Gomberg–Bachmann reaction explained

The Gomberg–Bachmann reaction, named for the Russian-American chemist Moses Gomberg and the American chemist Werner Emmanuel Bachmann, is an aryl-aryl coupling reaction via a diazonium salt.^{[1] [2]}

The arene compound (here benzene) is reacted with a diazonium salt in the presence of a base to provide the biaryl through an intermediate aryl radical. For example, p-bromobiphenyl may be prepared from 4-bromoaniline and benzene:

BrC₆H₄NH₂ + C₆H₆ → BrC₆H₄−C₆H₅

The reaction offers a wide scope for both diazonium component and arene component but yields are generally low following the original procedure (less than 40%), given the many side-reactions of diazonium salts. Several improvements have been suggested. One possibility is to employ diazonium tetrafluoroborates in arene solvent together with a phase-transfer catalyst,^[3] another is to use 1-aryl-3,3-dialkyltriazenes.^[4]

Pschorr reaction

One intramolecular variation which gives better results is the Pschorr cyclization:^{[5] [6]}

The group Z can be CH₂, CH₂CH₂, NH and CO (to fluorenone^[7]) to name just a few.

See also

• Graebe–Ullmann synthesis
• Meerwein arylation
• Sandmeyer reaction

References

1. The Synthesis of Biaryl Compounds by Means of the Diazo Reaction . . 1924 . 42 . 2339–2343 . 10.1021/ja01675a026 . 10 . Bachmann . W. E. . Werner Emmanuel Bachmann . M. . Gomberg . Moses Gomberg.
2. W. Pötsch. Lexikon bedeutender Chemiker (VEB Bibliographisches Institut Leipzig, 1989)
3. J. R. Beadle, S. H. Korzeniowski, D .E. Rosenberg, B. J. Garcia-Slanga, G. W. Gokel . . 1984 . 49 . 1594–603 . 10.1021/jo00183a021 . Phase-transfer-catalyzed Gomberg-Bachmann synthesis of unsymmetrical biarenes: a survey of catalysts and substrates . 9. Korzeniowski . Rosenberg . Garcia-Slanga . Gokel.
4. T. B. Patrick, R. P. Willaredt, D. J. DeGonia . . 1985 . 50 . 2232–2235 . 10.1021/jo00213a007 . Synthesis of biaryls from aryltriazenes . 13.
5. Robert . Pschorr . Neue Synthese des Phenanthrens und seiner Derivate . New Synthesis of Phenanthrene and Its Derivatives . German . . 1896 . 29 . 1 . 496–501 . 10.1002/cber.18960290198.
6. Kenneth K.. Laali. Mohammadreza. Shokouhimehr. The Pschorr Reaction, a Fresh Look at a Classical Transformation. Current Organic Synthesis. 2009. 6. 2. 193–202. 10.2174/157017909788167275.
7. Sandmeyer reactions. Part 5.1 Estimation of the rates of 1,5-aryl/aryl radical translocation and cyclisation during Pschorr fluorenone synthesis with a comparative analysis of reaction energetics . Stephen A. Chandler . Peter Hanson . Alec B. Taylor . Paul H. Walton . Allan W. Timms . . 2001 . 214–228 . 10.1039/b006184k . 2.