Büchner ring expansion

What is Büchner ring expansion?

The reaction known as the ring expansion, or Buchner reaction, involves the thermal or photochemical reaction of ethyl diazoacetate with benzene and its homologs, resulting in the production of isomeric esters of cycloheptatriene carboxylic acid.

This transformation was first discovered by Buchner and Curtius in 1885, who used the reaction to synthesize cycloheptatrienes from arene cyclopropanation via the electrocyclic ring opening of norcaradiene intermediates.

Büchner ring expansion
Büchner ring expansion

The Buchner reaction is useful for producing seven-membered carbocycles both inter- and intramolecularly. In the past, the product mixture was complicated, but with modern transition metal catalysts, such as rhodium-based catalysts, it has become much simpler. Rhodium(II)-trifluoroacetate catalysis, for instance, can produce a single isomer of cycloheptatriene with a 98 % yield. Interestingly, aromatic rings that are complexed with chromium tricarbonyl do not undergo the Buchner ring expansion either inter- or intramolecularly, which makes this reaction a convenient synthetic method.

References

  • Buchner, E. and Curtius, T. (1885), Synthese von Ketonsäureäthern aus Aldehyden und Diazoessigäther. [Synthesis of β-keto esters from aldehydes and diazoacetic acid.] Ber. Dtsch. Chem. Ges., 18: 2371-2377. https://doi.org/10.1002/cber.188501802118
  • Buchner, E. (1896), Ueber Pseudophenylessigsäure. [Pseudophenyl acetic acids.] Ber. Dtsch. Chem. Ges., 29: 106-109. https://doi.org/10.1002/cber.18960290121
  • Buchner, E. and Schottenhammer, K. (1920), Die Einwirkung von Diazo-essigester auf Mesitylen. [Action of diazoacetic ester on mesitylene.] Ber. dtsch. Chem. Ges. A/B, 53: 865-873. https://doi.org/10.1002/cber.19200530602
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