Allylic oxidation — ene mechanism
Riley oxidation (SeO₂ allylic oxidation)
How the Riley oxidation (SeO₂ allylic oxidation) 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: SeO₂ (cat. with t-BuOOH, or stoich.). Conditions: Dioxane or EtOH, 25–100 °C. Bonds formed: C–O (allylic alcohol), C=C shifted twice (net unchanged); bonds broken: allylic C–H, Se–C. Stereochemistry: Ene step is suprafacial; [2,3] via chair-like TS gives E-alkenes. Rate and selectivity: Ene reaction is rate-limiting; allylic C–H selective.
At a glance
Step 1: Ene reaction with SeO₂
Selenium dioxide's Se=O reacts with an alkene's allylic C–H through an ene reaction — the same six-membered, fully concerted group-transfer mechanism covered earlier — transferring a hydrogen to selenium while forming a new C–Se bond and shifting the alkene's position.
Step 2: [2,3]-Sigmatropic shift
This ene adduct undergoes a [2,3]-sigmatropic rearrangement — the same five-atom, closed-loop shift as the Mislow-Evans rearrangement — moving the oxygen from selenium onto the original allylic carbon while restoring the alkene to its original position.
Step 3: Hydrolysis
Hydrolysis of the resulting allylic selenous ester releases the allylic alcohol (often further oxidized in situ to the enal or enone) — this reaction selectively oxidizes an allylic C–H specifically because it's allylic, leaving ordinary saturated C–H bonds untouched.
Abbreviations and terms
- EtOH
- Ethanol. Polar protic solvent.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
How it compares with related reactions
| Riley oxidation (SeO₂ allylic oxidation) | Baeyer-Villiger oxidation of a ketone | Collins (Sarett) oxidation | |
|---|---|---|---|
| Reagents | SeO₂ (cat. with t-BuOOH, or stoich.) | mCPBA or CH₃CO₃H (peracid) | CrO₃·2 pyridine (Collins/Sarett reagent) |
| Conditions | Dioxane or EtOH, 25–100 °C | CH₂Cl₂, 25 °C; cat. acid helps | Anhydrous CH₂Cl₂, 25 °C |
| Stereochemistry | Ene step is suprafacial; [2,3] via chair-like TS gives E-alkenes | Retention at the migrating carbon | Neighbouring stereocentres retained |
| Rate / selectivity | Ene reaction is rate-limiting; allylic C–H selective | Migratory aptitude: tert > cyclohexyl ≈ sec > Ph > prim > Me | Alpha C–H cleavage rate-limiting; use 6 equiv reagent |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Riley oxidation (SeO₂ allylic oxidation)?
SeO₂ (cat. with t-BuOOH, or stoich.). Conditions: Dioxane or EtOH, 25–100 °C.
Which bonds are formed and which are broken?
Formed: C–O (allylic alcohol), C=C shifted twice (net unchanged). Broken: allylic C–H, Se–C.
What is the stereochemical outcome?
Ene step is suprafacial; [2,3] via chair-like TS gives E-alkenes
What controls the rate or the selectivity?
Ene reaction is rate-limiting; allylic C–H selective
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