svg|center|500px|Dritter Teil des Reaktionsmechanismus der Pschorr-Cyclisierung]]

Nitrogen is then cleaved from the aryldiazonium cation '''3''' by the use of the copper catalyst. The aryl radical thus formed reacts via ring closure to the intermediate stage '''4'''. Finally, [[Chemistry:Aromatization|rearomatization]] takes place using again the copper catalyst and phenanthrene is formed.

== Atom economy ==
The Pschorr cyclization has a relatively good [[Chemistry:Atom economy|atom economy]], since essentially only [[Chemistry:Nitrogen|nitrogen]] is produced as a waste material. For the diazotization, 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 low [[Chemistry:Yield|yields]].<ref name="Cepanec"/><ref name="Li"/>

== References ==
{{Reflist}}

[[Category:Name reactions]]

{{Sourceattribution|Pschorr cyclization}}