Carbene rearrangement
Wolff rearrangement
How the Wolff rearrangement 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: α-Diazoketone; Ag₂O (cat.) or hν or Δ; then nucleophile (H₂O, ROH, R₂NH). Conditions: Heat 60–100 °C, UV light, or Ag(I) at 25–70 °C. Bonds formed: ketene C=C; C–C (migrating R to carbene C); N₂ (gas); bonds broken: C–N₂; C–R (migrating). Stereochemistry: Migrating carbon retains configuration. Rate and selectivity: N₂ loss and shift are fast and probably concerted with Ag(I).
At a glance
Step 1: Loss of N₂ gives an acyl carbene
An α-diazo ketone loses N₂ on heating, with light, or with a silver catalyst (Ag₂O). The result is an α-keto carbene, a carbon with only six valence electrons next to a carbonyl.
Step 2: 1,2-R shift gives the ketene
The R group on the carbonyl carbon migrates to the carbene carbon in a 1,2-shift as the carbene pairs up with the carbonyl π bond. The result is a ketene, R(H)C=C=O, with the carbon skeleton rearranged.
Step 3: Nucleophile traps the ketene
Ketenes are electrophilic at the central carbon, so a nucleophile adds to it. Water gives a carboxylic acid, an alcohol gives an ester, and an amine gives an amide. This ketene step is what completes the Arndt-Eistert homologation, which makes an acid one carbon longer.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- Δ
- Heat. Reaction is run at elevated temperature.
- hν
- Light. Photochemical activation.
How it compares with related reactions
| Wolff rearrangement | Bamberger rearrangement | Beckmann rearrangement | |
|---|---|---|---|
| Reagents | α-Diazoketone; Ag₂O (cat.) or hν or Δ; then nucleophile (H₂O, ROH, R₂NH) | N-Phenylhydroxylamine, dilute H₂SO₄ (aq) | H₂SO₄, PCl₅, TsCl or PPA |
| Conditions | Heat 60–100 °C, UV light, or Ag(I) at 25–70 °C | H₂O, 25–60 °C | Acid, 100–130 °C (industrial: oleum) |
| Stereochemistry | Migrating carbon retains configuration | None; para-selective | Group anti to OH migrates, with retention |
| Rate / selectivity | N₂ loss and shift are fast and probably concerted with Ag(I) | Loss of water to give the nitrenium ion is rate-limiting | Migration is rate-limiting; leaving-group ability of N–OX matters |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Wolff rearrangement?
α-Diazoketone; Ag₂O (cat.) or hν or Δ; then nucleophile (H₂O, ROH, R₂NH). Conditions: Heat 60–100 °C, UV light, or Ag(I) at 25–70 °C.
Which bonds are formed and which are broken?
Formed: ketene C=C; C–C (migrating R to carbene C); N₂ (gas). Broken: C–N₂; C–R (migrating).
What is the stereochemical outcome?
Migrating carbon retains configuration
What controls the rate or the selectivity?
N₂ loss and shift are fast and probably concerted with Ag(I)
Study guides that help with this reaction
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti