Pericyclic — [3,3]-sigmatropic
Overman rearrangement
How the Overman 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: Allylic alcohol; Cl₃CCN, DBU (cat.); then heat (140 °C) or PdCl₂(MeCN)₂ / Hg(II) (25 °C). Conditions: Step 1: CH₂Cl₂, 0 °C. Step 2: xylene, 140 °C, or THF, 25 °C with Pd(II). Bonds formed: C–N; C=O (amide); bonds broken: C–O (allylic); C=N π. Stereochemistry: Suprafacial, chair TS: chirality transfer from C–O to C–N. Rate and selectivity: Thermal: slow (hours at 140 °C); Pd(II) catalysis lowers barrier ≈ 10⁸-fold.
At a glance
Step 1: Allylic alcohol → trichloroacetimidate
An allylic alcohol is converted to a trichloroacetimidate ester (via trichloroacetonitrile), setting up a nitrogen-containing leaving group positioned for sigmatropic rearrangement.
Step 2: [3,3]-Sigmatropic shift
Heat or a Pd(II) catalyst promotes a [3,3]-sigmatropic shift — the same closed-loop, three-arrow pattern as the Claisen and Cope rearrangements — moving the nitrogen from the imidate oxygen directly onto the far allylic carbon.
Step 3: Net 1,3-transposition
The product is an allylic trichloroacetamide with the double bond transposed — this is one of the most reliable ways to convert an allylic alcohol into an allylic amine with predictable 1,3-transposition of both the functional group and the alkene.
Abbreviations and terms
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- MeCN
- Acetonitrile. Polar aprotic solvent.
- DBU
- 1,8-Diazabicyclo[5.4.0]undec-7-ene. Strong, non-nucleophilic amidine base (pKaH ≈ 12 in water, 24 in MeCN) used for E2.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
- Δ
- Heat. Reaction is run at elevated temperature.
How it compares with related reactions
| Overman rearrangement | Claisen rearrangement | Cope rearrangement of 1,5-hexadiene | |
|---|---|---|---|
| Reagents | Allylic alcohol; Cl₃CCN, DBU (cat.); then heat (140 °C) or PdCl₂(MeCN)₂ / Hg(II) (25 °C) | Allyl vinyl ether (e.g. from allyl alcohol + enol ether, or Johnson–Claisen) | 1,5-Hexadiene (and substituted analogues) |
| Conditions | Step 1: CH₂Cl₂, 0 °C. Step 2: xylene, 140 °C, or THF, 25 °C with Pd(II) | Heat, 100–200 °C, no catalyst | Heat, ≈ 150–260 °C; or Pd(II) catalysis |
| Stereochemistry | Suprafacial, chair TS: chirality transfer from C–O to C–N | Chair TS; suprafacial; chirality transfer | Chair TS; suprafacial; E/Z-selective |
| Rate / selectivity | Thermal: slow (hours at 140 °C); Pd(II) catalysis lowers barrier ≈ 10⁸-fold | Concerted [3,3]; accelerated by polar solvents; driven by strong C=O | Degenerate here; ΔG° = 0, driven by substituents in general |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Overman rearrangement?
Allylic alcohol; Cl₃CCN, DBU (cat.); then heat (140 °C) or PdCl₂(MeCN)₂ / Hg(II) (25 °C). Conditions: Step 1: CH₂Cl₂, 0 °C. Step 2: xylene, 140 °C, or THF, 25 °C with Pd(II).
Which bonds are formed and which are broken?
Formed: C–N; C=O (amide). Broken: C–O (allylic); C=N π.
What is the stereochemical outcome?
Suprafacial, chair TS: chirality transfer from C–O to C–N
What controls the rate or the selectivity?
Thermal: slow (hours at 140 °C); Pd(II) catalysis lowers barrier ≈ 10⁸-fold
Study guides that help with this reaction
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti