The Pschorr reaction is a classic name reaction in organic chemistry, named after the German chemist Robert Pschorr. It involves the copper-salt-promoted decomposition of aromatic diazonium salts to generate aryl radicals, which subsequently undergo intramolecular aryl-aryl coupling to construct biaryl or fused polycyclic aromatic systems.

This reaction is particularly efficient for synthesizing polycyclic aromatic hydrocarbons (PAHs) such as phenanthrenes and fluorenes, making it a highly valuable tool in the synthesis of natural products, pharmaceuticals, and organic functional materials. As a seminal representative of early radical chemistry and copper-promoted single-electron transfer (SET) processes, the Pschorr reaction has profoundly influenced the development of modern radical cyclization and photoredox chemistry.

• Reagents: Nitrite salts, acids, copper salts (or copper powder)

• Substrates (Reactants): Arylamines (aromatic amines)

• Products: Polycyclic aromatic hydrocarbons (PAHs) or fused heterocycles

• Reaction Type: Intramolecular cyclization

Practical Experimental Tips

1. Strict Temperature Control during Diazotization:

   Because diazonium salts are thermally unstable and potentially explosive, the diazotization process must be conducted strictly in an ice bath (0-5℃) with the slow, dropwise addition of the nitrite reagent.

2. In Situ Generation of Diazonium Salts:

   Due to their limited stability, diazonium intermediates should not be stored for extended periods. It is highly recommended to generate them in situ and proceed directly to the subsequent cyclization step without isolation.

3. Mitigation of Radical Side Reactions:

   Aryl radical intermediates are highly reactive and prone to side reactions such as protodeamination (reductive denitrogenation) and intermolecular homocoupling. To suppress these bimolecular pathways, the reaction should be performed under high-dilution conditions or by carefully controlling the substrate concentration.

4. Transition to Sustainable Methodologies:

   Modern advancements are transforming the Pschorr reaction from a "classical copper-mediated radical cyclization" into a "photo- and electro-driven green aryl radical cyclization platform." Current research focuses heavily on enhancing the safety profile, regioselectivity, and functional group compatibility for complex molecule synthesis.

Mechanism

Original Literature

Robert Pschorr. Neue Synthese des Phenanthrens und seiner Derivate. Berichte der deutschen chemischen Gesellschaft. 1896, 29 (1), 496-501.

Top Citations

The Pschorr Reaction, a Fresh Look at a Classical Transformation. Current Organic Synthesis. 2009, 6(2), 193–202.

Soluble Catalysts for Improved Pschorr Cyclizations. J. Org. Chem. 1995, 60(1), 196-201.

Photocatalysis of the Pschorr reaction by tris-(2,2′-bipyridyl)ruthenium(II) in the phenanthrene series. J. Chem. Soc., Perkin Trans. 2. 1984, 68(21), 1093-1098.

Related Reactions:

Sandmeyer Reaction

Ullmann Reaction

Related Products:

Aromatic Amine Substrate

933045 Aniline, 99.5%, reagent grade CAS:62-53-3

147014 2-Aminobiphenyl, 98.5%, reagent grade CAS:90-41-5

222072 7-Methylquinoline, 98% CAS:612-60-2

Copper Salt

952467 Copper(I) chloride, 98% CAS:7758-89-6

929054 Copper(I) bromide, 99%, for synthesis CAS:7787-70-4

29-0075 Copper powder, spherical (99.9%) CAS:7440-50-8

936106 Copper(I) oxide, 99%, for synthesis CAS:1317-39-1

Photocatalyst

921652 Tris(2,2'-bipyridine)ruthenium dichloride, 98% CAS:14323-06-9

997766 Tris(2-phenylpyridine)iridium, 99%, for synthesis CAS:94928-86-6

344966 Eosin Y disodium salt, 80%, for analysis CAS:17372-87-1

Diazotizing Reagent

957737 Sodium nitrite, 98% CAS:7632-00-0

999641 tert-Butyl nitrite, 95%, for synthesis CAS:540-80-7

987981 Isopentyl nitrite, 98% CAS:110-46-3

Solvent

 954068 Ethanol, 99.5%, SuperDry, water≤50 ppm, J&KSeal CAS:64-17-5

918109 Acetic acid, 99.5%, AR CAS:64-19-7

187553 Water, HPLC grade CAS:7732-18-5

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