Carbene ring expansion
Buchner ring expansion
How the Buchner ring expansion works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Ethyl diazoacetate, Rh₂(OAc)₄ (cat.), benzene. Conditions: Benzene (solvent/substrate), 25–80 °C. Bonds formed: two C–C σ; C=C (triene); bonds broken: C=N₂; one C–C σ of norcaradiene; benzene aromaticity. Stereochemistry: Substituent at C7 of cycloheptatriene is exo-/pseudo-axial. Rate and selectivity: N₂ loss by Rh is rate-limiting; valence tautomerism is very fast.
At a glance
Step 1: Rh decomposes the diazo compound
A metal catalyst (often Rh or Cu) decomposes a diazo compound, releasing N₂ and generating a reactive metal carbenoid — the same carbenoid chemistry used in Simmons-Smith cyclopropanation.
Step 2: [2+1] addition across a benzene π bond
The carbenoid adds across one of benzene's π bonds in a formal [2+1] cycloaddition, momentarily forming a strained norcaradiene (a benzene ring fused to a cyclopropane).
Step 3: Electrocyclic ring opening
This bicyclic norcaradiene is in rapid equilibrium with its ring-expanded valence tautomer, cycloheptatriene — the six-membered aromatic ring effectively expands by one carbon, a rare way to build seven-membered rings directly from benzene.
Abbreviations and terms
- OAc
- Acetoxy. –OC(=O)CH₃, an acetate group.
How it compares with related reactions
| Buchner ring expansion | Arndt-Eistert homologation | Bamford-Stevens reaction | |
|---|---|---|---|
| Reagents | Ethyl diazoacetate, Rh₂(OAc)₄ (cat.), benzene | SOCl₂; CH₂N₂ (2 equiv.); Ag₂O (or hν / Δ), H₂O | Tosylhydrazone, strong base (NaOMe, NaH, or NaOEt) |
| Conditions | Benzene (solvent/substrate), 25–80 °C | Et₂O 0 °C, then H₂O / dioxane 50–70 °C | Aprotic solvent (diglyme, toluene), 100–200 °C |
| Stereochemistry | Substituent at C7 of cycloheptatriene is exo-/pseudo-axial | Wolff shift retains configuration at the migrating carbon | Mixture of E/Z; thermodynamic (more substituted) alkene favoured |
| Rate / selectivity | N₂ loss by Rh is rate-limiting; valence tautomerism is very fast | Ag(I) catalyses N₂ loss; second CH₂N₂ scavenges HCl | Diazo decomposition to carbene is rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Buchner ring expansion?
Ethyl diazoacetate, Rh₂(OAc)₄ (cat.), benzene. Conditions: Benzene (solvent/substrate), 25–80 °C.
Which bonds are formed and which are broken?
Formed: two C–C σ; C=C (triene). Broken: C=N₂; one C–C σ of norcaradiene; benzene aromaticity.
What is the stereochemical outcome?
Substituent at C7 of cycloheptatriene is exo-/pseudo-axial
What controls the rate or the selectivity?
N₂ loss by Rh is rate-limiting; valence tautomerism is very fast
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go