Inorganic — asymmetric catalysis (radical)
Jacobsen-Katsuki epoxidation
How the Jacobsen-Katsuki epoxidation 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: Chiral Mn(III)–salen (2–10 mol%), NaOCl (bleach) or mCPBA, alkene. Conditions: CH₂Cl₂, 0 °C, optional pyridine N-oxide. Bonds formed: C–O ×2 (epoxide); bonds broken: C=C π; Mn=O π. Stereochemistry: High ee for cis-alkenes; salen pocket selects the face. Rate and selectivity: Oxygen transfer to alkene is rate-limiting.
At a glance
Step 1: Bleach oxidises Mn(III) to Mn(V)=O
A chiral Mn(III)-salen complex is oxidized by a terminal oxidant (like bleach) to a high-valent Mn(V)-oxo species — an electrophilic, metal-bound oxygen held rigidly within the salen ligand's chiral pocket.
Step 2: Alkene transfers one electron: radical intermediate
The alkene approaches this oxo species, and a single electron transfers from the alkene to manganese, generating a short-lived radical intermediate rather than a fully concerted oxygen transfer — the chiral salen ligand's steric bulk controls which face of the alkene can approach closely enough to react.
Step 3: Radical collapse closes the epoxide
Radical collapse (ring closure) delivers the epoxide with high enantioselectivity, reducing manganese back to Mn(III) — unlike Sharpless epoxidation, this method works on simple, unfunctionalized alkenes that lack a directing hydroxyl group.
Abbreviations and terms
- Ph
- Phenyl. –C₆H₅.
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- mCPBA
- meta-Chloroperoxybenzoic acid. Peracid that transfers an electrophilic oxygen (epoxidation, Baeyer–Villiger).
- ee
- Enantiomeric excess. How much more of one enantiomer than the other is present.
How it compares with related reactions
| Jacobsen-Katsuki epoxidation | Asymmetric phase-transfer catalysis | Crabtree hydrogenation | |
|---|---|---|---|
| Reagents | Chiral Mn(III)–salen (2–10 mol%), NaOCl (bleach) or mCPBA, alkene | Chiral cinchona-derived Q*⁺Br⁻ (1–10 mol%), 50% aq. KOH, glycine imine tBu ester, BnBr | [Ir(cod)(PCy₃)(py)]⁺ PF₆⁻, H₂ (1 atm) |
| Conditions | CH₂Cl₂, 0 °C, optional pyridine N-oxide | Toluene/CH₂Cl₂, 0 °C, vigorous stirring | CH₂Cl₂, 25 °C |
| Stereochemistry | High ee for cis-alkenes; salen pocket selects the face | High ee: chiral ion pair shields one enolate face | Face selectivity set by the directing OH |
| Rate / selectivity | Oxygen transfer to alkene is rate-limiting | Alkylation of the ion pair is rate-limiting | Alkene insertion/oxidative addition; works for tetrasubstituted alkenes |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Jacobsen-Katsuki epoxidation?
Chiral Mn(III)–salen (2–10 mol%), NaOCl (bleach) or mCPBA, alkene. Conditions: CH₂Cl₂, 0 °C, optional pyridine N-oxide.
Which bonds are formed and which are broken?
Formed: C–O ×2 (epoxide). Broken: C=C π; Mn=O π.
What is the stereochemical outcome?
High ee for cis-alkenes; salen pocket selects the face
What controls the rate or the selectivity?
Oxygen transfer to alkene is rate-limiting
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam