Pericyclic — group transfer
Ene reaction
How the Ene reaction 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: Ene (alkene with allylic C–H) + enophile (e.g. CH₂=O, maleic anhydride). Conditions: Thermal 150–250 °C, or Lewis acid (Me₂AlCl) at low temp. Bonds formed: C–C σ; O–H; shifted C=C π; bonds broken: C=C π (ene); C=O π; allylic C–H. Stereochemistry: Suprafacial, concerted; endo/chair-like TS. Rate and selectivity: Higher barrier than Diels-Alder; Lewis acids accelerate.
At a glance
Step 1: Ene and enophile approach
An 'ene' component (an alkene with an allylic C–H) and an electrophilic 'enophile' (like an aldehyde) approach in a six-membered cyclic transition state.
Step 2: Six-electron cyclic transition state
Three things happen at once: a new σ bond forms between the ene's terminal carbon and the enophile, the ene's π bond shifts position, and the allylic C–H transfers directly to the enophile's other end.
Step 3: Homoallylic alcohol
Like the Diels-Alder, this is fully concerted with a predictable, cyclic transition state — but it transfers a hydrogen atom instead of forming a ring, making it a 'group-transfer' pericyclic reaction.
Abbreviations and terms
- Lewis acid
- Lewis acid. Electron-pair acceptor (AlCl₃, BF₃, TiCl₄…) that activates carbonyls and halides.
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
- π*
- Pi antibonding orbital. The empty orbital of a C=O or C=C that accepts nucleophiles.
How it compares with related reactions
| Ene reaction | 1,3-dipolar cycloaddition | 2-Aza-Cope rearrangement | |
|---|---|---|---|
| Reagents | Ene (alkene with allylic C–H) + enophile (e.g. CH₂=O, maleic anhydride) | Nitrile oxide (from hydroximoyl chloride + Et₃N, or ArCH=NOH + NCS/base), alkene | Homoallylic amine, aldehyde (to give the iminium), acid |
| Conditions | Thermal 150–250 °C, or Lewis acid (Me₂AlCl) at low temp | CH₂Cl₂ or toluene, 25–80 °C; no catalyst | Mild heat or RT (cationic variant), CH₂Cl₂ or MeCN |
| Stereochemistry | Suprafacial, concerted; endo/chair-like TS | Suprafacial, stereospecific (alkene geometry retained); 5-substituted regioisomer favored | Chair TS; stereo transferred |
| Rate / selectivity | Higher barrier than Diels-Alder; Lewis acids accelerate | Concerted, thermal; HOMO/LUMO of dipole and alkene both matter | Faster than all-carbon Cope (cationic N) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Ene reaction?
Ene (alkene with allylic C–H) + enophile (e.g. CH₂=O, maleic anhydride). Conditions: Thermal 150–250 °C, or Lewis acid (Me₂AlCl) at low temp.
Which bonds are formed and which are broken?
Formed: C–C σ; O–H; shifted C=C π. Broken: C=C π (ene); C=O π; allylic C–H.
What is the stereochemical outcome?
Suprafacial, concerted; endo/chair-like TS
What controls the rate or the selectivity?
Higher barrier than Diels-Alder; Lewis acids accelerate
Study guides that help with this reaction
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti