Alkene oxidation — syn-diol or cleavage
KMnO₄ oxidation of an alkene
How the KMnO₄ oxidation of an alkene 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: cold dilute KMnO₄, OH⁻ (diol); hot conc. KMnO₄ (cleavage). Conditions: 0–5 °C, basic, vs. heat, acid. Bonds formed: two C–O (syn diol) or C=O; bonds broken: C=C π (and C–C on cleavage). Stereochemistry: Syn addition (cis diol) in cold conditions. Rate and selectivity: Cycloaddition fast; Mn(VII) → Mn(V) → MnO₂.
At a glance
Step 1: [3+2] cycloaddition
The alkene's π electrons attack manganese in a concerted [3+2] cycloaddition, exactly like the OsO₄ mechanism in Sharpless dihydroxylation, forming a five-membered cyclic manganate ester across both alkene carbons.
Step 2: Cold, basic: hydrolysis to syn diol
Under cold, dilute, basic conditions, hydrolysis of this manganate ester simply releases a 1,2-diol with syn stereochemistry — the same outcome as OsO₄ dihydroxylation, just with a cheaper, more common oxidant.
Step 3: Hot, conc.: oxidative cleavage
Under hot, concentrated conditions, the initially formed diol is oxidized further, cleaving the C–C bond entirely — a disubstituted alkene carbon becomes a ketone, while a monosubstituted (terminal) alkene carbon is oxidized all the way to CO₂ and water. This harsher cleavage pathway is a classic way to determine an alkene's substitution pattern from its oxidation products.
Abbreviations and terms
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- OsO4
- Osmium tetroxide. Adds syn across alkenes to give diols (via a cyclic osmate ester).
- KMnO4
- Potassium permanganate. Strong oxidant: diols (cold, basic) or cleavage (hot).
How it compares with related reactions
| KMnO₄ oxidation of an alkene | Lemieux-Johnson oxidation | Baeyer-Villiger oxidation of a ketone | |
|---|---|---|---|
| Reagents | cold dilute KMnO₄, OH⁻ (diol); hot conc. KMnO₄ (cleavage) | Alkene, OsO₄ (cat., 1–5 mol%), NaIO₄ (≥ 4 equiv.), 2,6-lutidine (optional) | mCPBA or CH₃CO₃H (peracid) |
| Conditions | 0–5 °C, basic, vs. heat, acid | Dioxane/H₂O or THF/H₂O, 25 °C | CH₂Cl₂, 25 °C; cat. acid helps |
| Stereochemistry | Syn addition (cis diol) in cold conditions | Alkene geometry lost; α-stereocentres retained | Retention at the migrating carbon |
| Rate / selectivity | Cycloaddition fast; Mn(VII) → Mn(V) → MnO₂ | Osmylation is fast; NaIO₄ turns over Os(VI) → Os(VIII) | Migratory aptitude: tert > cyclohexyl ≈ sec > Ph > prim > Me |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the KMnO₄ oxidation of an alkene?
cold dilute KMnO₄, OH⁻ (diol); hot conc. KMnO₄ (cleavage). Conditions: 0–5 °C, basic, vs. heat, acid.
Which bonds are formed and which are broken?
Formed: two C–O (syn diol) or C=O. Broken: C=C π (and C–C on cleavage).
What is the stereochemical outcome?
Syn addition (cis diol) in cold conditions
What controls the rate or the selectivity?
Cycloaddition fast; Mn(VII) → Mn(V) → MnO₂
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